System-wide submerged aquatic vegetation model for Chesapeake Bay

A predictive model of submerged aquatic vegetation (SAV) biomass is coupled to a eutrophication model of Chesapeake Bay. Domain of the model includes the mainstem of the bay as well as tidal portions of major embayments and tributaries. Three SAV communities are modeled: ZOSTERA, RUPPIA, and FRESHWATER. The model successfully computes the spatial distribution and abundance of SAV for the period 1985–1994. Spatial distribution is primarily determined by computed light attenuation. Sensivitity analysis to reductions in nutrient and solids loads indicates nutrient controls will enhance abundance primarily in areas that presently support SAV. Restoration of SAV to areas in which it does not presently exist requires solids controls, alone or in combination with nutrient controls. For regions in which SAV populations exist at the refuge level or greater, improvements in SAV abundance are expected within 2 to 10 years of load reductions. For regions in which no refuge population exists, recovery time is unpredictable and will depend on propagule supply.     

Long-Term Trends in Submersed Aquatic Vegetation (SAV) in Chesapeake Bay, USA, Related to Water Quality

Chesapeake Bay supports a diverse assemblage of marine and freshwater species of submersed aquatic vegetation (SAV) whose broad distributions are generally constrained by salinity. An annual aerial SAV monitoring program and a bi-monthly to monthly water quality monitoring program have been conducted throughout Chesapeake Bay since 1984. We performed an analysis of SAV abundance and up to 22 environmental variables potentially influencing SAV growth and abundance (1984–2006). Historically, SAV abundance has changed dramatically in Chesapeake Bay, and since 1984, when SAV abundance was at historic low levels, SAV has exhibited complex changes including long-term (decadal) increases and decreases, as well as some large, single-year changes. Chesapeake Bay SAV was grouped into three broad-scale community-types based on salinity regime, each with their own distinct group of species, and detailed analyses were conducted on these three community-types as well as on seven distinct case-study areas spanning the three salinity regimes. Different trends in SAV abundance were evident in the different salinity regimes. SAV abundance has (a) continually increased in the low-salinity region; (b) increased initially in the medium-salinity region, followed by fluctuating abundances; and (c) increased initially in the high-salinity region, followed by a subsequent decline. In all areas, consistent negative correlations between measures of SAV abundance and nitrogen loads or concentrations suggest that meadows are responsive to changes in inputs of nitrogen. For smaller case-study areas, different trends in SAV abundance were also noted including correlations to water clarity in high-salinity case-study areas, but nitrogen was highly correlated in all areas. Current maximum SAV coverage for almost all areas remain below restoration targets, indicating that SAV abundance and associated ecosystem services are currently limited by continued poor water quality, and specifically high nutrient concentrations, within Chesapeake Bay. The nutrient reductions noted in some tributaries, which were highly correlated to increases in SAV abundance, suggest management activities have already contributed to SAV increases in some areas, but the strong negative correlation throughout the Chesapeake Bay between nitrogen and SAV abundance also suggests that further nutrient reductions will be necessary for SAV to attain or exceed restoration targets throughout the bay.     

Fish biomass size spectra in Chesapeake Bay

Biomass size spectra of pelagic fish were modeled to describe community structure, estimate potential fish production, and delineate trophic relationships in Chesapeake Bay. Spectra were constructed from midwater trawl collections each year in April, June–August, and October 1995–2000. The size spectra were bimodal: the first spectral dome corresponded to small zooplanktivorous fish, primarily bay anchovyAnchoa mitchilli; the second dome consisted of larger fish from several feeding guilds that are supported by multiple prey-predator linkages. Annual production estimates of pelagic fish, derived from a mean production to biomass ratio, varied nearly three-fold, ranging from 162 × 109 kcal (125 × 103 tons) in 1996 to 457 × 109 kcal (352 × 103 tons) in 2000. Seasonally, the biomass level and mean individual sizes of fish in the first dome increased from April to October, while the biomass level of the second dome was relatively stable. Regionally, biomass levels in the second dome were higher than biomasses in the first dome for the upper and lower Bay, but were minimal in the middle Bay where seasonal and episodic hypoxia occurs. To test a benthic-pelagic coupling hypothesis that could explain the higher biomass in the second domes for the lower and upper Bay, a cyclic size-spectrum model was fit that included only species in the zooplanktivorous-piscivorous fish guilds. The mean, normalized slope equaled ?1, indicating that zooplanktivorous fish may support piscivore production, but that a benthic-pelagic linkage is required to fully support fish production in the second dome. Interannual variability in slopes and intercepts of modeled size spectra was related to salinity, recruitment level of bay anchovy, and the primary axis of a correspondence analysis (salinity effect) on fish community structure. The spectral slope and intercept of normalized spectra were lowest in 1996, a near-record wet year. Results suggest that fish size spectra can be developed as useful indicators of ecosystem state and response to perturbations, especially if prey-predator relationships are explicitly represented.     

Seasonal-Scale and Decadal-Scale Sediment-Vegetation Interactions on the Subaqueous Susquehanna River Delta,Upper Chesapeake Bay

Submersed aquatic vegetation (SAV) have been a prominent feature on the Susquehanna Flats, the shallow, subaqueous delta of the Susquehanna River, Maryland. SAV were absent from the Flats between 1972 and 2000, but have since recovered. While it is well established that SAV can improve water quality by promoting sediment and nutrient retention, it is not well understood how SAV on the Flats modulate sediment input from the Susquehanna River into the Upper Chesapeake Bay over different timescales. This study evaluates sedimentation on the Flats over seasonal to decadal timescales, using naturally occurring radioisotopes (⁷Be, ²¹⁰Pb) within the context of SAV biomass and Flats geomorphology. Results indicate that sedimentation on the Flats is both spatially and temporally variable. Although this variability cannot be explained by relationships with grain size and SAV biomass, river discharge, sediment supply, and geometry over the SAV bed likely control sedimentation in this system. Decadal-scale sedimentation is influenced by both flood events and changes in SAV biomass abundance. Average annual sediment accumulation was higher when SAV were present than when SAV were absent. SAV bed area was strongly correlated with average annual accumulation rate. These results suggest that a positive feedback between SAV abundance and accumulation rate exists; however, sediment supply and transport pathways are also important factors.     

Standing crops of marsh vegetation of two tributaries of Chesapeake Bay

A comparative study of the standing crop of marsh vegetation was made of the Patuxent River and Parker Creek, two tributaries of Chesapeake Bay. The biomass of marsh vegetation in the tidal freshwater and brackish regions of the Patuxent was relatively uniform with regard to salinity, seasonally high concentrations of dissolved nitrogen, and phosphorus and nutrient gradient. Maximum values of biomass occurred in the tidal freshwater and slightly brackish water region of Parker Creek, a system whose nutrient concentrations approximated 20% of those of Patuxent River. Biomass values for the Patuxent River and Parker Creek averaged about 1417 and 895 g m^?2 dry weight, respectively. Estimates of total annual marsh production based on the maximum standing crop was 27×10³ and 519 metric tons, respectively, for the Patuxent River and Parker Creek.     

Suspended sediment discharge of the Susquehanna river to Northern Chesapeake Bay, 1966 to 1976

Between 1966 and 1976, the Susquehanna River at Conowingo, Maryland, discharged approximately 50 million metric tons of suspended sediment to Northern Chesapeake Bay. About 40 million metric tons were discharged in two floods associated with hurricanes: Agnes (24–30 June 1972) and Eloise (26–30 September 1975). In the six years with complete records and no major floods, the Susquehanna River discharged 0.9±0.3 million metric tons of suspended sediment per year at Conowingo Dam. Aout 50 to 60 percent of the annual sediment discharge is usually floods, suspended sediment transport at Harrisburg, Pennsylvania, exceeds sediment discharge at Conowingo, Maryland, probably due to sediment deposition in the lower river and behind three hydroelectric dams.     

Scales of nutrient-limited phytoplankton productivity in Chesapeake Bay

The scales on which phytoplankton biomass vary in response to variable nutrient inputs depend on the nutrient status of the plankton community and on the capacity of consumers to respond to increases in phytoplankton productivity. Overenrichment and associated declines in water quality occur when phytoplankton growth rate becomes nutrient-saturated, the production and consumption of phytoplankton biomass become uncoupled in time and space, and phytoplankton biomass becomes high and varies on scales longer than phytoplankton generation times. In Chesapeake Bay, phytoplankton growth rates appear to be limited by dissolved inorganic phosphorus (DIP) during spring when biomass reaches its annual maximum and by dissolved inorganic nitrogen (DIN) during summer when phytoplankton growth rates are highest. However, despite high inputs of DIN and dissolved silicate (DSi) relative to DIP (molar ratios of N∶P and Si∶P>100), seasonal accumulations of phytoplankton biomass within the salt-intruded-reach of the bay appear to be limited by riverine DIN supply while the magnitude of the spring diatom bloom is governed by DSi supply. Seasonal imbalances between biomass production and consumption lead to massive accumulations of phytoplankton biomass (often>1,000 mg Chl-a m^?2) during spring, to spring-summer oxygen depletion (summer bottom water <20% saturation), and to exceptionally high levels of annual phytoplankton production (>400 g m^?2 yr^?1). Nitrogen-dependent seasonal accumulations of phytoplankton biomass and annual production occur as a consequence of differences in the rates and pathways of nitrogen and phosphorus cycling within the bay and underscore the importance of controlling nitrogen inputs to the mesohaline and lower reaches of the bay.     

Effects of low dissolved oxygen events on the macrobenthos of the lower Chesapeake Bay

The effects of low dissolved oxygen or hypoxia (<2 mg l^?1) on macrobenthic infaunal community structure and composition in the lower Chesapeake Bay and its major tributaries, the Rappahannock, York, and James rivers are reported. Macrobenthic communities at hypoxia-affected stations were characterized by lower species diversity, lower biomass, a lower proportion of deep-dwelling biomass (deeper than 5 cm in the sediment), and changes in community composition. Higher dominance in density and biomass of opportunistic species (e.g., euryhaline annelids) and lower dominance of equilibrium species (e.g., long-lived bivalves and maldanid polychaetes) were observed at hypoxia-affected stations. Hypoxia-affected macrobenthic communities were found in the polyhaline deep western channel of the bay mainstem north of the Rappahannock River and in the mesohaline region of the lower Rappahannock River. No hypoxic effects on the infaunal macrobenthos were found in the York River, James River, or other deep-water channels of the lower Chesapeake Bay.     

Water quality associated with survival of submersed aquatic vegetation along an estuarine gradient

The decline of submersed aquatic vegetation (SAV) in tributaries of the Chesapeake Bay has been associated with increasing anthropogenic inputs, and restoration of the bay remains a major goal of the present multi-state “Bay Cleanup” effort. In order to determine SAV response to water quality, we quantified the water column parameters associated with success of transplants and natural regrowth over a three-year period along an estuarine gradient in the Choptank River, a major tributary on the eastern shore of Chesapeake Bay. The improvement in water quality due to low precipitation and low nonpoint source loadings during 1985–1988 provided a natural experiment in which SAV was able to persist upstream where it had not been for almost a decade. Mean water quality parameters were examined during the growing season (May–October) at 14 sites spanning the estuarine gradient and arrayed to show correspondence with the occurrence of SAV. Regrowth of SAV in the Choptank is associated with mean dissolved inorganic nitrogen <10 μM; mean dissolved phosphate <0.35 μM; mean suspended sediment <20 mg l^?1; mean chlorophylla in the water column <15 μg l^?1; and mean light attenuation coefficient (Kd) <2 m^?1. These values correspond well with those derived in other parts of the Chesapeake, particularly in the lower bay, and may provide managers with values that can be used as target concentrations for nutrient reduction strategies where SAV is an issue.     

Mechanisms of Storm-Related Loss and Resilience in a Large Submersed Plant Bed

There is a growing emphasis on preserving ecological resilience, or a system’s capacity to absorb or recover quickly from perturbations, particularly in vulnerable coastal regions. However, the factors that affect resilience to a given disturbance are not always clear and may be system-specific. We analyzed and synthesized time series datasets to explore how extreme events impacted a large system of submersed aquatic vegetation (SAV) in upper Chesapeake Bay and to identify and understand associated mechanisms of resilience. We found that physical removal of plants around the edge of the bed by high flows during a major flood event as well as subsequent wind-driven resuspension of newly deposited sediment and attendant light-limiting conditions were detrimental to the SAV bed. Conversely, it appears that the bed attenuated high flows sufficiently to prevent plant erosion at its inner core. The bed also attenuated wind-driven wave amplitude during seasonal peaks in plant biomass, thereby decreasing sediment resuspension and increasing water clarity. In addition, clear water appeared to “spill over” into adjacent regions during ebb tide, improving the bed’s capacity for renewal by creating more favorable growing conditions in areas where plant loss had occurred. These analyses demonstrate that positive feedback processes, whereby an SAV bed modifies its environment in ways that improve its own growth, likely serve as mechanisms of SAV resilience to flood events. Although this work focuses on a specific system, the synthetic approach used here can be applied to any system for which routine monitoring data are available.     

The response of fishes to submerged aquatic vegetation complexity in two ecoregions of the mid-Atlantic bight: Buzzards Bay and Chesapeake Bay

Estuarine seagrass ecosystems provide important habitat for fish and invertebrates and changes in these systems may alter their ability to support fish. The response of fish assemblages to alteration of eelgrass (Zostera marina) ecosystems in two ecoregions of the Mid-Atlantic Bight (Buzzards Bay and Chesapeake Bay) was evaluated by sampling historical eelgrass sites that currently span a broad range of stress and habitat quality. In two widely separated ecoregions with very different fish faunas, degradation and loss of submerged aquatic vegetation (SAV) habitat has lead to declines in fish standing stock and species richness. The abundance, biomass, and species richness of the fish assemblage were significantly higher at sites that have high levels of eelgrass habitat complexity (biomass >100 wet g m^?2; density <100 shotts m^?2) compared to sites that have reduced eelgrass (biomass <100 wet g m^?2; density <100 shoots m^?2) or that have completely lost eelgrass. Abundance, biomass, and species richness at reduced eelgrass complexity sites also were more variable than at high eelgrass complexity habitats. Low SAV complexity sites had higher proportions of pelagic species that are not dependent on benthic habitat structure for feeding or refuge. Most species had greater abundance and were found more frequently at sites that have eelgrass. The replacement of SAV habitats by benthic macroalgae, which occurred in Buzzards Bay but not Chesapeake Bay, did not provide an equivalent habitat to seagrass. Nutrient enrichment-related degradation of eelgrass habitat has diminished the overall capacity of estuaries to support fish populations.     

Sediment Accumulation Rates and Submersed Aquatic Vegetation (SAV) Distributions in the Mesohaline Chesapeake Bay, USA

This study assesses spatial and temporal sedimentological trends in four mesohaline Chesapeake Bay submersed aquatic vegetation (SAV) habitats, two with persistent SAV beds and two with ephemeral SAV beds, to determine their relationship to current and historical sediment characteristics??grain size, organic content, and accumulation rates. In general, grain size is similar among all sites, and subsurface sediment differs from surficial sediment only at one site where a thin surficial sand layer (??2?C3?cm) is present. This thin sand layer is not completely preserved in the longer-term sedimentary record even though it is critical to determining whether the sediment is suitable for SAV. Evidence for nearshore fining, similar to that observed in the deeper waters of the Bay, is present at the site where the shoreline has been hardened suggesting that locations with hardened shorelines limit exchange of coarser (sandy) material between the shore and nearshore environments. Whether the fining trend will continue to a point at which the sediment will become unsuitable for SAV in the future or whether some new type of equilibrium will be reached cannot be addressed with our data. Instead, our data suggest that SAV presence/absence is related to changes in sedimentary characteristics??persistent beds have relatively steady sediment composition, while ephemeral beds have finer sediments due to reduced sand input. Additionally, sediment accumulation rates in the persistent beds are ??9?mm/year, whereas rates in the ephemeral beds are ??3?mm/year. Thus, the ephemeral sites highlight two potential sedimentary controls on SAV distribution: the presence of a sufficiently thick surficial sand layer as previously postulated by Wicks (2005) and accumulation rates high enough to bury seeds prior to germination and/or keep up with sea-level rise.     

Relationship between Hypoxia and Macrobenthic Production in Chesapeake Bay

Human development has degraded Chesapeake Bay's health, resulting in an increase in the extent and severity of hypoxia (≤2 mg O₂ l^-1). The Bay's hypoxic zones have an adverse effect on both community structure and secondary production of macrobenthos. From 1996 to 2004, the effect of hypoxia on macrobenthic production was assessed in Chesapeake Bay and its three main tributaries (Potomac, Rappahannock, and York Rivers). Each year, in the summer (late July???early September), 25 random samples of the benthic macrofauna were collected from each system, and macrobenthic production in the polyhaline and mesohaline regions was estimated using Edgar's allometric equation. Fluctuations in macrobenthic production were significantly correlated with dissolved oxygen. Macrobenthic production was 90 % lower during hypoxia relative to normoxia. As a result, there was a biomass loss of ~7,320–13,200 metric tons C over an area of 7,720 km², which is estimated to equate to a 20 % to 35 % displacement of the Bay's macrobenthic productivity during the summer. While higher consumers may benefit from easy access to stressed prey in some areas, the large spatial and temporal extent of seasonal hypoxia limits higher trophic level transfer, via the inhibition of macrobenthic production. Such a massive loss of macrobenthic production would be detrimental to the overall health of the Bay, as it comes at a time when epibenthic and demersal predators have high-energy demands.     

Seasonal and long-term trends in the water quality of Florida Bay (1989–1997)

Analysis of 6 yr of monthly water quality data was performed on three distinct zones of Florida Bay: the eastern bay, central bay, and western bay. Each zone was analyzed for trends at intra-annual (seasonal), interannual (oscillation), and long-term (monotonic) scales. the variables TON, TOC, temperature, and TN∶TP ratio had seasonal maxima in the summer rainy season; APA and Chla, indicators of the size and activity of the microplankton tended to have maxima in the fall. In contrast, NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, turbidity, and DO_sat, were highest in the winter dry season. There were large changes in some of the water quality variables of Florida Bay over the study period. Salinity and TP concentrations declined baywide while turbidity increased dramatically. Salinity declined in the eastern, central, and western Florida Bay by 13.6‰, 11.6‰, and 5.6‰, respectively. Some of the decrease in the eastern bay could be accounted for by increased freshwater flows from the Everglades. In contrast to most other estuarine systems, increased runoff may have been partially responsible for the decrease in TP concentrations as input concentrations were 0.3–0.5 μM. Turbidity in the eastern bay increased twofold from 1991 to 1996, while in the central and western bays it increased by factors of 20 and 4, respectively. Chla concentrations were particularly dynamic and spatially heterogeneous. In the eastern bay, which makes up roughly half of the surface area of Florida Bay, Chla declined by 0.9 μg l⁻¹ (63%). The hydrographically isolated central bay zone underwent a fivefold increase in phytoplankton biomass from 1989 to 1994, then rapidly declined to previous levels by 1996. In western Florida Bay there was a significant increase in Chla, yet median concentrations of Chla in the water column remained modest (∼2 μg l⁻¹) by most estuarine standards. Only in the central bay did the DIN pool increase substantially (threefold to sixfold). Notably, these changes in turbidity and phytoplankton biomass occurred after the poorly-understood seagrass die-off in 1987. It is likely the death and decomposition of large amounts of seagrass biomass can at least partially explain some of the changes in water quality of Florida Bay, but the connections are temporally disjoint and the process indirect and not well understood.     

Regional and temporal variability in distribution and abundance of bay anchovy (Anchoa mitchilli) eggs, larvae, and adult biomass in the Chesapeake Bay

Patterns and variability in reproductive output of pelagic fish are seldom determined at the ecosystem scale. We examined temporal and spatial variability in spawning by bay anchovy (Anchoa mitchilli), and in distribution and abundances of its pelagic early-life stages, throughout Chesapeake Bay. On two cruises in June and July 1993, ichthyoplankton and zooplankton were collected on 15 transects at 18.5-km (10 nautical mile) intervals over the 260-km length of the bay. Finer-scale sampling was carried out in a grid of stations between two transects on each cruise. Regional abundance patterns of bay anchovy eggs and larvae in the lower, mid, and upper Bay were compared with zooplankton abundances, environmental variables, and biovolumes of two gelatinous predators—the scyphomedusa Chrysaora quinquecirrha and the lobate ctenophore Mnemiopsis leidyi. Abundances of anchovy eggs, and, especially, larvae were higher in July than in June. Baywide daily egg production increased from 4.25×10¹² in June to 8.43×10¹² in July. Concentrations of zooplankton that are potential anchovy prey nearly doubled on a baywide basis between June and July, while biovolumes of the ctenophore declined. Except for scyphomedusan biovolumes, all analyzed organisms differed regionally in abundance and were patchily distributed at 1-km to 10-km sampling scales. Negative correlations between larval anchovy abundances and gelatinous predator biovolumes suggested that predation may have controlled abundances of bay anchovy early-life stages. Biomasses of adult anchovy, estimated from daily egg productions, were higher in the lower Bay and remarkably similar—23,433 tons in June and 23,194 tons in July. Most spawning by bay anchovy occurred during July in the seaward third of Chesapeake Bay, emphasizing the importance of this region for recruitment potential of the Bay's most abundant fish.     

Multi-decade Responses of a Tidal Creek System to Nutrient Load Reductions: Mattawoman Creek,Maryland USA

We developed a synthesis using diverse monitoring and modeling data for Mattawoman Creek, Maryland, USA to examine responses of this tidal freshwater tributary of the Potomac River estuary to a sharp reduction in point-source nutrient loading rate. Oligotrophication of these systems is not well understood; questions concerning recovery pathways, threshold responses, and lag times remain to be clarified and eventually generalized for application to other systems. Prior to load reductions Mattawoman Creek was eutrophic with poor water clarity (Secchi depth <0.5 m), no submerged aquatic vegetation (SAV), and large algal stocks (50–100 μg L^?1 chlorophyll-a). A substantial modification to a wastewater treatment plant reduced annual average nitrogen (N) loads from 30 to 12 g N m^?2 year^?1 and phosphorus (P) loads from 3.7 to 1.6 g P m^?2 year^?1. Load reductions for both N and P were initiated in 1991 and completed by 1995. There was no trend in diffuse N and P loads between 1985 and 2010. Following nutrient load reduction, NO₂?+?NO₃ and chlorophyll-a decreased and Secchi depth and SAV coverage and density increased with initial response lag times of one, four, three, one, and one year, respectively. A preliminary N budget was developed and indicated the following: diffuse sources currently dominate N inputs, estimates of long-term burial and denitrification were not large enough to balance the budget, sediment recycling of NH₄ was the single largest term in the budget, SAV uptake of N from sediments and water provided a modest seasonal-scale N sink, and the creek system acted as an N sink for imported Potomac River nitrogen. Finally, using a comparative approach utilizing data from other shallow, low-salinity Chesapeake Bay ecosystems, strong relationships were found between N loading and algal biomass and between algal biomass and water clarity, two key water quality variables used as indices of restoration in Chesapeake Bay.     

Composition of particulate organic matter in the southern Chesapeake Bay: Sources and Reactivity

The distribution of two classes of lipid biomarker compounds (fatty acids and sterols) was used in conjunction with several bulk parameters (total suspended solids, chlorophyll a, and particulate carbon and nitrogen concentrations) to examine spatial and temporal variability in the sources of particulate organic matter (POM) important to southern Chesapeake Bay. Based on these geochemical parameters, we found that suspended and sedimentary organic matter in the southern Chesapeake Bay is derived from autochthonous sources including a mixture of fresh and detrital phytoplankton, zooplankton, and bacteria. The dominant factor contributing to temporal variability during our study was phytoplankton productivity. Enrichments in particulate organic carbon, chlorophyll a, total fatty acids, total sterols, and a number of biomarkers specific to phytoplankton sources were found in particles collected from surface (1 m) and deep (1 m above the bottom) portions of the water column at several sites during the spring bloom in March 1996 and during a localized bloom in July 1995. Comparison of sites at the mouths of two tributaries (York and Rappahannock rivers) to southern Chesapeake Bay with two sites located in the bay mainsterm indicates spatial variation in the composition of POM was not significant in this region of the bay. The energetic nature of this region of the Chesapeake Bay most likely contributes to the observed homogeneity. Comparison with biomarker studies conducted in other estuaries suggests the high levels of productivity characteristic of the Chesapeake Bay contribute to high background levels of POM.     

The effects of eelgrass habitat loss on estuarine fish communities of southern New England

We studied the late June–August fish community in extant and former eelgrass (Zostera marina L.) habitats in 15 estuaries of Buzzards Bay, and in Waquoit Bay, Massachusetts, U.S. Our objective was to quantify the effects of eelgrass habitat loss on fish abundance, biomass, species composition and richness, life-history characteristics, and habitat use by examining the response of the fish community to eelgrass loss in Waquoit and Buttermilk Bays over an 11-yr period (1988–1999) and in 14 other embayments of Buzzards Bay during 1993, 1996, and 1998. Sampling sites were located in present-day or historical eelgrass beds and were classified according to eelgrass habitat complexity (zero complexity: no eelgrass; low complexity: <100 eelgrass shoots or <100 g wet weight m⁻²; high complexity: ≥100 shoots and ≥100 g wet weight m⁻²). Habitats that had lost eelgrass included a variety of substratum types, from bare mud bottom to dense accumulations of red, brown, and green macroalgae (up to 7,065 g wet weight m⁻²). Contemporaneous sampling of fish (by otter trawl) and vegetated habitat (by divers) was conducted at each site. Overall, fish abundance, biomass, species richness, dominance, and life history diversity decreased significantly along the gradient of decreasing eelgrass habitat complexity. Loss of eelgrass was accompanied by significant declines in these measures of fish community integrity. Ten of the 13 most common species collected from 1988–1996 in Waquoit and Buttermilk Bays showed maximum abundance and biomass in sites with high eelgrass habitat complexity. All but two common species declined in abundance and biomass with the complete loss of eelgrass.     

Habitat Affects Survival of Translocated Bay Scallops, Argopecten irradians concentricus (Say 1822), in Lower Chesapeake Bay

Bay scallop (Argopecten irradians) populations existed in Chesapeake Bay until 1933, when they declined dramatically due to a loss of seagrass habitat. Since then, there have been no documented populations within the Bay. However, some anecdotal observations of live bay scallops within the lower Bay suggest that restoration of the bay scallop is feasible. We therefore tested whether translocated adults of the southern bay scallop, Argopecten irradians concentricus, could survive during the reproductive season in vegetated and unvegetated habitats of the Lynnhaven River sub-estuary of lower Chesapeake Bay in the absence of predation. Manipulative field experiments evaluated survival of translocated, caged adult scallops in eelgrass Zostera marina, macroalgae Gracilaria spp., oyster shell, and rubble plots at three locations. After a 3-week experimental period, scallop survival was high in vegetated habitats, ranging from 98% in their preferred habitat, Z. marina, to 90% in Gracilaria spp. Survival in Z. marina was significantly higher than that in rubble (76%) and oyster shell (78%). These findings indicate that reproductive individuals can survive in vegetated habitats of lower Chesapeake Bay when protected from predators and that establishment of bay scallop populations within Chesapeake Bay may be viable.     

Influences of Salinity and Light Availability on Abundance and Distribution of Tidal Freshwater and Oligohaline Submersed Aquatic Vegetation

Submersed aquatic vegetation (SAV) communities have undergone declines worldwide, exposing them to invasions from non-native species. Over the past decade, the invasive species Hydrilla verticillata has been documented in several tributaries of the lower Chesapeake Bay, Virginia. We used annual aerial mapping surveys from 1998 to 2007, integrated with spatial analyses of water quality data, to analyze the patterns and rates of change of a H. verticillata-dominated SAV community and relate them to varying salinity and light conditions. Periods of declining SAV coverage corresponded to periods where salinities exceeded 7 and early growing season (April to May) Secchi depths were <0.4 m. Increases were driven by the expansion of H. verticillata along with several other species into the upper estuary, where some areas experienced an 80% increase in cover. Field investigations revealed H. verticillata dominance to be limited to the upper estuary where total suspended solid concentrations during the early growing season were <15 mg l⁻¹ and salinity remained <3. The effect of poor early growing season water clarity on annual SAV growth highlights the importance of water quality during this critical life stage. Periods of low clarity combined with periodic salinity intrusions may limit the dominance of H. verticillata in these types of estuarine systems. This study shows the importance of the use of these types of biologically relevant episodic events to supplement seasonal habitat requirements and also provides evidence for the potential important role of invasive species in SAV community recovery.