Chesapeake Bay Ecosystem Research Articles

Simulation of high-frequency dissolved oxygen dynamics in a shallow estuary, the Corsica River, Chesapeake Bay

Understanding shallow water biogeochemical dynamics is a challenge in coastal regions, due to the presence of highly variable land-water interface fluxes, tight coupling with sediment processes, tidal dynamics, and diurnal variability in biogeochemical processes. While the deployment of continuous monitoring devices has improved our understanding of high-frequency (12 - 24 hours) variability and spatial heterogeneity in shallow regions, mechanistic modeling of these dynamics has lagged behind conceptual and empirical models. The inherent complexity of shallow water systems is represented in the Corsica River estuary, a small basin within the Chesapeake Bay ecosystem, where abundant monitoring data have been collected from long-term monitoring stations, continuous monitoring sensors, synoptic sensor surveys, and measurements of sediment-water fluxes. A state-of-the-art modeling system, the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), was applied to the Corsica domain with a high-resolution grid and nutrient loads from the most recent version of the Chesapeake Bay watershed model. The Corsica SCHISM model reproduced observed high-frequency variability in dissolved oxygen, as well as seasonal variability in chlorophyll-a and sediment-water fluxes. Time-series signal analyses using Empirical Model Decomposition and spectral analysis revealed that the diurnal and M2 tide frequencies are the dominant high-frequency modes and physical transport contributes a larger share to dissolved oxygen budgets than biogeochemical processes on an hourly time scale. Heterogeneity and patchiness in dissolved oxygen resulting from phytoplankton distributions and geometry-driven eddies amplify the physical transport effect, and on longer time scales oxygen is controlled more by photosynthesis and respiration. Our simulation demonstrates that interactions among physical and biological dynamics generate complex high-frequency variability in water quality and non-linear reposes to nutrient loading and environmental forcing in shallow water systems.

Non-native red alga Gracilaria vermiculophylla compensates for seagrass loss as blue crab nursery habitat in the emerging Chesapeake Bay ecosystem.

Non-native species can become deleterious or potentially beneficial as components of novel ecosystems. The non-native red macroalga Gracilaria vermiculophylla may provide nursery habitat where eelgrass Zostera marina has been extirpated in Chesapeake Bay. A mensurative experiment was conducted monthly May–October 2013 and 2014 in the York River, Chesapeake Bay, to evaluate hypotheses that Gracilaria (1) can compensate for the loss of seagrass nurseries by colonizing habitats where seagrass has been eliminated by environmental stress, and (2) is utilized by juvenile blue crabs (Callinectes sapidus) as nursery habitat. We quantified Gracilaria presence, percent cover, and biomass as a function of region (upriver, midriver, and downriver) and seagrass presence or absence using stratified random sampling, 20-m transects, and 0.0625-m2 quadrats. Gracilaria volume was measured and converted to dry weight. Effects of the factors and covariates temperature, salinity, dissolved oxygen, month, and year were analyzed using generalized linear models. Juvenile blue crab density was quantified in summer 2013 using suction sampling in Gracilaria and seagrass. A model with the collective effect of region and seagrass presence or absence (downriver seagrass, downriver unvegetated bottom, midriver unvegetated bottom) best predicted Gracilaria abundance. Gracilaria presence, percent cover, and biomass were highest in downriver seagrass, followed by downriver unvegetated bottom, and then midriver unvegetated bottom, where seagrass has been extirpated, supporting hypothesis (1). Gracilaria did not occur upriver, likely due to a lack of recruitment. Seagrass and Gracilaria housed similar densities of juvenile blue crabs, supporting hypothesis (2). We estimated that a single 40-ha cove system with Gracilaria could house 200,000 juvenile crabs as would a single 2.4-ha seagrass bed. Consequently, the numerous midriver and downriver cove systems in the York River could support millions of young juvenile blue crabs and thereby compensate for the loss of seagrass in the river and in other areas of Chesapeake Bay. At present, Gracilaria has no widespread negative impacts on seagrass in the York River or most regions of Chesapeake Bay, likely because percent cover and biomass are not excessively high at present. We posit that Gracilaria has become an important alternative nursery habitat for the blue crab in Chesapeake Bay and can potentially mitigate impacts of climate change on seagrass nursery habitats.

Metabolic scope and hypoxia tolerance of Atlantic croaker (Micropogonias undulatus Linnaeus, 1766) and spot (Leiostomus xanthurus Lacepède, 1802), with insights into the effects of acute temperature change

Dynamic conditions in estuaries result in fish experiencing acute changes in temperature and oxygen levels, the magnitude of which are predicted to increase due to directional climate change. Acute changes in temperature impose physiological stress on individuals through alterations in metabolism (often quantified as aerobic metabolic scope) that affect growth, reproduction, movement, and hypoxia tolerance; and their magnitude can thus have population-level consequences. Furthermore, increases in the magnitude of acute changes in temperature and oxygen levels in estuaries may reduce the availability of suitable habitat. We used intermittent-flow respirometry to determine the effects of acute temperature changes on the metabolic scope and hypoxia tolerance of Atlantic croaker and spot; two economically and ecologically important species in Chesapeake Bay. For Atlantic croaker, metabolic scope increased from 15 to 25 °C but did not change from 25 to 30 °C, whereas metabolic scope increased from 15 to 30 °C for spot. The metabolic scope of Atlantic croaker and spot was comparable at all temperatures examined. Hypoxia tolerance, and the effects of temperature on hypoxia tolerances, did not differ by species. Our results indicate that Atlantic croaker and spot are well-adapted to the conditions they currently experience in Chesapeake Bay. Decreased availability of suitable habitat as environmental conditions continue to change will, however, result in increased competition for resources. Our results suggest that under these conditions, spot will have a competitive advantage over Atlantic croaker which may alter the dynamics of the Chesapeake Bay ecosystem.

Investigating water use over the Choptank River Watershed using a multisatellite data fusion approach

AbstractThe health of the Chesapeake Bay ecosystem has been declining for several decades due to high levels of nutrients and sediments largely tied to agricultural production systems. Therefore, monitoring of agricultural water use and hydrologic connections between crop lands and Bay tributaries has received increasing attention. Remote sensing retrievals of actual evapotranspiration (ET) can provide valuable information in support of these hydrologic modeling efforts, spatially and temporally describing consumptive water use by crops and natural vegetation and quantifying response to expansion of irrigated area occurring with Bay watershed. In this study, a multisensor satellite data fusion methodology, combined with a multiscale ET retrieval algorithm, was applied over the Choptank River watershed located within the Lower Chesapeake Bay region on the Eastern Shore of Maryland, USA to produce daily 30 m resolution ET maps. ET estimates directly retrieved on Landsat satellite overpass dates have high accuracy with relative error (RE) of 9%, as evaluated using flux tower measurements. The fused daily ET time series have reasonable errors of 18% at the daily time step ‐ an improvement from 27% errors using standard Landsat‐only interpolation techniques. Annual water consumption by different land cover types was assessed, showing reasonable distributions of water use with cover class. Seasonal patterns in modeled crop transpiration and soil evaporation for dominant crop types were analyzed, and agree well with crop phenology at field scale. Additionally, effects of irrigation occurring during a period of rainfall shortage were captured by the fusion program. These results suggest that the ET fusion system will have utility for water management at field and regional scales over the Eastern Shore. Further efforts are underway to integrate these detailed water use data sets into watershed‐scale hydrologic models to improve assessments of water quality and inform best management practices to reduce nutrient and sediment loads to the Chesapeake Bay.

Long-Term Changes in Sediment and Nutrient Delivery from Conowingo Dam to Chesapeake Bay: Effects of Reservoir Sedimentation.

Reduction of suspended sediment (SS), total phosphorus (TP), and total nitrogen is an important focus for Chesapeake Bay watershed management. The Susquehanna River, the bay's largest tributary, has drawn attention because SS loads from behind Conowingo Dam (near the river's mouth) have been rising dramatically. To better understand these changes, we evaluated histories of concentration and loading (1986-2013) using data from sites above and below Conowingo Reservoir. First, observed concentration-discharge relationships show that SS and TP concentrations at the reservoir inlet have declined under most discharges in recent decades, but without corresponding declines at the outlet, implying recently diminished reservoir trapping. Second, best estimates of mass balance suggest decreasing net deposition of SS and TP in recent decades over a wide range of discharges, with cumulative mass generally dominated by the 75∼99.5th percentile of daily Conowingo discharges. Finally, stationary models that better accommodate effects of riverflow variability also support the conclusion of diminished trapping of SS and TP under a range of discharges that includes those well below the literature-reported scour threshold. Overall, these findings suggest that decreased net deposition of SS and TP has occurred at subscour levels of discharge, which has significant implications for the Chesapeake Bay ecosystem.

Atmospheric ammonia measurements at low concentration sites in the northeastern USA: implications for total nitrogen deposition and comparison with CMAQ estimates

We evaluated the relative importance of dry deposition of ammonia (NH3) gas at several headwater areas of the Susquehanna River, the largest single source of nitrogen pollution to Chesapeake Bay, including three that are remote from major sources of NH3 emissions (CTH, ARN, and KEF) and one (HFD) that is near a major agricultural source. We also examined the importance of nitrogen dioxide (NO2) deposition at one of these sites. Over the past decade, increasing evidence has suggested that NH3 deposition, in particular, may be an important contributor to total nitrogen deposition and to downstream nitrogen pollution. We used Ogawa passive samplers to measure NH3 concentrations over several years (2006–2011) for CTH, and primarily in 2008 and 2009 for the other sites. NO2 was measured at CTH mainly in 2007. Chamber calibration studies for NH3 and NO2, and field comparisons with annular denuders for NH3, validated the use of these passive samplers over a range of temperatures and humidity observed in the field, if attention is given to field and laboratory blank issues. The annual mean NH3 concentrations for the forested sites were 0.41 ± 0.03, 0.41 ± 0.06 and 0.25 ± 0.08 µg NH3/m3 for CTH, ARN and KEF, respectively. NO2 passive sampler mean annual concentration was 3.19 ± 0.42 µg NO2/m3 at CTH. Direct comparison of our measured values with the widely used Community Multiscale Air Quality (CMAQ) model (v4.7.1) show reasonably good agreement. However, the model-based estimates tend to be lower than our measured average NH3 concentration, by 8 % at our best studied site where we measured moderately low concentration, and up to 60 % at our site with the lowest concentrations and lowest sampling frequency. CMAQ NO2 concentration estimates were substantially higher than our measured values. Along a transect of sites near a source of NH3 emissions from animal agriculture, we found NH3 concentrations to be far higher than predicted for this area by the CMAQ model. This is not surprising, since the CMAQ model integrates over a relatively wide area. The higher NH3 concentrations we measured were generally within 1 km of the agricultural source. Such locally high atmospheric concentrations leading to locally high deposition may be ecologically significant. Analysis of such issues requires more locally scaled estimates than can be provided from the 12 km grid scale estimates of CMAQ used in this study. We estimated deposition of NH3 and NO2 using our concentration data and modified (concentration-weighted) deposition velocities derived from the CMAQ model. We estimate dry gaseous NH3 deposition as 2.0 ± 0.3 (CTH), 2.2 ± 0.4 (ARN) and 1.4 ± 0.7 kg N/ha-year (KEF). NO2 deposition at CTH is estimated to be 0.16 kg N/ha-year. NO2 deposition is a very small component of total nitrogen deposition at this site. On the other hand, NH3 deposition is either the largest or the second largest form of dry deposition at our sites, depending on how total N deposition is estimated. Based on total deposition best estimates of 9.2 kg N/ha for CTH and 8.6 kg N/ha for KEF, NH3 contributes between 16 and 22 % of total nitrogen deposition. Such deposition has normally not been measured through traditional national monitoring programs, yet is significant as a source of nitrogen pollution to areas such as the highly nitrogen-sensitive Chesapeake Bay ecosystem.

Predicting the Hypoxic-Volume in Chesapeake Bay with the Streeter-Phelps Model: A Bayesian Approach1

Liu, Yong, George B. Arhonditsis, Craig A. Stow, and Donald Scavia, 2011. Predicting the Hypoxic-Volume in Chesapeake Bay with the Streeter–Phelps Model: A Bayesian Approach. Journal of the American Water Resources Association (JAWRA) 47(6):1348–1363. DOI: 10.1111/j.1752-1688.2011.00588.x Abstract: Hypoxia is a long-standing threat to the integrity of the Chesapeake Bay ecosystem. In this study, we introduce a Bayesian framework that aims to guide the parameter estimation of a Streeter–Phelps model when only hypoxic volume data are available. We present a modeling exercise that addresses a hypothetical scenario under which the only data available are hypoxic volume estimates. To address the identification problem of the model, we formulated informative priors based on available literature information and previous knowledge from the system. Our analysis shows that the use of hypoxic volume data results in reasonable predictive uncertainty, although the variances of the marginal posterior parameter distributions are usually greater than those obtained from fitting the model to dissolved oxygen (DO) profiles. Numerical experiments of joint parameter estimation were also used to facilitate the selection of more parsimonious models that effectively balance between complexity and performance. Parameters with relatively stable posterior means over time and narrow uncertainty bounds were considered as temporally constant, while those with time varying posterior patterns were used to accommodate the interannual variability by assigning year-specific values. Finally, our study offers prescriptive guidelines on how this model can be used to address the hypoxia forecasting in the Chesapeake Bay area.

Annual, Seasonal, and Regional Variability in Diet of Atlantic Croaker (Micropogonias undulatus) in Chesapeake Bay

Atlantic croaker is one of the most abundant demersal fish in Chesapeake Bay. Until recently, when substantial declines in abundance have occurred, high biomass supported elevated fisheries landings. Therefore, study of the diet of Atlantic croaker is important to understand its own dynamics and its role in the Chesapeake Bay ecosystem. Patterns in the diet of croaker varied annually, seasonally, and spatially, but were not strongly correlated with any measured environmental variables. Although the majority of the diet of croaker consisted of polychaetes and other benthic items, about 20% of the croaker diet by weight consisted of anchovy and other fishes. Croaker consumption of anchovy is likely a result of crepuscular feeding that has not been captured in previous studies that sampled during the day and with bottom trawls. Thus, croaker influences both the benthic and pelagic components of the Chesapeake Bay food web and incorporating such diel patterns in diet may increase the reliability of fishery ecosystem models.

Reconstructing early 17th century estuarine drought conditions from Jamestown oysters

Oysters (Crassostrea virginica) were a central component of the Chesapeake Bay ecosystem in 1607 when European settlers established Jamestown, VA, the first permanent English settlement in North America. These estuarine bivalves were an important food resource during the early years of the James Fort (Jamestown) settlement while the colonists were struggling to survive in the face of inadequate supplies and a severe regional drought. Although oyster shells were discarded as trash after the oysters were eaten, the environmental and ecological data recorded in the bivalve geochemistry during shell deposition remain intact over centuries, thereby providing a unique window into conditions during the earliest Jamestown years. We compare oxygen isotope data from these 17th century oyster shells with modern shells to quantify and contrast estuarine salinity, season of oyster collection, and shell provenance during Jamestown colonization (1609-1616) and the 21st century. Data show that oysters were collected during an extended drought between fall 1611 and summer 1612. The drought shifted the 14 psu isohaline above Jamestown Island, facilitating individual oyster growth and extension of oyster habitat upriver toward the colony, thereby enhancing local oyster food resources. Data from distinct well layers suggest that the colonists also obtained oysters from reefs near Chesapeake Bay to augment oyster resources near Jamestown Island. The oyster shell season of harvest reconstructions suggest that these data come from either a 1611 well with a very short useful period or an undocumented older well abandoned by late 1611.

Using social network analysis tools in ecology: Markov process transition models applied to the seasonal trophic network dynamics of the Chesapeake Bay

Ecosystem components interact in complex ways and change over time due to a variety of both internal and external influences (climate change, season cycles, human impacts). Such processes need to be modeled dynamically using appropriate statistical methods for assessing change in network structure. Here we use visualizations and statistical models of network dynamics to understand seasonal changes in the trophic network model described by Baird and Ulanowicz [Baird, D., Ulanowicz, R.E., 1989. Seasonal dynamics of the Chesapeake Bay ecosystem. Ecol. Monogr. 501 (59), 329–364] for the Chesapeake Bay (USA). Visualizations of carbon flow networks were created for each season by using a network graphic analysis tool (NETDRAW). The structural relations of the pelagic and benthic compartments (nodes) in each seasonal network were displayed in a two-dimensional space using spring-embedder analyses with nodes color-coded for habitat associations (benthic or pelagic). The most complex network was summer, when pelagic species such as sea nettles, larval fishes, and carnivorous fishes immigrate into Chesapeake Bay and consume prey largely from the plankton and to some extent the benthos. Winter was the simplest of the seasonal networks, and exhibited the highest ascendency, with fewest nodes present and with most of the flows shifting to the benthic bacteria and sediment POC compartments. This shift in system complexity corresponds with a shift from a pelagic- to benthic-dominated system over the seasonal cycle, suggesting that winter is a mostly closed system, relying on internal cycling rather than external input. Network visualization tools are useful in assessing temporal and spatial changes in food web networks, which can be explored for patterns that can be tested using statistical approaches. A simulation-based continuous-time Markov Chain model called SIENA was used to determine the dynamic structural changes in the trophic network across phases of the annual cycle in a statistical as opposed to a visual assessment. There was a significant decrease in outdegree (prey nodes with reduced link density) and an increase in the number of transitive triples (a triad in which i chooses j and h, and j also chooses h, mostly connected via the non-living detritus nodes in position i), suggesting the Chesapeake Bay is a simpler, but structurally more efficient, ecosystem in the winter than in the summer. As in the visual analysis, this shift in system complexity corresponds with a shift from a pelagic to a more benthic-dominated system from summer to winter. Both the SIENA model and the visualization in NETDRAW support the conclusions of Baird and Ulanowicz [Baird, D., Ulanowicz, R.E., 1989. Seasonal dynamics of the Chesapeake Bay ecosystem. Ecol. Monogr. 501 (59), 329–364] that there was an increase in the Chesapeake Bay ecosystem's ascendancy in the winter. We explain such reduced complexity in winter as a system response to lowered temperature and decreased solar energy input, which causes a decline in the production of new carbon, forcing nodes to go extinct; this causes a change in the structure of the system, making it simpler and more efficient than in summer. It appears that the seasonal dynamics of the trophic structure of Chesapeake Bay can be modeled effectively using the SIENA statistical model for network change.

Communities recognition in the Chesapeake Bay ecosystem by dynamical clustering algorithms based on different oscillators systems

We have recently introduced [Phys. Rev. E 75, 045102(R) (2007); AIP Conference Proceedings 965, 2007, p. 323] an efficient method for the detection and identification of modules in complex networks, based on the de-synchronization properties (dynamical clustering) of phase oscillators. In this paper we apply the dynamical clustering tecnique to the identification of communities of marine organisms living in the Chesapeake Bay food web. We show that our algorithm is able to perform a very reliable classification of the real communities existing in this ecosystem by using different kinds of dynamical oscillators. We compare also our results with those of other methods for the detection of community structures in complex networks.

Oyster-sea nettle interdependence and altered control within the Chesapeake Bay ecosystem

Research on the effects of declining abundances of the Eastern oyster (Crassostrea virginica) in Chesapeake Bay and other estuaries has primarily focused on the role of oysters in filtration and nutrient dynamics, and as habitat for fish or fish prey. Oysters also play a key role in providing substrate for the overwintering polyp stage of the scyphomedusa sea nettle,Chrysaora quinquecirrha, which is an important consumer of zooplankton, ctenophores, and icthyoplankton. Temporal trends in sea nettle abundances in visual counts from the dock at Chesapeake Biological Laboratory, trawls conducted in the mesohaline portion of the Patuxent River, and published data from the mainstem Chesapeake Bay indicate that sea nettles declined in the mid 1980s when overfishing and increased disease mortality led to sharp decreases in oyster landings and abundance. Climate trends, previously associated with interannual variation in sea nettle abundances, do not explain the sharp decline. A potentially important consequence of declining sea nettle abundances may be an increase in their ctenophore prey (Mnemiopsis leidyi), and a resultant increase in predation on icthyoplankton and oyster larvae. Increased predation on oyster larvae by ctenophores may inhibit recovery of oyster populations and reinforce the current low abundance of oysters in Chesapeake Bay.

An index of biotic integrity based on the summer polyhaline zooplankton community of the Chesapeake Bay

A zooplankton index of biotic integrity was developed for the polyhaline waters of the Chesapeake Bay using data from a long-term environmental assessment program in which both zooplankton and water quality were regularly monitored. Summer (July to September) sampling events were classified as either coming from impaired or reference (least-impaired) conditions based on water quality conditions. Seventeen zooplankton community metrics were evaluated under these criteria and nine were chosen for a composite index. These were the Simpson diversity index, and abundance of barnacle larvae, rotifers, cladocerans, copepods, total mesozooplankton, and predators. The composite index of biotic integrity correctly classified about 94% of the impaired samples and about 82% of the reference samples. Average classification efficiency was 88%. This index appears to be an effective measure of eutrophication for the summer polyhaline waters of the Chesapeake Bay ecosystem.

Diet Composition of Young‐of‐the‐Year Bluefish in the Lower Chesapeake Bay and the Coastal Ocean of Virginia

AbstractThe lower Chesapeake Bay and coastal ocean of Virginia serve as an important nursery area for bluefish Pomatomus saltatrix. Describing the diet composition of young‐of‐the‐year (hereafter, age‐0) bluefish in this region is essential to support current Chesapeake Bay ecosystem modeling efforts and to contribute to the understanding of the foraging ecology of these fish along the U.S. Atlantic coast. The stomach contents of 404 age‐0 bluefish collected from the lower Chesapeake Bay and adjacent coastal zone in 1999 and 2000 were examined as part of a diet composition study. Age‐0 bluefish foraged primarily on bay anchovies Anchoa mitchilli, striped anchovies Anchoa hepsetus, and Atlantic silversides Menidia menidia. Other fishes such as striped bass Morone saxatilis, white perch Morone americana, Atlantic menhaden Brevoortia tyrannus, and bluefish were seasonally important. Crab zoea and megalope Callinectes spp. and amphipods Gammarus spp. were the most important invertebrate prey. A seasonal dietary shift from Atlantic silversides to anchovies was evident. Overall, small pelagic and littoral schooling fishes, particularly engraulids and atherinids, predominated in the diet of the age‐0 bluefish collected for this study. Although the results presented here were similar to the diet composition of age‐0 bluefish reported in previous studies, some notable differences were probably due to spatial and temporal differences in prey assemblages.

Production of bay anchovy Anchoa mitchilli in Chesapeake Bay: application of size-based theory

To evaluate and quantify productivity of abundant bay anchovy Anchoa mitchilli in the Chesapeake Bay ecosystem, daily stock biomass, production, and contribution to predators of young- of-the-year (YOY) bay anchovy were estimated from May 1995 to October 2000, based on tri-annual midwater trawl collections. Empirical allometric relationships were applied to develop size-depen- dent growth and mortality estimates. Estimated annual mean stock biomass of YOY bay anchovy was lowest in 1996 (17 × 10 3 metric tons (t) wet weight) and highest in 2000 (64 × 10 3 t). Estimated YOY production varied inter-annually by a factor of 4, ranging from 167 × 10 3 t in 1996 to 697 × 10 3 t in 2000. Daily stock biomass levels usually peaked in mid September, while daily production and con- tribution to predators peaked from mid July to late September. The annual production to mean bio- mass (P/ - B) ratio, which included the very productive larval stage, was high and ranged from 9.02 to 13.23. Analysis of environmental factors indicated that water temperature and dissolved oxygen were important controllers of growth rate. Inter-annual variability in length-specific mortality was inversely related to mean salinity in the summer and fall, suggesting that salinity controls abun- dances and spatial distributions of predators on bay anchovy. Results imply that annually variable hydrological conditions and secondary productivity may drive observed variability in recruitment and production of YOY bay anchovy by inducing small but decisive differences in growth and predation mortality on early-life stages.

Distribution of the euryhaline squid Lolliguncula brevis in Chesapeake Bay: effects of selected abiotic factors

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 226:235-247 (2002) - doi:10.3354/meps226235 Distribution of the euryhaline squid Lolliguncula brevis in Chesapeake Bay: effects of selected abiotic factors I. K. Bartol1,*, R. Mann2, M. Vecchione3 1Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, 621 Charles E. Young Drive South, Los Angeles, California 90095-1606, USA 2School of Marine Science, Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, Virginia 23062, USA 3National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Systematics Laboratory, National Museum of Natural History, 10th and Constitution NW, Washington, DC 20560, USA *E-mail: ikbartol@lifesci.ucla.edu ABSTRACT: The majority of cephalopods are thought to have limitations arising from physiology and locomotion that exclude them from shallow, highly variable, euryhaline environments. The brief squid Lolliguncula brevis may be a notable exception because it tolerates low salinities, withstands a wide range of environmental conditions, and swims readily in shallow water. Little is known about the distribution of L. brevis in Chesapeake Bay, a diverse and highly variable estuary. Therefore, a survey of L. brevis was conducted in the Virginia portion of Chesapeake Bay from 1993 to 1997 using a 9.1 m otter trawl, and the effects of selected factors on squid presence were assessed using logistic regression analysis. During spring through fall, L. brevis was collected over a wide range of bottom-water salinities (17.9 to 35.0 ‰), bottom-water temperatures (8.1 to 29.6°C), bottom-water dissolved oxygen levels (1.9 to 14.6 mg O2 l-1), and depths (1.8 to 29. but it was not present in trawls conducted during winter in. L. brevis, especially juveniles < 60 mm dorsal mantle length (DML), were abundant, frequently ranking in the upper 12% of overall annual nektonic trawl catches, and during the fall of some years, ranking second to anchovies. The probability of catching a squid increased in Chesapeake Bay at higher salinities and water temperatures, and was much greater in normoxic than in hypoxic waters; these variables had a profound influence on both annual and seasonal variability in distribution. Salinity had the largest influence on squid distribution, with squid being completely absent from the bay when salinity was <17.9” and most abundant in the fall when salinity was highest (despite declines in water temperature). Squid were most prevalent at depths between 10 and 15 m. The results of this study suggest that L. brevis is an important component of the Chesapeake Bay ecosystem when salinities and water temperatures are within tolerance limits and that unlike other squids, L. brevis may be well-equipped for an inshore, euryhaline existence. KEY WORDS: Squid · Estuaries · Salinity · Water temperature · Dissolved oxygen · Lolliguncula brevis Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 226. Online publication date: January 31, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Seasonal changes in the positional importance of components in the trophic flow network of the Chesapeake Bay

Community-level processes may shape food web structure. In this paper, a graph theoretical study of the weighted trophic flow network of the Chesapeake Bay ecosystem shows how important are positions in the energy (carbon) transport system. The positional importance of components is compared to the quantity of energy flowing through them. We suggest that the congruence between important network positions and large flows refers to the larger role of trophic interactions in community control. A seasonal dynamical analysis of the network has led us to the conclusion that winter is the season when the importance of predation is the highest.

Measuring the Health of the Chesapeake Bay: Toward Integration and Prediction

The health of an ecosystem is a function of its vigor (useful productivity), organization (complexity of interspecific interactions), and resilience (ability to maintain itself in the face of disturbance). The health of the Chesapeake Bay ecosystem has deteriorated largely as a result of nutrient overenrichment, concomitant reduction in light availability, and loss of habitats that provide complexity. This has resulted in an ecosystem that is a less vigorous producer of valuable fish and shellfish, less diverse and well organized, and more susceptible to and slower to recover from disturbances. It is not clear that degraded ecosystem health directly threatens human health; in fact sanitation and reductions in loadings of potentially toxic substances have reduced human health risks in recent decades. On the other hand, recently observed outbreaks of the toxin-producing dinoflagellate Pfiesteria piscicida could be a result of deteriorated ecosystem health and pose a human health risk. Monitoring of the environmental conditions, ecosystem health, and human health risks is critically important to the adaptive management of the Chesapeake Bay. Although this monitoring has produced very useful information, monitoring can be more effective if it more directly addressed the multiple uses of the resulting information, applied new technologies, and were more effectively integrated across environmental media, among resources, over space and time scales, and with modeling and research.

Nutrient controls on ecosystem dynamics: the Chesapeake mesohaline community

Ecological researchers have long borrowed concepts from the theory of chemical kinetics to describe nutrient dynamics in ecosystems. Contemporary ecology, however, is in the process of creating its own suite of ideas to quantify how whole ecosystems develop. In particular, the theory of ecosystem ascendency can be applied to data on the simultaneous flows of various chemical constituents to determine which element is limiting to each species via which individual input. That is, Liebig's law of the minimum appears a corollary to the broader description of whole-system development. Application of the method to networks of carbon, nitrogen and phosphorous flowing through the 36 major compartments of the Chesapeake Bay ecosystem reveals that, although nitrogen limits the production by most of the planktonic and benthic compartments, the nekton appear to be using phosphorus in limiting proportions. If one links together the ecosystem components via the controlling flows into each node, a new and sometimes dramatic picture of nutrient kinetics emerges. Not surprisingly, during the summer the root nutrient control on the system appears to be the recycle of nitrogen between particulate organic materials in the sediments and their attached bacteria. No coherent pattern of control is evident during the autumn turnover, whereas during winter and spring the ultimate control appears to be exerted by a feedback in the `microbial loop' that involves both N and P.

Oysters and the Chesapeake Bay Ecosystem: A Case for Exotic Species Introduction to Improve Environmental Quality?

Restoration of the Chesapeake Bay ecosystem has been a priority for residents and governments of the bay watershed for the past decade. One obstacle in the efforts to “save the bay” has been continuing nutrient enrichment from agricultural and sewer runoff. The attainability of a mandated 40% nutrient reduction goal has yet to be seen. Furthermore, disappearance of certain organisms may have had an adverse effect on the resilience of the ecosystem. The Eastern oyster (Crassostrea virginica), once abundant in Chesapeake Bay, was a vital part of the food web, processing excess phytoplankton and depositing materials on the bottom. Over harvesting and disease have decimated the native oyster population. The introduction of an exotic species, the Japanese oyster (Crassostrea gigas), may be a way to reestablish a robust oyster community in the bay. The literature on the role of bivalve molluscs in estuarine ecosystems shows that they are an essential part of healthy estuaries around the world. A comparison ofC. virginica andC. gigas in terms of temperature and salinity tolerance and resistance to disease shows thatC. virginica is ideally adapted to conditions in Chesapeake Bay, but it is unable to stave off the endemic diseases, whereasC. gigas is adapted to conditions in the lower bay only but is much less susceptible to the same diseases. We conclude that the potential introduction ofC. gigas to Chesapeake Bay would be limited by the Japanese species’ physiological requirements but that the revitalization of a bivalve population is imperative to the restoration of ecosystem function.