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1.
Identification of food web linkages is a major aim in ecology because it provides basic information on trophic flows and the potential for interspecific interactions. In addition, policy and restoration measures mandated to conform to ecosystem-based management principals can benefit from information on temporal and spatial variability in community-level interactions. Here, we analyzed guild structure of the demersal fish assemblage in Puget Sound, WA, a temperate estuarine system on the US west coast. Using diet information from 2,401 stomachs collected across three seasons (fall, winter, and summer), we identified guild membership for 21 fish species, examined seasonal guild switching, and tested for seasonal shifts in predation and for differences in the degree of diet overlap at the assemblage level. We accounted for ontogenetic variation in diet by dividing species into large (L) and small (S) size classes when appropriate. Using cluster analysis and a permutation approach, we identified seven significant guilds that were typified by predation on benthic invertebrates, pelagic invertebrates, and piscivory. Of the 18 species with more than one season of diet information, six switched guilds (Pacific sanddab L, sturgeon poacher, Pacific tomcod S, speckled sanddab, rex sole, and rock sole S). At the assemblage level, we tested for seasonal differences in prey use between seasons by performing an analysis of similarities based on Bray–Curtis diet similarities and found no significant difference. However, diet overlap was significantly higher in the summer than the fall and winter (with summer > fall > winter) indicating that diets within the assemblage converged in the summer. These results indicate that analyses of guild structure and diet overlap can reveal seasonal variation in community trophic structure and highlight intra-annual food web variation in the Puget Sound demersal fish community.  相似文献   

2.
Detritus from terrestrial ecosystems is the major source of organic matter in many streams, rivers, and estuaries, yet the role of detritus in supporting pelagic food webs is debated. We examined the importance of detritus to secondary productivity in the Sacramento and San Joaquin River Delta (California, United States), a large complex of tidal freshwater habitats. The Delta ecosystem has low primary productivity but large detrital inputs, so we hypothesized that de tritus is the primary energy source fueling production in pelagic food webs. We assessed the sources, quantity, composition, and bioavailability of organic matter among a diversity of habitats (e.g., marsh sloughs, floodplains, tidal lakes, and deep river channels) over two years to test this hypothesis. Our results support the emerging principle that detritus dominates riverine and estuarine organic matter supply and supports the majority of ecosystem metabolism. Yet in contrast to prevailing ideas, we found that detritus was weakly coupled to the Delta's pelagic food web. Results from independent approaches showed that phytoplankton production was the dominant source of organic matter for the Delta's pelagic food web, even though primary production accounts for a small fraction of the Delta's organic matter supply. If these results are general, they suggest that the value of organic matter to higher trophic levels, including species targeted by programs of ecosystem restoration, is a function of phytoplankton production.  相似文献   

3.
River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries. In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience.  相似文献   

4.
Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.  相似文献   

5.
Changes in seagrass food-web structure can shift the competitive balance between seagrass and algae, and may alter the flow of energy from lower trophic levels to commercially important fish and crustaceans. Yet, trophic relationships in many seagrass systems remain poorly resolved. We estimated the food web linkages among small predators, invertebrate mesograzers, and primary producers in a Chesapeake Bay eelgrass (Zostera marina) bed by analyzing gut contents and stable C and N isotope ratios. Though trophic levels were relatively distinct, predators varied in the proportion of mesograzers consumed relative to alternative prey, and some mesograzers consumed macrophytes or exhibited intra-guild predation in addition to feeding on periphyton and detritus. These findings corroborate conclusions from lab and mesocosm studies that the ecological impacts of mesograzers vary widely among species, and they emphasize the need for taxonomic resolution and ecological information within seagrass epifaunal communities.  相似文献   

6.
We compared the extent to which ancient and restoring wetlands in three estuary regions of San Francisco Bay support estuarine ecosystems through food web contributions. In comparison to mature marshes, we hypothesized that food webs of increasingly younger restoration sites would display increased dependency upon allochthonous subsidies due to nominal internal production. Using multiple stable isotopes (δ13C, δ15N, δ34S) in a mixing model, we traced links among primary producers and estuarine consumers. Results indicate that food webs of estuarine marshes are heavily dependent upon autochthonous marsh materials (76 ± 17%), even within the youngest restoration marshes (11 years). Nearly all sampled organisms relied upon autochthonous marsh materials, with the exception of Neomysis kadiakensis, a mysid shrimp, which derived the majority of its support from freshwater-produced phytoplankton. Marsh-derived organic matter (OM) support was consistent both temporally throughout the year and spatially along the three estuary regions, but evidence suggests that the specific type of OM supporting estuarine consumers depends on position along the estuarine gradient and on seasonal shifts in freshwater flow. These results indicate that wetland restoration rapidly provides important contributions to marsh consumers and potentially bolsters food web linkages in shallow-water ecosystems.  相似文献   

7.
Using a large-scale enclosed sea area in northern Hangzhou Bay as a case study, the trophic interactions, energy flows, and ecosystem properties of a coastal artificial ecosystem were analyzed by ecotrophic modeling using Ecopath with Ecosim software (EwE, 5.1 version). The model consists of 13 functional groups: piscivorous fish, benthic-feeding fish, zooplanktivorous fish, herbivorous fish, crabs, shrimp, mollusca, infauna, carnivorous zooplankton, herbivorous zooplankton, macrophytes, phytoplankton, and detritus. Input information for the model was gathered from published and unpublished reports and from our own estimates during the period 2006–2007. Results show that the food web in the enclosed sea area was dominated by a detritus pathway. The trophic levels of the groups varied from 1.00 for primary producers and detritus to 3.90 for piscivorous fish in the coastal artificial system. Using network analysis, the system network was mapped into a linear food chain, and five discrete trophic levels were found with a mean transfer efficiency of 9.8% from detritus and 9.4% from primary producer within the ecosystem. The geometric mean of the trophic transfer efficiencies was 9.6%. Detritus contributed 57% of the total energy flux, and the other 43% came from primary producers. The ecosystem maturity indices—total primary production/total respiration, Finn’s cycling index, and ascendancy—were 2.56, 25.0%, and 31.0%, respectively, showing that the coastal artificial system is at developmental stage according to Odum’s theory of ecosystem development. Generally, this is the first trophic model of a large-scale artificial sea enclosure in China and provides some useful insights into the structure and functioning of the system.  相似文献   

8.
The goal of this study was to determine the food web pathways supporting juvenile Chinook (Oncorhynchus tshawytscha) salmon in the Columbia River estuary through multiple stable isotope analysis (δ13C, δ15N, δ34S). Using this method, we distinguished the role of various organic matter sources in Chinook food webs and interpreted the dynamics of their use both spatially and temporally within the estuary. Our results indicate that subyearling Chinook are associated with fluvial, anthropogenic, estuarine, and marine organic matter sources, with hatchery food and vascular plant detritus being the most dominant sources in juvenile Chinook food webs. Although freshwater phytoplankton is involved in many food web pathways to subyearling Chinook, increased phytoplankton production from the impounded river has not replaced the loss of autochthonous marsh production to fish. Our results indicate that large-scale ecosystem alteration may have decreased the availability and quality of food webs in the estuary and potentially diminished the ability of the Columbia to support Chinook salmon.  相似文献   

9.
We established trophic guilds of macroinvertebrate and fish taxa using correspondence analysis and a hierarchical clustering strategy for a seagrass food web in winter in the northeastern Gulf of Mexico. To create the diet matrix, we characterized the trophic linkages of macroinvertebrate and fish taxa present inHalodule wrightii seagrass habitat areas within the St. Marks National Wildlife Refuge (Florida) using binary data, combining dietary links obtained from relevant literature for macroinvertebrates with stomach analysis of common fishes collected during January and February of 1994. Heirarchical average-linkage cluster analysis of the 73 taxa of fishes and macroinvertebrates in the diet matrix yielded 14 clusters with diet similarity ≥ 0.60. We then used correspondence analysis with three factors to jointly plot the coordinates of the consumers (identified by cluster membership) and of the 33 food sources. Correspondence analysis served as a visualization tool for assigning each taxon to one of eight trophic guilds: herbivores, detritivores, suspension feeders, omnivores, molluscivores, meiobenthos consumers, macrobenthos consumers and piscivores. These trophic groups, corss-classified with major taxonomic groups, were further used to develop consumer compartments in a network analysis model of carbon flow in this seagrass ecosystem. The method presented here should greatly improve the development of future network models of food webs by providing an objective procedure for aggregating trophic groups.  相似文献   

10.
To determine the role of mangroves for fisheries in the arid region of the Persian Gulf, we investigated fish community structure and trophic diversity in intertidal creeks with and without mangroves. Fish community abundances and biomass were compared across habitats and seasons. To identify variations in overall community trophic niches among habitats and seasons, we measured niches with size-corrected standard ellipse areas (SEAc) calculated from C and N stable isotope values. Although there was a slightly greater species richness occurred in mangrove creeks, we found a general similarity in the diversity patterns in creeks with and without mangroves. Also, there were no consistent differences in fish abundance or biomass for mangrove vs. non-mangrove fish collections. Community trophic diversity measured as SEAc also showed no significant difference between mangrove and non-mangrove sites. Instead, strong seasonal patterns were observed in the fish assemblages. Winter samples had consistently higher fish abundance and biomass than summer samples. Winter SEAc values were significantly higher, indicating that the fish community had a larger isotopic niche in winter than summer. Overall, we found that seasonality was much stronger than habitat in determining fish community structure and trophic diversity in the mangrove and non-mangrove ecosystems of Qeshm Island, Iran.  相似文献   

11.
The Skidaway River estuary, GA (USA), a tidally dominated subtropical system surrounded by extensive Spartina salt marshes, is experiencing steady increases in nutrients, chlorophyll, and particulate matter and decline in dissolved oxygen, associated with cultural eutrophication. A long-term study is documenting changes in these parameters: previous papers Verity (Estuaries 25:944–960, 2002a, Estuaries 25:961–975, b) reported on hydrography, nutrients, chlorophyll, and particulate matter during 1986–1996; plankton community responses are reported here. Phytoplankton, bacteria, heterotrophic nanoplankton and dinoflagellates, ciliates, and copepods exhibited strong seasonal cycles in abundance driven by temperature and resource availability, typically with summer maxima and winter minima. However, cultural eutrophication coincided with altered planktonic food webs as autotrophic and heterotrophic communities responded to increasing concentrations and changing ratios of inorganic and organic nutrients, potential prey, and predators. Small (<8 μm) photosynthetic nanoplankton increased in absolute concentration and also relative to larger cells. In contrast, diatoms did not show consistent increases in abundance, despite significant long-term increases in ambient silicate concentrations. Mean annual bacteria concentrations approximately doubled, and eukaryotic organisms in the microbial food web (heterotrophic and mixotrophic flagellates, dinoflagellates, ciliates, and metazoan zooplankton) also increased. All plankton groups except copepods showed trends of increasing annual amplitudes between seasonal high and low values, with higher peak concentrations each year. These observations suggest that the eutrophication signal was gradually becoming uncoupled from regulatory mechanisms. Theory and evidence from other more impacted waters suggest that, if these patterns continue, changes in the structure and function of higher trophic levels will ensue.  相似文献   

12.
Zooplankton are an important trophic link and a key food source for many larval fish species in estuarine ecosystems. The present study documents temporal and spatial zooplankton dynamics in Suisun Bay and the Sacramento–San Joaquin Delta—the landward portion of the San Francisco Estuary (California, USA)—over a 37-year period (1972–2008). The zooplankton community experienced major changes in species composition, largely associated with direct and indirect effects of introductions of non-native bivalve and zooplankton species. A major clam invasion and many subsequent changes in zooplankton abundance and composition coincided with an extended drought and accompanying low-flow/high-salinity conditions during 1987–1994. In the downstream mesohaline region, the historically abundant calanoid copepods and rotifers have declined significantly, but their biomass has been compensated to some extent by the introduced cyclopoid Limnothoina tetraspina. The more upstream estuary has also experienced long-term declining biomass trends, particularly of cladocerans and rotifers, although calanoid copepods have increased since the early 1990s due to the introduced Pseudodiaptomus spp. In addition, mysid biomass has dropped significantly throughout the estuary. Shifts in zooplankton species composition have also been accompanied by an observed decrease in mean zooplankton size and an inferred decrease in zooplankton food quality. These changes in the biomass, size, and possibly chemical composition of the zooplankton community imply major alterations in pelagic food web processes, including a drop in prey quantity and quality for foraging fish and an increase in the importance of the microbial food web for higher trophic levels.  相似文献   

13.
Using litter bag experiments in the Fraser River estuary in British Columbia, we tested for differences in the relative decomposition rates between leaves of purple loosestrife (Lythrum salicaria), an introduced exotic and a native sedge (Carex lyngbyei). The difference in the mean decay rate coefficient for the two species was significantly defferent (p<0.01) and the coefficient for purple loosestrife (0.0110 d?1) was nearly four times higher than for Lyngbyei's sedge (0.0028 d?1). This is the first estimate of the decay rate coefficient for purple loosestrife from an estuary. The rapid decay rate of loosestrife leaves suggests that they supply detritus to the ecosystem in autumn whereas the much slower decay rate of sedge implies that it supplies detritus throughout the winter and early spring. Consumer organisms important in juvenile salmon food webs appear to be adapted to take advantage of the detritus provided in these seasons. The findings have implications for habitat management because purple loosestrife has recently invaded estuaries of the northeast Pacific and may be outcompeting native sedges important in detrital-based food webs.  相似文献   

14.
The seasonal pattern of phytoplankton biomass (chlorophyll and particulate organic carbon) and the salinity-related pattern of phytoplankton biomass and size composition were determined in Apalachicola Bay, Florida, throughout 2004. Phytoplankton biomass was highest during summer and lowest during winter. During summer, phytoplankton biomass was highest in waters with salinity between about 5 and 23. In waters between 5 and 23, phytoplankton biomass was primarily (> 50%) composed of < 5 μm cells. The results from this study support the idea that a microbial food web characterizes mass and energy flow through the planktonic food web in Apalachicola Bay and other estuaries. During winter, the carbonxhlorophylla ratio averaged 56 ± 60 (standard deviation). During summer, the ratio ranged from 23 to 345, with highest values occurring in waters with salinity between about 8 and 22. The carbonxhlorophylla ratio was positively related to the percent of chlorophyll < 5 μm in size during summer.  相似文献   

15.
Physical and chemical characteristics of the Hooghly estuary during winter (December 1997–January 1998), summer (May 1998) and post-monsoon (November 1998) seasons have been studied. Salinity varied spatially and temporally and seasonally during ebb and flood tide conditions. Water temperature showed a difference of 10‡C in winter to summer. Temperature did not vary much vertically as it is a well-mixed estuary. Strong currents exceeding 100 cm S-1 were observed during peak ebb and flood tide conditions irrespective of the season. Longitudinal eddy diffusion coefficient (K x ) was estimated as 757m S-1 and 811m2 S-1 during summer and post-monsoon seasons, respectively. The vertical eddy diffusion coefficient (εv) was estimated as 0.0337 m2 S-1 during post-monsoon season. The salinity and current observations are compared with those obtained from models reported earlier. Values of pH, Dissolved Oxygen and Biological Oxygen Demand are within the threshold limits of the estuarine environment. Nutrients show seasonal variation in the estuarine environment. High values (160-2686 mg l-1) of total suspended matter were noticed both at surface and bottom in the study region showing the impact of fresh water and sediment transportation.  相似文献   

16.
Significant coherence among time series of environmental and biological production variables suggested mechanistic pathways through which climate contributed to the downward shift in estuarine production (biomass) in northern San Francisco Bay estuary, 1975–1993. Climate directly and indirectly affected physical processes in the estuary through precipitation and its subsequent impact on streamflow and physical variables affected by streamflow. Climate also directly influenced air temperature and wind velocity. The influence if climate was evaluated through a climate index based on sea level pressure. A shift in this climate index in the early 1980s coincided with changes in many environmental variables including water transparency, water temperature, wind velocity, and rainfall. These physical changes were accompanied by a decrease in diatom, total zooplankton, andNeomysis mercedis carbon at the base of the food web throughout the estuary. Box-Jenkins time series coherence analysis was used to quantify associations among these physical, chemical, and biological time series for nine regions of the estuary. These associations were used to develop a conceptual model of mechanistic pathways that directly linked food web carbon production to climate. Strong coherence among diatom, zooplankton, andN. mercedis carbon time series suggested climate also had an indirect impact on food web production through trophic cascade. Differing mechanistic pathways among the nine regions of the estuary suggested climate was an important contributor to the spatial variability in total food web production and trophic structure.  相似文献   

17.
Characterizing trophic flows is central to our understanding of energy transfer in marine ecosystems. The food webs of coastal systems are difficult to study because the proportion of autochthonous to allochthonous sources often varies seasonally and is often overlaid on a seasonal cycle of zooplankton composition. Here, we use a combination of fatty acids and stable isotopes to disentangle the trophic pathways in a productive coastal system (the Strait of Georgia (SoG), Canada). Over the span of a year, Metridia pacifica, a ubiquitous omnivorous copepod, can utilize a wide range of dietary items including diatoms, flagellates, bacteria, detritus, and microzooplankton. M. pacifica can switch from herbivory to carnivory in response to declining chlorophyll concentrations after the spring bloom and can occasionally utilize detrital sources. These findings are discussed in the context of previous knowledge of the SoG ecosystem, the current state of ecosystem modeling in the region, and the use of stable isotopes and fatty acids to assess trophic dynamics.  相似文献   

18.
Strong changes in stable isotope tracers commonly occur across estuarine salinity gradients from freshwater to the sea. The tracer gradients reflect the different geochemistries and mixing of freshwater and seawater, and these bottom-up geochemical influences are recorded in estuarine food webs in the isotopic compositions of animals. Conservative mixing calculations suggest that watershed-level inputs of freshwater and nutrients should exert strong influences on isotopic values of estuarine consumers, especially consumers such as bivalves that largely depend on phytoplankton production. Deviations from conservative isotope mixing also occur, and the magnitude of these deviations measures the strength of within-estuary organic matter cycling for estuarine food webs, especially inputs of non-phytoplankton foods such as macrophyte detritus and benthic algae. Measuring consumer isotopes across salinity gradients should be a relatively simple way to monitor effects of watershed nutrient loading and hydrologic flushing in supporting estuarine fisheries production.  相似文献   

19.
Neomysis americana, a common prey item in the diet of estuarine fish, is seasonally abundant in saltmarsh tidal creeks of North Inlet, South Carolina. Visual examination of mysids collected in winter revealed only diatom tests in their guts, with nearly all of the ingesta unidentifiable. Immunological diet analysis provided strong evidence that proteins fromSpartina alterniflora detritus were present in the guts of field-collected mysids. Laboratory determinations of weight-specific ingestion rates for male and female mysids fed particulateS. alterniflora were best described by a curvilinear quadratic function. Ingestion rates ranged from zero to 0.116 mg C ingested per mg C body weight per hr, or greater than 200% of body carbon per day if continuous feeding is assumed. These high ingestion rates may indicate low assimilation efficiency, but ifN. americana’s assimilation efficiency is similar to that of other mysid species, then this mysid may represent an efficient trophic link between saltmarsh macrophyte production and higher trophic levels. The occurrence ofSpartina detritus in the guts of winter mysids suggests that detritivory may be important during periods of scarce food.  相似文献   

20.
We used a mesocosm approach to examine howratch characteristics influenced predation and habitat selection in a tritrophic food web. Our experiments included juvenile red drum (Sciaenops ocellatus; RD), juvenile pinfish (Lagodon rhomboides; PF), and grass shrimp (Palaemonetes sp.; GS), members of a food web common in seagrass meadows of the northern Gulf of Mexico. We added an additional level of complexity to the experiment by including a predator that could feed at two different trophic levels. RD were top predators, PF were both prey items for RD and predators of GS, and GS were prey for RD and PF. We used 4 different artificial seagrass habitats that varied by size (0.049 and 0.203 m2) and shape (circular and stellate) to control for covariation between patch size and seagrass density. Predation on GS was measured in each habitat when PF, RD, and PF+RD were present, and predation on PF was measured when RD and RD+GS were present. Habitat selection by each of these 3 species was measured individually and in the presence of every other combination of the 3 species. Neither predation nor habitat selection were consistently influenced by patch characteristics (size, shape, or perimeter: area ratios) or the number of trophic levels. For GS, there was a significant negative relationship between patch size and predation rates in the GS+PF+RD treatment. Habitat selection by GS without the threat of predation suggested a preference for smaller habitats, but when in the presences of RD or RD+PF, GS preferred larger habitats. In predation experiments, PF predation by RD showed no significant relationships with patch characteristics or trophic structure. For our habitat selection experiments, PF preference was for larger habitats in the PF only and GS+PF+RD treatments. There were no significant relationships between patch size, shape, or trophic structure and RD habitat selection.  相似文献   

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