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1.
Understanding whether nutrient availability and grazing by consumers can control macroalgal growth is important to mitigate blooms. To assess the effect of long-term nitrogen loading on macroalgae, we ran a field experiment in which we measured growth of green and red macroalgae in estuaries where loads and eutrophication status differed. The relative abundances of consumers differed among estuaries with more grazers in non-eutrophied estuaries, an important interaction of bottom-up and top-down controls. In the estuary with the lowest nitrogen load, grazers controlled green macroalgal growth, but in higher nitrogen-loaded estuaries, where grazing was lower, growth of green macroalgae overwhelmed potential grazer control. The red macroalga was not controlled by grazers, even in the estuary where grazing pressure was highest. In the low-loaded estuary, invertebrate predators exerted top-down control over grazers, but predation effects did not cascade to macroalgae. Bottom-up mechanisms dominated control of macroalgae through an interaction of direct stimulation of growth and indirect alteration of consumer abundances, and thus, long-term nutrient regimes are likely determining potential for bloom formation in Waquoit Bay.  相似文献   

2.
Anthropogenic inputs of nutrients to coastal waters have rapidly restructured coastal ecosystems. To examine the response of macrophyte communities to land-derived nitrogen loading, we measured macrophyte biomass monthly for 6 years in three estuaries subject to different nitrogen loads owing to different land uses on the watersheds. The set of estuaries sampled had nitrogen loads over the broad range of 12 to 601 kg N ha−1 year−1. Macrophyte biomass increased as nitrogen loads increased, but the response of individual taxa varied. Specifically, biomass of Cladophora vagabunda and Gracilaria tikvahiae increased significantly as nitrogen loads increased. The biomass of other macroalgal taxa tended to decrease with increasing load, and the relative proportion of these taxa to total macrophyte biomass also decreased. The seagrass, Zostera marina, disappeared from the higher loaded estuaries but remained abundant in the estuary with the lowest load. Seasonal changes in macroalgal standing stock were also affected by nitrogen load, with larger fluctuations in biomass across the year and higher minimum biomass of macroalgae in the higher loaded estuaries. There were no significant changes in macrophyte biomass over the 6 years of this study, but there was a slight trend of increasing macroalgal biomass in the latter years. Macroalgal biomass was not related to irradiance or temperature, but Z. marina biomass was highest during the summer months when light and temperatures peak. Irradiance might, however, be a secondary limiting factor controlling macroalgal biomass in the higher loaded estuaries by restricting the depth of the macroalgal canopy. The relationship between the bloom-forming macroalgal species, C. vagabunda and G. tikvahiae, and nitrogen loads suggested a strong connection between development on watersheds and macroalgal blooms and loss of seagrasses. The influence of watershed land uses largely overwhelmed seasonal and inter-annual differences in standing stock of macrophytes in these temperate estuaries.  相似文献   

3.
Despite excessive growth of macroalgae in estuarine systems, little research has been done to examine the impacts of increased algal biomass that drifts into nearby salt marshes and accumulates on intertidal flats. The accumulation of macroalgal mats and subsequent decomposition-related releases of limiting nutrients may potentially alter marsh communities and impact multiple trophic levels. We conducted a 2-year in situ study, as well as laboratory mesocosm experiments, to determine the fate of these nutrients and any bottom-up impacts from the blooms on the dominant salt marsh plant (Spartina alterniflora) and herbivores. Mesocosm results showed that macroalgal decomposition had a positive impact on sediment nitrogen concentrations, as well as S. alterniflora growth rates. In contrast, our in situ results suggested that S. alterniflora growth was hindered by the presence of macroalgal mats. From our results, we suggest that macroalgal accumulation and subsequent release of nitrogen during decomposition may be beneficial in nitrogen limited areas. However, as marshes are becoming increasingly eutrophic, releasing lower marsh plants from nitrogen limitation, this accumulation of macroalgal biomass may hinder S. alterniflora growth through smothering and breakage of culms. As macroalgal blooms are predicted to intensify with rising temperatures and increased eutrophication, the ecological impacts associated with these changes need to be continuously monitored in order to preserve these fragile ecosystems.  相似文献   

4.
We examined the effect of nutrients and grazers on Thalassia testudinum in Jobos Bay, Puerto Rico by fertilizing sediment and manipulating grazer abundances. Bottom-up effects were variable: Added nutrients did not increase seagrass aboveground biomass, but decreased belowground biomass—perhaps as a result of less biomass being allocated to belowground structures in response to greater nutrient supply in porewater. Experimental fencing of 1.5 × 1.5 m plots provided shelter that attracted large aggregations of fish, including seagrass herbivores. Seagrass biomass and shoot density decreased with increasing abundance of herbivorous fish, indicating a significant top-down effect. There were interactions between nutrient supply, provision of shelter, and grazing pressure. Fertilization enhanced seagrass %N; however, %N also increased in unfertilized plots that were fenced, most likely due to uptake of N excreted from the large numbers of fish associated with the fences. Only plots where shelter was provided and fertilizer was applied to sediments exhibited evidence of heavy grazing, reducing both seagrass cover and aboveground biomass. In the unfertilized fenced plots, signs of grazing were fewer despite large abundances of fish and enhanced nutritional quality of seagrass leaves. This suggests the possibility that high nutrient availability in sediments lowered concentrations of chemical defense compounds in the seagrass and that cues other than %N may have been involved in stimulating grazing. This study highlights the complexity of bottom-up and top-down interactions in seagrass systems and the important role of refuge availability in shaping the relative strengths of these controls.  相似文献   

5.
Rapidly growing human populations have caused heavy modifications to the watersheds of many Mediterranean climate estuaries, subjecting them to excessive nutrient enrichment and harmful macroalgal blooms. Despite these impacts, comprehensive studies in these systems are rare and comparisons between systems are lacking. We surveyed five southern California estuaries that ranged in size from 93 to 1,000 ha and incorporated differing land usages and watershed sizes. We sampled environmental variables (sediment redox potential, organic content, total nitrogen and total phosphorus, water column nitrate, ammonium, and salinity) and macroalgal cover and biomass quarterly at three locations within each estuary over 15 months to compare spatial and wet vs. dry season patterns. Maximum mean water column nitrate concentration across all estuaries ranged from 47 to 1,700 μM, showing that all estuaries were highly enriched with nitrogen, at least at some times. Mean macroalgal biomass ranged from 0 to 1,500 g wet wt m?2. However, neither nutrient concentrations nor algal biomass showed consistent seasonal patterns as maximum values occurred in different seasons in different estuaries. Three-dimensional principal components analysis followed by regression analyses confirmed that macroalgal abundance was not directly related to water or sediment N concentrations. Rather each of these southern California estuaries showed individual patterns in all measured variables, which were most likely induced by a suite of physical modifications unique to each system and its watershed.  相似文献   

6.
Freshwater inputs often play a more direct role in estuarine phytoplankton biomass (chlorophyll a) accumulation than nitrogen (N) inputs, since discharge simultaneously controls both phytoplankton residence time and N loading. Understanding this link is critical, given potential changes in climate and human activities that may affect discharge and watershed N supply. Chlorophyll a (chla) relationships with hydrologic variability were examined in 3-year time series from two neighboring, shallow (<5?m), microtidal estuaries (New and Neuse River estuaries, NC, USA) influenced by the same climatic conditions and events. Under conditions ranging from drought to floods, N concentration and salinity showed direct positive and negative responses, respectively, to discharge for both estuaries. The response of chla to discharge was more complex, but was elucidated through conversion of discharge to freshwater flushing time, an estimate of transport time scale. Non-linear fits of chla to flushing time revealed non-monotonic, unimodal relationships that reflected the changing balance between intrinsic growth and losses through time and along the axis of each estuary. Maximum biomass occurred at approximately 10-day flushing times for both systems. Residual analysis of the fitted data revealed positive relationships between chla and temperature, suggesting enhanced growth rates at higher temperatures. N loading and system-wide, volume-weighted chla were positively correlated, and biomass yields per N load were greater than other marine systems. When combined with information on loss processes, these results on the hydrologic control of phytoplankton biomass will help formulate mechanistic models necessary to predict ecosystem responses to future climate and anthropogenic changes.  相似文献   

7.
Eutrophication and the development of persistent opportunistic macroalgal blooms are recognised as one of the main detrimental effects of increased anthropogenic pressures on estuarine and coastal systems. This study aimed to highlight the interplay between pressures and controlling physical factors on ecosystem functioning. The hypothesis that hydrological regime can control the growth of opportunistic macroalgae was tested with the study of two Irish estuaries, the Argideen and the Blackwater, with similar nutrient loading sources but divergent hydrological regimes. Seasonal monitoring data was initially examination, while the application of a pre-existing box model allowed a further analysis of the influence of residence time and nutrient load modifications on macroalgal growth. Seasonal oscillations in monitored river flow rates altered nutrient transfer from the catchments to the estuaries in both cases, as is shown through differences between winter and summer nutrient concentrations. In the Argideen, however, the relative contribution of phosphorus (P) from adjacent marine waters was high due to the shorter residence times and greater influx of marine water into the estuary. Modelling studies showed that in the Argideen Estuary, P load reduction would have potentially minimal impact on macroalgal growth due to the shorter residence time which increased the influx of P from marine sources. Nitrogen (N) load reduction of 60 % had a significant, albeit limited, impact on macroalgae and was insufficient in achieving the environmental objectives for this waterbody. For the more river-dominated Blackwater Estuary, modelled reductions in P resulted in a considerable decrease in biomass. Any further P decreases would accentuate the existing disparity in estuarine N:P ratios with possible repercussions for N transport to the coastal system. Hence, the hydrological complexity of estuarine systems demonstrated dictates that a portfolio of separate, but complimentary, management approaches may be required to address eutrophication in these estuaries.  相似文献   

8.
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   

9.
The effects of advection, dispersion, and biological processes on nitrogen and phytoplankton dynamics after a storm event in December 2002 are investigated in an estuary located on the northern New South Wales coast, Australia. Salinity observations for 16 d after the storm are used to estimate hydrodynamic transports for a one-dimensional box model. A biological model with nitrogen limited phytoplankton growth, mussel grazing, and a phytoplankton mortality term is forced by the calculated transports. The model captured important aspects of the temporal and spatial dynamics of the bloom. A quantitative analysis of hydrodynamic and biological processes shows that increased phytoplankton biomass due to elevated nitrogen loads after the storm was not primarily regulated by advection or dispersion in spite of an increase in river flow from <1 to 928×103 m3 d−1. Of the dissolved nitrogen that entered the surface layer of the estuary in the 16 d following the storm event, the model estimated that 28% was lost through exchange with the ocean or bottom layers, while 15% was removed by the grazing of just one mussel species,Xenostrobus securis, on phytoplankton, and 50% was lost through other biological phytoplankton loss processes.X. securis grazing remained an important loss process even when the estimated biological parameters in the model were varied by factors of ± 2. The intertidal mangrove pneumatophore habitat ofX. securis allows filtering of the upper water column from the lateral boundaries when the water column is vertically stratified, exerting top-down control on phytoplankton biomass.  相似文献   

10.
The combined effects of nutrient enrichment and grazing by isopods and amphipods on abundances of seagrass epiphytes were tested inZostera marina L. (eelgrass) microcosms. Using epifluorescence microscopy, densities of epiphytic diatoms, cyanobacteria, heterotrophic flagellates, and heterotrophic bacteria were enumerated after 1 mo and 2 mo of treatment. In general, numbers of diatoms decreased, in the presence of grazers and showed little response to nutrient enrichment, whereas numbers of cyanobacteria increased with nutrient enrichment and showed little response to grazing. Thus, macrofaunal grazing maintained a photoautotrophic community domainated by cyanobacteria, particularly under nutrient enriched conditions. Following 2 mo of treatment, dense macroalgal growth under nutrient-enriched conditins with grazers absent appeared to limit populations of both epiphytic autotrophs. Patterns of abundance of heterotrophic bacteria suggested that the original bacteria population was nutrient limited. Bacteria populations may have been limited by organic carbon supplies at the end of the experiment. Abundances of heterotrophic flagellates and bacteria were strongly correlated on both sampling dates. Results suggest that heterotrophic flagellates might serve as a link between heterotrophic bacterial production and higher trophic levels in seagrass epiphyte food webs.  相似文献   

11.
We examined the spatial and temporal variability in drift macroalgal abundance in two seagrass dominated estuarine systems on the Texas coast: Redfish Bay (in the Copano-Aransas Estuary) and Lower Laguna Madre. Measurements of benthic macroalgal variability were made in conjunction with a suite of biotic (seagrass biomass, percent cover, blade width and length, shoot density, epiphyte biomass, seagrass blade C:N ratios, and drift macroalgal abundance and composition) and abiotic (inorganic nitrogen and phosphorus concentrations, chlorophylla, total suspended solids, light attenuation, salinity, temperature, total organic carbon and porewater NH4 +) indicators. All parameters were measured at 30 sites within each estuary semiannually from July 2002 to February 2004. Principal components analysis (PCA) was used to examine relationships between drift macroalgal abundance and biotic and abiotic parameters. In both Redfish Bay and Lower Laguna Madre, drift macroalgal distribution was widespread, and during three of four sampling periods, abundance was equal to abovegro und biomass ofThalassia testudinum, the dominant seagrass. Drift macro algal abundance was highly variable within sites, between sites, and between seasons in both estuaries. No significant differences in drift macroalgal abundance were found between Redfish Bay and Lower Laguna Madre. In Redfish Bay, drift macroalgae (90.1±10.2 gm−2) tended to accumulate in bare patches within seagrass beds. In Lower Laguna Madre, drift macroalgae (72.7±10.7 gm−2) tended to accumulate in areas of dense seagrass cover rather than in bare areas. We found no relationship between drift macroalgal abundance and low (<2μM) water column nutrient concentrations, and although several of our measured parameters were related to drift macroalgal abundance, none alone sufficiently explained the variability in abundance noted between the two estuarine systems. The contrasting patterns of macroalgal accumulation between Redrish Bay and Lower Laguna Madre likely reflect differences in water circulation characteristics between the two regions as dictated by local physiography, in cluding the shape and orientation of the lagoons, with seasonal variations in macroalgal abundance related to changes in freshwater inflow and nutrient loading.  相似文献   

12.
Macroalgal bloom dynamics in a highly eutrophic southern California estuary   总被引：3，自引：0，他引：3  
A 16-mo long monitoring study was carried out in Upper Newport Bay estuary (UNB), Orange County, California, to quantify the macroalgal community of a southern California estuary. Quarterly sampling began December 1996 at 8 stations along the main channel and tidal creeks ranging from the head to the lower end of UNB. At each station, two strata (one at high and one at low elevation) were surveyed. Macroalgal species abundance (% cover and biomass) and algal tissue nitrogen (N) and phosphorus (P) were measured. The algal community changed from sparse macroalgal cover during winter 1996 to larger patches dominated byEnteromorpha intestinalis in spring 1997. The community was characterized by a thick cover of macroalgae comprised ofE. intestinalis andUlva expansa in summer 1997 andU. expansa andCeramium spp. in fall 1997. UNB returned to sparse macroalgal cover by spring 1998. In summer and fall 1997, biomass ofE. intestinalis andCeramium reached over 1,000 g wet wt m−2 each, andU. expansa biomass exceeded 700 g wet wt m−2. Tissue N was high inE. intestinalis andU. expansa collected from UNB (≈3% dry wt) and higher inCeramium (≈3.5% dry wt). Tissue P in all three algae ranged from 0.24–0.28% dry wt. Tissue N∶P (molar) ratios inE. intestinalis andU. expansa ranged from 16.4 to 30.0 and inCeramium from 21.8 to 40.1. A field experiment was conducted in whichE. intestinalis was used as a bioassay of N and P availability. Algal tissue was cultured under known conditions and samples were deployed throughout the estuary and left for 24 h. Tissue N of algae from these bags showed a nominal increase in N with proximity to the primary nutrient input to the system, San Diego Creek (p=0.0251; r2=0.200). Our data indicate that UNB is already a highly eutrophic estuary, but macroalgal blooms in UNB may increase if more N is added to the system.  相似文献   

13.
Subtropical estuaries have received comparatively little attention in the study of nutrient loading and subsequent nutrient processing relative to temperate estuaries. Australian estuaries are particularly susceptible to increased nutrient loading and eutrophication, as 75% of the population resides within 200 km of the coastline. We assessed the factors potentially limiting both biomass and production in one Australian estuary, Moreton Bay, through stoichiometric comparisons of nitrogen (N), phosphorus (P), silicon (Si), and carbon (C) concentrations, particulate compositions, and rates of uptake. Samples were collected over 3 seasons in 1997–1998 at stations located throughout the bay system, including one riverine endmember site. Concentrations of all dissolved nutrients, as well as particulate nutrients and chlorophyll, declined 10-fold to 100-fold from the impacted western embayments to the eastern, more oceanic-influenced regions of the bay during all seasons. For all seasons and all regions, both the dissolved nutrients and particulate biomass yielded N:P ratios <6 and N:Si ratios <1. Both relationships suggest strong limitation of biomass by N throughout the bay. Limitation of rates of nutrient uptake and productivity were more complex. Low C:N and C:P uptake ratios at the riverine site suggested light limitation at all seasons, low N:P ratios suggested some degree of N limitation and high N:Si uptake ratios in austral winter suggested Si limitation of uptake during that season only. No evidence of P limitation of biomass or productivity was evident.  相似文献   

14.
An assessment of developing eutrophic conditions in small temperate lagoons along the coast of Rhode Island suggests that in such shallow, macrophyte based systems the response to nutrient enrichment differs from that described for plankton based systems. The nitrogen loadings per unit area of the salt ponds are 240–770 mmol N per m2 per year. Instead of the high nutrient concentrations, increased phytoplankton biomass and turbidity, leading to eventual loss of benthic macrophytes described for such systems as the Chesapeake, Patuxent and Appalachicola Bay, nutrient enrichment of the Rhode Island lagoons has led to increased growth of marine macroalgae. The increased macroalgal growth appears to alter the benthic habitat and a shift from a grazing to detrital food chain appears to be impacting important shellfisheries. As more extensive areas of organic sediments develop, geochemical cycling changes, resulting in higher rates of nitrogen remineralization and accelerated eutrophication. The major sources of nitrogen inputs to the salt ponds have been identified and a series of management initiatives have been designed to limit inputs from present and potential development within the watersheds of the lagoons.  相似文献   

15.
A simple model of annual average response of an estuary to mean nitrogen loading rate and freshwater residence time was developed and tested. It uses nitrogen inputs from land, deposition from the atmosphere, and first-order calculations of internal loss rate and net export to perform a steady-state analysis over a yearly cycle. The model calculates the fraction of total nitrogen input from land and the atmosphere that is exported and the fraction that is denitrified or lost to other processes within the estuary. The model was tested against data from the literature for 11 North American and European estuaries having a wide range of physical characteristics, nitrogen loading rates, and geographical and climatic settings. The model shows that the fraction of nitrogen entering an estuary that is exported or denitrified can be predicted from the freshwater residence time. The first-order rate constant for nitrogen loss within an estuary, as a fraction of total nitrogen in the water column, is 0.30 mo−1. Denitrification typically accounts for 69–75% of the total annual net nitrogen removal from the water column by processes within the estuary. The model makes explicit the dependence of nitrogen concentration in the water column on the loading rate of nitrogen, water residence time, estuary volume, and the rate constant for loss within the estuary.  相似文献   

16.
The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various14C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m?2 h?1/mg C produced m?2 h?1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.  相似文献   

17.
Hydrologists have long been concerned with the interface of groundwater flow into estuaries, but not until the end of the last century did other disciplines realize the major role played by groundwater transport of nutrients to estuaries. Mass balance and stable isotopic data suggest that land-derived NO3, NH4, and dissolved organic N do enter estuaries in amounts likely to affect the function of the receiving ecosystem. Because of increasing human occupancy of the coastal zone, the nutrient loads borne by groundwater have increased in recent decades, in spite of substantial interception of nutrients within the land and aquifer components of watersheds. Groundwater-borne nutrient loads have increased the N content of receiving estuaries, increased phytoplankton and macroalgal production and biomass, decreased the area of seagrasses, and created a cascade of associated ecological changes. This linkage between land use and eutrophication of estuaries occurs in spite of mechanisms, including uptake of land-derived N by riparian vegetation and fringing wetlands, “unloading” by rapid water removal, and direct N inputs to estuaries, that tend to uncouple the effects of land use on receiving estuaries. It can be expected that as human activity on coastal watersheds continues to increase, the role of groundwater-borne nutrients to the receiving estuary will also increase.  相似文献   

18.
Eutrophication and species introductions have resulted in increased macroalgal biomass in coastal ecosystems around the globe. Macroalgal mats may compete with microphytobenthos (MPB) for light and nutrients and, due to their position in the canopy, have a negative impact on MPB biomass. We tested this effect by conducting a meta-analysis of prior experiments, as well as a comparative survey, and a macroalgal-removal manipulation in the coastal lagoons of the Virginia Coastal Reserve (VCR) on the eastern shore of Virginia (USA). In all cases, MPB biomass was estimated using benthic chlorophyll as a proxy. While prior individual studies documented impacts of macroalgae, when effect sizes were averaged across studies, there was no consistent effect of macroalgal biomass on MPB biomass. In the VCR, a non-native red macroalga, Gracilaria vermiculophylla, dominates intertidal mats and attains high biomasses at some sites. Nevertheless, MPB biomass was unrelated to macroalgal mass based on a survey of mudflats. Further, when macroalgae were experimentally manipulated on a mudflat using a before and after impact design, there was no change in MPB. Based on the meta-analysis, survey, and manipulation we conducted, macroalgal mats do not have a generalizable effect on MPB, interactions seem context-dependent, and in the VCR, the effects on MPB appear neutral. This finding is important given the significance of MPB in supporting food webs and other estuarine ecosystem functions, as well as the increasing frequency and intensity of macroalgal blooms.  相似文献   

19.
Nutrient additions represent an important anthropogenic stress on coastal ecosystems. At moderate levels, increased nutrients may lead to increased primary production and, possibly, to increased biomass of consumers although complex trophic interactions may modify or mask these effects. We examined the influence of nutrient additions and interactive effects of trophic interactions (predation) on benthic infaunal composition and abundances through small-scale field experiments in 2 estuaries that differed in ambient nutrient conditions. A blocked experimental design was used that allowed an assessment of direct nutrient effects in the presence and absence of predation by epibenthic predators as well as an assessment of the independent effects of predation. Benthic microalgal, production increased with experimental nutrient additions and was greater when infaunal abundances were lower, but there were no significant interactions between these factors. Increased abundances of one infaunal taxa,Laeonereis culveri, as well as the grazer feeding guild were observed with nutrient additions and a number of taxa exhibited higher abundances with predator exclusion. In contrast to results from freshwater systems there were no significant interactive effects between nutrient additions and predator exclusion as was predicted. The infaunal responses observed here emphasize the importance of both bottom-up (nutrient addition and primary producer driven) and top-down (predation) controls in structuring benthic communities. These processes may work at different spatial and temporal scales, and affect different taxa, making observation of potential interactive effects difficult.  相似文献   

20.
The biomass of phytoplankton, microzooplankton, copepods, and gelatinous zooplankton were measured in two tributaries of the Chesapeake Bay during the springs of consecutive dry (below average freshwater flow), wet (above average freshwater flow), and average freshwater flow years. The potential for copepod control of microzooplankton biomass in the dry and wet years was evaluated by comparing the estimated grazing rates of microzooplankton by the dominant copepod species (Acartia spp. andEurytemora affinis) to microzooplankton growth rates and by calculating the percent of daily microzooplanton standing stock removed through copepod grazing. There were significant increases in phytoplankton and copepod biomass, but not for microzooplankton biomass in the wet year as compared to the dry year. The ctenophoreMnemiopsis leidyi was present during the dry year but was absent during the sampling period of the wet and average freshwater flow years. Grazing pressure on microzooplankton was greatest in the wet year, withAcartia spp. andE. affinis ingesting 0.21–2.64 μg of microzooplankton C copepod−1 d−1 and removing up to 60% of the microzooplankton standing stock per day. In the dry year, these copepod species ingested 0.10–0.73 μg of microzooplankton C copepod−1 d−1 with a maximum daily removal of approximately 3% of the microzooplankton standing stock. Potential copepod grazing pressure was significantly less than microzooplankton growth in the dry year, but was equivalent to microzooplankton growth in the wet year, implying strong top-down control of the microzooplankton community in the wet year. These results suggest that increased grazing control of microzooplankton populations by more copepods in the wet year released top-down control of phytoplankton. Reduced microzooplankton grazing, in conjunction with increased nutrient availability, resulted in large increases in phytoplankton biomass in the wet year. Increased freshwater flow has the potential to influence trophic cascades and the partitioning of plankton production in estuarine systems.  相似文献   

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