Search

Along the northwest coast of Sweden, over 50% of Zostera marina L. (eelgrass) meadows have vanished since the 1980s. With the improvement of conditions, there is a growing interest to restore lost habitats, but methods are lacking for restoration of eelgrass beds at high latitudes where long winters create special challenges. We assessed if seed planting could be used for large-scale restoration, with the aim to identify the major causes of seed and seedling loss and to determine which planting method best increases restoration success. In the laboratory, we identified optimal conditions for long-term seed storage and demonstrated that eelgrass seeds can be successfully stored for 8 mo before being planted in the spring. However, field studies did not find an increased seedling establishment in seeds planted in the spring of 2013 compared to those planted in the fall of 2012. Field results suggest that the main causes for the seed loss were seed predation from crabs, seed transport by currents and bioturbation by lugworms, while the main processes affecting shoot development were light availability and physical disturbance. Covering the seeds with a layer of sand increased seedling establishment 2 to 6 times compared to uncovered seeds, probably through the reduction of seed predation and seed transport, and could potentially be developed as a method to reduce losses during restoration. In addition, positive feedback mechanisms (i.e. sediment resuspension and drifting macroalgae mats) may also prevent natural recovery and restoration success. However, high seed loss (on average 98.6%) and high shoot mortality pose a challenge that need to be addressed before restoration using seeds can be recommended for large-scale restoration.

Blooming filamentous algae recurrently overgrow macroalgae and seagrass in many coastal ecosystems, leading to potential losses in the functions and services these habitats provide. The seasonal reoccurrence of algal blooms is partly caused by excessive input of nutrients into coastal ecosystems. At the same time, overfishing of top predators has led to increased densities of smaller predators that reduce herbivore abundance. Managers of coastal ecosystems are in urgent need of information on how overfishing may act synergistically with eutrophication to promote algal blooms. In the present study, we model the interaction between the opportunistic filamentous algae Ulva spp. and the effective mesograzer (small invertebrate herbivore) Gammarus locusta L. under different nutrient regimes and predation rates by intermediate-size fish (4 to 12 cm). The aim is to as- sess how productivity, herbivory and predation interact to regulate seasonal algal growth under a range of scenarios, including temporal matching or mismatching between trophic groups. The model is parameterised with nutrient and demographic data from a seagrass ecosystem on the west coast of Sweden. Model simulations show that both nutrient level and herbivore abundance regulate seasonal algal biomass in the seagrass meadow. Their relative importance is dynamic and sensitive to the arrival times of herbivores in relation to the start of algal development in spring. Thus, herbivore presence and timing are crucial factors for the control of algal biomass, particularly during times when resource levels are ambient to slightly elevated. Simulations also show that it is mainly the predation rate and timing of predators, rather than food limitation, that limit herbivore abundance. As predation rates by invertebrates and smaller fish can be linked to the presence of larger and commercially targeted fish, overfishing may have similar effects to eutrophication in the seagrass ecosystems.

This study aims to improve estimates of dispersal by including information on larval traits, and in particular to explore how larval depth distribution affects connectivity and MPA (marine protected area) functionality in the Baltic Sea. A field survey showed that both invertebrates and fish differed in their larval depth distribution, ranging from surface waters to >100 m. A biophysical model of larval dispersal in the Baltic Sea showed that decreased depth distribution increased average dispersal distance 2.5-fold, decreased coastal retention and local recruitment, and substantially increased connectivity. Together with pelagic larval duration (PLD), depth distribution explained 80% of total variation in dispersal distance, whereas spawning season, and geographic and annual variations in circulation had only marginal effects. Median dispersal distances varied between 8 and 46 km, with 10% of simulated trajectories dispersing 30 to 160 km depending on drift depth and PLD. In the Baltic Sea, the majority of shallow Natura 2000 MPAs are 10% within MPAs of this size. Connectivity between MPAs was expected to be low for most larval trait combinations. Our simulations and the empirical data suggest that the MPA size within the Natura 2000 system is considerably below what is required for local recruitment of most sessile invertebrates and sedentary fish. Future designs of MPA networks would benefit from spatially explicit biophysical models that consider dispersal and connectivity for complex circulation patterns and informed larval traits.

Algal grazers are thought to play a critical role in preventing overgrowth by ephemeral algae in seagrass beds. We studied the interaction between the omnivorous grass shrimp Palaemon elegans, the algal grazer Gammarus locusta and the filamentous green alga Ulva sp. (syn. Entero- morpha sp.) to assess whether the shrimp has a positive or negative effect on the growth of ephemeral algae in eelgrass beds on the Swedish west coast. Laboratory experiments demonstrated that P. ele- gans were effective predators on gammarids 5 mm TL, but negatively affected when G. locusta was smaller. In a field cage experiment, enclosed P. elegans decreased the abundance of gammarids by on average 68%, causing a trophic cascade that increased the biomass of filamentous Ulva sp. 7-fold. These results suggest that P. elegans on the Swedish northwest coast feed mainly on amphipods, including the algal grazer G. locusta, resulting in a positive net trophic effect on the growth of filamentous Ulva sp. This study indicates that P. ele- gans may be an important link between overexploitation of cod in northwest Sweden and the recent increase of algal mats and loss of eelgrass in the area.

We compared eelgrass Zostera marina communities in 3 regions in Sweden believed to be affected by eutrophication and overfishing, to determine whether bottom-up or top-down processes control the biomass of epiphytic macroalgae and grazers. Nitrogen and carbon isotope signatures were analyzed to explore the food webs and to identify the grazing species feeding on filamentous algae and/or eelgrass. Mixing model (IsoSource version 1.3.1) analysis of the isotope signatures indicated that the amphipods Gammarus locusta and Microdeutopus gryllotalpa fed primarily on filamentous algae and that only 2 small gastropod species consumed eelgrass. Moreover, the grass shrimp Palaemon elegans and F adspersus were ca. 1 trophic level above amphipods and algae, but according to the mixing model played different trophic roles in the different areas. The highest biomass of filamentous algae was found in the west coast beds housing grazers with the lowest biomass and mean size (predominantly G. locusta and M. gryllotalpa, 0.5 to 3 mm). In contrast, the Baltic Sea beds had low algal biomass, but the grazers (mostly G. locusta and Idotea baltica) had higher biomass and were significantly larger (mean size ca. 10 mm). An overall negative correlation was found between grazer biomass and biomass of filamentous algae. The significantly smaller grazers and absence of isopod grazers on the west coast may be due to substantial consumption by small predatory fish. This supports the suggestions that, in Swedish eelgrass beds, grazers are top-down controlled, and overexploitation. of large predators and eutrophication play an important role in the recent increases in algal biomass.

We assessed the relative role of 3 potentially important processes affecting the distribu- tion of young juvenile blue crabs Callinectes sapidus Rathbun: (1) habitat selection at settlement, (2) selection of habitats by dispersing juveniles, and (3) habitat-specific predation rates, using cage experiments. The results suggest that active habitat selection by postlarvae and young juvenile crabs determines the habitat-specific distribution of juvenile blue crabs. Densities of blue crab settlers (megalopae and first instar crabs) in caged habitat patches were high and similar in artificial sea- grass, live shoal grass Halodule wrightii, and live oyster habitats (on average 69 to 82 settlers m -2 ), but significantly lower in mud (26 settlers m -2 ), indicating active selection for any structurally com- plex habitat at settlement. Second and third instar juvenile blue crabs (J2 and J3 crabs) also colonized the structurally complex habitats in higher numbers (34 to 115 crabs m -2 ), compared to mud (14 crabs m -2 ), demonstrating that young juvenile blue crabs are highly mobile and redistribute soon after metamorphosis. Densities of J2 and J3 blue crabs in uncaged treatments were significantly lower in all habitats than in caged treatments (on average 44% loss), suggesting high predation mortality. However, the loss of settlers and juvenile crabs was similar in all habitats, and had no significant effect on the juvenile crab distribution. Densities of potential predators were on average 5 times higher in the structurally complex habitats than in mud. An aggregation of predators in the refuge habitats, coupled with a refuge at low prey densities in unstructured habitats, appears to decrease the proximate effect of predation on the distribution of juvenile crabs. Concurrent high settlement of a small xanthid crab (cf. Eurypanopeus sp.) occurred almost exclusively in the oyster habitat, sug- gesting that species with a limited ability to redistribute after metamorphosis may display a more specific selection of the adult habitat at settlement.

In marine organisms with dispersing larval stages, current ecological theory suggests that recruitment of benthic juveniles may be limited by juvenile habitats and regulated by early post- settlement processes. However, few studies have attempted to identify the density-dependent mech- anisms responsible for these demographic bottlenecks. I conducted a series of experiments to assess if interference competition for space, within nursery habitats, could result in density-dependent juve- nile growth and dispersal from refuge habitats, and regulate local populations of juvenile shore crabs Carcinus maenas in shallow nursery areas in Sweden. In laboratory mesocosms, increased natural densities of similarly sized crabs resulted in decreased growth and increased per capita emigration from mussel habitats, even though food was provided in excess, suggesting that competition for space and mutual interference mediated the density-dependent growth and dispersal. This was supported in a field experiment where a density-dependent relation was found between the abundance of juve- nile crabs in nursery areas and their migration rates to experimental mussel patches. The results sug- gest that interference competition for space and density-dependent emigration from refuge habitats, coupled with habitat-specific mortality, is an important regulating mechanism for juvenile shore crab populations, in particular for older juvenile cohorts that have escaped inter-cohort cannibalism through a size refuge. Density-dependent effects on growth were found only at unusually high nat- ural densities and may, therefore, be less important as a regulating mechanism. However, negative effects on feeding rates and growth at high conspecific densities may represent an important selec- tive force in juvenile migration behavior. Density-dependent dispersal was found at juvenile densi- ties that are regularly found in nursery areas, suggesting that nursery habitats become saturated with juveniles during the recruitment season and represent a limiting resource for local populations, con- sistent with field observations of shore crab populations in the study area.

In contrast to general settlement models of invertebrate larvae, recent Swedish studies of settlement of the shore crab Carcinus maenas indicate that many postlarvae close to metamorphosis emigrate from their preferred settlement habitats. In the present study, we assessed physical and biological factors affecting shore crab settlement and emigration from nursery habitats, and explored possible ultimate factors in this behavior. Postlarval abundance in the field was not affected by the tidal phase, but varied significantly according to the light conditions. Settlement densities on artificial substrates were significantly higher during the day compared to at night. In contrast, densities of planktonic postlarvae were highest at night. This result suggests that the postlarvae cling to shallow benthic habitats during the day and swim in the plankton at night. Mesocosm experiments demonstrated that most postlarvae remained in the benthos during the day, even in suboptimal habitats, but that a large proportion (on average 58%) actively emigrated at dusk, irrespective of settlement conditions. Emigration rates were significantly higher and metamorphosis rates lower in open sand compared to mussel habitats, whereas food levels and settlement densities had only small effects on the settlement process. Emigrated postlarvae were at a significantly earlier developmental stage and metamorphosed on average 1 d later than the megalopae that remained in the benthic habitat. Post-larvae >2 d from metamorphosis emigrated even from optimal settlement conditions. These results suggest that settlement in shore crabs is not an irreversible process once a late-stage postlarva has found a settlement habitat, but involves 2 separate behavioral components: (1) the selection of a benthic habitat during the day, which is mainly affected by the availability of refuges from predation; and (2) the choice to stay or emigrate the following night, where emigration is induced by a decrease in light and determined mainly by the development stage of the postlarvae. We propose a conceptual model of settlement in the shore crab where the dynamic behavior of the postlarvae is an adaptation to diel differences in predation risk between the settlement habitat and the plankton.

Tide-related vertical migration behaviours which assist in horizontal transport of marine larvae have been documented in many species from meso-tidal systems; however, little is known about larval behaviour in areas where the tides do not produce predictable changes in water cur- rents. We assessed the vertical migration behaviour in larvae of the shore crab Carcinus maenas (L.) from the Swedish west coast, to explore possible transport mechanisms in a micro-tidal system. Repli- cated plankton samples were collected at a fixed station from 2 depths during flood and ebb tides, and analysed for effects of tides and light on temporal and vertical patterns in abundance. In a com- plementary experiment, first-stage zoeae were hatched in the laboratory from field-collected females and studied in constant darkness for endogenous circa-tidal or circa-diel rhythms. The abundance of different larval stages decreased from the first to the fourth zoeal stage by a factor 10, whereas mega- lopal densities were similar to those of the first zoea. This pattern in abundance indicates an off- shore/onshore transportation of larvae during ontogeny, suggesting a behavioural component in larval dispersal. However, contrary to what has been demonstrated in larvae from meso-tidal areas, none of the larval stages displayed a vertical or temporal pattern in abundance related to the tide, although a clear semi-diurnal tidal change of approximately 0.3 m was registered throughout the sampling periods. Instead, the highest densities of all larval stages were consistently sampled during low light conditions, indicating a diel pattern in swimming behaviour. This behaviour appears to be controlled by exogenous cues, since the laboratory experiment with first zoeae in constant darkness did not indicate any consistent tidal or diel rhythm in vertical migration. The lack of an endogenous circa-tidal vertical migration rhythm suggests that Swedish shore crabs are reproductively isolated from the British Isles populations, where this behaviour has been demonstrated to be inherited. The possibility that C. maenas larvae in micro-tidal systems may use predictable sea and land breezes as a mechanism for cross-shelf transport is discussed.

In marine benthic organisms with a pelagic larval phase, assessment of recruitment reg- ulation necessitates accurate estimates of larval supply and initial settlement densities. We assessed 2 commonly used methods for estimating decapod larval supply, plankton net tows and artificial settle- ment substrates (ASS), together with a new technique using passive-migration traps. The aim was to evaluate how these methods estimate larval supply and settlement of 3 common decapod species (the shore crab Carcinus maenas L., the brown shrimp Crangon crangon L. and the grass shrimps Palae- mon spp.) in a microtidal nursery area on the Swedish west coast, and to assess how these estimates relate to the juvenile recruitment of these species. Discrete plankton net tows outside the shallow nursery area collected a higher proportion of larvae at an early development stage, compared to the other methods, and produced only a snapshot of the variable water-column abundance of shore crab and grass shrimp larvae that correlated poorly with numbers collected with ASS at the same location. Artificial settlement substrates appeared to produce good, integrated relative estimates of shore crab larval supply and settlement. The number of larvae collected from ASS correlated significantly with larval abundance estimated by passive-migration traps in shallow nursery areas, and reflected changes in shore crab settlement densities in caged mussel habitats. We did not detect any effect of predation on ASS placed in nursery areas. However, a large proportion (an estimated 83%) of shore crab postlarvae appeared to emigrate from ASS immersed for periods longer than 12 h, possibly during the night. All grass shrimp larvae appeared to emigrate from the collectors within 24 h, suggesting that ASS immersed longer than 12 h do not produce useful integrated estimates of larval supply for this species. These results demonstrate the importance of assessing postlarval emigration patterns from ASS to optimize immersion and collection time, and to avoid confounded estimates of larval abun- dance and settlement. A new method using replicated passive migration traps that fished continuously in opposite directions (on-shore and off-shore) gave promising integrated estimates of net fluxes (total number immigrating minus number emigrating per unit time) of both pelagic postlarvae and early benthic juvenile stages of crabs and shrimp. The majority of the brown shrimp recruits were young juveniles, demonstrating the importance of incorporating juvenile migration in recruitment studies of motile benthic species. High numbers of shore crab and grass shrimp larvae were found to emigrate from the bay, indicating that many decapod postlarvae found in nursery areas may be transitional. The emigrating larvae were on average in an earlier development stage than those migrating to the bay. These short-term experiments demonstrate the importance of assessing larval development stage and migratory behavior when estimating larval supply and settlement for recruitment studies.

Settlement and early juvenile stages are considered a bottleneck in the Life history of many epibenthic organisms because of high predation mortality. Nursery habitats may play an important role in mitigating settlement and post-settlement mortality by providing refuge from predation. We examined these relationships in postlarvae and early juvenile stages of the shore crab Carcinus maenas L. in laboratory and field tethering experiments. We studied habitat and size related habitat mortality using postlarvae and young juvenile crabs as prey, and various predators, including juvenile conspecifics, in several habitats common in shallow (0 to 1 m) soft bottom nursery areas on the Swedish west coast. Settling mortality was high in open sand (80 to 90%), whereas a significant habitat refuge was obtained in mussel beds, eelgrass and filamentous green algae, the latter yielding the lowest mortality (13 to 14%). Small differences in structural complexity of ephemeral macroalgae dramatically affected predation mortality of first instar crabs, with a significant refuge obtained only in algae of medium complexity. Predation rate on tethered crabs in the field was high (52 to 67%) only on the smallest crabs (