Hypoxia in Chesapeake Bay Tributaries: Worsening effects on Macrobenthic Community Structure in the York River

We assessed the effects of hypoxia on macrobenthic communities in the York and Rappahannock Rivers, Chesapeake Bay, in box-core samples before and after hypoxic episodes in 2003 and 2004. Hypoxia occurred in both years and was associated with a decrease in biomass and a shift in community structure toward opportunistic species in both rivers. Long-term data indicate that the frequency of hypoxia in the York has increased over the last 22 years. In previous work from ∼20 years ago, the macrobenthic community structure did not change in response to hypoxia in the York; however, in the present study hypoxia was associated with a reduction in community biomass and a change in community structure. We conclude that currently hypoxia is a more important environmental problem in the York than in previous years. Hypoxia likely negatively affects the estuarine food web, as lower macrobenthic biomass could decrease food availability to epibenthic predators.     

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.     

Hypoxia and salinity in Virginia estuaries

Hypoxia, periods of reduced dissolved oxygen concentrations, has been observed not only in the Chesapeake Bay but also in the deeper waters of the Virginia estuaries that are tributaries to the Chesapeake Bay. When water temperature exceeded 20°C, minimum oxygen concentrations were observed to be <50% of saturation concentrations in 75%, 50% and 2% of the surveys in the estuaries of the Rappahannock, York and James rivers, respectively. The observation that hypoxia rarely occurred in the James River is surprising, given the fact that it receives the greatest amount of wastewater. Analysis of the oxygen budgets in these estuaries indicates that the variations in the frequency, duration, and severity of hypoxia are related to the net movement of bottom waters. This relationship has significant implications for the management of water quality and marine fisheries.     

Egg extrusion as a kepone-clearance route in the blue crab,Callinectes sapidus

The concentration of Kepone in extruded egg masses and the backfin muscles of the blue crab,Callinectes sapidus, collected from seven stations in the lower James River and lower Chesapeake Bay was determined. Crabs from the lower James River were generally more contaminated than those from the lower Chesapeake Bay. Extruded eggs contain approximately three times more Kepone than backfin muscle. Egg extrusion is concluded to be a major route of Kepone clearance from female blue crabs.     

Influence of land use on macrobenthic communities in nearshore estuarine habitats

Macrobenthic community indices were examined for their ability to characterize the influence of shoreline alteration and watershed land use in nearshore estuarine environments of the Chesapeake Bay, U.S.A. Twenty-three watersheds were surveyed in 2002 and 2003 for nearshore macrobenthic assemblages, environmental parameters (i.e., dissolved oxygen, pH, total suspended solids, salinity, and sediment composition), shoreline condition, and land use. Two indices of macrobenthic biological integrity, benthic index of biological integrity in the nearshore (B-IBI_N) and abundance biomass comparison (W-value), were evaluated for associations with environmental and shoreline condition, and riparian and watershed land use. Comparisons between nearshore measures of the B-IBI with offshore values (>2 m; Chesapeake Bay benthic index of biological integrity [B-IBI_CB]) were conducted to assess the ability of the index to reflect land use patterns at near and far proximities to shore. Nearshore macrobenthic communities were represented by a total of 94 species (mean number of species =9.2 ± 0.4 sample⁻¹), and were dominated by the phyla Arthropoda, Annelida, and Mollusca. Temporal variability in environmental conditions and macrobenthic abundance and biomass may be attributable to the notable increase in precipitation in 2003 that led to nutrient influxes and algal blooms. For the biotic indices applied in the nearshore, the highest scores were associated with forested watersheds (W-value, B-IBI_N). Ecological thresholds were identified with nonparametric change-point analysis, which indicated a significant reduction in B-IBI_N and W-value scores when the amount of developed shoreline exceeded 10% and developed watershed exceeded 12%, respectively.     

The relationship between trends in a condition index of the American oyster,Crassostrea virginica,and environmental parameters in three Virginia estuaries

Oyster Condition Index (CI) was partitioned, using a moving average filter, into seasonal cycles and long-term trends in the James, York, and Rappahannock rivers for the period 1970–1983. Seasonal cyclic fluctuations in CI could be explained partially by changes in salinity and number of days within various temperature regimes. Long-term trends in the James River show a steady increase in CI over the study period, while a concurrent decline was noted in the Rappahannock River. Superimposed on these trends is a 4 to 5 yr cycle that is in synchrony with river discharge (salinity). In the York River, CI peaked in 1975–1976 at all stations. Measured environmental parameters do not sufficiently explain the trends. We speculate that the differences in the Rappahannock and James rivers may be due to a decline in bottom oxygen as a result of gravitational circulation differences.     

The Influence of Storms on Water Quality and Phytoplankton Dynamics in the Tidal James River

Due to the unpredictable nature of intense storms and logistical constraints of sampling during storms, little is known about their immediate and long-term impacts on water quality in adjacent aquatic ecosystems. By combining targeted experiments with routine monitoring, we evaluated immediate impacts of two successive storm events on water quality and phytoplankton community response in the tidal James River and compared these findings to a non-storm year. The James River is a subestuary of the Chesapeake Bay and sampling was conducted before, during, and after Hurricane Irene and Tropical Storm (TS) Lee in 2011 and during the same time period (late summer/early fall) in 2012 when there were no storms. We collected and compiled data on nutrient and chlorophyll a concentrations, phytoplankton abundance, nitrogen uptake, primary productivity rates, and surface salinity, temperature, and turbidity in the meso- and polyhaline segments of the James River. Hurricane Irene introduced significant amounts of freshwater over the entire James River and Chesapeake Bay watersheds, while rainfall from TS Lee fell primarily on the tidal fresh region of the James River and headwaters of the Chesapeake Bay. Dinoflagellates dominated the algal community in the meso- and polyhaline segments prior to the storms in 2011, and a mixed diatom community emerged after the storms. In the mesohaline river segment, cyanobacteria abundance increased after TS Lee when salinities were depressed, likely due to washout from the oligohaline and tidal fresh regions of the river. In 2012, dinoflagellates dominated the community in both segments of the river during late summer but diatoms were also abundant and their biomass fluctuated throughout the summer and fall. Cyanobacteria were not present in either segment. Overall, we observed that the high-intensity rainfall from Hurricane Irene combined with high flushing in the headwaters as a result of TS Lee likely reduced primary productivity and altered community composition in the mesohaline segment but not the more estuarine-influenced polyhaline segment. Understanding the influence of high freshwater flow with a short residence time associated with storms is key to the planning and management of estuarine restoration as such disturbances are projected to increase as a result of climate change.     

Kepone concentration in juvenile anadromous fishes

Young-of-the-year alewife, American shad, blueback herring, and striped bass were analyzed for Kepone contamination. Samples were collected from the Potomac, Rappahannock, Pamunkey, Mattaponi, Chickahominy, and James rivers during the period 1977–79. Concentrations of Kepone ≥0.3 ppm occurred in all four species collected in the James River nursery zone between km 65 and 120, and in the lower Chickahominy River. Concentrations of Kepone <0.3, ppm were also present in samples from the Mattaponi and Pamunkey rivers. Four possible explanations, for the occurrence of Kepone in these samples from the upper York River system (Mattaponi and Pamunkey rivers) were, evaluated. Kepone was not detectable in samples from the Rappahannock and Potomac rivers.     

Epifaunal communities thrive in an estuary with hypoxic episodes

We characterized the abundance and species composition of sessile and mobile epifaunal assemblages in the York River, a tributary of the Chesapeake Bay, U.S., during the summer hypoxia seasons in 1996 and 1997. We collected communities on artificial substrates in two areas of the river that have historically experienced different exposure to hypoxia. Despite frequent hypoxic stress, epifauna formed dense communities in both areas. Dominant species comprised a range of phyla and included the polychaetesPolydora cornuta andSabellaria vulgaris, the bryozoansMembranipora tenuis andConopeum tenuissimum, the tunicateMolgula manhattensis, the barnacleBalanus improvisus, the anemoneDiadumene leucolena, and the hydroidsEctopleura dumortieri andObelia bicuspidata. Common mobile species included the nudibranchsCratena kaoruae andDoridella obscura, the amphipodsMelita nitida andParacaprella tenuis, the polychaeteNereis succinea, and the flatwormStylochus ellipticus. We found few differences in species composition between the two areas, even though one area usually experienced lower oxygen concentrations during hypoxic events, suggesting that hypoxia does not exclude any epifaunal species, in the York River. We did find differences between the two study areas in percent cover and abundance of some species. While tunicates, hydroids, and anemones were equally abundant in both areas during both study years, bryozoans and the polychaeteS. vulgaris were more abundant in the area with gereally higher oxygen, suggesting that they may be less tolerant of hypoxic stress. The polychaeteP. cornuta was more abundant in the area that usually had lower oxygen. These results suggest that many epifaunal species have high hypoxia tolerance, and most epifaunal species found in the lowr York River are able to survive in hypoxic areas. Epifaunal species are not necessarily more susceptible to hypoxia than infaunal species in the York River. Epifaunal communities in areas with brief hypoxic episodes and moderate hypoxia (0.5–2 mg O₂ I⁻¹) can persist with little change in species composition, and with few changes in abundance, as oxygen concentrations fall.     

Partial Migration Across Populations of White Perch (<Emphasis Type="Italic">Morone americana</Emphasis>): A Flexible Life History Strategy in a Variable Estuarine Environment

We evaluated the prevalence of partial migration, coexisting resident and migratory life history types, within six white perch (Morone americana) populations in sub-estuaries (Upper Bay, and Potomac, Choptank, Nanticoke, James, and York Rivers) of the Chesapeake Bay. Otolith stable isotope (δ¹⁸O) values were used to resolve fish habitat use along an estuarine salinity gradient and define resident or migratory behavior. The majority of adults within Upper Bay and Potomac River populations were resident, whereas individuals from the Choptank, Nanticoke, James, and York Rivers were predominantly migratory. Beyond population differences, large interannual variability in life history types was observed, likely due to differences in estuarine conditions that influence growth rate of individuals (e.g., temperature, zooplankton density). Because we observed partial migration in all study populations, we suggest that this trait is characteristic of this species, permitting plastic responses to variation in the estuarine environment.     

The distribution of heavy metals in the hard clam,Mercenaria mercenaria,in the lower Chesapeake Bay region

Populations of the hard clam,Mercenaria mercenaria, were sampled at thirty sites in the lower Chesapeake Bay region in 1972 and 1973. Subsamples were taken for analysis for the trace metals cadmium, copper and zinc. Emphasis was placed on samples from the York and James rivers, two very productive and commercially utilized clam grounds. The levels of metals determined were comparable to those reported from other regions except Southampton Water in the United Kingdom. Differences in levels between the York and James rivers were statistically significant (P<0.001) which indicates that the James River probably suffers from contamination by these metals. One metal, copper, varied significantly with the age of the organism whereas the other two, cadmium and zinc, varied with salinity. InMercenaria the salinity of the sampling site must be considered before cadmium and zinc data can be interpreted. A standard action-level to denote pollution from copper would be adequate in this species.     

Patterns and Rates of Recovery of Macrobenthic Communities in a Polyhaline Temperate Estuary Following Sediment Disturbance: Effects of Disturbance Severity and Potential Importance of Non-local Processes

Community-level responses of soft sediment macrobenthos to two relatively large-scale disturbance events associated with dredged material (DM) disposal are examined for subtidal (>10 m) lower Chesapeake Bay. Disturbance severity (DM thickness on initial sampling date following disposal) and date of sampling were important factors explaining the patterns and rates of recovery for species richness, abundance, biomass, and community composition, but sediment disposal had minimal effects when DM thickness was ≤15 cm. It took 1.5 years or less following the cessation of disposal activities for richness, abundance, biomass and community composition at high disposal severity (DM > 15 cm) to attain levels measured at reference stations representing the ambient community of the region. Positive correlations of community structure metrics between the disposal area and reference stations provide evidence that non-local processes influenced patterns of recovery in this estuarine setting. Species interactions and food limitation may also have been important at local scales.     

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Fishes

The structure of the fish community associated with eelgrass beds in the lower Chesapeake Bay was studied over a 14 month period. A total of 24,182 individuals in 48 species was collected by otter trawl with Leiostomus xanthurus (spot) comprising 63% of the collection, Syngnathus fuscus (northern pipefish) 14%, Anchoa mitchilli (bay anchovy) 9%, and Bairdiella chrysoura (silver perch) 5%. The density and diversity of fishes were higher in vegetated areas compared to unvegetated areas; fishes were more abundant in night collections Fish abundance and species number increased in the spring and early summer as both water temperature and eelgrass biomass increased and decreased in the fall and winter as temperature and eelgrass biomass decreased. Gill netting revealed some of the top predators in the system, especially the sandbar shark, Carcharhinus milberti. The fish community in the Chesapeake Bay was quite different from North Carolina eelgrass fish communities. Most notable was the rarity of the pinfish, Lagodon rhomboides, which may be a very important predator in the structuring of the epifaunal communities.     

Analysis of the abundance of submersed aquatic vegetation communities in the Chesapeake Bay

A procedure was developed using aboveground field biomass measurements of Chesapeake Bay submersed aquatic vegetation (SAV), yearly species identification surveys, annual photographic mapping at 1∶24,000 scale, and geographic information system (GIS) analyses to determine the SAV community type, biomass, and area of each mapped SAV bed in the bay and its tidal tributaries for the period of 1985 through 1996. Using species identifications provided through over 10,000 SAV ground survey observations, the 17 most abundant SAV species found in the bay were clustered into four species associations: ZOSTERA, RUPPIA, POTAMOGETON, and FRESHWATER MIXED. Monthly aboveground biomass values were then assigned to each bed or bed section based upon monthly biomass models developed for each community. High salinity communities (ZOSTERA) were found to dominate total bay SAV aboveground biomass during winter, spring, and summer. Lower salinity communities (RUPPIA, POTAMOGETON, and FRESHWATER MIXED) dominated in the fall. In 1996, total bay SAV standing stock was nearly 22,800 metric tons at annual maximum biomass in July encompassing an area of approximately 25,670 hectares. Minimum biomass in December and January of that year was less than 5,000 metric tons. SAV annual maximum biomass increased baywide from lows of less than 15,000 metric tons in 1985 and 1986 to nearly 25,000 metric tons during the 1991 to 1993 period, while area increased from approximately 20,000 to nearly 30,000 hectares during that same period. Year-to-year comparisons of maximum annual community abundance from 1985 to 1996 indicated that regrowth of SAV in the Chesapeake Bay from 1985–1993 occurred principally in the ZOSTERA community, with 85% of the baywide increase in biomass and 71% of the increase in are a occurring in that community. Maximum biomass of FRESHWATER MIXED SAV beds also increased from a low of 3,200 metric tons in 1985 to a high of 6,650 metric tons in 1993, while maximum biomass of both RUPPIA and POTAMOGETON beds fluctuated between 2,450 and 4,600 metric tons and 60 and 600 metric tons, respectively, during that same period with net declines of 7% and 43%, respectively, between 1985 and 1996. During the July period of annual, baywide, maximum SAV biomass, SAV beds in the Chesapeake Bay typically averaged approximately 0.86 metric tons of aboveground dry mass per hectare of bed area.     

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.     

Kepone distribution in selected tissues of blue crabs,Callinectes sapidus,collected from the James River and lower Chesapeake Bay

Male, ovigerous female and nonovigerous female blue crabs were collected from seven stations in the lower Chesapeake Bay and lower James River, Virginia. The Kepone concentration in muscle, gonad and hepatopancreas was determined by GC analysis. All crabs from the lower Bay contained little Kepone, while many of those from the lower James River were contaminated. Male crabs contained more Kepone in the backfin muscle than females. Female crabs concentrated Kepone into the gonad, and, to a lesser extent, the hepatopancreas whereas the males did not. Ovigerous female crabs with spent ovaries, had less Kepone in the ovary than the corresponding egg masses. Further, ovigerous females, especially those with spent ovaries, had more Kepone in backfin muscle than nonovigerous females. It is concluded that the differences in Kepone partitioning among tissues of male and female crabs may explain the previously reported observation that males contain more Kepone in the backfin muscle than do females.     

The nursery role of seagrass meadows in the upper and lower reaches of the Chesapeake Bay

A two-year trawling and gill-netting study of vegetated and unvegetated bottoms near Parson’s Island, Maryland and near the mouth of the York River, Virginia was carried out to assess the nursery function of submerged vegetation for populations of fishes and decapod crustaceans in the Chesapeake Bay. Results revealed that vegetated bottoms supported substantially larger numbers of decapods, but not fishes, than unvegetated substrates. The lower Bay grassbed was an important nursery area for juvenile blue crabs, although neither of the grassbeds functioned as a nursery for commercially or recreationally valuable fishes. Our results suggest that: (1) further decreases in lower Bay Seagrass biomass would result in reduced numbers of adult blue crabs, but should not substantially affect populations of valuable fish species; (2) additional decreases in Upper Bay submerged vegetation should not produce dramatic change in the population sizes of either adult blue crabs or fishes.     

The effect of aquaculture practices on the benthic macroinvertebrate community of a lagoon system in the Bay of Cádiz (southwestern Spain)

Monthly quantitative Ekman-Birge grab sampling was used to characterize and compare the composition and structure of the benthic macroinvertebrate community inhabiting semi-enclosed polyculture lagoons (SPL) (three sampling sites) and enclosed monoculture ponds (EMP) (two sampling sites) of a lagoonal system of the Bay of Cádiz. The two areas differed considerably in habitat characteristics and aquaculture management. The SPL area was characterized by low rates of water exchange, low fish densities, and the presence of a macroalgal cover. In the EMP area, there was a complete exchange of water daily (by pumping) and a supply of food pellets, density of fish was high, and no vegetative cover was present. There were considerable differences in species composition between habitats with different culture methods: 11 of the 21 most abundant species were exclusive to one or the other. Several epibenthic species were abundant in the polyculture lagoon but were low in density or were absent in monoculture ponds. Some infaunal species, on the other hand, were more abundant in the monoculture ponds. Univeriate measures of community structure (abundance and biomass, Margalef’s species richness, Shannon-Wiener diversity, and Pielou’s evenness indices) did not indicate significant differences between the SPL and EMP areas. Conversely, the abundance-biomass comparison (ABC) method indicated that, on average, the macrobenthic community was moderately disturbed in the SPL and undisturbed in the EMP areas. Multidimensional scaling (MDS) ordination and hierarchical cluster analysis (Bray-Curtis similarity measure) revealed the occurrence of two main benthic assemblages that corresponded to the aquaculture methods. The different rates of water exchange for the two aquaculture practices seem to have contributed to differences in the composition and structure of the benthic communities.     

Environmental and Physical Controls on the Formation and Transport of Blooms of the Dinoflagellate <Emphasis Type="Italic">Cochlodinium polykrikoides</Emphasis> Margalef in the Lower Chesapeake Bay and Its Tributaries

Massive blooms of the harmful alga Cochlodinium polykrikoides Margalef occurred in the lower Chesapeake Bay and its tributaries during the summers of 2007 and 2008. The Lafayette and Elizabeth Rivers appeared to act as initiation grounds for these blooms during both years. However, in 2008 there were also localized sites of initiation and growth of populations within the mesohaline portion of the James River. Bloom initiation appeared to be correlated with intense, highly localized rainfall events during neap tides. Subsequent spring tides increased tidal flushing and transport of C. polykrikoides from the Lafayette and Elizabeth Rivers into the lower James River where it was transported upriver by local estuarine circulation. Blooms dissipated in response to increased wind-driven mixing associated with frontal systems moving through the region. A combination of physical factors including, seasonal rainfall patterns, increased stratification, nutrient loading, spring-neap tidal modulation, and complex estuarine mixing and circulation allowed C. polykrikoides to spread and form massive blooms over large portions of the tidal James River and the lower Chesapeake Bay.     

Vertical distribution of infauna in sediments of a subestuary of central Chesapeake Bay

The vertical distribution of infauna was quantified in eight strata from 0–35 cm in sand and mud sediments of a lower mesohaline subestuary of Chesapeake Bay. Large numbers of small polychaetes, amphipods, and clams occurred in the upper 5 cm of both sediment types, whereas large clams (Macoma balthica in mud andMya arenaria in sand) extended down to 30 cm and comprised most of the biomass in their respective sediment types. There was extensive overlap of the species inhabiting both sediment types. Vertical stratification within and among species apparently reflected constraints on burrowing depth related to body size rather than resource partitioning among competitors. The maximal sediment penetration of 35 cm, which was exhibited byHeteromastus filiformis, was considerably less than the maximal penetration for deep burrowing species in some marine infaunal communities. Several species which burrowed deeper than 5 cm exhibited significant temporal shifts in their vertical distribution.