Status, Trends, and Conservation of Eelgrass in the Northeast

Workshop invitation. The objective of the workshop was the timely dissemination of information regarding the science and conservation of eelgrass among federal, state, and provincial agencies and environmental organizations. Invited presentations and discussions highlighted status and trends in eelgrass distribution and abundance, factors controlling ecosystem change, current management issues, and regional examples of eelgrass conservation efforts. Participants were invited from all sectors of eelgrass science and management throughout eastern Canada and the northeastern United States. This event was hosted by Gulf of Maine Council on the Marine Environment, Friends of Casco Bay, Casco Bay Estuary Partnership, and Bay of Fundy Ecosystem Partnership. Additional sponsors included James W. Sewall Co., Maine Coastal Program, and U.S. Geological Survey

Eelgrass Habitat in Casco Bay: Past, Present, and Future? (2015 State of the Bay Presentation)

https://digitalcommons.usm.maine.edu/cbep-presentations/1018/thumbnail.jp

Relative effects of nutrient enrichment and grazing on epiphyton-macrophyte (Zostera marina L.) dynamics

Dissolved nutrient concentrations and invertebrate grazing activity regulate epiphytic biomass. Because epiphyton may limit light and carbon at leaf surfaces and the consequent productivity of submerged macrophytes, factors which influence epiphytic biomass may indirectly affect macrophyte abundance. I measured the simultaneous effects of water column nutrients (ambient or 3x ambient concentrations of nitrogen and phosphorus) and grazing (presence or absence of epifaunal community) on epiphyton and macrophytes seasonally in eelgrass (Zostera marina L.) microcosms on lower Chesapeake Bay. Grazing was more important than nutrients in controlling accrual of total epiphytic biomass, although effects on epiphytic components varied; numbers of diatoms responded to grazing, whereas numbers of cyanobacteria responded to nutrients. Numbers of heterotrophic microflagellates mimicked those of bacteria. The indirect effects of nutrients and grazing on macrophytes depended upon the relative magnitude of each factor and the physiological demands of the macrophyte. Under low grazer densities of early summer, macrophyte production (g m&\sp{lcub}-2{rcub}& d&\sp{lcub}-1{rcub}&) was reduced with grazer removal and nutrient enrichment independently. In contrast, under high densities of late summer, production was reduced by enrichment with grazers absent only. There were no macrophyte responses to treatment during the spring and fall, regardless of differences in epiphytic biomass; this may have been related to comparatively low light requirements of eelgrass at low temperatures. I used a simulation model to extrapolate microcosm results to predictions for community persistence. The model included ranges of environmental variables specific to lower Chesapeake Bay, where declines in eelgrass abundance in recent decades were correlated with nutrient enrichment, reduced grazer populations, and increased turbidity. Simulations indicated that neither nutrient enrichment nor loss of grazers alone would limit eelgrass survival, but together would cause community instability. Simulations indicated further that with grazers present, nutrient enrichment with a slight decrease in submarine irradiance would cause macrophyte loss. Measured rates of epiphytic accrual on artificial substrata in situ suggested that with grazers present, light reduction actually reduced the absolute rates of biomass accumulation despite nutrient enrichment. Predictions for macrophyte community stability must thus consider the relative effects of both direct (acting on macrophytes) and indirect (acting via epiphyton) environmental controls

Zostera marina (eelgrass) growth and survival along a gradient of nutrients and turbidity in the lower Chesapeake Bay

Survival of transplanted Zostera marina L. (eelgrass), Z. marina growth, and environmental conditions were studied concurrently at a number of sites in a southwestern tributary of the Chesapeake Bay to elucidate the factors limiting macrophyte distribution in this region. Consistent differences in survival of the transplants were observed, with no long-term survival at any of the sites that were formerly vegetated with this species but that currently remain unvegetated. Therefore, the current distribution of Z. marina likely represents the extent of suitable environmental conditions in the region, and the lack of recovery into historically vegetated sites is not solely due to lack of propagules. Poor long-term survival was related to seasonally high levels of water column light attenuation. Fall transplants died by the end of summer following exposure to levels of high spring turbidity (K-d \u3e 3.0) Accumulation of an epiphyte matrix during the late spring (0.36 to 1.14 g g(-1) dry wt) may also have contributed to this stress. Differences in water column nutrient levels among sites during the fall and winter (10 to 15 mu M dissolved inorganic nitrogen and 1 mu M dissolved inorganic phosphates) had no observable effect on epiphyte accumulation or macrophyte growth. Salinity effects were minor and there were no symptoms of disease. Although summertime conditions resulted in depressions in growth, they did not alone limit long-term survival. It is suggested that water quality conditions enhancing adequate seagrass growth during the spring may be key to long-term Z. marina survival and successful recolonization in this region

Mechanism and Inhibition of the Enoyl-ACP Reductases from Biodefense and Emerging Opportunistic Pathogens

The enoyl-acyl-carrier-protein reductase (ENR) catalyzes the last reaction in the elongation cycle in the fatty acid biosynthesis type II (FAS-II) pathway. To date, there are four known ENR isoenzymes: FabI, FabK, FabL, and FabV. We have rigorously characterized the FabV ENR from Burkholderia mallei (BmFabV) and have shown that this enzyme catalyzes substrate reduction via an ordered bi-bi mechanism, in which NADH binds first to the enzyme followed by the enoyl substrate [Lu, H. (2010) Biochemistry 49, 1281-1289]. However, this pathogen contains both FabI and FabV ENRs, and mechanistic insights into ENR substrate recognition are lacking in pathogens that solely express the FabV ENR. Thus, we extended our mechanistic studies to the FabV ENR from Yersinia pestis(YpFabV). Here, steady-state kinetic analysis revealed that YpFabV catalyzes substrate reduction via a random bi-bi mechanism. Site-directed mutagenesis at the N-terminal end of the helical substrate binding loop revealed that residue T276 plays a key role in substrate specificity and catalytic efficiency. Kinetic analysis and X-ray crystallographic structures demonstrated that the hydroxyl side chain of T276 is essential for hydrogen bonding interactions with NADH, while the methyl group provides favorable hydrophobic interactions with the acyl-coenzyme A (CoA) substrate. Our studies also revealed that alteration of the substrate binding mechanism through site-directed mutagenesis may affect the mode of inhibition of YpFabV. Structure-activity relationship (SAR) studies on the FabI ENR isoenzymes have been used as a platform to determine how slow binding inhibitors effect the transition and ground states of the drug-target binary complex. In turn, we were able to use rational inhibitor design to translate the slow-onset inhibition mechanism from FabI to FabV. Steady-state kinetic analysis of the P142W YpFabV mutant revealed a gain of slow-onset inhibition for an inhibitor (PT156) that displays rapid-reversible binding kinetics for the wild-type enzyme. This is the first example of slow-onset inhibition of a FabV ENR. | 171 page

Consumer identity, abundance and nutrient concentration affect epiphyte diversity in an experimental eelgrass system

Conceptual models predict a unimodal effect of consumer abundance on prey diversity with the highest diversity at intermediate consumer abundance (intermediate disturbance hypothesis). Consumer selectivity and prey productivity are assumed to be further important determinants. Preferential grazing on dominant prey species favoured by high nutrient supply is supposed to increase prey diversity, whereas the effect of consumers on prey diversity may be negative under low nutrient conditions (grazer reversal hypothesis). We tested the effect of four common consumers the isopod Idotea baltica, the amphipod Gammarus oceanicus, and the gastropods Littorina littorea and Rissoa membranacea on diversity and composition of epiphytes growing on eelgrass Zostera marina. Consumer density was manipulated (four levels: grazer free control, low, medium, high) based on abundances observed in eelgrass systems. Additionally, we manipulated nutrient supply (three levels) and the presence of Idotea in a factorial experiment. The impact of consumer abundance on epiphyte diversity varied depending on consumer identity and epiphyte evenness was affected rather than species number in this short-term experiment. Idotea reduced epiphyte diversity (Shannon-Wiener index H') and Gammarus increased epiphyte diversity. Littorina had no effect at low and medium abundance, but a negative effect in the high density treatment. Only Rissoa supported the conceptual models as it caused the proposed unimodal pattern in epiphyte diversity. The varying species-specific selectivity of the studied consumers is likely to explain their diverse impact on epiphyte diversity. Nutrients enhanced epiphyte diversity at medium enrichment, whereas higher nutrient supply reduced epiphyte diversity. The effect of Idotea changed from negative at low nutrient concentration to positive at higher nutrient supply, supporting the grazer reversal hypothesis. This study implies that consumer species identity and nutrient concentrations are important in controlling prey diversity and composition. Different consumer selectivity and changes in selectivity with growing consumer abundance and nutrient concentration are the causal factors for this effect

Nutrients increase epiphyte loads: broad-scale observations and an experimental assessment

The original publication can be found at www.springerlink.comThere is a global trend towards elevated nutrients in coastal waters, especially on human-dominated coasts. We assessed local- to regional-scale relationships between the abundance of epiphytic algae on kelp ( Ecklonia radiata) and nutrient concentrations across much of the temperate coast of Australia, thus assessing the spatial scales over which nutrients may affect benthic assemblages. We tested the hypotheses that (1) percentage cover of epiphytic algae would be greater in areas with higher water nutrient concentrations, and (2) that an experimental enhancement of nutrient concentrations on an oligotrophic coast, to match more eutrophic coasts, would cause an increase in percentage cover of epiphytic algae to match those in more nutrient rich waters. Percentage cover of epiphytes was most extensive around the coast of Sydney, the study location with the greatest concentration of coastal chlorophyll a (a proxy for water nutrient concentration). Elevation of nitrate concentrations at a South Australian location caused an increase in percentage cover of epiphytes that was comparable to percentage covers observed around Sydney’s coastline. This result was achieved despite our inability to match nutrient concentrations observed around Sydney (<5% of Sydney concentrations), suggesting that increases to nutrient concentrations may have disproportionately larger effects in oligotrophic waters.Bayden D. Russell, Travis S. Elsdon Bronwyn M. Gillanders and Sean D. Connel