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1.
Geochemical (total nitrogen, total organic carbon, total phosphorus, total sulfur, and carbon and nitrogen stable isotopes) and selected biotic (diatom, foraminifera, polychaete) indicators preserved in two estuarine sediment cores from the mesohaline Chesapeake Bay provide a history of alterations in the food web associated with land-use change. One core from the mouth of the Chester River (CR) (collected in 2000) represents a 1,000-year record. The second core (collected in 1999), from the Chesapeake Bay’s main stem opposite the Choptank River (MD), represents a 500-year record. As European settlers converted a primarily forested landscape to agriculture, sedimentation rates increased, water clarity decreased, salinity decreased in some areas, and the estuarine food web changed into a predominantly planktonic system. Representatives of the benthic macrofaunal community (foraminifera and the polychaetes Nereis spp.) were affected by local changes before there were widespread landscape alterations. Nitrogen stable isotope records indicated that land-use changes affected nitrogen cycling beginning in the early 1700s. Extreme changes were evident in the mid-nineteenth century following widespread deforestation and since the mid-twentieth century reflecting heightened eutrophication as development increased in the Chesapeake Bay watershed. Results also demonstrate how paleoecological records vary due to the degree of terrestrial inputs of freshwater runoff and nutrients at core locations within the Chesapeake Bay.  相似文献   

2.
A hypothesis on the formation and seasonal evolution of Atlantic menhaden (Brevoortia tyrannus) juvenile nurseries in coastal estuaries is described. A series of cruises were undertaken to capture postmetamorphic juvenile menhaden and to characterize several biological and physical parameters along estuarine gradients. The two study systems, the Neuse and Pamlico rivers in North Carolina, contain important menhaden nursery grounds. Juvenile menhaden abundance was found to be associated with gradients of phytoplankton biomass as evidenced by chlorophylla levels in the upper water column. Fish abundances were only secondarily associated with salinity gradients as salinity was a factor that moderated primary production in the estuary. The persistence of spatial and temporal trends in the distribution of phytoplankton in the Neuse and Pamlico estuaries was reviewed. The review suggested that postmetamorphic juvenile menhaden modify their distribution patterns to match those created by phytoplankton biomass, which in turn makes them most abundant in the phytoplankton maxima of estuaries. Because the location of these maxima varies with the mixing and nutrient dynamics of different estuaries, so will the location of the nursery.  相似文献   

3.
To compare natural variability and trends in a developed estuary with human-influenced patterns, stable isotope ratios (δ13C and δ15N) were measured in sediments from five piston cores collected in Chesapeake Bay. Mixing of terrestrial and algal carbon sources primarily controls patterns of δ13Corg profiles, so this proxy shows changes in estuary productivity and in delivery of terrestrial carbon to the bay. Analyses of δ15N show periods when oxygen depletion allowed intense denitrification and nutrient recycling to develop in the seasonally stratified water column, in addition to recycling taking place in surficial sediments. These conditions produced 15N-enriched (heavy) nitrogen in algal biomass, and ultimately in sediment. A pronounced increasing trend in δ15N of +4‰ started in about A.D. 1750 to 1800 at all core sites, indicating greater eutrophication in the bay and summer oxygen depletion since that time. The timing of the change correlates with the advent of widespread land clearing and tillage in the watershed, and associated increases in erosion and sedimentation. Isotope data show that the region has experienced up to 13 wet-dry cycles in the last 2700 yr. Relative sea-level rise and basin infilling have produced a net freshening trend overprinted with cyclic climatic variability. Isotope data also constrain the relative position of the spring productivity maximum in Chesapeake Bay and distinguish local anomalies from sustained changes impacting large regions of the bay. This approach to reconstructing environmental history should be generally applicable to studies of other estuaries and coastal embayments impacted by watershed development.  相似文献   

4.
In this paper we assemble and analyze quantitative annual input-export budgets for total nitrogen (TN) and total phosphorus (TP) for Chesapeake Bay and three of its tributary estuaries (Potomac, Patuxent, and Choptank rivers). The budgets include estimates of TN and TP sources (point, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cycling rates (zooplankton excretion and net sediment-water flux), and net downstream exchange. Annual terrestrial and atmospheric inputs (average of 1985 and 1986 data) of TN and TP ranged from 4.3 g TN m?2 yr?1 to 29.3 g TN m?2 yr?1 and 0.32 g TP m?2 yr?1 to 2.42 g TP m?2 yr?1, respectively. These rates of TN and TP input represent 6-fold to 8-fold and 13-fold to 24-fold increases in loads to these systems since the precolonial period. A recent 11-yr record for the Susquehanna River indicates that annual loads of TN and TP have varied by about 2-fold and 4-fold, respectively. TN inputs increased and TP inputs decreased during the 11-yr period. The relative importance of nutrient sources varied among these estuaries: point sources of nutrients delivered about half the annual TN and TP load to the Patuxent and nearly 60% of TP inputs to the Choptank; diffuse sources contributed 60–70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet-fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct, although damped, relationships between annual rates of nutrient input, water-column and sediment nutrient stocks, and nutrient losses via burial in sediments and denitrification. Our budgets indicate that the annual mass balance of TN and TP is maintained by a net landward exchange of TP and, with one exception (Choptank River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terrestrial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the literature were resolved by the inclusion of sediments derived from shoreline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmospheric dry-fall, contributions of nutrients via groundwater, and the sedimentation rates used to calculate nutrient burial rates.  相似文献   

5.
A 2,500-year history of anoxia and eutrophication in Chesapeake Bay   总被引：2，自引：0，他引：2  
Ongoing monitoring programs and historical data are not sufficient to establish anthropogenic effects on the ecology of Chesapeake Bay. However, stratigraphic records preserved in the sediments can be used to reconstruct both prehistoric and historic sedimentation and water conditions of the bay, including anoxia and eutrophication. Pollen, diatoms, total organic carbon (TOC), nitrogen, total sulfur, and an estimate of the degree of pyritization of iron (DOP) are being used as paleoecological indicators in dated sediment cores for the purpose of reconstructing a long-term environmental history of the bay. Analysis of the data indicates that sedimentation rates, anoxic conditions, and eutrophication have increased in the Chesapeake Bay since the time of European settlement. For example, since initial land clearance around 1760, sedimentation rates have increased from as low as 0.02 cm yr?1 to an average 0.22 cm yr?1, and TOC from 0.14 mg cm?2 yr?1 to a high 4.96 mg cm?2 yr?1. Diatom community structure shows a steady decrease in overall diversity since 1760 and the centric:pennate ratio has increased significantly since 1940.  相似文献   

6.
Organisms and chemicals preserved in sediment cores from the Chesapeake estuary in mid-Atlantic USA are consistent with a precolonial landscape covered with a diversity of forests and marshes, large and small. During the past 300 years, many of the wetlands have been drained, and the landscape was converted to agricultural fields and urban and suburban development. During this time, sources of nitrogen have diversified, and loadings have increased. Since precolonial time, the mesohaline estuary has become increasingly eutrophic and anoxic. Estuaries and coastal regions throughout the world have experienced similar conditions in their recent history. These changes are recorded in Chesapeake sediment cores by increases in ragweed pollen, dry taxa, sedimentation rates, nitrogen influxes, and a major change in estuarine autotrophs from benthic to planktonic. In many areas, attempts to reverse estuarine eutrophication and anoxia have centered on restoring streams and riparian areas and reducing fertilizer use on agricultural lands. However, data from soils and historical reports and the paleoecological record suggest that to reduce the effects of modern nitrogen inputs, it may be necessary to locate and enhance denitrifying areas throughout the watershed.  相似文献   

7.
Environmentally sensitive benthic foraminifera (protists) from Chesapeake Bay were used as bioindicators to estimate the timing and degree of changes in dissolved oxygen (DO) over the past five centuries. Living foraminifers from 19 surface samples and fossil assemblages from 11 sediment cores dated by210Pb,137Cs,14C, and pollen stratigraphy were analyzed from the tidal portions of the Patuxent, Potomac, and Choptank Rivers and the main channel of the Chesapeake Bay.Ammonia parkinsoniana, a facultative anaerobe tolerant of periodic anoxic conditions, comprises an average of 74% of modern Chesapeake foraminiferal assemblages (DO-0.47 and 1.72 ml l−1) compared to 0% to 15% of assemblages collected in the 1960s. Paleoecological analyses show thatA. parkinsoniana was absent prior to the late 17th century, increased to 10–25% relative frequency between approximately 1670–1720 and 1810–1900, and became the dominant (60–90%) benthic formaniferal species in channel environments beginning in the early 1970s. Since the 1970s, deformed tests ofA. parkinsoniana occur in all cores (10–20% ofAmmonia), suggesting unprecedented stressful benthic conditions. These cores indicate that prior to the late 17th century, there was limited oxygen depletion. During the past 200 years, decadal scale variability in oxygen depletion has occurred, as dysoxic (DO=0.1–1.0 ml l−1), perhaps short-term anoxic (DO<0.1 ml l−1) conditions developed. The most extensive (spatially and temporally) anoxic conditions were reached during the 1970s. Over decadal timescales, DO variability seems to be linked closely to climatological factors influencing river discharge; the unprecedented anoxia since the early 1970s is attributed mainly to high freshwater flow and to an increase in nutrient concentrations from the watershed.  相似文献   

8.
Long records of monthly salinity observations along the axis of Chesapeake Bay, Delaware Bay, and Long Island Sound are used to test a simple advection–dispersion model of the salt distribution in linearly tapered estuaries developed in a previous paper. We subdivide each estuary into three to five segments, each with linear taper allowing a distributed input of fresh water, and evaluate the dispersion in each segment. While Delaware Bay has weak dispersion and a classical sigmoidal salinity structure, Long Island Sound and Chesapeake Bay are more dispersive and have relatively small gradients in the central stretches. Long Island Sound is distinguished by having a net volume and salt flux out of its low-salinity end resulting in a smaller range of salinity and increasing axial gradients at its head rather than the usual asymptotic approach to zero salinity. Estimates of residence times based on model transport coefficients show that Long Island Sound has the most rapid response to fresh-water flux variations. It also has the largest amplitude cycle in river discharge fluctuation. In combination, these cause the large seasonal variation in the salinity structure relative to interannual variability in Long Island Sound as compared with Chesapeake Bay and Delaware Bay.  相似文献   

9.
The decline of eelgrass (Zostera marina) in Chesapeake Bay in the 1960s and 1970s has been studied in the context of changes in water quality and habitat suitability; little effort has focused on the importance of reproductive ecology in understanding current and potential recovery of these populations. The spatial variability of seed-bank characteristics ofZ. marina in Chesapeake Bay was explored by a reproductive shoot and seed-bank sampling effort. Seed banks were sampled from 105 beds of submerged aquatic vegetation among 12 zones throughout the lower and middle Chesapeake Bay. Number of viable seeds was highly variable among and within zones, with seeds found in all but one zone and also found in cores not containing anyZ. marina shoots. Number of reproductive shoots was also highly variable among and within zones, with differences probably driven by different local environmental conditions. Bay-wide, viable seeds were found in more monospecificZ. marina cores than in mixed species or monospecificRuppia maritima cores suggesting local biological and environmental control on sexual reproduction. Lower densities of viable seeds in the middle Chesapeake Bay region reflect the lower abundance ofZ. marina in these regions and provide context for discussion of historical changes inZ. marina in Chesapeake Bay. While this study focused on a snap shot of the seed bank immediately after establishment, we highlight critical questions for future study that may be important for their conservation and restoration.  相似文献   

10.
The purpose of this study was to quantify the nitrogen (N) inputs to 34 estuaries on the Atlantic and Gulf Coasts of the United States. Total nitrogen (TN) inputs ranged from 1 kg N ha−1 yr−1 for Upper Laguna Madre, Texas, to 49 kg N ha−1 yr−1 for Massachusetts Bay, Massachusetts. TN inputs to 11 of the 34 estuaries were dominated by urban N sources (point sources and septic systems) and nonpoint source N runoff (5% of total); point sources accounted for 36–86% of the TN inputs to these 11 urban-dominated estuaries. TN inputs to 20 of the 34 estuaries were dominated by agricultural N sources; N fertilization was the dominant source (46% of the total), followed by manure (32% of the total) and N fixation by crops (16% of the total). Atmospheric deposition (runoff from watershed plus direct deposition to the surface of the estuary) was the dominant N source for three estuaries (Barnegat Bay, New Jersey: 64%; St. Catherines-Sapelo, Georgia: 72%; and Barataria Bay, Louisiana: 53%). Six estuaries had atmospheric contributions ≥30% of the TN inputs (Casco Bay, Maine: 43%; Buzzards Bay, Massachusetts: 30%; Great Bay, New Jersey: 40%; Chesapeake Bay: 30%; Terrebonne-Timbalier Bay, Louisiana: 59%; and Upper Laguna Madre: 41%). Results from our study suggest that reductions in N loadings to estuaries should be accomplished by implementing watershed specific programs that target the dominant N sources.  相似文献   

11.
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.  相似文献   

12.
Two laboratory microcosm experiments were conducted to mimic an annual spring diatom bloom in South San Francisco Bay by isolating the phytoplankton community from the benthic grazing pressure to induce a phytoplankton bloom. The purpose of these experiments was to isolate the impact of a spring diatom bloom on the nutrient and trace metal geochemical cycling. Microcosms were created in 2.5 L incubation bottles and subjected to one of 4 treatments (control, copper [Cu] addition, manganese [Mn] addition, and both Cu and Mn addition) to investigate the toxicity of Cu on the resident plankton and the potential antagonistic effects of Mn on reducing Cu toxicity. Dissolved macronutrient (nitrate + nitrite, phosphate, and silicate), and dissolved and particulate trace metal (Cu, Ni, Mn) concentrations were monitored in the grow-out incubations on a daily basis. Chlorophylla concentrations were also monitored over the course of the experiment and used to calculate diatom-specific growth rates. In the experiments containing ambient South San Francisco Bay surface waters, average specific growth rates were on the order of 1.1 d?1. The induced diatom blooms resulted in significant removal of macronutrients from the microcosms over the course of the experiments. Our research supports previous suggestions that dissolved Ni and Cu concentrations in South San Francisco Bay have a very low biological availability as a result of organic chelation. Ni(EDTA)2? has been found to be the dominant dissolved Ni species by other researchers and Cu speciation analyses from this study and others indicate that > 99% of the dissolved Cu in South San Francisco Bay is strongly chelated as CuL1. The free cupric ion concentration was on the order of 10?12 M. Marked removal of dissolved Mn was observed in the control treatments, well exceeding expected dissolved Mn removal by diatom uptake. Additions of 375 nM Cu resulted in the complete titration of the chelating ligand (L1) concentrations. The elevated [Cu2+] (≈10?8MM) appeared to have a toxic effect on the diatom community observed in the significant decreases in their specific growth rates (μ=0.4 d?1). The suppression of dissolved Mn removal from solution was also observed in treatments spiked with high levels of dissolved Cu, providing support that Mn precipitation was due to biologically mediated oxidation not phytoplankton assimilation. The observed geochemical behavior in the concurrent Cu and Mn addition treatments provide evidence in support of Mn alleviation of Cu toxicity. The biological role in the ambient short-term biogeochemical cycling of Cu and Ni in South San Francisco Bay appears to be minimal due to the inert character of the organic ligand-metal complexes. A significant portion of the annual macronutrient and Mn cycling occurs as a result of spring diatom blooms in South San Francisco Bay.  相似文献   

13.
Hurricane Floyd in September 1999 caused disastrous flooding from South Carolina to Massachusetts in the United States, with particularly severe and prolonged flooding in eastern North Carolina resulting in record flood-flow loadings of freshwater and contaminants to Pamlico Sound, North Carolina. The inland flooding, water quality, and loadings to Pamlico Sound were determined as part of a multi-agency response to the floods and in an effort to understand the effects of the floods on the greater Pamlico Sound Basin. All major river basins draining to Pamlico Sound experienced floods at the 500-yr recurrence level. The volume of flood waters entering Pamlico Sound during September–October 1999 was estimated to be equivalent to about 95% of the volume of Pamlico Sound, meaning that flood waters could have essentially displaced most of the water present in Pamlico Sound. Nitrogen and phosphorus loads to the Pamlico River estuary and Neuse River estuary, the two principal estuaries draining to Pamlico Sound, in a 36-d period during the flooding were between 50–90% of the long term average annual loads. Pesticide concentrations in flood waters were surprisingly high, given the amount of dilution produced by the floodwaters.  相似文献   

14.
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.  相似文献   

15.
We calculated Chesapeake Bay (CB) sediment and carbon fluxes before and after major anthropogenic land clearance using robust monitoring, modeling and sedimentary data. Four distinct fluxes in the estuarine system were considered including (1) the flux of eroded material from the watershed to streams, (2) the flux of suspended sediment at river fall lines, (3) the burial flux in tributary sediments, and (4) the burial flux in main CB sediments. The sedimentary maximum in Ambrosia (ragweed) pollen marked peak land clearance (~1900 a.d.). Rivers feeding CB had a total organic carbon (TOC)/total suspended solids of 0.24?±?0.12, and we used this observation to calculate TOC fluxes from sediment fluxes. Sediment and carbon fluxes increased by 138–269% across all four regions after land clearance. Our results demonstrate that sediment delivery to CB is subject to significant lags and that excess post-land clearance sediment loads have not reached the ocean. Post-land clearance increases in erosional flux from watersheds, and burial in estuaries are important processes that must be considered to calculate accurate global sediment and carbon budgets.  相似文献   

16.
The use of multiple stable isotopes in the study of trophic relationships in temperate estuaries has usually been limited to euhaline systems, in which phytoplankton, benthic microalgae, andSpartina alterniflora are major sources of organic matter for consumers. Within large estuaries such as Chesapeake Bay, however, many species of consumers are found in the upper mesohaline to oligohaline portions. These lower salinity wetlands have a greater abundance of macrophytes that use C3 photosynthesis to fix carbon, in addition toS. alterniflora, which fixes carbon via the C4 photosynthetic pathway. In a broad survey of the biota and sediments of a brackish tidal creek tributary to Chesapeake Bay, combined δ13C and δ34S measurements disclosed a balanced contribution to secondary production from phytoplankton, C3 macrophytes,Spartina sp., and benthic microalgae. Surface sediment δ13C suggested that the organic matter from C3 plants was derived both from allochthonous sources (terrestrial runoff) and from autochthonous production (marsh macrophytes). Unlike most estuarine systems studied to date, which are dominated by algae (phytoplankton and benthic microalgae) and C4 macrophytes, C3 plants are of greater importance in the diets of consumers in this low-salinity creek system.  相似文献   

17.
Since its discovery in natural estuarine habitat of North Carolina in 1991, the widespread impact of the toxic dinoflagellate, Pfiesteria piscicida (gen. et sp. nov.), popularly called the “phantom” dinoflagellate, on North Carolina fish stocks has been established, yet little is known about its influence outside of North Carolina estuaries. Here, we document the presence of P. piscicida in Chesapeake Bay. A fish kill was observed after inoculating an aquarium containing mummichogs with sediment samples from Jenkins Creek, a brackish creek (salinity 11‰) of the Chesapeake Bay system. P. piscicida was the cause of the kill, as supported by morphological, physiological, and histological evidence. The appearance and behavior of the algae and symptoms associated with fish mortality were consistent with those previously observed in P. piscicida-associated aquaria fish kills in North Carolina. The discovery of P. piscicida in Chesapeake Bay supports the speculation that these toxic dinoflagellates have a dramatic and far-reaching impact on fish stocks in shallow, eutrophic estuaries along the eastern United States.  相似文献   

18.
Sedimentation rates were determined with the 210Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm?2 y?1 in the central part to 0.13 g cm?2 y?1 in the eastern part of the lake and then increases rapidly towards the Rhine delta. In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the 210Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production. Dating with 137Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones. A positive correlation of 210Pb fluxes and sedimentation rates indicates that 210Pb flux into sediments follows the distribution pattern of solids. 210Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long-range particulate input variations remains to be proved. Sedimentation rates determined with the 210Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.  相似文献   

19.
In order to establish paleoenvironmental conditions during the late Quaternary, four cores from the Basin of Mexico (central Mexico) were drilled in Chalco Lake, located in the southeastern part of the basin. The upper 8 m of two parallel cores were studied, using paleomagnetic, loss-on-ignition, pollen, and diatom analyses. Based on 11 14C ages, the analyzed record spans the last 19,000 14C yr B.P. Volcanic activity has affected microfossil abundances, both directly and indirectly, resulting in absence or reduction of pollen and diatom assemblages. Important volcanic activity took place between 19,000 and 15,000 yr B.P. when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry, cold climate. During this interval, abrupt environmental changes with increasing moisture occurred. From 15,000 until 12,500 yr B.P. the lake level increased and the pollen indicates wetter conditions. The highest lake level is registered from 12,500 to ca. 9000 yr B.P. The end of the Pleistocene is characterized by an increase in humidity. From 9000 until ca. 3000 yr B.P. Chalco Lake was a saline marsh and the pollen record indicates warmer conditions. After 3000 yr B.P. the lake level increased and human disturbance dominates the lacustrine record.  相似文献   

20.
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.  相似文献   

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