The planktonic food web structure of a temperate zone estuary,and its alteration due to eutrophication

Hydrobiologia - Current conventional wisdom argues that human-induced excesses in nutrient loadings to estuaries often stimulate ‘excess’ algal production leading to hypoxia, via...     

Typical features of particulate phosphorus in the Seine estuary (France)

During the 2001–2002 hydrological cycle, 8,000 tons P year⁻¹ (44% as particulate phosphorus) originating from the Seine basin entered the Seine estuary. P content in suspended sediments (SS) is 2.9 g P kg⁻¹ (80% as inorganic form) at Poses (the upstream limit of the Seine estuary). During the transfer from Poses to Caudebec (the limit of saline water intrusion), Particulate Inorganic Phosphorus (PIP) in SS decreases by 40% whereas Particulate Organic Phosphorus (POP) remains stable. This decrease is explained as the result of (i) a dilution by SS poorer in P, originating from storage zones (mudflats) within the estuary, and (ii) a loss of P by sedimentation, especially in the Rouen harbour where 15% to 20% of SS are yearly trapped and extracted. Downstream, in the turbidity maximum of the estuary, P content in SS is twice as low as at Poses (1.5 g P kg⁻¹, 70% as inorganic form). PIP content is fairly homogeneous both vertically and throughout an annual survey. On the other hand, POP varies by season. Higher POP content is observed during vernal period with phytoplankton accumulation. In a salinity gradient from 0 to 30 (PSS78), PIP content further decreases by 30%. POP does not vary much in this gradient, leading to the assumption that PIP is submitted to desorption in response to the dilution by marine waters (poor in orthophosphates). Using ³²P isotopic method, we establish a mathematical formulation of P exchangeable between the suspended solid and water phases of the Seine estuary. Particulate P in the turbidity maximum zone is shown to represent a possible source of dissolved P, available for algal growth in the Seine Bight. Instead of playing a role of nutrient retention, the turbidity maximum zone of the Seine estuary could favour coastal eutrophication.     

Modelling Pb and Cd dynamics in the Seine estuary

The Seine estuary (France) is currently one of the world’s most contaminated estuaries, due in particular to its high cadmium and lead content. Proper understanding of contaminant transfer, transformation and retention mechanisms throughout the estuary and up to the adjacent marine zone require a range of studies involving data collections, experiments, and modeling tool. A multivariable transport model (SiAM-3D) was used to simulate dissolved and particulate transport and it is applied to several calculation grids; a speciation model (MOCO) was used to select key species and obtain a schematic system representation. The coupled model for Cd and Pb was compared with field measurements. The complementary character of various tools (model applied to annual time scale, in situ measurements and experiments) allowed to explore and quantify various hypotheses on the high dissolved cadmium concentrations observed during low river flow. The target is to achieve a compromise between acceptable computing times and adequate result accuracy. Although particle and contaminant behaviour is globally well-reproduced by the coarse grid, calculation errors relating to bayward fluxes and stocks deposited inside the estuary were highlighted after comparison with the fine grid.     

An integrated modelling approach to forecast the impact of human pressure in the Seine estuary

Within the framework of the European Water Framework Directive, the Seine-Normandie Water Agency has defined prospective scenarios describing possible trends of evolution of the pressures on water resources. In order to evaluate the resulting water quality improvement or degradation of water bodies in the Seine river basin, an integrated modelling was proposed. The approach consisted in coupling three models, the seneque model for upstream sub-basins, the ProSe model for the Seine river and main tributaries and finally the s i am1d model for the downstream estuary. After consistency verification, the integrated model was applied to scenarios proposed by the Seine-Normandie Water Agency. As a result of improvement in the nitrogen treatment by waste water treatment plants, the annual load of ammonia at the basin scale will be reduced by 65%. The oxygen and ammonia criteria in the estuary will improve from “bad” to “good”. However the nitrate criteria will remain “poor”, given the strong influence of non-point sources. Despite a 70–75% drop of the point orthophosphate loads, the criteria for this variable will also remain “poor”. The nutrient levels will be high enough to maintain eutrophication in the system; a general trend to a shift from N-limitation to P-limitation will be accentuated.     

Organic matter transport across a macrotidal estuary grandient: the Seine estuary,Fance

The macrotidal Seine estuary receives mineral and organic fluxes from a strongly industrialized basin. Upstream-downstream analysis of POC, polysaccharides and phenolic compounds in the particulate fraction indicated clearly the continental inflow. Within the maximum turbidity zone, the organic matter/suspended particulate matter ratio is lower than elsewhere which results from the dilution effect. Particulate tracers were quantified using grain size analysis and S.E.M. techniques. These sedimentary data defined the ratio of marine to continental particulate flux and mechanisms controlling the suspended particulate matter load within the estuarine mixing zone (resuspension, frontal enrichment and turbulent mixing processes). Organic parameters confirm the data on the mineral matter and were used to distinguish between the marine and continental inputs, from the upstream and downstream samples. Between the two endmembers, variations in organic parameters were controlled mainly by the hydrodynamics in the estuarine zone, rather than by salinity changes.     

Convergent signs of degradation in both the capacity and the quality of an essential fish habitat: state of the Seine estuary (France) flatfish nurseries

The Bay of Seine is a potentially important nursery ground, especially for flatfish. This area, however, is also strongly anthropogenically influenced. A composite approach was chosen to compare the flatfish nursery function of the Seine estuary with other coastal and estuarine areas and to determine the impact of anthropogenic disturbances. It emerges that alteration of the nursery function in the Seine estuary is related to the loss of nursery habitat and to a lower quality of the residual surfaces. These observations stressed the importance of habitat destruction and decreased quality of remaining habitat for fish stocks renewal.     

Kinetics of zooplankton in an aquatic continuum: from the Marne River and reservoir to the Seine estuary

A study was carried out within a 700-km river sector, including three types of ecosystems (a reservoir, a river and its estuary) to characterise the major features of zooplankton communities in the Seine Basin. In rivers, zooplankton biomass becomes significant only when the growth rate of the organisms is higher than the dilution rate (4-5th orders rivers, according to River Continuum Concept). Upstream, short residence times favour the development of small species (Rotifers) with low individual body weight and biomass. Conversely, larger species (microcrustaceans) develop more downstream, where increased residence time leads to autochthonous production (Riverine Productivity Model). Such a pattern is greatly modified by human impact. Zooplankton input from the Marne reservoir represents one type of disruption in the general upstream-downstream trend (according to the Serial Discontinuity Concept). This reservoir is a source of microcrustaceans; they rapidly disappear mainly through fish predation, and therefore have little impact on the river phytoplankton. Discontinuities, such as confluences, have a relatively small effect on the stock of zooplankton with regard to the water release from the reservoir, but they persist more downstream, because they have the same lotic origin. A few microhabitats with macrophytes play a small role for this canalised river, but they can modify locally the plankton community structure and composition. As a whole, the flux of zooplankton rises exponentially, whereas discharge increases linearly from upstream (4th order) to downstream (8th order). In the canalised sectors, Dreissena larvae build up an important biomass, adding to that of the zooplankton sensu stricto. Especially abundant in the downstream sector of the Marne and Seine Rivers, the larvae show a widespread colonisation of the benthic substrates by the adult Dreissena. One of the largest mussel colonies in the middle estuary can contribute to a rapid decrease of zooplankton. Estuary ecosystems form a transitional zone between freshwater and seawater, with zooplankton dynamics closely linked to the particular conditions on this part of the river system.     

Production rates of Eurytemora affinis in the Elbe estuary,comparison of field and enclosure production estimates

Production rates of the calanoid copepod Eurytemora affinis were estimated from field studies in the Elbe estuary and from an enclosure experiment. As one basic parameter of production rates, the body length, was compared between both investigations. Most of the copepodid stages in the enclosure experiment reached a significant greater length than the copepodids in the estuary. The differences in length between copepods from the field and the experiment could mainly be explained by a four times higher chlorophyll-a level in the enclosure experiment. The better food supply also results in a higher individual growth rate for all instars in the enclosure experiment. Therefore the population of Eurytemora affinis in the Elbe estuary was regarded as food limited during certain times of the year, especially in late spring and summer.Maximum daily production rate in the enclosure experiment (40 µg dw l^–1 d^–1) was four times higher than in the estuary (12 µg dw l^–1 d^–1). The mean daily P:B ratio in the enclosure was 0.301 d^–1 compared to 0.11 d^–1 in the estuary.     

Benthic variability in intertidal soft-sediments in the mesohaline part of the Schelde estuary

The benthic (zoobenthos and microphytobenthos) and physical characteristics of intertidal sediments were studied in April and September 1997 on 10 locations, differing in elevation and exposure to tidal currents, situated on three mudflats in the mesohaline part of the Schelde estuary. Sediment characteristics were spatially and temporally relatively homogeneous among the sampling locations, all characterized by a high proportion of mud. It emerged that the nature of the sediment (in terms of granulometric characterization, organic matter) did not fully explain the substantial spatial and temporal (seasonal) variability in the benthos (pigment contents and benthic fauna), but that other environmental conditions explained the observed variability. Chlorophyll a and fucoxanthin contents in the top layer (0–0.5 cm) of the sediment were about 7 times higher in April than in September, but a large spatial variation was observed, especially in April, which was mainly attributed to a difference in elevation of the mudflats. The high pigment contents in April were accompanied by a relative low abundance of the benthic fauna. In April sub-surface deposit feeders, mainly Oligochaeta, were numerically most important in the 1000 and 500 m fraction of the zoobenthos, and Nematodes in the 250 m fraction. The surface deposit feeders Manayunkia aestuarina and Copepoda were locally very abundant, showing a strong positive correlation with elevation and pigment contents. The critical shear stress for erosion, measured in situ with the SedErode device, varied between 0.26 and 0.43 N m^–2 in April. The critical shear stress for erosion was lower in September when pigment contents were significantly lower and benthic fauna abundance was higher. In September Corophium volutator, Heteromastus filiformis, Nereis diversicolor and Macoma balthica dominated the 1000 m fraction, Oligochaeta and C. volutator the 500 m fraction, and Nematodes, M. aestuarina, Oligochaeta and Copepoda the 250 m fraction. It is argued that in estuarine systems with high seasonal variability in river flow rate and therefore salinity, the upper-middle estuarine fauna may switch each year between an oligohaline and a mesohaline fauna, which result in communities that seldom progress beyond early benthic-community succession. The majority of the benthic infauna was found in the upper 3 cm in both seasons, with surface deposit feeders (e.g. C. volutator, M. aestuarina) mainly occurring in the top 1 cm of the sediment, showing a strong correlation with both elevation and pigment contents and sub-surface deposit feeders (e.g. H. filiformis, Oligochaeta) occurring deeper in the sediment, showing a positive correlation with the mud content of the sediment.     

Effects of the non-indigenous cladoceran Cercopagis pengoi on the lower food web of Lake Ontario

1. In North America, the invasive predatory cladoceran Cercopagis pengoi was first detected in Lake Ontario. We explored the impact of Cercopagis on the lower food web of Lake Ontario through assessments of historical and seasonal abundance of the crustacean zooplankton, by conducting feeding experiments on the dominant prey of the invader, and by estimating its food requirements. 2. Between 1999 and 2001, a decrease in the abundance of dominant members of the Lake Ontario zooplankton community (Daphnia retrocurva, Bosmina longirostris and Diacyclops thomasi) coincided with an increase in the abundance of Cercopagis. Daphnia retrocurva populations declined despite high fecundity in all 3 years, indicating that food limitation was not responsible. Chlorophyll a concentration generally increased, concomitant with a decline in the herbivorous cladoceran zooplankton in the lake. 3. Laboratory experiments demonstrated that Cercopagis fed on small‐bodied species including D. retrocurva and B. longirostris. 4. Consumption demand of mid‐summer populations of Cercopagis, estimated from a bioenergetic model of the confamilial Bythotrephes, was sufficient to reduce crustacean abundance, although the degree of expected suppression varied seasonally and interannually. 5. Predatory effects exerted by Cercopagis on the Lake Ontario zooplankton, while initially very pronounced, have decreased steadily as the species became established in the lake.     

The relative importance of herbivory and carnivory on the distribution of energy in a stochastic tri-trophic food web

A three-state, discrete-time Markov chain is used to model the dynamics of energy flow in a tri-trophic food web. The distribution of energy in the three trophic levels is related to the rates of flow between the trophic levels and calculated for the entire range of possible flow values. These distributions are then analysed for stability and used to test the idea that plants are resource-limited and herbivores are predation-limited. Low rates of death and decomposition, when coupled with low rates of herbivory and carnivory, tend to destabilize this food web. Food webs with higher rates of death and decomposition are relatively more stable regardless of rates of herbivory and carnivory. Plants are more prone to resource-limitation and herbivores are, in general, limited by their predators, which supports Hairston et al. (Am. Nat. 94 (1960) 421). The rate of decomposition often mediates the roles of top-down and bottom-up control of energy flow in the food web.     

Zooplankton in the Schelde estuary, Belgium and the Netherlands: long-term trends in spring populations

A compilation of available data in between 1967 and 2002 on spring zooplankton abundance was made for the brackish and the freshwater zone of the Schelde estuary. The general picture is a significant increase of 1–2 orders of magnitude in abundance for Rotifera, Copepoda and Branchiopoda (mainly Cladocera) in the freshwater zone, while zooplankton abundance in the brackishwater zone remained more constant. Possible natural and management related causes for this increase in zooplankton abundance are briefly discussed.     

Fluxes of polycyclic aromatic hydrocarbons in the Seine estuary,France: mass balance and role of atmospheric deposition

Measurements of polycyclic aromatic hydrocarbons (PAHs) were carried out from 2001 to 2003 in various environmental compartments (ambient air, atmospheric deposition, soils, sediments, runoff, and surface waters) in the Seine Estuary. The goal of this study was to identify the importance of atmospheric fluxes to occurrence of PAHs in the estuary, and to estimate the major pathways of their transport and accumulation within this ecosystem. The annual atmospheric inputs in the estuary ranged from 2.5 to 16 kg for the first sampling year, which is high when compared to those calculated for other European locations. In parallel, PAH transport from the atmosphere to the watershed outlet was investigated for two rural and urban small experimental watersheds within the Seine Estuary. In rural areas, atmospheric inputs are higher than exported loads by surface water and PAHs are accumulating in soils. The inverse occurs in urban areas, where the contribution of atmospheric deposition is negligible compared with PAH exported loads by runoff. These results allowed us to establish a mass balance for PAHs on the scale of the Seine Estuary, and to determine the dynamics of PAH transport. This study highlights the role of the atmospheric compartment in the transfer of contaminants and the importance of the local PAH inputs within the Seine Estuary.

Linearity in the aggregate effects of multiple predators in a food web

Theory in community ecology often assumes that predator species have similar indirect effects and thus can be treated mathematically as a single functional unit (e.g. guild or trophic level). This assumption is questionable biologically because predator species typically differ in their effects, creating the potential for nonlinearities when they coexist. We evaluated the nature of indirect effects caused by three species of hunting spider predators, singly and in multiple species combinations, on grass and herb plants in experimental old-field food webs. Despite the potential for nonlinearity, indirect effects in different multiple predator combinations consistently did not differ significantly from the respective means of the single species effects. Thus, for this experimental system, the whole was simply the average of the parts. Consequently, models which abstract predator species as single trophic levels would successfully predict indirect effects in this system regardless of the composition of the predator fauna.     

Macrobenthic community structure before, during, and after implementation of the Clean Water Act in the Neches River estuary (Texas)

Community structure of macrobenthos in the industrialized, tidal Neches River was studied 12 years after implementation of phase III of the Clean Water Act (CWA). Result were compared with a 1971–72 study conducted before implementation of the CWA and a 1984–85 study conducted after implementation of phases I & II of the CWA. The permitted BOD waste load decreased 93% between the 1971–72 and 1984–85 studies, then increased 19% between 1984–85 and the current study. A total of 50 taxa were collected during the 1971–72 study, 104 during the 1984–85 study, and 110 during the current study. Numbers of taxa per collection at each station increased by a factor of at least two between the 1971–72 and 1984–85 studies and were significantly different at the 0.0001 level. Numbers of taxa per collection at each station were not significantly different between the 1984–85 and the current study (p=0.286). Minimum collection densities in 1984–85 were higher than maximum densities in 1971–72 and were significantly different at the 0.0001 level. Collection densities were not significantly different between the 1984–85 study and the current study (p=0.374). Shannon's annual and station collection diversity values significantly increased (p=<0.05) between the 1971–72 and 1984–85 studies, but not between the 1984–85 and the current study (p=>0.05). Dendrograms of Sorenson's similarity index between all pairs of stations were more alike between the 1971–72 and the current study than between the 1984–85 and the current study. Evidence of some organic enrichment at the upper stations was indicated by the dominance of Limnodrilus hoffmeisteri and depressed oxygen concentrations. Environmental quality in the river has greatly improved since implementation of the CWA in 1972. Most of the improvments were due to phases I & II of the CWA, with little apparent change since implementation of phase III.     

Impact of a Flood Event on the Planktonic Food Web of the Schelde Estuary (Belgium) in Spring 1998

To evaluate the effect of short-term changes in discharge on plankton dynamics, a station in the upper reaches of the Schelde estuary was monitored during 1 month in spring 1998 with a sampling frequency of 1–2 days. During this monitoring period, a flood event occurred during which discharge increased about 5-fold. This flood event had a strong effect on the composition of the planktonic community. Rotifers were strongly negatively affected by the flood event while densities of heterotrophic nanoflagellates and scuticociliates increased. Chlorophyll a concentration and abundance of bacteria, oligotrich ciliates and crustacean zooplankton did not respond clearly to the flood event. Although the flood event lasted only a few days it took more than 2 weeks for the planktonic community to return to its original composition.     

Roles of food web and heterotrophic microbial processes in upper ocean biogeochemistry: Global patterns and processes

The growth and dynamics of plankton in the ocean vary with natural cycles, global climate change and the long-term evolution of ecosystems. The ocean is a large reservoir for CO₂ and the food webs in the upper ocean play critical roles in regulating the global carbon cycle, changes in atmospheric CO₂ and associated global warming. Microheterotrophs are a key component of the upper ocean food webs. Here, we report on the results of an analysis of the distribution of bacteria and related properties in the World Ocean. We found that, for the data set as a whole, there is a significant latitudinal gradient in all field-measured and computed bacterial properties, except growth rate. Gradients were, for the most part, driven by an equator-ward increase in the Southern Hemisphere. The biomass, rates of production and respiration and dissolved organic carbon concentrations were significantly higher in the Northern than the Southern hemispheres. In contrast, growth rates were the same in the two hemispheres. We conclude that the lower biomass and production in the Southern Hemisphere reflects greater top-down control by microbial grazers, which would be due to a lower abundance or activity of omnivorous zooplankton in the Southern than Northern Hemispheres. These large spatial differences in dynamics, structure and activity of the bacterial community and the microbial food web will be reflected in different patterns of carbon cycling, export and air–sea exchange of CO₂ and the potential ability of the ocean to sequester carbon.     

Organic contaminant exposure in the Lake St. Clair food web

Chemical concentrations and distributions in an aquatic food web were studied to quantify the relative importance of chemical properties versus food web processes in determining exposure dynamics of organic contaminants in aquatic ecosystems. Five organochlorines were measured (Pentachlorobenzene QCB, Hexachlorobenzene HCB, Octachlorostyrene OCS, Dichlorodiphenyldichloroethylene DDE and Polychlorinated Biphenyls PCBs) in the food web of Lake St. Clair. Levels of QCB in aquatic organisms ranged from 1.0 to 25 µg kg^–1 lipid, and levels of HCB ranged from 10 to 410 µg kg^–1 lipid. More elevated concentrations of OCS (13 to 392 µg kg^–1 lipid), DDE (162 to 11 986 µg kg^–1 lipid) and PCB (650 to 64 900 µg kg^–1 lipid) were observed. Organism — water equilibrium ratios were calculated for all species sampled to quantify the importance of food web processes in regulating contaminant exposure dynamics. Correlations of organism — water equilibrium ratios with body size were not significant for QCB, HCB and OCS (P>0.1), but were found to be significant for DDE and PCB (P<0.01).Results support the conclusion that both chemical properties and food web dynamics regulate the distribution and concentration of organochlorines in aquatic ecosystems. Food web processes are important, however, for chemicals, that are not metabolized and have octanol — water partition coefficients (log K _ow) greater than 5.5.