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1.
Estuaries are defined by mixing of freshwater from rivers and saltwater from seas. Water resource development can reduce river flows to the coast, but it is difficult to predict effects on estuaries. The Lavaca-Colorado Estuary is a major estuarine system along the Texas coast that provides major economic benefit to the region by supporting a variety of agricultural, residential, industrial, and recreational functions. New water projects could divert freshwater from Matagorda Bay. So, what environmental effects could result from further changes to inflow patterns in the Matagorda Bay system? To answer this question, a bioenergetic model, calibrated using a long-term dataset of benthic biomass, was run to investigate dynamics of macrobenthic biomass related to salinity regimes in the estuary. The model simulation results were interpreted to assess the role of freshwater inflow in controlling benthic productivity. Simulations, based on calibrated parameters (1988–1999), were run for a long-term period from 1988 to 2005. The model performance was found to be promising with the best percent root mean square (RMS) difference being 63% and worst being 92%. Sensitivity tests for the benthic responses to changes in salinity show that, in general, when salinity increased with decreasing nutrient concentrations, deposit feeder biomass increased while suspension feeder biomass decreased. Estuary-wide comparison predicts that reducing freshwater inflow may cause the upper and lower bay communities to respond in different ways. Reduced inflow to Lavaca Bay would result in decreasing benthic biomass; whereas, in Matagorda Bay, biomass would increase. Also, functional diversity would decrease in both bays with decreasing inflow. These effects are probably due to the benthic community acclimating to different salinity regimes, or more (or less) salt tolerant species populating the area. It is concluded that freshwater inflow plays an important role in maintaining the observed character of estuarine productivity through the combined effects of the frequency, duration, timing, and magnitude of inflow, particularly during droughts or low-flow periods.  相似文献   

2.
The plankton community composition comprising heterotrophic bacteria, pro-/eukaryotes, heterotrophic nanoflagellates, microzooplankton and mesozooplankton was assessed during the spring bloom and at non-bloom stations in the English Channel and Celtic Sea between 6 and 12 April 2002. Non-bloom sites were characterised by a dominance of pro-/eukaryotic phytoplankton <20 μm, higher abundance of heterotrophic nanoflagellates, microzooplankton standing stocks ranging between 60 and 380 mg C m−2, lower mesozooplankton diversity and copepod abundance of between 760 and 2600 ind m−3. Within the bloom, the phytoplankton community was typically dominated by larger cells with low abundance of pro-/eukaryotes. Heterotrophic nanoflagellate cell bio-volume decreased leading to a reduction in biomass whereas microzooplankton biomass increased (360–1500 mg C m−2) due to an increase in cell bio-volume and copepod abundance ranged between 1400 and 3800 ind m−3. Mesozooplankton diversity increased with an increase in productivity. Relationships between the plankton community and environmental data were examined using multivariate statistics and these highlighted significant differences in the abiotic variables, the pro-/eukaryotic phytoplankton communities, heterotrophic nanoflagellate, microzooplankton and total zooplankton communities between the bloom and non-bloom sites. The variables which best described variation in the microzooplankton community were temperature and silicate. The spatial variation in zooplankton diversity was best explained by temperature. This study provides an insight into the changes that occur between trophic levels within the plankton in response to the spring bloom in this area.  相似文献   

3.
The inter-annual variability in phytoplankton summer blooms in the upper reaches of the Schelde estuary was investigated between 1996 and 2005 by monthly sampling at 10 stations. The large inter-annual variations of the chlorophyll a concentration in the freshwater tidal reaches were independent from variations in chlorophyll a in the tributary river Schelde. Summer mean chlorophyll a concentrations were significantly negatively correlated with flushing rate (Spearman correlation: r = −0.67, p = 0.05, n = 9) but not with temperature, irradiance and suspended particulate matter or dissolved silica (DSi) concentrations. During dry summers, low flushing rates permitted the development of dense phytoplankton populations in the upper part of the estuary, while during wet summers high flushing rates prevented the development of dense phytoplankton blooms. Flushing rate was also found to be important for the phytoplankton community composition. At low flushing rates, the community was dominated by diatoms that developed within the upper estuary. At high flushing rates, chlorophytes imported from the tributary river Schelde became more important in the phytoplankton community. The position of the chlorophyll a maximum shifted from the head of the estuary when flushing rates were low, to more downstream when flushing rates were high. Although DSi concentrations tended to be lower during years of high phytoplankton (mainly diatom) biomass, the relation with flushing rate was not significant.  相似文献   

4.
The biogeochemistry of Tapong Bay, one of the major lagoons in southern Taiwan, was studied from 1999 to 2004, encompassing a period in 2003 in which aquaculture activities were terminated and the associated structures removed. Removal of the maricultural structures resulted in the reduction of the mean time for water exchange time in Tapong Bay from about 10+/-2d to 6+/-2d. The annual mean concentration of measured nutrients (DIN, DIP and DSi) also decreased significantly, likely due to improved water exchange, ceased feeding and increased biological utilization. An overall high primary production was maintained and likely to be constrained by temperature, light availability and turbidity rather than by abundant nutrients throughout the study period. The change in stoichiometric ratios among inorganic and organic nutrients (C/N, Si/N, N/P) may be attributed to the shift of plankton community between the two periods. The annual mean of Delta POC/Delta PON was 8.1, a little bit larger than that (7.3) before removal, also possibly resulting from the shift of planktonic community (from diatom-dominated to flagellate-dominated and increase of total phytoplankton and zooplankton) and removal of periphyton and oysters. The Tapong Bay shows a 37% increase (from 5.6 to 7.7 mol C m(-2)yr(-1)) in net ecosystem production (NEP) after structure removal, although the increase was not statistically significant. The change in environmental conditions has therefore influenced profoundly the carbon and nutrient biogeochemical processes and budgets in the semi-enclosed ecosystem.  相似文献   

5.
An integrated mass balance and modelling approach for analysis of estuarine nutrient fluxes is demonstrated in the Swan River Estuary, a microtidal system with strong hydrological dependence on seasonal river inflows. Mass balance components included estimation of gauged and ungauged inputs to the estuary and losses to the ocean (outflow and tidal exchange). Modelling components included estimation of atmospheric (N fixation, denitrification) and sediment–water column nutrient exchanges. Gross and net denitrification derived using two independent methods were significantly correlated (r2 = 0.49, p < 0.01) with net rates averaging 40% of gross. Annual nitrogen (N) and phosphorus (P) loads from major tributaries were linearly correlated with annual freshwater discharge and were 3-fold higher in wet years than in dry years. Urban drains and groundwater contributed, on average, 26% of N inputs and 19% of P inputs, with higher relative contributions in years of low river discharge. Overall, ungauged inputs accounted for almost 35% of total nitrogen loads. For N, elevated loading in wet years was accompanied by large increases in outflow (7x) and tidal flushing (2x) losses and resulted in overall lower retention efficiency (31%) relative to dry years (70%). For P, tidal flushing losses were similar in wet and dry years, while outflow losses (4-fold higher) were comparable in magnitude to increases in loading. As a result, P retention within the estuary was not substantially affected by inter-annual variation in water and P loading (ca. 50% in all years). Sediment nutrient stores increased in most years (remineralisation efficiency ca. 50%), but sediment nutrient releases were significant and in some circumstances were a net source of nutrients to the water column.  相似文献   

6.
Freshwater flow is generally held to be one of the most influential factors affecting community structure and production in estuaries. In coastal Louisiana, the Caernarvon Freshwater Diversion (CFD) is managed to control freshwater discharge from the Mississippi River into Breton Sound basin. Operational since 1991, CFD has undergone several changes in management strategy including pulsed spring flooding, which was introduced in 2001. We used a 20-yr time series of fisheries-independent data to investigate how variation in freshwater inflow (i.e., pre- and post-CFD, and pre and post spring pulsing management) influences the downstream nekton community (abundance, diversity, and assemblage). Analyses of long-term data demonstrated that while there were effects from the CFD, they largely involved subtle changes in community structure. Spatially, effects were largely limited to the sites immediately downstream of the diversion and extended only occasionally to more down-estuary sites. Temporally, effects were 1) immediate (detected during spring diversion events) or 2) delayed (detected several months post-diversion). Analysis of river management found that pulsed spring-time inflow resulted in more significant changes in nekton assemblages, likely due to higher discharge rates that 1) increased marsh flooding, thus increasing marsh habitat accessibility for small resident marsh species, and 2) reduced salinity, possibly causing displacement of marine pelagic species down estuary.  相似文献   

7.
Three estuaries with differing catchment use and freshwater input were investigated in terms of their nutrient status, phytoplankton biomass, freshwater inflow and salinity between 1993 and 1995. The nutrients analysed include phosphate, nitrate, nitrite, ammonia and total particulate nitrogen. All the parameters were investigated for their relationship with land-use and freshwater abstraction. The Kromme River catchment area is relatively pristine, the river is impounded for ca. 133% of its mean annual runoff, and consequently, freshwater input into the estuary is only episodic. Nutrient and chlorophyll-a concentrations are low, but become elevated when freshwater does reach the estuary. The Geelhoutboom tributary contributes nutrients to the Kromme estuary during high freshwater inflow conditions, but is not a viable nutrient contributor during low flow conditions. Freshwater abstraction from the Swartkops River catchment is limited, and it is characterised by urbanisation and industrial development. The Swartkops River was the main source of phosphate in the estuary, whereas other small tributaries along the estuary were additional point sources for nitrate, ammonia and nitrite. The third system, the Sundays estuary, has no tributaries or other point sources except the Sundays River, where the catchment is extensively used for agriculture and freshwater input relatively high. The phytoplankton biomass (in terms of chlorophyll-a) was highest in the Sundays estuary, although phosphate concentrations were as low as in the Kromme estuary. Trends over time indicated a decrease in phosphate concentrations and showed variations for inorganic dissolved nitrogen concentrations since the previous 15 years in all the three estuaries. Nutrient stochiometry had changed in favour of inorganic dissolved nitrogen.  相似文献   

8.
Water temperature, salinity, nutrient concentrations and the composition of the plankton community were recorded at three stations in inner Tokyo Bay over a period of 328 days (from June 8, 1995 to April 30, 1996) with a nominal sampling frequency of once per day. Inspection of the results revealed that the data could be divided into two blocs as an aid to analysis: the period from June to October was characterized by the development of stratification of temperature and salinity (stratification period), and November to March was characterized by uniform temperature and salinity in the water column due to vertical mixing (mixing period). Oxygen-depleted water forms in the bottom layer during the stratification period, but vertical mixing of the water column, due to changing wind and rainfall conditions caused by passing weather fronts, results in the breakdown of the oxygen-depleted water mass. Nutrient loads are high in the surface water due to the freshwater supply, but occasional pulses of primary production cause a depletion of phosphate in the surface water, suggesting that the phosphorus becomes a limiting nutrient for phytoplankton growth in this period. Several short-term peaks of plankton abundance (blooms) occurred as responses to temporal changes in water quality from June to November, with consequent species succession. Significant fluctuations in the densities of the diatom Skeletonema costatum and several species of ciliates corresponded to the daily changes in the physical and chemical characteristics of the coastal environment. During the mixing period, when water temperature and solar radiation decreased, there were no short-term variations in water quality and although nutrient concentrations gradually increased from November to February, primary production remained low. This study shows that the short-term dynamics of the phytoplankton community are closely coupled to fluctuations in environmental forcing, and that the degree of coupling is stronger during periods when solar radiation is greater. The results provide a novel typological understanding of seasonal plankton dynamics in a shallow, eutrophicated marine embayment, and suggest how such systems may be treated in simulation modeling.  相似文献   

9.
The phytoplankton ecology of Great South Bay, New York, was studied over a 1-year period. The study area, a large barrier island estuary (coastal lagoon with estuarine circulation), was characterized by high levels of inorganic nutrients, high turbidity and a shallow euphotic zone (<2 m). Net annual primary production by phytoplankton was high—450 g C m?2 year?1—and accounted for approximately 85% of the total ecosystem primary production. Chlorophyll a-specific productivity was dependent on mean photic zone light intensity in areas of the bay <1 m in depth from September 1979 through June 1980; 65–95% of the total light extinction in those areas was attibutable to suspended solids. Nitrogenous nutrient concentration did not limit phytoplankton productivity. Diatom and dinoflagellate cell densities varied greatly over time, while cryptomonad and chlorophyte species were abundant throughtout the year. Chlorophytes of 2–4 μm (‘small forms’) were numerically dominant, and contributed approximately half of the total phytoplankton biomass. Dilution of bay water by intruding ocean water appeared to control the spatial distribution of chlorophyll a on the south side of the bay; in other areas, growth appeared to exceed the rate of dilution by flushing. Waters entrained in eelgrass beds were significantly higher in salinity and mean photic zone light intensity, and had lower phytoplankton standing stock and depth-integrated primary production than control areas; waters in the sediment plume of active clamdigging boats were statistically similar to control areas with respect to water quality and phytoplankton community characteristics.  相似文献   

10.
To elucidate iron regeneration and organic iron(III)-binding ligand formation during microzooplankton and copepod grazing on phytoplankton, incubation experiments were conducted in the western subarctic Pacific. During 8 days of dark incubation of ambient water and that amended with plankton concentrate, dissolved iron and organic iron(III)-binding ligands accumulated, approximately proportionally to the decrease in chlorophyll a. The observed increases in dissolved iron concentration were much greater than those expected from the consumption of phytoplankton biomass and previously reported Fe:C value of cultured algal cells, suggesting resolution from colloidal or particulate iron adsorbed onto the algal cell surface. When copepods were added to the ambient water, organic iron(III)-binding ligands accumulated more rapidly than in the control receiving no copepod addition, although consumed phytoplankton biomass was comparable between the two treatments. Bioassay experiment using filtrates collected from the incubation experiment showed that organic ligands formed during microzooplankton grazing reduced the iron bioavailability to phytoplankton and suppressed their growth. Moreover, picoplankton Synechococcus sp. and Micromonas pusilla were more suppressed by the organic ligands than the diatom Thalassiosira weissflogii. In conclusion, through microzooplankton and copepod grazing on phytoplankton, organic iron(III)-binding ligands as well as regenerated iron are released into the ambient seawater. Because the ligands lower iron bioavailability to phytoplankton through complexation and the degree of availability reduction varies among phytoplankton species, grazing by zooplankton can shift phytoplankton community structure in iron-limited waters.  相似文献   

11.
During late winter and spring of 2002 and 2003, 24 two- to three-day cruises were conducted to Dabob Bay, Washington State, USA, to examine the grazing, egg production, and hatching success rates of adult female Calanus pacificus and Pseudocalanus newmani. Here, we discuss the results of our grazing experiments for P. newmani. Each week, we conducted traditional microzooplankton dilution experiments and “copepod dilution” experiments, each from two different layers. Grazing was measured by changes in chlorophyll concentration and direct cell counts. Clearance rates on individual prey species, as calculated by cell counts, showed that Pseudocalanus are highly selective in their feeding, and may have much higher grazing rates on individual taxa than calculated from bulk chlorophyll disappearance. The grazing rates of the copepods, however, are typically an order of magnitude lower than the grazing rates of the microzooplankton community, or the growth rates of the phytoplankton. P. newmani ingested diatoms, but, at certain times fed preferentially on microzooplankton, such as ciliates, tintinnids, and larger dinoflagellates. Removal of the microzooplankton may have released the other phytoplankton species from grazing pressure, allowing those species’ abundance to increase, which was measured as an apparent “negative” grazing on those phytoplankton species. The net result of grazing on some phytoplankton species, while simultaneously releasing others from grazing pressure resulted in bulk chlorophyll-derived estimates of grazing which were essentially zero or slightly negative; thus bulk chlorophyll disappearance is a poor indicator of copepod grazing. Whether copepods can significantly release phytoplankton from the grazing pressure by microzooplankton in situ, thus causing a trophic cascade, remains to be verified, but is suggested by our study.  相似文献   

12.
Estuaries exhibit a large range in their responses to nitrogen loadings determined in part by characteristics of the driver, such as magnitude and frequency, but also by such intrinsic characteristics as physical/chemical factors (e.g., depth, volume, hypsometry, salinity, turbidity) and biological factors (e.g., nature of ecological communities, trophic interactions). To address the richness of estuarine response to driver variables, the aim ultimately is to establish a simple estuarine classification scheme, beginning with a river-dominated subset of estuarine systems and focusing on the role of water residence time in the estuary. Residence time (or flushing time) is related to other drivers (streamflow, nutrient, and sediment loads) and drives much of the biological response of estuaries because of flushing effects on plankton, temperature, nutrients, and light. Toward this goal, nutrient–phytoplankton–zooplankton (NPZ) models have been used to examine a range of subjects including effects of nutrient limitation and zooplankton predation on phytoplankton dynamics and fish predation. This class of model can admit a wide range of behavior, including multiple steady-states and oscillatory behavior. The NPZ equations include terms for nutrient recycling, phytoplankton settling, benthic regeneration, and zooplankton mortality. Analysis of the equations suggests that both the nature of nitrogen loading (i.e., whether it is correlated with discharge or independent of it) and residence time are critical in determining the steady-state response of the system.  相似文献   

13.
The seasonality of primary productivity plays an important role in nutrient and carbon cycling. We quantify the seasonality of satellite-derived, oceanic net primary production (NPP) and its interannual variability during the first decade of the SeaWiFS mission (1998 to 2007) using a normalized seasonality index (NSI). The NSI, which is based upon production half-time, t(1/2), generally becomes progressively more episodic with increasing latitude in open ocean waters, spanning from a relatively constant rate of primary productivity throughout the year (mean t(1/2) ~5 months) in subtropical waters to more pulsed events (mean t(1/2) ~3 months) in subpolar waters. This relatively gradual, poleward pattern in NSI differs from recent estimates of phytoplankton bloom duration, another measure of seasonality, at lower latitudes (~40°S–40°N). These differences likely reflect the temporal component of production assessed by each metric, with NSI able to more fully capture the irregular nature of production characteristic of waters in this zonal band. The interannual variability in NSI was generally low, with higher variability observed primarily in frontal and seasonal upwelling zones. The influence of the El Niño–Southern Oscillation on this variability was clearly evident, particularly in the equatorial Pacific, where primary productivity was anomalously episodic from the date line east to the coast of South America in 1998. Yearly seasonality and the magnitude of annual production were generally positively correlated at mid-latitudes and negatively correlated at tropical latitudes, particularly in a region bordering the Pacific equatorial divergence. This implies that increases of annual production in the former region are attained over the course of a year by shorter duration but higher magnitude NPP events, while in the latter areas it results from an increased frequency or duration of similar magnitude events. Statistically significant trends in the seasonality, both positive and negative, were detected in various patches. We suggest that NSI be used together with other phenomenological characteristics of phytoplankton biomass and productivity, such as the timing of bloom initiation and duration, as a means to remotely quantify phytoplankton seasonality and monitor the response of the oceanic ecosystem to environmental variability and climate change.  相似文献   

14.
The Bonnet Carré Spillway, located 28 miles northwest of New Orleans, was constructed in the early 1930s as part of an integrated flood-control system for the lower Mississippi River system. From 11 April to 8 May 2008, Mississippi River water was diverted through the spillway into the 629-square-mile Lake Pontchartrain, which is hydraulically connected to the Gulf of Mexico. On 8 April, prior to the opening of the spillway, water-quality instruments were deployed and recorded hourly measurements of water temperature, dissolved oxygen, specific conductance, pH, and nitrate. Discrete water-quality and phytoplankton (algae) samples were collected in Lake Pontchartrain from 8 April to 3 October 2008 to assess the water-quality nutrient enrichment effects of the diversion on the lake. The maximum influence of river water in the southern portion of the lake was captured with continuous (hourly) monitoring of nitrate concentrations, and field measurements such as of specific conductance during the critical period in late April to early May. By late May, the deployed instruments had recorded the arrival, peak, and decline of selected constituents associated with the freshwater influx from the Mississippi River/Bonnet Carré Spillway diversion. The continuous monitoring data showed the short-term interactions of high-nitrate, low-specific conductance river water and low-nitrate, high-specific conductance lake water. The phytoplankton community composition, as an indicator of water quality, illustrated an extended response from the river water evident even after the continuous and discrete samples indicated that the lake had returned to pre-diversion conditions. The initial phytoplankton community response to nutrient increases was related to accumulations of diatoms. During periods of low nutrient concentrations, accumulations of blue-greens occurred by July and August. As blue-green algae cell densities and biovolumes increased in the summer, so did the species richness of blue-green algae, particularly the harmful algae bloom taxa. Cell densities and biovolume of the phytoplankton lake indicator taxa Skeletonema costatum, Anabaena sp., and Cylindrospermopsis raciborskii were highest and dominated the diatom and blue-green algae communities during the period of most river water influence on the lake and immediately following the freshwater inflows. The dominance and recession of these indictor taxa reflect the dramatic changes that occurred in the phytoplankton community in response to an increase in nutrient-rich freshwater from the diversion into the lake, and not normal seasonal phytoplankton compositional differences. Water-quality data indicated a gradual reversion to pre-diversion lake conditions by June to July, but shifts in the phytoplankton composition were still evident through August 2008. Observations from this study were similar to results from previous studies of Mississippi River/Bonnet Carré Spillway diversion opening in 1997.  相似文献   

15.
Freshwater fraction method is popular for cost-effective estimations of estuarine flushing time in response to freshwater inputs. However, due to the spatial variations of salinity, it is usually expensive to directly estimate the long-term freshwater fraction in the estuary from field observations. This paper presents the application of the 3D hydrodynamic model to estimate the distributions of salinity and thus the freshwater fractions for flushing time estimation. For a case study in a small estuary of the North Bay in Florida, USA, the hydrodynamic model was calibrated and verified using available field observations. Freshwater fractions in the estuary were determined by integrating freshwater fractions in model grids for the calculation of flushing time. The flushing time in the North Bay is calculated by the volume of freshwater fraction divided by the freshwater inflow, which is about 2.2 days under averaged flow conditions. Based on model simulations for a time series of freshwater inputs over a 2-year period, a power regression equation has been derived from model simulations to correlate estuarine flushing time to freshwater inputs. For freshwater input varying from 12 m3/s to 50 m3/s, flushing time in this small estuary of North Bay changes from 3.7 days to 1.8 days. In supporting estuarine management, the model can be used to examine the effects of upstream freshwater withdraw on estuarine salinity and flushing time.  相似文献   

16.
To improve our understanding of the trophic link between micro-zooplankton and copepods in Gyeonggi Bay, Yellow Sea, the diet composition, ingestion rates, and prey selectivity of Acartia hongi, known as the most abundant and widespread copepod species, was estimated by conducting in situ bottle incubation throughout the different seasons. The results showed that A. hongi preferentially grazed on ciliate and heterotrophic dinoflagellate of a size ranging from 20 to 100 μm rather than phytoplankton. Although micro-zooplankton comprised only an average 13.7% of the total carbon available in the natural prey pool, micro-zooplankton accounted for >70% of the total carbon ration ingested by A. hongi throughout the year, except for winter diatom blooming periods when A. hongi obtained about 60% of its carbon ration from phytoplankton. Our results demonstrated that A. hongi modified their diet composition and feeding rates in response to change in composition and size of prey available to them, and that A. hongi preferentially ingested micro-zooplankton over phytoplankton. Feeding activity of A. hongi could therefore affect the species composition and size structure of natural plankton communities in this study area, particularly the micro-zooplankton. Strongly selective feeding and high grazing pressure by A. hongi on micro-zooplankton shows the role of trophic coupling between copepods and the microbial food web in the pelagic ecosystem of Gyeonggi Bay.  相似文献   

17.
This study investigates a freshwater budget in the Yellow and East China Seas (YECS) using a global f general circulation model with a regional focus on the YECS. A freshwater budget analysis finds that major freshwater contributors over the YECS change seasonally. In summer, freshwater inflow from Changjiang and positive precipitation minus evaporation (P-E) dominates freshwater outflow across the boundaries around the YECS, resulting in net freshwater gain in the YECS. In winter, evaporation, intensified by strong winds, dominates freshwater inflow from Changjiang and precipitation, while net freshwater transport across the boundaries around the YECS is negligible, causing freshwater loss in total over the YECS. Although P-E has often been assumed to be negligible by supposing that the annual mean of precipitation is nearly equal to that of evaporation, this study suggests that P-E needs to be included in the seasonal freshwater budget in the YECS.  相似文献   

18.
莱州湾夏季浮游桡足类的摄食研究   总被引：23，自引：8，他引：15  
于１９９７年７月有（１９－２４日）和小湖（２６－３０日）期间在莱州湾对不同大小桡足类自然群体（大型：〉５００ｍｍ,小型：２００－５００ｍｍ）的分布及肠道色素含量进行了现场测定,并在一个４８ｈ连续观测站（Ａ４）进行了肠道排空率和摄食节律实验。结果表明,浮动物肠道色素含量随个体的增大而增加,大型桡足类存在着一定的昼夜摄食节律,摄食高峰出现在夜间,小型桡足类摄食切律不明显。Ａ４站测得的桡足类摄食结果表明  相似文献   

19.
Copepods are considered to be a vital component connecting the unique macrotidal environment to the high productivity and high biodiversity of the Ariake Sea. To examine the spatiotemporal succession of copepod communities, we conducted monthly sampling (vertical hauls of a 100-μm mesh plankton net) in three neighboring macrotidal estuaries between 2005 and 2006. Irrespective of the season, three copepod communities were recognized in relation to the relatively long gradients of salinity and turbidity along the Chikugo and Midori River estuaries. The oligohaline community (salinity 1–10) was observed at higher turbidities (>100 NTU), whereas the freshwater (salinity <1) and meso/polyhaline (salinity >10) communities were associated with lower turbidities (<100 NTU). The oligohaline calanoid Sinocalanus sinensis occurred only in the Chikugo River estuary, maintaining a large biomass (dry weight >10 mg m?3) in or close to the well-developed estuarine turbidity maximum (ETM) throughout the year. In the Midori River estuary, the oligohaline community lacked S. sinensis and showed a minimum biomass during winter (<10 mg m?3). In both estuaries, the freshwater community always remained at a small biomass (<1 mg m?3), whereas the meso/polyhaline community showed marked seasonal changes in biomass (0.1–657 mg m?3). The prevalence of higher salinities allowed only the meso/polyhaline community to occur in the Kuma River estuary. In summary, S. sinensis characterized the copepod community distinctive of the well-developed ETM, potentially serving as an important link to higher trophic levels during winter when copepods are scarce in other areas.  相似文献   

20.
Laboratory experiments were performed on the food ecology of four congeneric species of free-living plathelminths, Promesostoma caligulatum, P. marmoratum, P. rostratum, and P. meixneri, all inhabiting an intertidal sandflat near the island of Sylt (North Sea). Their prey spectrum is within the microcrustaceans: P. caligulatum preferred ostracods, while the other three species favoured copepods, with species-specific differences for copepod species and size classes. Daily consumption of prey species varied with the size of both the predator and the prey. On average, P. marmoratum consumed 0.76 Harpacticus flexus per day while this rate decreased to 0.06 in P. meixneri, the smallest predator. When these Promesostoma species were fed with Tachidius discipes, a smaller prey species, their predation rates were about 25% higher. While the larger predators preferred the larger harpacticoids as prey, the small P. meixneri preferred small cyclopoids over larger harpacticoids. In terms of biomass, P. marmoratum's mean consumption of T. discipes per day was about half the predator's own weight. This average varied with prey density and temperature. A comparison of these consumption rates with the field densities of the predators and their prey shows that the plathelminth predators may consume as much as 10% per day of their copepod prey populations, thus strongly influencing these prey populations on these sandflats. The predation pressure of P. caligulatum on ostracods was about 1% per day of the prey population. Since ostracods usually have fewer generations per year, the total effect on the population dynamics may be similar to that on copepods. Therefore, nocturnal swimming of copepods in the water column may be interpreted as an attempt to escape plathelminth predators.  相似文献   

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