The balance between microzooplankton grazing and phytoplankton growth in a highly productive estuarine fjord

During 24, three-day cruises to Dabob Bay, Washington State, USA, from February 4 to April 26, 2002, and February 4 to May 1 2003, we examined the relative growth and grazing rates of phytoplankton and microzooplankton using dilution experiments. Experiments were conducted over two time intervals: 8–10 h during the nighttime only, or 24 h from noon to noon. We used water from two depths during each cruise: from the surface mixed layer, and from a deep layer below the seasonal thermocline. During 2002, there was one mid-sized bloom consisting mainly of Thalassiosira spp. in early February, and a larger bloom in April comprised of two Chaetoceros spp. and Phaeocystis sp. During 2003, there were also two blooms, one in early February, which was again dominated by Thalassiosira spp., and a second larger bloom in mid-April, comprised mainly of Thalassiosira spp. and Chaetoceros spp. During all four of these blooms, and for both water source depths, specific grazing rates of microzooplankton were most often as high or higher than the calculated phytoplankton specific growth rates. The major microzooplankton categories that could have accounted for this were (1) a large Gyrodinium spp., (2) a group of fusiform-shaped mid-sized Protoperidinium species, and (3) three loosely defined taxonomic groups consisting of naked ciliates, tintinnids, and unidentified heterotrophic dinoflagellates. Based on our measurements, it appears that the microzooplankton community grazing pressure can often exert significant control on phytoplankton biomass, even during the extremely productive spring bloom periods and under several different diatom-dominated bloom types. These results suggest that even in highly productive estuarine ecosystems, which are often nurseries to economically important fisheries species, microzooplankton play a critical role and may significantly alter the availability and efficiency of transfer of energy to higher trophic levels.     

The distribution of brachyuran crustacean megalopae in the waters of the York river, lower Chesapeake Bay and adjacent shelf: Implications for recruitment

Vertical and horizontal distributions are described for megalopae of 11 brachyuran taxa common to the lower Chesapeake Bay and adjacent shelf. Three distribution patterns are apparent from horizontal distribution.

1. (1) More than 75% of the megalopae of estuarine adults, such as Hexapanopeus angustifrons, Neopanope sayi, Panopeus herbstii, Rhithropanopeus harrisii and Pinnotheres ostreum, are retained in estuarine waters.

2. (2) The megalopae of three estuarine taxa, Callinectes sapidus, Uca spp. and Pinnixa sp., are most abundant on the shelf.

3. (3) More than 90% of the megalopae of the shelf species, Portunus sp., Cancer irroratus and Libinia spp. remained in shelf waters.

Vertical distributions indicate the megalopae of a majority of estuarine crabs are epibenthic when waters are stratified. Exceptions include the megalopae of both Rhithropanopeus which are found slightly shallower in well mixed waters and Callinectes sapidus which are generally most abundant in the neuston. The megalopae of two shelf species, Portunus sp. and Cancer irroratus, reach highest densities in surface waters, while those of Libinia spp. are common in the epibenthos.Megalopal distributions analyzed with respect to water column stratification show megalopae never reached highest densities within the layer of the pycnocline. The megalopae of five species show significant shifts in vertical distributions between stratified and homogeneous water columns.     

Structure, biomass distribution and trophodynamics of the pelagic ecosystem in the Oyashio region, western subarctic Pacific

Biomass distribution and trophodynamics in the oceanic ecosystem in the Oyashio region are presented and analyzed, combining the seasonal data for plankton and micronekton collected at Site H since 1996 with data for nekton and other animals at higher trophic levels from various sources. The total biomass of biological components including bacteria, phytoplankton, microzooplankton, mesozooplankton, micronekton, fishes/squids and marine birds/mammals was 23 g C m⁻², among which the most dominant component was mesozooplankton (34% of the total), followed by phytoplankton (28%), bacteria (15%) and microzooplankton (protozoans) (14%). The remainder (9%) was largely composed of micronekton and fish/squid. Marine mammals/birds are only a small fraction (0.14%) of the total biomass. Large/medium grazing copepods (Neocalaus spp., Eucalanus bungii and Metridia spp.) accounted for 77% of the mesozooplankton biomass. Based on information about diet composition, predators were assigned broadly into mean trophic level 3–4, and carbon flow through the grazing food chain was established based on the estimated annual production/food consumption balance of each trophic level. From the food chain scheme, ecological efficiencies as high as 24% were calculated for the primary/secondary production and 21% for the secondary/tertiary production. Biomass and production of bacteria were estimated as 1/10 of the respective values for phytoplankton at Site H, but the role of the microbial food chain remains unresolved in the present analysis. As keystone species in the oceanic Oyashio region, Neocalanus spp. are suggested as a vital link between primary production and production of pelagic fishes, mammals and birds.     

Geographical and seasonal variations in biomass and estimated production rates of net zooplankton in Yatsushiro Bay, Japan

The biomass and production rate of net zooplankton were studied at eight stations in Yatsushiro Bay, Japan, monthly from May 2002 to April 2003. Based on environmental conditions, the bay was divided into three regions, viz. northern (average depth, salinity and chlorophyll a concentration: 11 m, 31.8 and 6.5 μg l⁻¹, respectively), central (30 m, 32.8 and 3.2 μg l⁻¹, respectively) and southern (43 m, 33.4 and 1.9 μg l⁻¹, respectively). Net zooplankton biomass was high in warm months and low in cold ones, with annual averages of 20.2, 38.8 and 16.4 mg C m⁻³ in the northern, central and southern regions, respectively. Copepods were the most important constituent (>ca. 70% of net zooplankton biomass) in all regions. The northern region was characterized by the dominance of Oithona spp. in summer and Acartia spp. in winter-spring. In the central region, Microsetella norvegica was most pronounced in summer-fall. In both central and southern regions, Calanus sinicus and Eucalanus spp. dominated in winter-spring and fall, respectively. The annual average net zooplankton secondary production rate was 4.4, 7.5 and 3.9 mg C m⁻³d⁻¹ in the northern, central and southern region, respectively. Combining the results from the present study with those from other collaborative works on microzooplankton allowed us to determine the trophic interactions in Yatsushiro Bay. If the secondary producers depend entirely on phytoplankton for food, their daily carbon requirement is equivalent to 12.5, 21.6 and 19.1% of the phytoplankton biomass in the respective regions.     

Seasonal variation in the community and size structure of nano- and microzooplankton in Gyeonggi Bay,Yellow Sea

To investigate the seasonal variation and community structure of nano- and microzooplankton in Gyeonggi Bay of the Yellow Sea, the abundance and carbon biomass of nano- and microzooplankton were evaluated at 10-day intervals from January 1997 to December 1999. Four major groups of nano- and microzooplankton communities were classified: heterotrophic ciliates, heterotrophic dinoflagellates (HDF), heterotrophic nanoflagellates (HNF), and copepod nauplii. The total carbon biomass of nano- and microzooplankton ranged from 10.2 to 168.8 μg C L⁻¹ and was highest during or after phytoplankton blooms. Nano- and microzooplankton communities were composed of heterotrophic ciliates (7.4–81.4%; average 41.7% of total biomass), HDF (0.1–70.3%; average 26.1% of total biomass), copepod nauplii (1.6–70.6%; average 20.7% of total biomass), and HNF (0.8–59.5%; average 11.5% of total biomass). The relative contribution of individual components in the nano- and microzooplankton communities appeared to differ by seasons. Ciliates accounted for the most major component of nano- and microzooplankton communities, except during summer and phytoplankton blooming seasons, whereas HDF were more dominant during the phytoplankton blooming seasons. The abundance and biomass of nano- and microzooplankton generally followed the seasonal dynamics of phytoplankton. The size and community distribution of nano- and microzooplankton was positively correlated with size-fractionated phytoplankton. The carbon requirement of microzooplankton ranged from 60 to 83% of daily primary production, and was relatively high when phytoplankton biomass was high. Therefore, our result suggests that the seasonal variation in the community and size composition of nano- and microzooplankton appears to be primarily governed by phytoplankton size and concentration as a food source, and their abundance may greatly affect trophic dynamics by controlling the seasonal abundance of phytoplankton.     

A Linked Physical and Biological Framework to Assess Biogeochemical Dynamics in a Shallow Estuarine Ecosystem

The littoral zone of Chesapeake Bay contains a mosaic of shallow vegetated and nonvegetated habitats with biotic components that are sensitive to changes in biological and physical driving factors. Static and dynamic modelling frameworks provide an integrative way to study complex hydrodynamic and biogeochemical processes in linked estuarine habitats. In this study we describe a spatial simulation model developed and calibrated relative to a specific littoral zone, estuarine ecosystem. The model consisted of four distinct habitats that contained phytoplankton, sediment microalgae, Zostera marina (eelgrass), and Spartina alterniflora. There was tidal exchange of phytoplankton, particulate and dissolved organic carbon and dissolved inorganic nitrogen between the littoral zone ecosystem and the offshore channel. Physical exchange and biogeochemical transformations within the habitats determined water column concentrations in each habitat. Predicted subtidal water column concentrations and Z. marina and S. alterniflora biomass were within the variability of validation data and the predicted annual rates of net primary production were similar to measured rates. Phytoplankton accounted for 17%, sediment microalgae 46%, the Z. marina community 24% and S. alterniflora 13% of the annual littoral zone primary production. The linked habitat model provided insights into producer, habitat and ecosystem carbon and nitrogen properties that might not have been evident with stand-alone models. Although it was an intra-ecosystem sink for particulate carbon, the seagrass habitat was a DOC source and responsible for over 30% of the littoral zone carbon and nitrogen primary production. The model predicted that the Goodwin Islands littoral zone was a sink of channel derived POC, but a source of DOC to the surrounding estuary. The framework created in this study of estuarine ecosystem dynamics is applicable to many different aquatic systems over a range of spatial and temporal scales.     

Dissolved organic carbon in the freshwater tidal reaches of the Schelde estuary

To unravel the factors that regulate DOC dynamics in the freshwater tidal reaches of the Schelde estuary, DOC concentration and biodegradability were monitored in the upper Schelde estuary and its major tributaries. Although the Schelde estuary possesses a densely populated and industrialized catchment, our data suggest that the bulk of DOC in the freshwater tidal reaches is not derived from waste water. This was concluded from the low biodegradability of DOC (on average 9%), DOC concentrations that are close to the mean for European rivers (4.61 mg l⁻¹) and the absence of an inverse relationship between DOC and discharge. Most DOC originating from waste water being discharged in tributaries of the estuary appears to be remineralised before these tributaries reach the main estuary. Although dense phytoplankton blooms were observed in the upper estuary during summer (up to 700 μg chl a l⁻¹), these blooms did not appear to produce large quantities of DOC in the freshwater tidal reaches as DOC concentrations were low when phytoplankton biomass was high. The fact that DOC concentrations were high in winter and decreased in summer suggests a predominantly terrestrial source of DOC in the freshwater tidal reaches of the Schelde estuary.     

A sediment budget for the Choptank River estuary in Maryland,U.S.A.

A sediment budget for the Choptank River, one of the three largest estuaries on the eastern shore of Chesapeake Bay, was developed from measurements of sediment carried in upland runoff, shore erosion, sedimentation, and levels of suspended sediments in estuarine waters. Shore erosion was the major source of sediment (340 × 10⁶ kg y^?1), contributing seven times more sediment than upland runoff. Low relief, the rural character of the Coastal Plain drainage basin, and the susceptibility of poorly consolidated shoreline materials to erosion contributed to the dominance of shore erosion over runoff as a sediment source. Box modelling indicated a net annual flux (14–44 × 10⁶ kg y^?1) of sediment from the Choptank River to Chesapeake Bay. A mass balance estimate of sedimentation, calculated as the difference between total inputs and loss at the mouth of the estuary, (350 × 10⁶ kg y^?1) agreed well with an estimate based on ²¹⁰Pb profiles (340 × 10⁶ kg y^?1) measured along the longitudinal axis of the estuary. Lead-210 sedimentation rates correspond to accumulation rates of 1·5–7·9 mm y^?1.     

Diatom assemblages from surface sediments of the Río de la Plata estuary,Uruguay

The Río de la Plata estuary (RdlP) exhibits environmental gradients associated with the freshwater input and oceanic water intrusion. The aim of this study was to assess diatom species distribution in surface sediment samples related to such environmental gradients. The internal section of RdlP was dominated by Aulacoseira spp., Eunotia spp., Staurosirella martyi, Actinocyclus normanii and Thalassiosira baltica, indicatives of low salinity levels and high trophic conditions, associated with the riverine and estuarine regimes. The external section was dominated by Coscinodiscus radiatus, Thalassiosira spp., Paralia sulcata, Cyclotella striata, among other marine taxa, indicatives of high salinity and low trophic conditions, associated with the influence of the Southwestern Atlantic Ocean. Furthermore, the intermediate section presents a mixture of both diatom groups representing mixing conditions. The observed diatom species groups capture fairly well the RdlP environmental variability and can be reliably used for paleoenvironmental studies in this and other similar estuarine systems.     

Ctenophores-invaders and their role in the trophic dynamics of the planktonic community in the coastal regions off the Crimean coasts of the Black Sea (Sevastopol Bay)

The abundances, biomasses, and population structures of two introduced ctenophore species—Mnemiopsis leidyi and Beroe ovata—were monitored along with mesoplankton in the near-shore waters of the northern Black Sea (Sevastopol Bay and adjacent regions) over a period of four years (2000–2003), after the B. ovata invasion. The annual dynamics of the M. leidyi population were similar in these years: very low abundances and biomass values were observed during the major part of the year (unlike previous years) with a shortterm peak in the summer-early autumn. B. ovata development during the growth in the M. leidyi biomass resulted in a sharp fall in the M. leidyi biomass down to extremely low values. The interannual differences in the populations of both ctenophore species were reflected by their quantitative parameters: the maximum biomass of M. leidyi varied from 790 g/m² in 2001 to 211–266 g/m² in other years. The maximum biomass values of B. ovata (38.9 and 32.5 g/m²) were observed in 2001 and 2003, respectively. In 2000–2003, from July to September, during the peak in mnemiopsis development, the population consumed from 1.9 ± 0.4 to 13.4 ± 5.7% of the mesoplankton biomass per day, while in the years of B. ovata absence, these values were as high as 30–40%. For the first time, the grazing rate of microzooplankton by M. leidi larvae was estimated. In August 2003, the maximum daily consumption rate was as great as 23–25% of the microzooplankton biomass. The daily rations of the mnemiopsis larvae on microzooplankton were close or even higher than those on mesoplankton.     

中肋骨条藻-裸甲藻-双毛纺锤水蚤营养传递的级联效应

中型浮游动物因摄食微型浮游动物,释放了微型浮游动物对浮游植物的摄食压力,这种营养级联效应会增加浮游植物丰度和降低中型浮游动物对浮游植物的摄食率,从而弱化浮游生物网营养传递过程中的下行控制作用。本研究在实验室模拟了食物链中肋骨条藻-裸甲藻-双毛纺锤水蚤的营养传递过程,发现在中肋骨条藻低生物量时,双毛纺锤水蚤偏好于选择摄食裸甲藻;高生物量时,双毛纺锤水蚤偏好选择摄食中肋骨条藻。营养传递过程中存在正的级联效应(0.018~0.12 d^-1),级联效应的大小与裸甲藻的摄食率和双毛纺锤水蚤对裸甲藻的摄食选择指数呈现显著的正相关关系。双毛纺锤水蚤对中肋骨条藻的直接摄食死亡率大于营养级联效应,从而导致中肋骨条藻生物量的降低。因此,营养级联效应对中型浮游动物摄食浮游植物的影响要弱于中型浮游动物的直接摄食作用。     

Grazing by meso- and microzooplankton on phytoplankton in the upper reaches of the Schelde estuary (Belgium/The Netherlands)

In contrast with the marine reaches of estuaries, few studies have dealt with zooplankton grazing on phytoplankton in the upper estuarine reaches, where freshwater zooplankton species tend to dominate the zooplankton community. In spring and early summer 2003, grazing by micro- and mesozooplankton on phytoplankton was investigated at three sites in the upper Schelde estuary. Grazing by mesozooplankton was evaluated by monitoring growth of phytoplankton in 200 μm filtered water in the presence or absence of mesozooplankton. In different experiments, the grazing impact was tested of the calanoïd copepod Eurytemora affinis, the cyclopoid copepods Acanthocyclops robustus and Cyclops vicinus and the cladocera Chydorus sphaericus, Moina affinis and Daphnia magna/pulex. No significant grazing impact of mesozooplankton in any experiment was found despite the fact that mesozooplankton densities used in the experiments (20 or 40 ind. l⁻¹) were higher than densities in the field (0.1–6.9 ind. l⁻¹). Grazing by microzooplankton was evaluated by comparing growth of phytoplankton in 30 and 200 μm filtered water. Microzooplankton in the 30–200 μm size range included mainly rotifers of the genera Brachionus, Trichocerca and Synchaeta, which were present from 191 to 1777 ind. l⁻¹. Microzooplankton had a significant grazing impact in five out of six experiments. They had a community grazing rate of 0.41–1.83 day⁻¹ and grazed up to 84% of initial phytoplankton standing stock per day. Rotifer clearance rates estimated from microzooplankton community grazing rates and rotifer abundances varied from 8.3 to 41.7 μl ind.⁻¹ h⁻¹. CHEMTAX analysis of accessory pigment data revealed a similar phytoplankton community composition after incubation with and without microzooplankton, indicating non-selective feeding by rotifers on phytoplankton.     

Factors controlling the temporal and spatial variations in Synechococcus abundance in a monsoonal estuary

Temporal and spatial variations in Synechococcus abundance were investigated over an annual cycle (February'10–January'11) along a salinity gradient (0–35) in the tropical Zuari estuary, influenced by south-west monsoons. Synechococcus exhibited salinity preferences with phycoerythrin-rich cells at salinities >2 (Synechococcus-PEI), >20 (Synechococcus-PEII) and <1 (Synechococcus-PEIII) whereas phycocyanin-rich (Synechococcus-PC) dominant at lower salinities. Downstream stratification during monsoon caused Synechococcus group segregation in the surface and near-bottom waters. During monsoon-break and non-monsoon period stabilized waters, increased salinity, temperature, solar radiation and low rainfall favored high Synechococcus abundance whereas unstable waters, increased turbidity and low solar radiation during active monsoon lowered abundance. SYN-PC positively co-related with nitrate and phosphate and SYN-PEI with phosphate. Synechococcus contribution to phytoplankton carbon biomass ranged from 9 to 29%. In monsoonal estuaries, rainfall intensity regulates freshwater runoff which modulates the estuarine environment, creating temporal–spatial niche segregation of Synechococcus groups thereby serving as indicator organisms of the estuarine hydrodynamics.     

Spatial and Temporal Distribution of Coloured Dissolved Organic Matter (CDOM) in Narragansett Bay,Rhode Island: Implications for Phytoplankton in Coastal Waters

One indicator of health in estuarine and coastal ecosystems is the ability of local waters to transmit sunlight to planktonic, macrophytic, and other submerged vegetation for photosynthesis. The concentration of coloured dissolved organic matter (CDOM) is a primary factor affecting the absorption of incident sunlight in coastal and estuarine waters. In estuaries, CDOM concentrations vary due to changes in salinity gradients, inflows of industrial and domestic effluents, and the production of new dissolved organic matter from marine biologic activity. CDOM absorption data have been collected from a variety of waters. However, there are a limited number of measurements along the US east coast and a general lack of data from New England waters.This study characterized the temporal and spatial variability of CDOM absorption over an annual cycle in Narragansett Bay and Block Island Sound (Rhode Island). Results suggested that, in Narragansett Bay, the magnitude of CDOM absorption is related to the seasonal variability of freshwater input from surrounding watersheds and new CDOM production from in situ biologic activity. The data show that the average CDOM absorption coefficient at 412 nm was 0·45 m⁻¹ and the average spectral slope was 0·020 nm⁻¹.     

Effects of atrazine and linuron on photosynthesis and growth of the macrophytes, Potamogeton perfoliatus L. and Myriophyllum spicatum L. in an estuarine environment

Phytotoxicities of the herbicides, atrazine and linuron, were evaluated for two species of submersed vascular plants (Potamogeton perfoliatus, L. Myriophyllum spicatum, L.) which, until the late 1960s, had been abundant in Chesapeake Bay. Plants were grown in 50-liter laboratory microcosms, containing filtered estuarine water and sediments for a period of 5 weeks and then treated with atrazine or linuron at initial concentrations of 0, 5, 50, 100, 500 and 1000 gmg/liter. Plant responses were measured primarily in terms of apparent O₂ production, P₃, and above-ground biomass for 4 weeks post treatment. In general, at ≥ 50 gmg/liter there was a significant depression in P_a for both species and herbicides. However, M. spicatum appeared to be less sensitive, with a significant enhancement in P_a of this species at 5 gmg/liter, and linuron was slightly more effective than atrazine at reducing P_a for both species. Treatment effects on biomass generally paralleled those for P_a. In spite of relatively constant atrazine concentrations (84–89 % remaining at termination), both species exhibited evidence of photosynthetic recovery 2–3 weeks after treatment at concentrations ≤ 100 gmg/liter. Using an exponential dose-response model, I₅₀ (concentration for 50 % photosynthetic inhibition), ranged from 45–117 gmg/liter for all experiments. In general, in situ concentrations of atrazine and linuron in Chesapeake Bay and its tributaries appear to be sufficient to result in small reductions in P_a (2–10%, estimated from dose-response model) during a typical growing season. While such effects may be important for the survival of otherwise stressed plant populations, they suggest that these herbicides, per se, were not the cause of the general decline in abundance of these plants.     

Trophic efficiency of the planktonic food web in a coastal ecosystem dominated by Phaeocystis colonies

The trophic efficiency of the planktonic food web in the Phaeocystis-dominated ecosystem of the Belgian coastal waters was inferred from the analysis of the carbon flow network of the planktonic system subdivided into its different trophodynamic groups. A carbon budget was constructed on the basis of process-level field experiments conducted during the spring bloom period of 1998. Biomass and major metabolic activities of auto- and heterotrophic planktonic communities (primary production, bacterial production, nanoproto-, micro- and mesozooplankton feeding activities) were determined in nine field assemblages collected during spring at reference station 330. In 1998, the phytoplankton spring flowering was characterised by a moderate diatom bloom followed by a massive Phaeocystis colony bloom. Phaeocystis colonies, contributing 70% to the net primary production, escaped the linear food chain while the early spring diatom production supplied 74% of the mesozooplankton carbon uptake. The rest of mesozooplankton food requirement was, at the time of the Phaeocystis colony bloom, partially fulfilled by microzooplankton. Only one-third of the microzooplankton production, however, was controlled by mesozooplankton grazing pressure. Ungrazed Phaeocystis colonies were stimulating the establishment of a very active microbial network. On the one hand, the release of free-living cells from ungrazed colonies has been shown to stimulate the growth of microzooplankton, which was controlling 97% of the nanophytoplankton production. On the other hand, the disruption of ungrazed Phaeocystis colonies supplied the water column with large amounts of dissolved organic matter available for planktonic bacteria. The budget calculation suggests that ungrazed colonies contributed up to 60% to the bacterial carbon demand, while alternative sources (exudation, zooplankton egestion and lysis of other organisms) provided some 30% of bacterial carbon requirements. This suggests that the spring carbon demand of planktonic bacteria was satisfied largely by autogenic production. The trophic efficiency was defined as the ratio between mesozooplankton grazing on a given source and food production. In spite of its major contribution to mesozooplankton feeding, the trophic efficiency of the linear food chain, restricted to the grazing on diatoms, represented only 5.6% of the available net primary production. The trophic efficiency of the microbial food chain, the ratio between mesozooplankton grazing on microzooplankton and the resource inflow (the bacterial carbon demand plus the nanophytoplankton production) amounted to only 1.6%. These low trophic efficiencies together with the potential contribution of ungrazed Phaeocystis-derived production to the bacterial carbon demand suggest that during spring 1998 most of the Phaeocystis-derived production in the Belgian coastal area was remineralised in the water column.     

Copepod grazing during spring blooms: Does Calanus pacificus avoid harmful diatoms?

During late winter and spring of 2002 and 2003, 24, 2–3 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. The results of the copepod grazing experiments for C. pacificus are discussed here. Each week, copepod grazing incubation experiments from two different depth layers were conducted. Grazing was measured by both changes in chlorophyll concentration and cell counts. In 2002, there was one moderate bloom consisting mainly of Thalassiosira spp. in early February, and a larger bloom in April comprised of two Chaetoceros species and Phaeocystis sp. Similarly, in 2003, there were two blooms, an early one dominated by Thalassiosira spp., and a later one consisting of Chaetoceros spp. and Thalassiosira spp. Clearance rates on individual prey species, as calculated by cell counts, showed that C. pacificus are highly selective in their feeding, and may have much higher clearance rates on individual taxa than rates calculated from bulk chlorophyll disappearance. During weeks of high phytoplankton concentration, the copepods generally ate phytoplankton. However, they often rejected the most abundant phytoplankton species, particularly certain Thalassiosira spp., even though the rejected prey were often of the same genus and similar size to the preferred prey. It is speculated that this avoidance may be related to the possible deleterious effects that certain of these diatom species have on the reproductive success of these copepods. During weeks of medium to low phytoplankton concentration, the copepods selectively ate certain species of phytoplankton, and often had high electivity for microzooplankton. The selection mechanism must consist of active particle rejection most likely based on detection of surface chemical properties, since the diatoms that were selected were of the same genus, nearly the same size, and at lower numerical abundance than those cells that were avoided. The grazing choices made by these copepods may have important consequences for the overall ecosystem function within coastal and estuarine systems through changes in the transfer efficiency of energy to higher trophic levels.     

Potential climate-change impacts on the Chesapeake Bay

We review current understanding of the potential impact of climate change on the Chesapeake Bay. Scenarios for CO₂ emissions indicate that by the end of the 21^st century the Bay region will experience significant changes in climate forcings with respect to historical conditions, including increases in CO₂ concentrations, sea level, and water temperature of 50–160%, 0.7–1.6 m, and 2–6 °C, respectively. Also likely are increases in precipitation amount (very likely in the winter and spring), precipitation intensity, intensity of tropical and extratropical cyclones (though their frequency may decrease), and sea-level variability. The greatest uncertainty is associated with changes in annual streamflow, though it is likely that winter and spring flows will increase. Climate change alone will cause the Bay to function very differently in the future. Likely changes include: (1) an increase in coastal flooding and submergence of estuarine wetlands; (2) an increase in salinity variability on many time scales; (3) an increase in harmful algae; (4) an increase in hypoxia; (5) a reduction of eelgrass, the dominant submerged aquatic vegetation in the Bay; and (6) altered interactions among trophic levels, with subtropical fish and shellfish species ultimately being favored in the Bay. The magnitude of these changes is sensitive to the CO₂ emission trajectory, so that actions taken now to reduce CO₂ emissions will reduce climate impacts on the Bay. Research needs include improved precipitation and streamflow projections for the Bay watershed and whole-system monitoring, modeling, and process studies that can capture the likely non-linear responses of the Chesapeake Bay system to climate variability, climate change, and their interaction with other anthropogenic stressors.     

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

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

Dissolved oxygen dynamics in a eutrophic estuary,Upper Newport Bay,California

Eutrophication often causes hypoxia in estuarine and coastal systems, but the mechanisms that control hypoxic events vary among estuaries and are often difficult to discern. We monitored surface and bottom dissolved oxygen (DO) in the Upper Newport Bay (UNB), a tidally mixed estuary in southern California subject to anthropogenic nutrient loading, eutrophication and hypoxia. Our goal was to identify the environmental factors regulating DO dynamics. Six hypoxic events occurred between June and November and were associated with a combination of low solar radiation, increased freshwater discharge following precipitation, and enhanced haline stratification during reduced tidal range periods. At the head of the estuary, high macroalgal biomass and pronounced haline stratification resulted in high DO in the surface layer and low DO in the bottom layer. Oxygen-rich and oxygen-poor waters were transported down-estuary by ebb tides, resulting in DO heterogeneity throughout the UNB. Cross-wavelet analysis illustrated the down-estuary propagation of high/low DO signal correlated with the phases of diurnal photosynthetic and semi-diurnal tidal cycles.