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.     

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

Otter trawl collections of eelgrass habitats in the lower Chesapeake Bay during 1976–1977 produced 14 species of decapod crustaceans. These collections were dominated by palaemonid shrimp (Palaemonetes spp.), blue crabs (Callinectes sapidus), and sand shrimp (Crangon septemspinosa), each of which exhibited unimodal seasonal abundance curves with large summer peaks. Decapod abundance was positively correlated with plant biomass throughout the year. Decapod densities on vegetated bottoms were greater than on unvegetated bottoms, and nighttime abundance on each bottom type was greater than corresponding daytime abundance. Total decapod abundances in Chesapeake Bay eelgrass meadows appear to be much greater than those reported in North Carolina eelgrass or Gulf of Mexico turtlegrass habitats.     

Secondary production within a seagrass bed (Zostera marina andRuppia maritima) in lower Chesapeake Bay

Monthly sampling of a 140-ha seagrass bed in the lower Chesapeake Bay, Virginia, revealed that 13 numerically and trophically important species, representing about 20% of the total community densities over the year-long study period, accounted for the production of ≈42 g dry wt m^?2 yr^?1. This estimate is likely conservative due to our assumptions on voltinism and fixed size at maturity regardless of season for the species studied. The isopodErichsonella attenuata accounted for 17.6 g dry wt m^?2 yr^?1 or 42% of the calculated total production, while the portunid decapodCallinectes sapidus and the amphipodGammarus mucronatus each accounted for 7.7 g dry wt m^?2 yr^?1. The 10 remaining species (4 peracarids, 4 molluscs, and 2 decapods) each produced less than 2 g dry wt m^?2 yr^?1. Total seagrass-associated secondary production was estimated to equal or exceed 200 g dry wt m^?2yr^?1. By applying this estimate to the entire 140-ha grassbed, we projected that, on average, 4.8 metric tons dry wt of invertebrate standing stock and 55.9 metric tons of invertebrate production occur over the year.     

Feeding ecology of the northern pipefish,Syngnathus fuscus,in a seagrass community of the lower Chesapeake Bay

Examination of gut contents of the northern pipefish,Syngnathus fuscus, revealed that gammarid amphipods, caprellid amphipods, isopods, and calanoid copepods were the dominant food items during the sevenmonth study period.Gammarus mucronatus, calanoid copepods, andErichsonella attenuata were the seasonally dominant prey items in the spring, summer, and fall, respectively.G. mucronatus and calanoid copepods were consumed in approximate proportion to their numerical abundance in the environment, whileE. attenuata, present in rather uniform densities throughout the study period, was extensively consumed only in the late summer and fall. An ontogenetic pattern of prey consumption was evident, in addition to the seasonal pattern. Comparison ofG. mucronatus andE. attenuata size ranges from the field and in pipefish guts revealed thatS. fuscus preyed upon the smaller size classes of each species, and that mean size of prey consumed was positively related to fish size.     

Wave-forced resuspension of upper Chesapeake Bay muds

Moored instruments were used to make observations of near bottom currents, waves, temperature, salinity, and turbidity at shallow (3.5 m and 5.5 m depth) dredged sediment disposal sites in upper Chesapeake Bay during the winters of 1990 and 1991 to investigate time-varying characteristics of resuspension processes over extended periods. Resulting time series data show the variability of two components of the suspended sediment concentration field. Background suspended sediment concentrations varied inversely with salinity and in direct relation to Susquehanna River flow. Muddy bottom sediments were also resuspended locally by both tidal currents and wind-wave forcing, resulting in short-term increases and decreases in suspended concentration, with higher peak concentrations near the bottom. In both years, episodes of wave-forced resuspension dominated tidal resuspension on an individual event basis, exceeding most tidal resuspension peaks by a factor of 3 to 5. The winds that generated the waves responsible for the observed resuspension events were not optimal for wave generation, however. Application of a simple wind-wave model showed that much greater wave-forced resuspension than that observed might be generated under the proper conditions. The consolidated sediments investigated in 1990 were less susceptible to both tidal and wave-forced resuspension than the recently deposited sediments investigated in 1991. There was also some indication that wave-forced resuspension increased erodibility of the bottom sediments on a short-term basis. Wave-forced resuspension is implicated as an important part of sediment transport processes in much of Chesapeake Bay. Its role in deeper, narrower, and more tidally energetic estuaries is not as clear, and should be investigated on a case-by-case basis.     

The effect of bridge piles on stratification in lower Chesapeake Bay

Tidally-forced flow beneath the existing trestle of the Chesapeake Bay Bridge-Tunnel causes significant distortion of the ambient density field in the immediate vicinity of the trestle due to flow over the scour zone around the pilings. Effects are minimal a short distance away from the trestle. Contrary to previous studies, stratification was not appreciably stronger during neap as compared to spring tide periods at the time of these observations, but a tendency toward such a state was detected. Estimates of piling-induced destratification based on direct observations of temperature, salinity, and currents near the pilings indicate that piling effects are substantially less than naturally occurring destratification due to bottom and wind stresses.     

Clay mineral distributions and source discrimination of upper Quaternary sediments,lower Chesapeake Bay,Virginia

Discriminant function analysis of clay mineral data from five cores of upper Quaternary sediment indicates that marine inputs are a significant source of fine-grained sediment near Thimble Shoal Channel, a major natural channel in lower Chesapeake Bay. Illite, of predominantly marine origin, is found to be enriched in clay fractions of three cores adjacent to the channel, relative to two cores near the mouth of the James River estuary. Thimble Shoal Channel is a significant repository and conduit for fine-grained sediment from the shelf. This result is analogous to previous results showing that the shelf is the dominant source of fine sand in the lower bay. Sediment in Thimble Shoal Channel reflects a shelf source even at the landward end, an area which was previously thought to be dominated by deposition from the James River.     

Temporal patterns of feeding by blue crabs (Callinectes sapidus) in a tidal-marsh creek and adjacent seagrass meadow in the lower Chesapeake Bay

A 24-h study of blue crab feeding periodicity was conducted concurrently in a tidal marsh creek and adjacent seagrass meadow in the lower Chesapeake Bay. Crabs from the grassbed tended to have fuller guts than crabs from the marsh creek. In the grassbed, a weak trend toward nocturnal feeding was observed, with an apparent peak at dusk. During the day, crabs were not easily observed and were assumed to be feeding beneath the eelgrass canopy; at night crabs fed in the canopy. In the marsh creek, feeding was related to the tidal cycle, with guts being fullest at high tide and decreasing to lows just prior to the next high tide. This study suggests the potential importance of habitat on blue crab feeding patterns.     

Epiphyte-grazer relationships in seagrass meadows: Consequences for seagrass growth and production

Studies of seagrass meadows have shown that the production of algal epiphytes attached to seagrass blades approaches 20% of the seagrass production and that epiphytes are more important as food for associated fauna than are the more refractory seagrass blades. Since epiphytes may compete with seagrasses for light and water column nutrients, excessive epiphytic fouling could have serious consequences for seagrass growth. We summarize much of the literature on epiphytegrazer relationships in seagrass meadows within the context of seagrass growth and production. We also provide insights from mathematical modeling simulations of these relationships for a Chesapeake BayZostera marina meadow. Finally we focus on future research needs for more completely understanding the influences that epiphyte grazers have on seagrass production.     

Responses of upper Chesapeake Bay to variations in discharge of the Susquehanna River

Upper Chesapeake Bay can be defined as the segment of the bay extending from its head seaward to the mouth of the Potomac Estuary. The Susquehanna River Estuary provides approximately 87% of the total freshwater input to this reach of the bay. With infrequent exceptions, resulting from summer and early fall hurricanes, the Susquehanna River has a well-defined seasonal flow pattern typical of mid-latitude rivers: high discharge in late winter and early spring, followed by low-to-moderate discharge throughout the summer and autumn. Within this general seasonal framework, there are large year-to-year variations in the magnitude of the early spring freshet and of the summer-fall period of low flow. The large seasonal variations in river flow produce significant responses to this fluctuating signal which are manifested in variations in the milieu of upper Chesapeake Bay. Stratification, circulation, sedimentation, nutrient levels, dissolved oxygen levels, and a number of other important and characteristic properties and processes of the upper bay all are closely coupled to variations in discharge of the Susquehanna River. Effects are not limited to the bay proper. Renewal of the tributary estuaries of the upper bay is controlled by variations in flow of the Susquehanna River.     

The relationship of physical factors and biological response in coastal seagrass meadows

Continuous, long-term studies of coastal grassbed assemblages in the N.E. Gulf of Mexico indicate complex relationships between physical controlling factors and biological response. Such seagrass systems are physically unstable over short periods. Seasonal ranges of temperature, salinity, and natural water quality conditions are considerable with periodic, recurrent “catastrophic” events such as floods and cold winters. These factors control the distribution and productivity of the seagrasses and algae which constitute the habitat and organic substrate for diverse assemblages of organisms. In addition, the benthic plants mediate predator-prey relationships and competitive interactions. Despite the physical instability, timed sequences of distinct ontogenetic feeding populations are generally stable from year to year as are other population and community characteristics. Thus, physical processes determine overall habitat conditions and productivity cycles whereas biological processes such as predation and competition define specific community relationships. However, seemingly minor changes in the physical environment due to anthropogenous activities can lead to major reorganization of the biological system; the observed biological stability of the seagrass beds can be ephemeral if important habitat features are altered in a way that exceeds the adaptive response of the system. Concepts are discussed which relate observed sequences of ontogenetic feeding units to food web patterns and geographic differences of population-niche relationships from one estuary to another.     

Distribution and abundance of the cownose ray,Rhinoptera bonasus,in lower Chesapeake Bay

Aerial surveys were conducted in the lower Chesapeake Bay during 1986–1989 to estimate abundance and examine the distribution of the cownose ray,Rhinoptera bonasus, during its seasonal residence, May–October. Most of the survey effort was concentrated in the lower and mid-bay regions. Cownose rays appeared uniformly distributed across the bay during mid-summer, but were more abundant in the eastern portion of the bay during migration. North-south distribution varied and reflected the general seasonal migration pattern. Mean abundance increased stepwise monthly from June through September and declined dramatically in October with their emigration from the bay. Abundance estimates from individual surveys varied. The greatest range of individual survey abundance estimates occurred in September (0–3.7×10⁷ cownose rays0 due to high variation in school size and abundance between surveys. Monthly mean cownose ray abundance ranged from 0 in May and November to an estimated maximum of 9.3×10⁶ individuals in September. The magnitude of the population suggests that the cownose ray plays an important role in the trophic dynamics of the Chesapeake Bay ecosystem. The historical data were insufficient to determine whether the population has increased, but these surveys provided the baseline data which would allow future investigation of cownose ray population dynamics in lower Chesapeake Bay.     

Meteorological influences on sea level and water temperature in the lower Chesapeake Bay: 1992

The influence of atmospheric forcing on the flow and heat transports in the lower Chesapeake Bay and the adjacent coastal ocean were studied by comparing nontidal sea level and sea surface temperature variations in this region with meteorological data for 1992. Northeasterly and southwesterly winds caused the greatest changes in mean sea level (greater than 0.25 m) throughout the year. Northeastely winds caused a more rapid response than southwesterly winds, causing sea-level rises in less than 6 h. Barometric pressure changes typically contributed approximately 10% to extreme sea-level variations and were less influential than wind stress in most cases. Wind forcing was also responsible for summer events in which the horizontal water temperature gradient between two near-surface locations in the vicinity of the bay mouth vanished. These zero-gradient events corresponded to inflows and outflows at the bay's entrance caused by northeasterly and southwesterly winds, respectively. Wind-induced advection outside the lower Chesapeake Bay was additionally responsible for extreme heat flux variations. Heat gains and losses during the spring and fall occurred in pulsating events related to wind direction but were probably not connected to lower bay processes.     

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.     

Predators of juvenile blue crabs outside of refuge habitats in lower Chesapeake Bay

Blue crabsCallinectes sapidus in lower Chesapeake Bay are subject to high rates of predation during the late summer of their first year of growth as they migrate out of vegetated nursery habitats. Predators, potentially contributing to this pattern, were identified in video-recorded field observations of tethered juvenile crabs (20–25 mm carapace width). Predators were also tested in large laboratory tanks containing similarly-sized untethered crabs as prey. Seven different predators attacked tethered crabs in the field. Only two predators, larger blue crabs and northern puffers,Sphoeroides maculatus, consistently succeeded in preying on crabs in both field and laboratory settings. These results confirm the importance of cannibalism on juvenile blue crabs and identify puffers as a potentially overlooked source of predation pressure.     

Distribution and abundance of overwintering blue crabs,Callinectes sapidus,in the lower Chesapeake Bay

A study was conducted to define winter distribution patterns of blue crabs,Callinectes sapidus, in the lower Chesapeake Bay and to relate these patterns to environmental variation. During February 1986 a stratified random survey was conducted to examine the distribution of blue crabs with respect to three major habitat types: 1) high energy, wave- and tide-dominated, spits and shoals; 2) moderate energy, tide-dominated basins; and 3) variable energy, tide-dominated or quiescent channels (natural or cut). Each major habitat type was further stratified on the basis of location (to account for possible salinity effects), resulting in a total of 17 habitat-stratum combinations. Blue crabs exhibited significant differences in abundance among habitats. Crabs were most abundant in the basin habitat and least abundant in the shoal and spit habitat. A posteriori evaluations of abundance patterns in relation to sediment type and depth showed that crabs were significantly more abundant where sediments contained between 41 and 60% sand and at depths exceeding 9 m. The sampled population of blue crabs was dominated by mature females. There were no significant differences in crab sex ratios between habitats, but significant differences between two fixed sites sampled through the winter showed that there were proportionately more male crabs at the western site than there were at the eastern site. The observed patterns indicate that some differential habitat utilization occurs and that overwintering female crabs are found preferentially in areas characterized by moderate energy regimes and fine, but sandy sediments.     

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.     

The occurrence of the anemone Peachia parasitica as a symbiont in the scyphozoan Cyanea capillata in the lower Chesapeake Bay

Juveniles of the burrowing anemone Peachia parasitica, living on the scyphozoan Cyanea capillata, were obtained from Virginia, thus extending the southern limit for this species. Anemones were easily maintained in the laboratory; one lived for over four years. The feeding behavior is described.     

Food habits and feeding behavior of the cownose ray,Rhinoptera bonasus,in lower Chesapeake Bay

The most important food item of the cownose ray,Rhinoptera bonasus, in the Virginian tributaries of lower Chesapeake Bay is the soft shell clam,Mya arenaria. The Baltic macoma,Macoma balthica, ranks a distant second. Adult rays feed on deep burrowing mollusks, juveniles on shallow- or non-burrowing bivalves. Foraging schools of rays invade tidal flats during the flood tide. Stirring motions of the pectoral fins combined with suction from the expansive orobranchial chamber are probably used to excavate deep burrowing bivalves.     

A comparison of phytoplankton assemblages and environmental relationships in three estuarine rivers of the lower Chesapeake Bay

A 16-month data set of phytoplankton assemblages and environmental parameters were studied in the lower James, York (-Pamunkey), and Rappahannock rivers using several exploratory statistical approaches. Based on species composition and river station relationships, three site groups were established and subsequently identified as predominantly tidal fresh, oligo-mesohaline, and mesohaline sites. Phytoplankton assemblages within these rivers were influenced and subsequently augmented by the onset of the spring freshet which was different in 1986 and 1987. Five temporal assemblages of phytoplankters were also identified and designated into seasonal groupings of spring 1986, summer-fall, summer-winter, fall-winter, and winter-spring 1987. Discriminant analysis (MANOVA) evaluations were made for water quality parameters to site and seasonal phytoplankton assemblages and these relationships are discussed. Moving downstream along an oligohaline-mesohaline gradient, the nitrogen and phosphorus levels decreased and the phytoplankton composition was more similar at several corresponding site locations in the different rivers than at stations relatively close to each other in the same river. Within these data sets approximately 58% of the explained variance was associated with site (spatial) effects, 30% with temporal effects, and 12% with site-temporal interactions. A transition from dominant bloom-producing freshwater diatoms to estuarine species occurs from the tidally influenced freshwater zone downstream. This change may be rapid as the decline ofSkeletonema potamos, or more gradual, as withCyclotella striata andCyclotella meneghiania. These are replaced downstream bySkeletonema costatum, Cyclotella caspia, andLeptocylindrus minimus as dominant species.