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Links between hydrologic modifications, flow and salinity regimes, and submerged aquatic vegetation (SAV) species composition and abundance were assessed with an empirical analysis of 33 years of monitoring data collected at nine sites in Florida’s Caloosahatchee River Estuary (CRE). Freshwater inflows to the estuary (30-day means) were often outside the previously recommended envelope of 12.74 to 79.29 m3 s−1. Discharges from Lake Okeechobee through a synthetic hydrologic link were responsible for 43% of the above-envelope flows, but reduced the incidence of below-envelope flows by 30%. A salinity model and salinity stress indices developed for each SAV species indicated that the observed flows generated variable salinity conditions likely to harm both seagrasses and freshwater SAV in the estuary. Regression modeling of SAV abundance generally confirmed the flow and salinity responses expected for each species: Halodule wrightii and Thalassia testudinum in the lower estuary were both harmed by high-flow, low-salinity conditions, while Vallisneria americana in the upper estuary was decimated by low-flow, high-salinity conditions. There was a species-specific effect of the seasonal timing of high flows—T. testudinum was more negatively correlated with high flows in the dry season; H. wrightii in the wet season. The regression analyses also highlighted strong, year-to-year autocorrelations in SAV abundance, indicating reduced resilience after severe losses, particularly for V. americana. Large residual variation in some regression models suggested that factors other than salinity (e.g., optical water quality or grazing impacts) may also influence the system dynamics and should be incorporated in continuing research. This analysis suggests that use of artificial water management infrastructure to reduce extreme high and low flows to the Caloosahatchee and other estuaries could help maintain SAV health in light of intensifying climate variability and degraded watershed flow regulation capacity.

A growing number of examples indicate that large predators can alter seagrass ecosystem structure and processes via top-down trophic interactions. However, the nature and strength of those interactions varies with biogeographic context, emphasizing the need for region-specific investigations. We investigated spatial and temporal variation in predatory fish and seagrass communities across a Marine Protected Area (MPA) boundary in the Banana River Lagoon, Florida (USA), assessing trophic roles of intermediate consumers, and performing a large-consumer exclusion experiment in the MPA. Large, predatory fishes were most abundant within the MPA, while some mid-sized fishes were more abundant outside it. Small, seagrass-resident fishes, epifaunal invertebrates, and macrophytes also differed across the MPA boundary, but varied more among individual sites and seasonally. We cannot conclusively attribute these patterns to MPA status because we lack data from prior to MPA establishment and lack study replication at the level of MPA. Nevertheless, other patterns among our data are consistent with hypothesized mechanisms of top-down control. E.g., inverse seasonal patterns in the abundance of organisms at adjacent trophic levels, coupled with stable C and N isotope and gut contents data, suggest top-down control of crustacean grazers by seasonal recruitment of small fishes. Large-consumer exclosures in the MPA increased the abundance of mid-sized predatory and omnivorous fishes, but had few impacts on lower trophic levels. Results suggest that large-scale variation in large, predatory fish abundance in this system does not strongly affect seagrass-resident fish, invertebrate, and algal communities, which appear to be driven more by habitat structure and seasonal variation in small fish abundance.

Form–function relationships in plants underlie their ecosystem roles in supporting higher trophic levels through primary production, detrital pathways, and habitat provision. For widespread, phenotypically-variable plants, productivity may differ not only across abiotic conditions, but also from distinct morphological or demographic traits. A single foundation species, eelgrass Zostera marina, typically dominates north temperate seagrass meadows, which we studied across 14 sites spanning 32–61°N latitude and two ocean basins. Body size varied by nearly two orders of magnitude through this range, and was largest at mid-latitudes and in the Pacific Ocean. At the global scale, neither latitude, site-level environmental conditions, nor body size helped predict productivity (relative growth rate 1–2% day-1 at most sites), suggesting a remarkable capacity of Z. marina to achieve similar productivity in summer. Furthermore, among a suite of stressors applied within sites, only ambient leaf damage reduced productivity; grazer reduction and nutrient addition had no effect on eelgrass size or growth. Scale-dependence was evident in different allometric relationships within and across sites for productivity and for modules (leaf count) relative to size. Zostera marina provides a range of ecosystem functions related to both body size (habitat provision, water flow) and growth rates (food, carbon dynamics). Our observed decoupling of body size and maximum production suggests that geographic variation in these ecosystem functions may be independent, with a future need to resolve how local adaptation or plasticity of body size might actually enable more consistent peak productivity across disparate environmental conditions.

Most studies evaluating the effects of biodiversity on ecosystem functioning are conducted at a single location, limiting our understanding of how diversity-function relationships may change when measured across different spatial scales. We conducted a species-removal experiment at three sites nested in each of three regions along the rocky intertidal coastline of the Gulf of Maine, USA, to evaluate the potential for scale-dependent effects of species loss on the biomass of intertidal seaweed assemblages. We randomly assigned 50 plots in the mid-intertidal zone at each site to one of five treatments (n = 10 plots each): an unmanipulated control, a polyculture plot that contained our three target seaweed species, and three monoculture plots. We manipulated diversity by removing all non-target species from monoculture and polyculture plots, then removing additional biomass from polyculture plots, proportionate to species' relative abundances, so that the average amount removed from monoculture and polyculture plots was equivalent at each site. At the largest spatial scale, all sites considered together, after accounting for region and site nested within region seaweed diversity had consistent, positive effects on seaweed cover. Diverse polyculture plots always had higher cover than was predicted by the average performance of the component seaweed species and usually had higher cover than even the best-performing component species. Diversity effects weakened and became less consistent at smaller spatial scales, so that at the scale of individual sites, diverse polycultures only performed better than the average of monocultures ~40% of the time. Hence, our results indicate that weak and/or inconsistent biodiversity effects at the level of individual sites may scale up to stronger, more consistent effects at larger spatial scales. Quantitative summaries of biodiversity experiments conducted at the scale of individual sites do not capture this spatial aspect of biodiversity effects and may therefore underestimate the functional consequences of biodiversity loss.

Changes in seagrass food-web structure can shift the competitive balance between seagrass and algae, and may alter the flow of energy from lower trophic levels to commercially important fish and crustaceans. Yet, trophic relationships in many seagrass systems remain poorly resolved. We estimated the food web linkages among small predators, invertebrate mesograzers, and primary producers in a Chesapeake Bay eelgrass (Zostera marina) bed by analyzing gut contents and stable C and N isotope ratios. Though trophic levels were relatively distinct, predators varied in the proportion of mesograzers consumed relative to alternative prey, and some mesograzers consumed macrophytes or exhibited intra-guild predation in addition to feeding on periphyton and detritus. These findings corroborate conclusions from lab and mesocosm studies that the ecological impacts of mesograzers vary widely among species, and they emphasize the need for taxonomic resolution and ecological information within seagrass epifaunal communities.

We characterized the seasonal and interannual variation in macrophytes, epiphytes, invertebrate herbivores, small demersal predators, and physicochemical characteristics of an eelgrass (Zostera marina) bed in Chesapeake Bay, Virginia, over 10 yr, to explore the relative importance of abiotic and biotic forcing on community composition and abundance. Our hypotheses were (1) physicochemical drivers affect community structure directly, (2) bottom-up trophic control is evidenced by positive covariance among trophic levels, (3) top-down control generates inverse patterns of abundance at adjacent trophic levels, and (4) species diversity among herbivores contributes to temporal stability. Composition and abundance of eelgrass-associated species varied strongly among seasons and years. Much of this variation correlated with temperature and salinity anomalies, and multivariate analysis grouped communities roughly by season, supporting our first hypothesis. Severe seagrass loss during the hot summer of 2005 shifted the community toward a novel composition, but community structure rebounded within a year. Evidence for trophic control was mixed: selected taxa showed patterns consistent with top-down or bottom-up control, but these patterns generally disappeared at the level of whole years and entire trophic levels. Our ability to detect trophic effects may have been limited, however, by consumer movement or changing behavioral responses to resource availability and predation. There was also little evidence that diversity stabilized total herbivore abundance. Although consumer effects on lower levels were inconsistent, the strong physicochemical forcing of community structure supports suggestions that eelgrass communities are highly vulnerable to natural and anthropogenic changes in climate and hydrography.

Eutrophication and fishing are common perturbations in aquatic ecosystems that have pervasive effects on community structure, including species diversity and abundance. While sedi- ment biogeochemical processes probably respond to these stressors, the linkages to ecosystem func- tioning remain poorly understood. To explore these linkages, we experimentally manipulated water column nutrient levels and food web composition (i.e. predator and grazer presence and absence) in a factorial design using field enclosures situated in a natural eelgrass Zostera marina bed. After 28 d, we quantified sediment organic matter (SOM) abundance and composition using measures of total organic carbon and nitrogen as well as fatty acid (FA) biomarkers. Nutrient enrichment led to a rapid increase of epiphytes and a decline in Z. marina biomass. Responding to the available algae, grazers reduced epiphytes and the abundance of microalgal FAs in the sediment. Predators reduced Z. marina abundance and possibly its ability to trap particulate organic matter (OM), leading to lower sediment organic carbon content and total FA abundance. There was evidence of a trophic cascade as FA contributions to sediments from epiphytes and diatoms were higher in treatments with both grazers and predators than in treatments with grazers only. Predators increased contributions of labile diatom-derived OM, which probably resulted in higher proportions of bacterial FA. Inter- actions between nutrient additions and food web composition indicated that SOM responses were complex and not predictable from single variables. Changes in SOM composition, combined with a rapid heterotrophic bacterial response, suggest that resource levels and aboveground community structure are important to sediment biogeochemistry in natural seagrass systems.

Interacting changes in predator and prey diversity likely influence ecosystem properties but have rarely been experimentally tested. We manipulated the species richness of herbivores and predators in an experimental benthic marine community and measured their effects on predator, herbivore and primary producer performance. Predator composition and richness strongly affected several community and population responses, mostly via sampling effects. However, some predators survived better in polycultures than in monocultures, suggesting complementarity due to stronger intra- than interspecific interactions. Predator effects also differed between additive and substitutive designs, emphasizing that the relationship between diversity and abundance in an assemblage can strongly influence whether and how diversity effects are realized. Changing herbivore richness and predator richness interacted to influence both total herbivore abundance and predatory crab growth, but these interactive diversity effects were weak. Overall, the presence and richness of predators dominated biotic effects on community and ecosystem properties.

Nutrient loading can dramatically alter benthic communities and has been implicated in the worldwide decline of seagrass beds. Ongoing changes in food webs caused by overfishing could also contribute to seagrass decline. However, the interaction of these factors and the role of small invertebrate grazers in mediating them are poorly understood. We examined the relative impacts of nutrient loading and food web alteration on eelgrass Zostera marina L. community structure in Chesapeake Bay by manipulating nutrients, predatory crabs, and invertebrate grazers in field enclo- sures over 28 d in summer. Nutrient loading increased epiphyte accumulation early in the experi- ment, decreased eelgrass biomass, and reduced the abundance of the colonial tunicate Botryllus schlosseri. Grazers decreased epiphyte accumulation, altered the recruitment of sessile inverte- brates, and sometimes damaged eelgrass via overgrazing. Crabs reduced the abundance of eelgrass, and changed the species composition and abundance of grazers and sessile invertebrates. On aver- age, the impacts of food web alterations and nutrient loading were comparable in magnitude and tended to be additive, rather than interactive. However, the distinct responses of different taxa in the community to the experimental treatments indicated that food web structure interacted with both bottom-up and top-down forces to determine overall community organization. These results high- light the importance of incorporating food web dynamics into seagrass conservation and manage- ment efforts.

Nutrient pollution and reduced grazing each can stimulate algal blooms as shown by numerous experiments. But because experiments rarely incorporate natural variation in environmental factors and biodiversity, conditions determining the relative strength of bottom-up and top-down forcing remain unresolved. We factorially added nutrients and reduced grazing at 15 sites across the range of the marine foundation species eelgrass (Zostera marina) to quantify how top-down and bottom-up control interact with natural gradients in biodiversity and environmental forcing. Experiments confirmed modest top-down control of algae, whereas fertilisation had no general effect. Unexpectedly, grazer and algal biomass were better predicted by cross-site variation in grazer and eelgrass diversity than by global environmental gradients. Moreover, these large-scale patterns corresponded strikingly with prior small-scale experiments. Our results link global and local evidence that biodiversity and top-down control strongly influence functioning of threatened seagrass ecosystems, and suggest that biodiversity is comparably important to global change stressors. National Science Foundation [OCE-1031061] info:eu-repo/semantics/publishedVersion

Dinucleoside polyphosphates act as agonists on purinergic P2Y receptors to mediate a variety of cellular processes. Symmetrical, naturally occurring purine dinucleotides are found in most living cells and their actions are generally known. Unsymmetrical purine dinucleotides and all pyrimidine containing dinucleotides, however, are not as common and therefore their actions are not well understood. To carry out a thorough examination of the activities and specificities of these dinucleotides, a robust method of synthesis was developed to allow manipulation of either nucleoside of the dinucleotide as well as the phosphate chain lengths. Adenosine containing dinucleotides exhibit some level of activity on P2Y(1) while uridine containing dinucleotides have some level of agonist response on P2Y(2) and P2Y(6). The length of the linking phosphate chain determines a different specificity; diphosphates are most accurately mimicked by dinucleoside triphosphates and triphosphates most resemble dinucleoside tetraphosphates. The pharmacological activities and relative metabolic stabilities of these dinucleotides are reported with their potential therapeutic applications being discussed.

While most of the conclusions about diversity effects in Douglass, J.G., Duffy, J.E. & Bruno, J.F. [Ecol. Lett., 11, 2008, 1] are upheld, correction of a statistical miscalculation indicates that grazer diversity and predator diversity had combined effects on responses, but did not have interactive effects as initially reported.