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Population dynamics and life history traits of the ‘giant’ limpetScutellastra laticostataon intertidal limestone platforms at Rottnest Island, Western Australia, were recorded by interannual (January/February) monitoring of limpet density and size structure, and relocation of marked individuals, at 3 locations over periods of 13-16 yr between 1993 and 2020. Limpet densities ranged from 4 to 9 ind. m-2on wave-swept seaward margins of platforms at 2 locations and on a rocky notch at the landward margin of the platform at a third. Juvenile recruits (25-55 mm shell length) were present each year, usually at low densities (-2), but localized pulses of recruitment occurred in some years. Annual survival rates of marked limpets varied among sites and cohorts, ranging from 0.42 yr-1at the notch to 0.79 and 0.87 yr-1on the platforms. A mass mortality of limpets on the platforms occurred in 2003, likely mediated by thermal stress during daytime low tides, coincident with high air temperatures and calm seas. Juveniles grew rapidly to adult size within 2 yr. Asymptotic size (L∞, von Bertalanffy growth model) ranged from 89 to 97 mm, and maximum size from 100 to 113 mm, on platforms. Growth rate and maximum size were lower on the notch. Our empirical observations and simulation models suggest that these populations are relatively stable on a decadal time scale. The frequency and magnitude of recruitment pulses and high rate of adult survival provide considerable inertia, enabling persistence of these populations in the face of sporadic climatic extremes.

Coral reefs are critically important marine ecosystems that are threatened worldwide by cumulative impacts of global climate change and local stressors. The Solomon Islands comprise the southwestern boundary of the Coral Triangle, the global center of coral diversity located in the Indo-Pacific, and represent a bright spot of comparatively healthy coral reef ecosystems. However, reports on the status of coral reefs in the Solomon Islands are based on monitoring conducted at 5 stations in 2003–2004 and 2006–2007, with no information on how corals in this region have responded to more recent global bleaching events and other local stressors. In this study, we compare reef condition (substrate composition) and function (taxonomic and morphological diversity of hard corals) among 15 reefs surveyed in the Western Province, Solomon Islands that span a range of local disturbance and conservation histories. Overall, we found high cover of live hard coral (15–64%) and diverse coral assemblages despite an unprecedented 36-month global bleaching event in the three years leading up to our surveys in 2018. However, there was significant variation in coral cover and diversity across the 15 reefs surveyed, suggesting that impacts of global disturbance events are moderated at smaller scales by local anthropogenic factors (fisheries extraction, land-use impacts, marine management) and environmental (hydrodynamics) conditions. Our study provides evidence that relatively healthy reefs persist at some locations in the Solomon Islands and that local stewardship practices have the potential to impact reef condition at subregional scales. As coral reef conservation becomes increasingly urgent in the face of escalating cumulative threats, prioritising sites for management efforts is critical. Based on our findings and the high dependency of Solomon Islanders on coral reef ecosystem services, we advocate that the Western Province, Solomon Islands be considered of high conservation priority.

El Niño Southern Oscillation events (ENSO) and the subsequent opposite weather patterns in the following months and years (La Niña) have major climatic impacts, especially on oceanic habitats, affecting breeding success of both land and sea birds. We assessed corticosterone concentrations from blood samples during standardized protocols of capture, handling and restraint to simulate acute stress from 12 species of Galapagos Island birds during the ENSO year of 1998 and a La Niña year of 1999. Plasma levels of corticosterone were measured in samples collected at capture (to represent non-stressed baseline) and subsequently up to 1 h post-capture to give maximum corticosterone following acute stress, and total amount of corticosterone that the individual was exposed to during the test period (integrated corticosterone). Seabird species that feed largely offshore conformed to the brood value hypothesis whereas inshore feeding species showed less significant changes. Land birds mostly revealed no differences in the adrenocortical responses to acute stress from year to year with the exception of two small species (

The invasive seagrass Halophila stipulacea has spread throughout the eastern Caribbean since it was first recorded in Grenada in 2002. We quantified the distribution and abundance of H. stipulacea , and its associated macroinvertebrate fauna, in sampling stations and transects around the island of Carriacou (a nearby dependency of Grenada) in early 2016. Halophila stipulacea occurred in extensive monospecific stands (average bottom cover, 62%), or smaller mixed stands with native seagrass ( Thalassia testudinum , Syringodium filiforme , Halodule wrightii ), at 1–5 m depth in large bays along the leeward (west) coast. It was sparsely distributed on the more wave-exposed east and south coasts, usually in mixed patches with native seagrass. In leeward bays, H. stipulacea has largely replaced the native seagrass H. wrightii , providing a novel biogenic habitat for various filter-feeding invertebrates living within the turf-like leaf canopy (e.g., sponges, ascidians, bivalves, ophiuroids), and sea urchins (mainly Tripneustes ventricousus ) and a microphagous sea star ( Oreaster reticulatus ) that graze upon it. Populations of the sea star consisted mainly of juveniles indicating the seagrass may serve as a nursery habitat for this endangered species. The spread of H. stipulacea along the leeward coast of Carriacou in recent years represents a community-level shift in the shallow subtidal zone, with attendant changes in habitat structure, species composition, and trophic interactions.

Understanding processes that drive sudden shifts in ecosystem structure and function has become an important research focus for coastal management. In kelp bed ecosystems, regime shifts occur when high densities of sea urchins destructively graze kelp and create coralline algal barrens. While the importance of predation and disease in mediating shifts between kelp beds and barrens on shallow rocky reefs has been well documented, little is known about the role of deep-living urchins in these alternative stable-state dynamics. In this study we test the hypothesis that deep-living urchins along the central Atlantic coast of Nova Scotia move onshore and trigger shifts from kelp beds to barrens on shallow rocky reefs. We documented urchin distribution and abundance using tow-camera surveys down to 140 m depth and spanning 140 km of coast and created a predictive species-distribution model using these observations and spatial data on environmental factors that likely delineate suitable habitat for urchins. We used a random forest model to generate our predictions, which correctly classified 91% of observations into a positive or negative occurrence of urchins. Sea urchins predominantly occurred within 1.5 km of shore, in depressions and flat habitats between 40 and 85 m depth. We found that shallow regions where destructive grazing fronts have been documented over the past 4 decades were closer to deep-living sea urchin habitats compared to regions that remained in a kelp bed state during the same period. This supports our prediction that deep-living urchins play an important role in driving shallow regime shift dynamics, and indicates that their distribution can help identify areas of coast that are most vulnerable to a collapse to barrens. This article is protected by copyright. All rights reserved.

During mass spawning of tropical corals, extremely dense accumulations of eggs develop throughout the water column, the fate of which depends on physical conditions, such as topography, currents, and prevailing winds. We measured egg transport [mainly acroporid corals and the palolo worm, Eunice viridis (Gray, 1847)] during a mass spawning at Ningaloo Reef, Western Australia, in April 2010. We tracked changes in egg and embryo abundance in the lagoon over five consecutive days with plankton tows at the sea surface immediately after spawning and four and 16 hours later, and characterized the flow regime with drifter deployments. Egg dispersal was rapid with a pronounced decrease in concentration within four hours of spawning, from approximately 106 to 101 and 104 m–3 for corals and E. viridis, respectively, on the peak date; few eggs or embryos remained within the lagoon after 16 hrs. Changes in concentration of eggs and embryos along transects inside the lagoon and perpendicular to the reef flat reflected the local hydrodynamics and high flushing rate, providing empirical support for a biophysical model from the same site.

Small invertebrate grazers can disproportionately reduce plant fitness by discriminately consuming valuable tissues, but the context and attendant consequences of this activity at higher levels of ecological organization rarely are considered. To assess the impact of a gastropod mesograzer Lacuna vincta on fecundity and potential reproductive output of the habitat-forming kelp Saccharina latissima, we measured the intensity and distribution of grazing damage on kelp blades at five sites of varying kelp density, during the annual reproductive peak (October-November) in Nova Scotia. We found most grazing damage on reproductive individuals consisted of superficial excavations, and was concentrated on the central sorus (region where sporangia develop) compared to the vegetative blade margins. Grazing intensity on sori (percent grazed) averaged 29.6% across sites and sampling periods. The distribution of grazing on non-reproductive individuals was opposite to that of reproductive ones, indicating that snails shift feeding from blade margins to the center as sori develop. Choice and no-choice feeding assays in the laboratory revealed that focused grazing on sori is likely due to an active feeding preference for sporogenous over vegetative tissue. This preference was correlated with the distribution of chemical defense between tissues (phlorotannin content was ~2.5 times higher in vegetative tissue than sori), but not nutritional quality (no difference in C/N ratio). We deduce, with support from histological observations, that consumption of sorus tissue by L. vincta reduces fecundity of S. latissima. Extrapolating our results to estimate potential reproductive output within kelp beds suggests that spore supply and recruitment limitation may be predominantly imposed by the scarcity of reproductive individuals in the most degraded kelp beds. However, loss of reproductive output to grazing could extend recruitment limitations that impede recovery of waning kelp populations in Nova Scotia.

The movement of resource subsidies across natural systems can have important effects on recipient communities and has emerged as a key research area in ecology. Detrital subsidies are critical in marine ecosystems where communities are reliant on external sources of primary production, yet few studies have quantified the spatial extent of drift algae at coastal scales. Using observations of the seafloor (up to 140 m depth) from tow-camera surveys along 145 km of Nova Scotia coast, and bathymetric data of this region, we created the first predictive map of drift subsidy in a marine ecosystem. We used a random forest model to generate our predictions, which correctly classified 95 % of observations into a presence or absence of drift. Distance from source, slope, and bathymetric position index (elevation relative to surrounding landscape) was the main predictor variables of the occurrence of drift. Drift algae occurred across a range of benthic habitats within our study area, but most frequently within 1.4 km of the coast on flat bottoms or in regions with zero or negative bathymetric position index. Such areas were coincident with seafloor depressions and flat low-energy habitats. Repeated observations at some locations indicated that areas with steep slopes or large curvature tended to have variable patterns of drift compared to areas with little or no slope or curvature. We predict that deep subtidal environments receiving drift subsidy will be impacted by the declines in kelp biomass projected for this region (and others) due to changes in ocean climate.

Low recruitment due to limitations of propagule supply or post-settlement survival reinforces dominance of turf algal assemblages that replace canopy algae following large-scale losses. However, post-recruitment processes that hinder juvenile growth and survival (epiphytic overgrowth, grazing, physical stress) also could impede recovery. To evaluate the contribution of recruitment, growth, and survival of young sporophytes to recovery of degraded kelp populations and key factors driving post-recruitment tissue loss and mortality, we followed cohorts of juvenile kelp Saccharina latissima at two defoliated sites in Nova Scotia. We also monitored kelp recruitment, abundance, size structure, and macroalgal composition for 5.5 years. Recruit densities were an order of magnitude lower compared to previous studies in the region. Large decreases in blade area of juveniles were related to cover by the invasive bryozoan Membranipora membranacea, grazing by small snails Lacuna vincta, and warm seawater temperatures. Cohort survival was low (time to 50% mortality 2.5–5.5 months) and increased risk of death was directly related to bryozoan encrustation. Modest seasonal or interannual gains in kelp abundance were lost during periods of peak temperature, which showed a warming trend during the study, favouring persistence of widespread turf-forming, opportunistic and invasive algae. We conclude that low recruitment success, high rate of tissue loss relative to growth, and high mortality, inhibited kelp recovery. Impacts of epiphytic overgrowth, grazing, and warm temperatures on these processes highlight the need to protect intact kelp populations, growing in favourable conditions, to maintain positive interactions that increase resilience to undesirable regime shifts.

Allostichaster polyplax is a fissiparous (capable of reproducing asexually by fission) sea star found in intertidal and shallow subtidal habitats in New Zealand and Australia. A localized population of A. polyplax occurred at a density of 2.7 individuals m−2 in an intertidal boulder field at Otago Harbour, South Island, New Zealand. The percentage of sea stars undergoing fission, recorded from January 2010 to March 2011, peaked in January (midsummer) in both years. The frequency of fission decreased for larger sea stars (mean arm length, R > 25 mm), suggesting an ontogenic shift from asexual to sexual reproduction. Effects of fission (i.e. arm regeneration) and food availability (small gastropods) on growth, storage and sexual reproduction were examined in a factorial laboratory experiment. There was no effect of arm regeneration or food on total arm growth. Storage capacity, measured as pyloric caeca index (PCI), was greater in full-armed than regenerating sea stars, and greater in fed than starv...

Along the Atlantic coast of Nova Scotia sea urchin disease outbreaks are statistically linked to North Atlantic hurricanes and warm sea temperatures. The amoebic pathogen Paramoeba invadens, which causes these disease outbreaks, is unable to withstand typical minimum sea temperatures along this coast, suggesting that it is reintroduced during periods of peak temperatures. Here, we examine hypotheses for mechanisms of introduction or persistence of P. invadens using data on sea urchin disease outbreaks from a 5-yr field experiment (2010–2014), in combination with high-temporal-resolution oceanographic and meteorologic data. Disease outbreaks were observed in 4 yr, with the onset of mass mortality (≥50% morbidity or mortality) ranging from mid August to mid October. Physical data suggest that P. invadens originates in warm offshore surface waters that are transported to the coast during strong storms. Disease outbreaks were associated with passing hurricanes in 2 yr (2010 and 2011). In 2012, a disease outbreak occurred in the absence of a storm and following a strong positive anomaly in winter sea temperature, suggesting survival of the pathogen from 2011. In 2014, a disease outbreak occurred in association with a strong storm (nor'easter) that was not a hurricane. Our findings indicate that changing environmental conditions of increasing minimum sea temperatures and increasing intensity of storms may be altering the dynamics of this disease. These inferences remain equivocal, however, indicating the need for high-resolution dynamical modeling of the physical environment and rapid identification of P. invadens in the environment using genetic tools.

article i nfo Disease outbreaks in the sea urchin Strongylocentrotus droebachiensis on the Atlantic coast of Nova Scotia have increased in frequency over the last 35 years, in association with increasing sea temperatures and strong storm events. Previous studies suggest that the causative agent of disease, the pathogenic amoeba Paramoeba invadens, cannot withstand winter sea temperatures on this coast, and may be an alien species. Consequently, recurrent epizootics result from repeated introductions of this pathogen, possibly due to advection or vertical mixing asso- ciated with storm events. Here we test an alternative hypothesis that P. invadens has established resident popula- tions, either through physiological adaptation to low winter temperatures, or because winter minima have increased since 1980. To test this hypothesis, we manipulated culture temperatures of P. invadens isolated from moribund sea urchins during a disease outbreak in 2012. Specific growth rates of P. invadens in culture at 0.5 to 18.0 °C were similar to those observed in the early 1980s, providing no evidence of physiological adaptation. P. invadens declined to extinction in culture after ~1 month at 0.5 °C and 2.0 °C, but survived for 3 months at 3.5 °C, as indicated by recovery of cultures following transfer to optimal temperature. These results indicate a lower temperature tolerance threshold around 2-3 °C, which is consistent with observational data of recurrent mass mortalities of sea urchins in 1983 and 2012 in the absence of a storm but following relatively warm winters. A trend of increasing minimum temperatures in winter indicates that P. invadens could undergo a range expansion to establish a resident population in Nova Scotia within 1-2 decades. This is expected to increase the resilience of the kelp-bed state of the benthic ecosystem but result in the complete collapse of the sea urchin fishery.

We examined the potential for a trait-mediated indirect interaction (TMII), whereby a behavioural response to a predator (crab Cancer borealis ) alters the per capita effect of a reacting species (sea urchin Strongylocentrotus droebachiensis ) on its algal food (kelp Saccharina latissima ), in field and laboratory experiments. We used time-lapse video to monitor sea urchin movement in response to either an attractant food cue (kelp) or a repellant predation-risk cue (crabs) placed in small cages at the centre of 7 m 2 circular plots in a wave-exposed sea urchin barrens off Nova Scotia. We found that sea urchins generally moved randomly over the rock substratum and showed a limited response to cues emanating from a cage with kelp, with a few individuals exhibiting directed movement towards kelp and greater proportion of urchins in contact with the cue cage (with kelp) compared to the control (without kelp) at the end of the experiment. Similarly, only a few individuals that came in close contact with a cage containing crabs showed a clear flight response, compared to the control. In experiments with individual kelp thalli that were systematically dispersed around the central cage in the same plots, we found that the presence of crabs in the cage did not reduce sea urchin grazing rates on kelp compared to a control without crabs. We observed similar results using a similar experimental design in large tanks (900 L) under controlled flow conditions in the laboratory. We conclude that C. borealis does not significantly inhibit the foraging activity of S. droebachiensis under turbulent flow conditions in nature, and that a TMII is unlikely to contribute to a trophic cascade in the shallow rocky subtidal zone of the Northwest Atlantic.

The coralline red algae, Lithothamnion glaciale, Phymatolithon laevigatum, P. rugulosum, and Corallina officinalis, induced >85% of laboratory-reared larvae of Strongylocentrotus droebachiensis to metamorphose. Larvae must contact live L. glaciale or its spores for metamorphosis to occur; the inducer is not sensed in the water column. However, aqueous extracts of L. glaciale can induce metamorphosis, suggesting that the inducing factor is chemical. Neither ashed nor boiled L. glaciale induces metamorphosis, indicating that the factor is heat-labile and that thigmotaxis, per se, is not important in the response. The amino-acid, γ-aminobutyric acid (GABA), which induces settlement of other marine invertebrate larvae, also induces significant rates of metamorphosis of S. droebachiensis at concentrations ≥ 10-4 M. A reduction (with antibiotics) in the number of live bacteria on the surface of L. glaciale does not affect the rate of metamorphosis of larvae.

We examined how large-scale distur- bances that defoliate kelp beds (outbreaks of an invasive bryozoan, hurricanes) alter local-scale grazing dynamics of an abundant herbivore, the gastropod Lacuna vincta, on the Atlantic coast of Nova Scotia, Canada. From field observations and a 5 wk kelp-thinning experiment that simulated dis- turbance, we found that snail density and grazing intensity on the kelp Saccharina latissima increased non-linearly with decreasing kelp biomass, as it varied within a site. Grazing intensity on S. latissima also increased non-linearly with decreasing stand- ing kelp biomass across 5 sites spanning 40 km (linear distance) of coast and 2 yr, but we did not find strong support for this relationship for the kelp Laminaria digitata. Intensification of grazing aug- ments the indirect effect of L. vincta on S. latissima (increased blade erosion and fragmentation), and drives it beyond a threshold for further losses of kelp biomass with subsequent storms. This positive feedback between large-scale disturbances and local-scale grazing could reinforce the depletion of kelp and facilitate the establishment of turf-forming algae on Nova Scotian rocky reefs. We conclude that interactions of large external perturbations with local natural perturbations must be considered to under stand how drivers of ecosystem change collec- tively disrupt the balance of top-down and bottom- up forces to cause shifts to unexpected community states.

Highly productive kelp beds off Nova Scotia, Canada export detrital material to adja- cent sedimentary habitats in deeper waters. We evaluated the importance of this subsidy to sea urchins Strongylocentrotus droebachiensis in less productive habitats. Gonad index and gut con- tent of urchins on sediments at 60 m depth were measured monthly from September 2010 to May 2013 in a large semi-protected bay, and compared to concurrent measurements of sea urchins at 8 m depth off a nearby headland. Detrital algal cover in the bay was measured using a towed cam- era system. Seasonal patterns in algal deposition and gut contents of deep-living urchins indicated that detritus was the predominant food source from March to October. We observed lower gonad indices in deep-living sea urchins compared to those at 8 m, but deep-living urchins spawned out- side of the main spring season, suggesting periodic detrital subsidy enhances their reproductive condition. Food availability and temperature appear to control the timing of gametogenesis, and phytodetritus may cue gamete release in these deep habitats. To evaluate the response of deep- living urchins to detrital subsidy, and turnover time of this material, we deployed lines baited with kelp thalli at 45 and 60 m depth and measured rates of encounter and consumption of kelp detritus by sea urchins, using the towed camera. Sea urchins aggregated on kelp within hours of deploy- ment, and consumed it after 3 to 5 mo. Our findings suggest kelp detritus represents an important energy source that can support a high reproductive capacity in deep-living urchins.

Predation is an important agent of post-settlement mortality of sea urchins that is mediated by the availability and suitability of spatial refuges, particularly during the vulnerable juvenile stage. In laboratory and field caging experiments, we show that holdfasts of a dominant kelp, Saccharina latissima, provide a spatial refuge for juvenile sea urchins Strongylocentrotus droebachiensis (< 20 mm, test diameter) from crabs Cancer borealis and C. irroratus, considered to be the dominant predators of sea urchins in kelp-bed ecosystems in the northwestern Atlantic. In treatments with individual crabs of either species, the presence of holdfasts reduced predation on juvenile sea urchins by ~20 to 30% compared to treatments with no refuge. Crabs consumed juve- niles (from 5 to 19 mm) in each of three 5 mm size classes in proportion to their abundance, regard- less of treatment. In kelp beds in St. Margarets Bay, Nova Scotia, Canada, the number of juvenile sea urchins per holdfast ranged from 0.3 to 0.9, with juveniles in holdfasts accounting for two- thirds of the total urchin population density at one site. Up to 4 juveniles occurred within a single holdfast, and there was a significant positive relationship between juvenile size (but not number) and holdfast volume. Small adult sea urchins were not found within holdfasts in kelp beds and rarely occupied holdfasts presented to them in laboratory cages. Our findings indicate an ontoge- netic shift in sea urchin-kelp interactions, whereby kelp facilitates recruitment of its major grazer.

Outbreaks of disease in herbivorous sea urchins have led to ecosystem phase shifts from urchin barrens to kelp beds (forests) on temperate rocky reefs, and from coral to macroalgal-dominated reefs in the tropics. We analyzed temporal patterns in epizootics that cause mass mortality of sea urchins, and consequent phase shifts, based on published records over a 42-year period (1970–2012). We found no evidence for a general increase in disease outbreaks among seven species of ecologically important and intensively studied sea urchins. Periodic waves of recurrent amoebic disease of Strongylocentrotus droebachiensis in Nova Scotia coincide with periods when the system was in a barrens state and appear to have increased in frequency. In contrast, following a major epizootic that decimated Diadema antillarum throughout the Caribbean in 1983, subsequent outbreaks of disease were highly localized and none have been reported since 1991. Epizootics of Strongylocentrotus in the NW Atlantic and NE Pacific, and Paracentrotus and Diadema in the eastern Atlantic, have been linked to climate change and overfishing of sea urchin predators. The spatial extent of recurrent disease outbreaks in these species, and the frequency of phase shifts associated with these epizootics, has decreased over time due to the expansion of the macroalgal state and its stabilization through positive feedback mechanisms. Longitudinal studies to monitor disease outbreaks in sea urchin populations and improved techniques to identify causative agents are needed to assess changes in the frequency and extent of epizootics, which can profoundly affect the structure and functioning of coastal marine ecosystems.

The stalked tunicate Boltenia ovifera is widely distributed in the Arctic and North Atlantic oceans on rocky substrata at 10–300 m depth, although its ecological role in benthic communities is poorly understood. The distribution and abundance of B. ovifera were recorded at 10–100 m depth in towed-video transects in November 2011 and in June, July, and November 2012 off a wave-exposed headland near Halifax, Nova Scotia. Specimens also were collected at 30 m depth using SCUBA (44°26.88′N, 63°31.59′W) in September 2012. Areas dominated by B. ovifera had densities of 10–100 individuals m−2 on rocky substrata at 30–60 m depth. These tunicate beds often were associated with sparse kelp (Agarum clathratum) and turf-forming red algae. A generalized additive model indicated that depth, substrate type, and benthic algal type were strong predictors of tunicate abundance. Twenty-two epibiotic species were found on specimens of B. ovifera, including juvenile conspecifics. Filter-feeding macroinvertebrates, including anemones and soft corals, were more abundant in areas with B. ovifera than in areas without these tunicates. Our findings suggest that beds of B. ovifera can act as biogenic habitat to enhance local species richness in the rocky subtidal zone of Nova Scotia.

In Nova Scotia, Canada, periodic outbreaks of amoebic disease (paramoebiasis) cause mass mortality of sea urchins Strongylocentrotus droebachiensis in subtidal barrens. How- ever, in kelp beds, where sea urchins are cryptic and generally less dense than in barrens, disease outbreaks are not readily observed and the importance of disease in regulating these populations is unknown. To determine whether sea urchin populations in kelp beds are controlled by disease, we analyzed population data from kelp beds at a single location (St. Margarets Bay) across a span of 44 yr (1968 to 2012) to compare changes in size structure and density in relation to the timing of disease outbreaks in adjacent sea urchin aggregations and barrens. We found that sea urchin den- sity, maximum test diameter and percentage of adults decreased following disease outbreaks and increased during intervening periods without disease, indicating that disease regulates the popu- lation in kelp beds by limiting survival to adulthood. Our results suggest that disease has replaced predation as a major agent controlling sea urchin populations in Nova Scotian kelp beds.

The exchange of energy and nutrients between ecosystems (i.e., resource subsidies) plays a central role in ecological dynamics over a range of spatial and temporal scales. Little attention has been paid to the role of anthropogenic impacts on natural systems in altering the magnitude, timing, and quality of resource subsidies. Kelp ecosystems are highly productive on a local scale and export over 80% of kelp primary production as detritus, subsidizing consumers across broad spatial scales. Here, we generate a model of detrital production from a kelp bed in Nova Scotia to hindcast trends in detrital production based on temperature and wave height recorded in the study region from 1976 to 2009, and to project changes in detrital production that may result from future climate change. Historical and projected increases in temperature and wave height led to higher rates of detrital production through increased blade breakage and kelp dislodgment from the substratum, but this reduced kelp biomass and led to a decline in detrital production in the long-term. We also used the model to demonstrate that the phase shift from a highly productive kelp bed to a low-productivity barrens, driven by the grazing activity of sea urchins, reduces kelp detrital production by several orders of magnitude, an effect that would be exacerbated by projected increases in temperature and wave action. These results indicate that climate-mediated changes in ecological dynamics operating on local scales may alter the magnitude of resource subsidies to adjacent ecosystems, affecting ecological dynamics on regional scales.

Sea urchin barrens are benthic communities on rocky subtidal reefs that are dominated by urchins and coralline algae; in the absence of intense herbivory by urchins, these barrens support luxuriant seaweed communities such as kelp beds (or forests). Barrens can extend over 1000s of km of coastline or occur in small patches (10s to 100s of m) within a kelp bed. They are characterized by low primary productivity and low food-web complexity relative to kelp communities and are generally considered a collapsed state of the kelp ecosystem. To assess the stability of sea urchin barrens and potential for return to a kelp-dominated state, we document temporal and spatial patterns of occurrence of barrens along temperate and polar coasts. We examine the various drivers of phase (or regime) shifts in these areas, the threshold levels of urchin abundance that trigger abrupt changes in ecosystem state, and the feedback mechanisms that stabilize each state. Although longitudinal (decadal) studies are limited, we find evidence in several regions that transitions between barrens and kelp beds are characterized by discontinuous phase shifts, with different thresholds for forward (to barrens) and reverse (to kelp beds) shifts, in accordance with alternative stable-state dynamics. In other areas, barrens may reflect regime shifts associated with large-scale oceanographic changes. Accelerating climate change and increasing anthropogenic impacts play important roles in altering alternative stable-state dynamics and triggering phase shifts. Recovery of the kelp state may be possible through management or remediation measures, but this necessitates a clear understanding of the thresholds and stabilizing factors for a given system.

The epiphytic bryozoan Membranipora membranacea encrusts the surface of kelp blades, causing recurrent large-scale defoliation events in kelp beds off the Atlantic coast of Nova Scotia, Canada. The gastropod Lacuna vincta grazes kelp, creating perforations that weaken blade tissues and increase the fragmentation rate. We assess the interaction between M. membranacea and L. vincta by measuring the grazing rate of snails on bryozoan-encrusted and non-encrusted kelp (Saccharina latissima) in no-choice and choice experiments in the laboratory conducted in November and December 2010. There was no effect of diet on grazing rate in no-choice experiments. In choice experiments, however, L. vincta grazed significantly more non-encrusted than encrusted kelp (7.1 versus 1.1 mg snail−1 d−1), and grazing rate of non-encrusted kelp was almost twice that in the no-choice experiment (3.8 mg snail−1 d−1), indicating that snails may avoid colonies of M. membranacea on partially encrusted kelp blades. We found no effect of diet on growth, reproduction and survival of snails maintained for four weeks on encrusted or non-encrusted kelp. By concentrating grazing damage on non-encrusted areas of blades, L. vincta may act synergistically with M. membranacea to increase the likelihood of blade breakage and canopy loss. This indirect effect of the invasive bryozoan could augment its direct effect on the standing biomass of native kelp beds and detrital export to adjacent communities.

The large amount of fish feed involved in salmon aquaculture results in high nutrient loading to the surrounding environment. The near-field effects of this loading are well documented but the full zone of influence, or far-field effect, remains poorly understood. We investigated the potential of the carotenoid canthaxanthin, a common additive in farmed Atlantic salmon diets, as a biochemical tracer to identify this zone of influence in two benthic invertebrate species that are locally abundant around salmon farms: the blue mussel Mytilus edulis (or Mytilus trossulus ), a suspension feeder, and the green sea urchin Strongylocentrotus droebachiensis , an omnivorous grazer. To measure persistence of canthaxanthin in the digestive gland of mussels or the gonad of sea urchins, both species were individually fed three levels of salmon feed (6, 13, 26 mg d − 1 for mussels; 70, 150, 300 mg d − 1 for sea urchins) for 13 d to measure pigment uptake rates, followed by a non-pigmented diet for 7 d to measure pigment loss rates. Mussels exhibited a relatively rapid uptake of canthaxanthin (1 to 3 d), which subsequently declined to zero within 3 d following cessation of the pigmented diets. The sea urchins exhibited slower initial uptake (4 to 10 d) and the pigment signal lasted up to 46 d, suggesting retention of canthaxanthin. To examine the scale of dispersion of feed-derived particulate material in nature, canthaxanthin uptake by sea urchins was measured at fixed intervals along a transect extending 1 km from a salmon cage in Passamaquoddy Bay, New Brunswick, Canada. Pigment concentration in the gonad dropped from ~ 5 to 0.5 μg g − 1 within the first 100 m from the cage and remained at this low level along the remainder of the transect, likely reflecting low background levels of salmon farm-derived particles within the bay. Our study demonstrates the potential of canthaxanthin as an organic tracer for salmon aquaculture, and the use of blue mussel digestive gland for short-term detection and sea urchin gonad for long-term detection, in accordance with the rapid ingestion and nutrient storage functions of the respective tissues.

The flow of detritus between habitats is an important form of connectivity that affects regional productivity and the spatial organization of marine ecosystems. Kelps form highly productive beds or forests that produce detritus through incremental blade erosion, fragmentation of blades, and dislodgement of whole fronds and thalli. Rates of detrital production range from 8 to 2657 g C m !2 yr !1 for blade erosion and fragmentation, and from 22 to 839 g C m !2 yr !1 for loss of fronds and thalli. The estimated global average rate of detrital production by kelps is 706 g C m !2 yr !1 , accounting for 82% of annual kelp productivity. Detrital production rates are regulated by current and wave-driven hydrodynamic forces and are highest during severe storms and follow- ing blade weakening through damage by grazers and encrusting epibionts. Detritus settles within kelp beds or forests and is exported to neighboring or distant habitats, including sandy beaches, rocky intertidal shores, rocky and sedimentary subtidal areas, and the deep sea. Exported kelp detritus can provide a significant resource subsidy and enhance secondary production in these communities ranging from tens of meters to hundreds of kilometers from the source of production. Loss of kelp biomass is occurring worldwide through the combined effects of climate change, pol- lution, fishing, and harvesting of kelp, which can depress rates of detrital production and subsidy to adjacent communities, with large-scale consequences for productivity.

Invasive species have the potential to alter the dynamics of detrital subsidy from high to low productivity areas through changes in quantity and nutritional quality of detrital material. We examined the effect of the invasive alga Codium fragile ssp. fragile on the nature of detrital export from subtidal algal beds off Nova Scotia, Canada, by comparing changes in mass, nutri- tional quality (%C, %N, C/N ratio), concentration of dimethylsulfoniopropionate (DMSP, a sec- ondary metabolite that deters grazers), and isotopic composition (δ 13 C and δ 15 N) between C. frag- ile and the native kelp Saccharina latissima at 4 wk intervals over 16 wk of degradation in cages on a sand bottom at 19 m depth. C. fragile degraded more slowly, and had a consistently lower C/N and higher DMSP concentration, than S. latissima. Isotope signatures did not differ between algal species: δ 15 N became slightly enriched (~1‰) after 16 wk of degradation, with no change in δ 13 C. We also compared macrofaunal communities associated with degrading thalli of each algal species and found significantly higher abundances of invertebrates (mainly capitellid poly- chaetes) on S. latissima after 8 wk of degradation, resulting in lower evenness (J') and diversity (H') on S. latissima compared to C. fragile. Macrofaunal community composition became similar between algal species at 12 and 16 wk in concordance with decreases in C/N ratio in S. latissima and DMSP concentration in C. fragile. Our results indicate that differences in biochemical compo- sition and the rate of degradation between C. fragile and native kelps result in community-level effects in areas linked to shallow algal beds via the transfer of detritus.

Severe storm events are important agents of disturbance that can transport large quantities of algal detritus from highly productive kelp beds (or forests) in shallow water to deeper, more food-limited areas. We measured canopy cover in shallow kelp beds (5 to 15 m depth) and the cover of detrital kelp in sedimentary habitats directly offshore of these beds (20 to 45 m) before and after Hurricane Earl, which struck the Atlantic coast of Nova Scotia in September 2010. The storm resulted in large losses of kelp canopy cover (from 71.0 to 38.7%, averaged across sites) and significantly increased the cover of detrital kelp deposits below the kelp beds (from 1.5 to 3.4%). Detrital deposits were more commonly found in a semi-protected bay than off an exposed headland and persisted in the bay for at least 6 wk. Sea urchins Strongylocentrotus droe- bachiensis were associated with detrital kelp deposits in offshore habitats. At sites with the great- est amount of detrital kelp, we estimated that sea urchins could consume this material within 2 mo, indicating that storm-generated detrital pulses may be an important form of trophic connectivity between adjacent ecosystems off this coast.

Fatty acid (FA) analysis is a well- established tool for studying trophic interactions in marine habitats. However, its application to benthic food webs poses 2 particular challenges. First, unlike pelagic zooplankton, benthic con- sumers have access to different sizes and func- tional groups of primary producers and may con- sume a highly mixed diet. Classes of benthic primary producers are distinct in their overall FA composition, but most do not possess unique marker FAs that can be used to identify their con- tribution to higher trophic levels. Second, unlike mammalian predators, benthic invertebrates have the capacity to significantly modify their dietary FAs and thereby obscure markers for food sources. Controlled feeding studies have been used to dis- tinguish dietary tracer FAs from those that are modified by the consumer in several benthic invertebrates, but more such studies are needed. Despite these challenges, FAs have been used to study trophic structure in a variety of benthic habitats including the deep sea, polar regions, estuaries, and the rocky subtidal zone. However, the complexity of benthic food webs and lack of unique markers impose uncertainties in the inter- pretation of FA data from field studies. Multivari- ate analyses are necessary for analyzing FA datasets, although univariate tests can be useful for comparing levels of informative FAs among food sources and consumers. Combining FA analysis with other lines of evidence, such as sta- ble isotope analysis, offers a more reliable approach to examining trophic interactions in benthic systems.

The frequency of epizootics causing mass mortality of sea urchins (Strongylocentrotus droebachiensis) along the coast of Nova Scotia, Canada has increased over the past three decades. Laboratory and field studies show that outbreaks of disease are caused by the amoeba Paramoeba invadens, and are associated with hurricane activity and warm seawater temperatures. A statistical model indicates that the probability of a mass mortality event increases with the proximity of a hurricane to the coast and the maximum sustained wind speed, and is greater when post-storm seawater temperature is above a threshold for disease propagation. To assess the reliability of the model in predicting mass mortality events on an annual scale, and to examine spatial variability in mortality (in the event of a disease outbreak) on a local scale (meters to kilometers), we transplanted sea urchins into cages in kelp beds at 6 sites around St. Margarets Bay, Nova Scotia, where localized outbreaks of paramoebiasis have been observed following hurricanes in the past. On 4 September 2010 a Category 1 hurricane (Earl) made landfall 110 km SSW of the experimental area. Based on the parameters of the storm, the model forecasted a 43% probability of a disease outbreak. Morbidity of caged animals was first documented on 6 September 2010, and morbidity and mortality in the cages was ~ 50% by the end of September and ~ 85% two months after the storm. Laboratory experiments indicated that the temperature-dependent transmission or induction of morbidity was consistent with paramoebiasis. Our findings provide support for the efficacy of the model to predict the occurrence of disease outbreaks, although the source population(s) of the pathogenic agent and oceanographic mechanisms affecting its introduction and spread along the coast of Nova Scotia remain poorly understood.

Grazing aggregations of the sea urchin Strongylocentrotus droebachiensis drive the transition between alternative ecosystem states in Nova Scotia, from productive kelp beds to less productive barrens. This transition can be initiated by the formation of gaps within a kelp bed, containing dense aggregations of sea urchins. We examined the importance of local density of sea urchins and pre-existing gaps in a kelp canopy in mediating the formation of destructive grazing aggregations of sea urchins in a kelp bed. We transplanted 14 000 adult sea urchins from a barrens on the Atlantic coast of Nova Scotia into ~4.5 m 2 plots within a nearby kelp bed, at densities above and below a predicted threshold value for destructive grazing, and simulated disturbance to the kelp bed by removing the kelp canopy in half of the plots. Sea-urchin abundance and gap formation and expansion (as loss of kelp canopy cover) were monitored in and around plots weekly for 9 wk. Grazer-mediated gap formation began 3 wk after sea urchins were introduced, and increased for the remainder of the exper- iment. Our results indicate a direct linear relationship between sea-urchin abundance and increase in gap area within undisturbed treatments. Gaps expanded in the kelp bed at sea- urchin densities below the putative threshold for destructive grazing, indicating that the kelp bed was less resilient to grazing than predicted. Our findings provide insights into mechanisms controlling the stability of the kelp-bed ecosystem state and mediating shifts from kelp beds to barrens in Nova Scotia.

Increased breakage of macroalgal fronds during large wave events can significantly reduce canopy cover and biomass. We examined the effects of encrustation by the invasive bryozoan Membranipora membranacea and damage by the snail Lacuna vincta on the ability of kelp blades ( Saccharina longicruris , Laminaria digitata , and Laminaria complanata ) to withstand wave forces. Using standard materials testing procedures, we documented significant reductions in the maximum stress before breakage, toughness, and extensibility of blade material following bryozoan encrustation. Histological sections of blade tissue indicated a significant degradation of the outer layers of cells following prolonged encrustation by M. membranacea as a likely cause of weakening. Full-thickness perforations and partial-thickness grazing scars also reduced blade strength, suggesting that grazing damage can initiate cracks that lead to blade breakage. Our findings provide a mechanistic link between the damaging effects of mesograzers and encrusting bryozoa on their algal hosts and the export of detrital material from subtidal kelp beds.

We used a spatial model of destructive grazing of kelp beds by dense aggregations (fronts) of sea urchins Strongylocentrotus droebachiensis, together with measures of sea urchin absorption efficiency and fecal degradation rate, to predict rates of fecal production and accumu- lation during the transition from a kelp bed to a barrens state in a rocky subtidal ecosystem of Nova Scotia, Canada. The rate of fecal production as dry mass reached 9.17 g m -2 d -1 at 10 yr after the formation of a grazing front, with associated rates of carbon and nitrogen production of 2.12 g C m -2 d -1 and 0.21 g N m -2 d -1 . Fecal production rate exceeded the rates of physical, chemical, and microbial degradation, such that up to 193.6 g of feces, 70.2 g of C, and 4.0 g of N accumulated per linear m of front after 10 yr. The extent of sea urchin grazing fronts along the Atlantic coast of Nova Scotia was estimated at 280 km in 2000, which translates to a total fecal production rate of 6423 kg of feces d -1 and an accumulation of 54 200 kg of feces after 10 yr of front formation, based on our model. This temporally variable flux of organic matter likely has a profound effect on adja- cent deeper-water macrobenthic communities utilizing the feces as a food source.

We tracked changes in community composition that occurred with state shifts in the rocky subtidal ecosystem in Nova Scotia, Canada, from 1992 to 2008. At the beginning of our study, a dense aggregation of sea urchins was destructively grazing a kelp bed, leaving coralline- algae- dominated barrens in its wake. In subsequent years, the system underwent a series of state shifts mediated by mass mortality of sea urchins due to amoebic disease, defoliation of kelp by an invasive bryozoan Membranipora membranacea, invasion of the green alga Codium fragile ssp. fragile, and finally decline of C. fragile and recolonisation by kelps. Using multidimensional scal- ing (MDS), we found 4 macroalgal assemblage types (dominated by kelp, coralline algae, C. frag- ile, or a transitional mixture of species), each associated with a distinct invertebrate assemblage. Sea stars and kelp-grazing gastropods were associated with the kelp-dominated state; sea urchins, chitons and scale worms were most abundant in the barrens state; and small bivalves and amphipods were most abundant in the C. fragile-dominated state. Invertebrate diversity remained relatively constant despite dramatic shifts in the algal assemblage. Although the system was vul- nerable to multiple perturbations between 1992 and 2002, the partial recovery of kelp beds and associated fauna by 2008 demonstrates some resilience in the longer term.

Population increases of the gastropod Lacuna vincta have been associated with sig- nificant damage to kelp blades and decreases in kelp biomass in subtidal kelp beds off Nova Sco- tia, Canada. We measured the total level and along-blade distribution of grazing damage by Lacuna vincta on the dominant kelp species at 5 sites in Nova Scotia, Canada, over a 15 mo period. Grazing was typically low or absent in the basal regions of blades, consistent with seasonal fluc- tuations in growth and physical properties of blade tissues. Grazing was largely concentrated in middle and distal sections, although this distribution varied with site exposure and over time. The cover of the invasive bryozoan Membranipora membranacea on the surface of kelp blades did not have a strong effect on grazing by L. vincta. The total level of grazing damage (max. 1% of blade area for Saccharina longicruris and 1.5% for Laminaria digitata) varied seasonally, with peaks in September at some sites. Spatial variation was driven in part by a negative relationship with site exposure. In a field experiment, simulated grazing damage that exceeded a threshold value of 0.5 to 1.0% of blade area caused a significant increase in blade loss during a period of heavy wave action due to a passing hurricane. Our results show that direct reductions in kelp biomass through grazing by L. vincta are relatively small, but can indirectly lead to significant losses of kelp bio- mass during large wave events.

Evaluating the efficacy of artificial structures in enhancing or sustaining biodiversity on tropical coral reefs is key to assessing their role in reef conservation or management. Here, we compare spatial and temporal patterns of colonization and succession of the benthic assemblage on settlement collectors (ceramic tiles) in a 13-mo mensurative experiment on a suspended artificial reef, a seafloor artificial reef, and two nearby natural reefs at Eilat, Gulf of Aqaba. We also conducted a concurrent 7-mo manipulative experiment on the suspended reef and one of the natural reefs, and monitored fish feeding behaviour on experimental collectors, to examine effects of large mobile consumers on these patterns. In both experiments, taxonomic composition as percent planar cover for the whole community or biomass for the invertebrate component differed between collector topsides, dominated by a filamentous algal matrix, and shaded undersides with a profuse assemblage of suspension- or filter-feeding invertebrates. In the mensurative experiment, we found differences in final community and invertebrate composition between sites, which clustered according to reef type (artificial vs. natural) for collector undersides. Invertebrate biomass was greater at both artificial reefs than at one (undersides) or both (topsides) natural reefs. In the manipulative experiment, we found similar differences in composition between sites/reef types as well as between treatments (exclusion vs. control), and the invertebrate biomass was greater on the artificial reef. Invertebrate biomass was greater in the exclusion treatment than the control on collector undersides, suggesting mobile consumers can affect community composition and abundance. Predominant fish species observed interacting with collectors differed between artificial and natural reefs, likely contributing to differences in patterns of colonization and succession between sites and reef types. Our findings suggest artificial reefs have the potential to enhance cover and biomass of certain reef-associated assemblages, particularly those occupying sheltered microhabitats.

Connectivity via the transport of detrital material from areas of high to low productivity may be an important determinant of secondary productivity and biodiversity in receiving communi- ties. On the Atlantic coast of Nova Scotia, detritus exported from subtidal kelp beds contributes to food webs in communities inhabiting deeper waters offshore. To estimate the amount of energy available via this pathway, we measured rates of detrital production via erosion of kelp blades in kelp beds at 5 sites varying in wave exposure. Specifically, we measured productivity and erosion of the 2 dominant species of kelp, Laminaria digitata and Saccharina longicruris, over 16 mo. We also examined the effect of environmental and biological factors on erosion rates, including temperature, wave exposure, grazing by snails and cover by the invasive bryozoan Membranipora membranacea. We observed clear seasonal and spatial patterns in productivity, with the highest production in spring. Erosion rate was highly variable across sites and seasons, and was strongly related to the cover of M. membranacea and the intensity of snail grazing at the distal end of kelp blades. Both factors cause tissue degradation, abrasions and perforations that weaken blades. Erosion rate was also positively related to water temperature and site exposure. Annual detrital production from the erosion of kelp blades increased linearly with kelp bed biomass, and ranged from 0.5 to 1.71 kg dry weight m -2 (150-513 g C m -2 ) across sites. These rates equal or exceed annual phytoplankton production off the Atlantic coast of Nova Scotia and estimates of detrital production in seagrass beds in North America.

The methionine-derived secondary metabolite dimethylsulfoniopropionate (DMSP) is believed to function in cryoprotection, defense against herbivory, and reduction of oxidative stress in some seaweeds. The ability to produce DMSP, and modify its concentration according to environ- mental conditions, may confer adaptive advantages to these algae. We studied the spatial and tem- poral distribution of DMSP content in Codium fragile off the Atlantic coast of Nova Scotia. Levels of DMSP in C. fragile were highest in boreal spring (~2% of total dry wt) and lowest in fall (~1%), and negatively related to seawater temperature. DMSP content differed among sites and tended to decrease or remain constant with depth. In tide pools, partially bleached algae contained less DMSP than undamaged individuals, consistent with the putative function of DMSP in response to oxidative stress. Results of field- and laboratory-based experiments, designed to examine plasticity in DMSP production by C. fragile, indicate that it increases with light intensity, exposure to sea urchin grazing, and decreasing temperature.

An increase in the incidence of disease in various marine organisms over the past few decades has been linked to ocean climate change. In Nova Scotia, Canada, mass mortalities of sea urchins, due to an amoebic disease, are associated with tropical cyclones of relatively high intensity that pass close to the coast when water temperature is above a threshold for disease propagation. These conditions increase the likelihood of introduction and spread of a nonindigenous water-borne pathogen through turbulent mixing. Our analysis shows that the most deadly storms, in terms of the probability of a sea urchin mass mortality, have become more deadly over the past 30 years. We also found that storms have been tracking closer to the coast and that surface temperature has increased during the hurricane season. These trends are likely to continue with climate warming, resulting in a regional shift to a kelp bed ecosystem and the loss of the urchin fishery.

Field observations and laboratory experiments support the hypothesis that disease- induced mass mortality of the sea urchin Strongylocentrotus droebachiensis can be associated with hurricane events that introduce a pathogenic amoeba Paramoeba invadens to coastal waters. The temporal pattern of morbidity and mortality of sea urchins observed in a large embayment on the Atlantic coast of Nova Scotia following Hurricane Juan in September 2003 and Hurricane Bill in August 2009, and in laboratory infection experiments during the 2009 event, closely conformed to that expected based on the known temperature-dependent dynamics of this amoebic disease.

On the Atlantic coast of Nova Scotia, transitions between alternative states of the subti- dal ecosystem, kelp beds and sea urchin barrens, occur on a decadal scale. To explore the process of urchin aggregation within kelp beds that leads to the shift to barrens, we developed a coupled map lattice model to simulate the spatial dynamics of kelp and green sea urchin Strongylocentrotus droe- bachiensis abundance over time. Our simulations show that the following factors can cause sea urchins to form grazing aggregations that create gaps in a kelp bed: (1) random movement by >60% of sea urchins residing in the bed, (2) moderate to high levels of spatial variability in sea urchin recruitment (30 to 90 (urchins m -2 ) 2 ), (3) localized aggregation of sea urchins (150 urchins m -2 ) amid a moderate to high background density of sea urchins within the kelp bed (>10 urchins m -2 ), with or without a chemotactic response of sea urchins to kelp, and (4) removal of kelp from areas >20 m 2 (to simulate physical or biological disturbance, or harvesting). Gaps formed at random locations within the spatial domain and expanded and coalesced to form barrens in which sea urchins were randomly distributed. Sea urchins formed circular fronts around gaps in the kelp bed. The rate of advance of fronts (and increase in gap size) was linearly related to the density of sea urchins at the front. The duration of the transition to the barrens state decreased with increases in (1) the proportion (Pm) of sea urchins moving (from >6 yr for Pm = 0.8 to 5 yr for 30 (urchins m -2 ) 2 to

To examine the role of fecal production by the sea urchin Strongylocentrotus droe- bachiensis in native and invaded algal assemblages in Nova Scotia, Canada, we examined the quan- tity (production rate by urchin feeding aggregations, or 'fronts') and quality (organic carbon and nitro- gen content) of feces egested on 3 diets: (1) a native kelp Saccharina longicruris, (2) kelp encrusted with the invasive bryozoan Membranipora membranacea, and (3) an invasive green alga Codium fragile ssp. fragile. Relatively low absorption of clean and encrusted kelp resulted in high fecal output from urchins grazing native and invaded kelp beds (74 and 81 g feces m -1 front d -1 , respectively). In contrast, high absorption of C. fragile resulted in low fecal output from urchins grazing meadows of the invasive alga (31 g feces m -1 front d -1 ). Low C:N ratios of feces obtained from grazing C. fragile (8.5) or encrusted kelp (11.4), compared to clean kelp (30.7), suggest that feces from invaded assemblages are a higher quality food source for microbes and detritivores. To compare dispersal characteristics of feces from each diet, we measured pellet shape, size, density, and settling and critical erosion veloci- ties. Using a wave model, we estimated the depth at which feces would be deposited under seasonally varying wave conditions. We found that pellets of C. fragile are likely deposited at the greatest depths (33 to 55 m), pellets of clean kelp at intermediate depths (28 to 47 m), and pellets of encrusted kelp at the shallowest depths (22 to 40 m). Our findings suggest a smaller amount of higher quality feces enters the detrital food web at greater depths from Codium meadows than from native kelp beds.

Since its introduction in 1989, the invasive green alga Codium fragile ssp. fragile (formerly Codium fragile ssp. tomentosoides) has spread rapidly in Atlantic Canada. Although its spread has likely been facilitated by human transport, C. fragile possesses diverse modes of natural dispersal. In addition to parthenogenetically developing swarmer cells, it can propagate through the release of vegetative buds, thallus fragments, and entire, dislodged thalli. We examined the natural dispersal potential of these propagules using a combination of field and laboratory experiments. Vegetative buds were most abundant on thalli in late August, coinciding with increasing late summer wave activity. At two locations within two field sites, we examined the effects of tidal state, wave action and topography on the dispersal of fragments and intact thalli. Over a 4-hour period, retention rate was generally lower and dispersal distance of retained propagules higher: during flood than ebb tide; at the more wave-exposed locations; and for fragments than thalli. Dispersal direction corresponded with topography or flood tide currents at certain locations, but was often bi-directional or random, suggesting an added role of wave action. Over periods of weeks, the retention rate of marked detrital thalli was negatively related to the magnitude of wave activity. In the laboratory, settling rate was negatively related to propagule length, and large buds had higher critical shear erosional velocities than either small buds or fragments. Our results indicate that large propagules of C. fragile, such as intact adult thalli, have a generally low dispersal potential (meters), but may be transported longer distances (kilometers) during storms or when positively buoyant. Fragments and buds are regularly transported 10's of meters at a time at average flow velocities. However, they also become resuspended in average flows, and probably disperse longer distances over multiple resuspension events, or when positively buoyant. Once settled, the smallest propagules may be less easily resuspended or transported along the bottom as they become entrapped by small-scale topographic features or turf algae. The wide variety of propagules produced by C. fragile and the variable distances over which different propagule types may be transported give C. fragile the advantage of both short- and long-distance dispersal, and have likely played a role in the invasive success of this alga.

In the Northwest Atlantic, overgrowth of the competitively dominant, native kelps by an invasive bryozoan Membranipora membranacea increases frond erosion, which has facilitated the establishment and spread of the invasive macroalga Codium fragile ssp fragile. To document the spread of both introduced species along the Atlantic coast of Nova Scotia from initial introduction points (the ‘epicentre’) southwest of Halifax, we conducted video-surveys of shallow rocky habitats along the southwestern shore of Nova Scotia (100 km linear distance, encompassing the range of M. membranacea) in 2000, and then along the entire Atlantic coast in 2007 (650 km). Membranipora membranacea was observed continuously throughout the surveyed ranges in 2000 and 2007, wherever kelps were present, suggesting natural dispersal via planktonic larvae. Codium fragile was observed along 95 km of the surveyed range in 2000 and along 445 km in 2007, with a relatively patchy distribution beyond the epicentre, suggesting a combination of natural and anthropogenic dispersal mechanisms. Rockweed-dominated (Fucus spp.) or mixed algal assemblages common outside the epicentre may alter the interaction between M. membranacea and C. fragile, since seaweeds other than kelp are not subject to defoliation by the bryozoan. Percent cover of kelp at the epicentre generally increased from 2000 to 2007, while that of C. fragile generally decreased. Codium fragile was the dominant canopy alga at 54% of sites in 2000 and at only 15% of sites in 2007. These findings indicate that, at near decadal timescales, C. fragile does not prevent re-colonization by native kelps.

We assessed the potential of fatty acid (FA) markers for tracing primary production from an invasive green alga (Codium fragile ssp. fragile) and a native kelp (Saccharina longicruris) through 2 trophic levels in an experimental food web: a primary consumer, the green sea urchin Strongylo- centrotus droebachiensis, and 2 secondary consumers, the native rock crab Cancer irroratus and invasive green crab Carcinus maenas. Sea urchins fed the 2 algal species had distinct gonadal FA compositions, and contained markers of each alga. Crabs of each species were then fed the gonads of sea urchins which had been fed either S. longicruris or C. fragile, and their hepatopancreases were analyzed. We were able to distinguish crabs of both species with C. fragile at the base of the food chain using the marker FAs 16:3n-3 and 18:3n-3. The overall FA composition of the hepatopancreas differed with diet in the green crab but not the rock crab. Our results suggest that 16:3n-3 may be a useful marker for tracing C. fragile production in grazers and some secondary consumers in rocky subtidal habitats in the Northwest Atlantic, but signal attenuation with each trophic transfer will limit the utility of this approach in higher consumers.

We used underwater videography to examine seasonal and interannual patterns in the cover (on kelp) of the encrusting epiphytic bryo- zoan Membranipora membranacea, and associated changes in the structure and abundance of native kelp (Saccharina longicruris) populations, at 2 sites on the Atlantic coast of Nova Scotia and over 4 to 11 yr since initial introduction of this invasive species around 1992. We show that (1) changes in the cover of M. membranacea on kelp, and in the cover of kelp on the seabed, are reciprocal and seasonal; (2) ther- mal history during the summer/fall period of bryo- zoan colony growth explains a large proportion (83%) of the interannual variation in peak cover of M. membranacea on kelp; and (3) annual decreases in kelp cover and blade size are related to the degree of infestation by M. membranacea, and not to wave action alone. Particularly severe outbreaks of M. membranacea, resulting in extensive defoliation of kelp beds, occurred in 1993, 1997 and 1999. Our field observations indicate that recurrent seasonal outbreaks of this invasive bryozoan can have a devastating effect on native kelp populations in Nova Scotia, which, in turn, facilitates the establish- ment and growth of the invasive green alga Codium fragile ssp. fragile.

Aim Introduced macroalgae are widespread in the world’s oceans and, despite increasing awareness and attempts to limit the phenomenon, the number of species introductions in coastal waters has increased exponentially over time. Little is known about the rates and mechanisms of spread, even among species that have received the most attention. We compare patterns of range expansion for nine species of invasive algae across eight geographic regions. Location World-wide. Methods We compiled records of introduced algae from the scientific literature, herbaria, and by contacting experts to reconstruct chronologies for 22 algal invasions. These were used to map patterns of spread at a regional scale (thousands of km). Results Range size tended to increase linearly with time, often after an initial lag. Range expansion occurred at rates of tens to hundreds of kilometres per year, often with large infrequent increases. Rates of range expansion differed significantly between species within the same region, and between regions for the same species. Main conclusions Our results suggest that anthropogenic vectors likely play a key role in the spread of introduced macroalgae at a regional scale, although natural long-distance dispersal also may be important for some species. The lack of consistency in the rates within individual species and regions suggests that multiple interacting factors (e.g. algal traits, characteristics of invaded communities, environmental conditions and anthropogenic activities) determine where propagules of introduced algae are delivered and whether they become established.