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...     

Combined stable isotope and gut contents analysis of food webs in plant-dominated, shallow lakes

Summary 1. To determine feeding links between primary producers, invertebrates and fish, stable isotope analyses and gut content analyses of fish were conducted on the components of four shallow, eutrophic to hypertrophic, plant-dominated lakes.
2. Although separation of basal resources was possible, the diets of both fish and invertebrates were broad, comprising food from different compartments (planktonic, epiphytic/benthic), as well as from different trophic levels.
3. Mixing models were used to determine the extent to which periphyton production supported higher trophic levels. Only one species of invertebrate relied upon periphyton production exclusively.
4. Fish density affected the diets of invertebrates. The response was different for planktonic and epiphytic/benthic invertebrates. The proportion of periphyton production in the diets of zooplankton appeared to increase with fish density, whilst it decreased for other invertebrates.
5. As all zooplankton samples were collected in the open water at dusk, these results are further evidence for the diurnal horizontal migration of zooplankton. Although not conclusive, they are consistent with a behavioural response by invertebrates and zooplankton in the presence of fish.     

Biota of Lake St. Clair: habitat evaluation and environmental assessment

As a shallow, productive lake in the drainage system between Lake Huron and Lake Erie, Lake St. Clair provides habitat for a diverse biota including significant populations of fish and wildlife that are of use to man. Of the more than 70 species of native and migrant fishes, 43 use the lake for spawning. Peak numbers of waterfowl utilizing the lake and adjoining wetlands have been estimated at 60 000 in spring and 150 000 in autumn. In addition to recreational fishing and hunting, the lake is also used for swimming, boating and as a source of municipal water. It is located downstream from an industrial centre and adjacent to a population estimated at about four million. Mercury contamination closed the fisheries in 1970 and concentrations of the metal persist above consumption guidelines in some species. Almost all of the Michigan shoreline is urbanized and much of it altered through dyking and bulkheading. Coastal wetlands of the lake have declined 41 percent in the past century and only about one-half of the remaining area is open to the lake.Despite impacts from the large urban population and users, ecosystem quality remains reasonably good. The flushing action of relatively clean water from Lake Huron has slowed the eutrophication process. Major habitat problems are toxic substances from industries located on the St. Clair River and the continued loss of shoreline and wetlands to urbanization and agriculture. Further research and monitoring of sources, fates and impacts of toxic substances in sediments and biota are required. In addition, there is a need for environmental and economic evaluations of shorelines and wetlands to prevent further losses of these important habitats.Contribution No. 90-16 of the Ontario Ministry of Natural Resources, Research Section, Fisheries Branch, Box 5000, Maple, Ontario.     

Nutrient dynamics and pelagic food web interactions in oligotrophic and eutrophic environments: an overview

The concept of limiting nutrients is a cornerstone of theories concerning the control of production, structure and dynamics of freshwater and marine plankton. The current dogma is that nitrogen is limiting in most marine environments while freshwater ecosystems are mostly phosphorus-limited, although evidence of phytoplankton limitation by either N or P has been found in both environments.However, the same considerations apply to the availability of phosphorus in freshwater as to nitrogen in oceans. In resource-limited environments the plankton dynamics depend mostly on the internal mechanisms which act to recycle the limiting nutrient many times over within the surface waters. As the overall productivity increases, this dependence on nutrient regeneration decreases.The relationship between the stock of limiting nutrient, rates of supply and plankton dynamics must therefore be seen in the light of the processes operating within the entire food chain over quite different time scales. There is strong evidence that process-rates are mostly size-dependent and that food web interactions at the microbial level (picophytoplankton, bacteria, microheterotrophs) strongly effect the production of carbon and the regeneration of nutrients in the pelagic zone.     

Effects of different N- and P-loading on primary and secondary production in an experimental marine ecosystem

The effects of nutrient loading on phytoplankton, zooplankton and macrozoobenthos in experimental ecosystems was studied in a 7-month experiment. The mesocosms were designed to mimic the major physical characteristics (irradiance, temperature, mixing) of the Dutch coastal zone in the river Rhine plume. Three different nutrient loading scenarios were used, representing present and future conditions. The level of the spring phytoplankton bloom was determined by phosphorus loading, whereas during summer the nitrogen loading determined phytoplankton biomass. The differences in nutrient loading did not result in shifts in phytoplankton species composition. With exception of the early phase of the spring bloom, diatoms dominated phytoplankton biomass in all nutrient treatments. This was ascribed to microzooplankton grazing on smaller algal species. Microzooplankton biomass showed a positive correlation with primary production, and also significant differences between nutrient treatments. Copepod development was limited, probably due to competition with microzooplankton and predation by benthic fauna. Macrobenthos biomass correlated with primary production, and was lower in the lowest nutrient treatment.     

Spatial and temporal patterns of microcrustacean assemblage structure and secondary production in a wetland ecosystem

1. In contrast to extensive studies of zooplankton in lakes, the role of microcrustaceans in wetlands is not well studied. In this study, spatial and temporal patterns of microcrustacean assemblage structure and secondary production were quantified over a 2-year period in a southeastern U.S.A. wetland.
2. Thirty-two species, including 19 cladocerans, 10 copepods and three ostracods, generated different temporal patterns of density and production between vegetated ( Nymphaea ) and non-vegetated (open-water) zones reflecting species-specific differences in life histories.
3. Summer assemblages were dominated by small, planktonic filter-feeders, typified by high annual production/biomass ( P / B ) and daily production. In contrast, winter assemblages were dominated by larger, epibenthic detritivores with low P / B and high biomass. Seasonal shifts in the relative importance of planktonic species in the warmer months to benthic and epiphytic species in the cooler months suggest that energy flow pathways through microcrustaceans may vary seasonally.
4. Total annual production was higher during both years in the Nymphaea zone (13.0 g and 13.6 g DM m⁻² year⁻¹) than the open-water (8.2 and 6.3 g DM m⁻² year⁻¹), and was similar between years for the entire wetland pond (12.3 and 12.2 g DM m⁻² year⁻¹).
5. Although wetland ecosystems have been the subject of considerable ecological research in the past 20 years, our study is one of the few to demonstrate a highly diverse and relatively productive microcrustacean assemblage. Such comprehensive production studies can be used to quantify the ecological importance of microcrustaceans in freshwater wetland ecosystems.     

Population structure and secondary productivity of Limnodrilus hoffmeisteri (Claparede) and Branchiura sowerbyi Beddard in the profundal zone of Lake Naivasha,Kenya

Lake Naivasha has been well studied since the 1930s but attempts to understand its ecological functioning have had to wait until enough was known about its structure. The energetics of the lake has only been studied to date at primary producer level. Following the identification of the invertebrate components of the littoral and profundal benthos, this study was initiated. The absence of native fish species in Lake Naivasha, combined with a fishery based only on three, introduced species, added an applied dimension to the work. The introduction of additional fish species which will utilize unexploited ecological niches has been suggested. The benthic invertebrates form one such niche. Two oligochaetes dominated the community, Branchiura sowerbyi Beddard and Limnodrilus hoffmeisteri Claparade. The former had a productivity of 7.4 g m^–2 ann^–1 (as dry weight), the latter 0.6. These figures are not particularly high and do not support the introduction of a new fish species on their own.     

Vertical and seasonal distributions of micro-organisms,zooplankton and phytoplankton in a eutrophic lake

The vertical distributions of bacteria, picoalgae,protozoan and metazoan zooplankton, and phytoplanktonin the highly eutrophic Lake Köyliönjärvi(SW Finland) were studied monthly during the period ofice-cover in January-April 1996. For comparison, wealso provide some data on the distributions of theplankton during the summer. The whole watercolumn remained oxic during the ice-covered period,although the near-bottom oxygen concentrations werealways very low. The heterotrophic nanoflagellateswere more abundant in winter than in summer, butciliates, picoalgae and bacteria were more numerous insummer. In general both zooplankton and phytoplanktonhad low biomass during the ice-covered period.However, the biomass of the diatom Aulacoseiraislandica ssp. islandica was high under the icein April. The calanoid copepod Eudiaptomusgraciloides was the dominant zooplankton species fromJanuary to March, but had almost disappeared by thebeginning of April and did not increase again until inJune. The dominant rotifer species in winterwere Keratella cochlearis, Filinia terminalis,and Filinia longiseta in the surface water andRotaria neptunia near the bottom.     

The edibility of Staurastrum chaetoceras and Cosmarium abbreviatum (desmidiaceae) for Daphnia galeata/hyalina and the role of desmids in the aquatic food web

A laboratory clone of Daphnia galeata/hyalina was fed with two different planktic desmid taxa: Staurastrum chaetoceras and Cosmarium abbreviatum var. planctonicum, being about equal in cell size. Whereas Staurastrum chaetoceras was readily ingested and assimilated to a high degree, Cosmarium was hardly incorporated. This could be partly due to the presence of an extracellular mucilaginous envelope in the latter species. When decapsulated by mild sonification, Cosmarium cells were significantly better ingested but digestion was still inferior to that of Staurastrum, presumably because of the more compact cell shape of Cosmarium.From literature it appears that small-sized planktic desmid species occasionally may constitute a main food source for zooplankton, especially in eutrophic lakes. Most likely however, desmids, particularly large-sized species, play a much more important role in the food chain in the benthic compartment of shallow, oligotrophic water bodies where they serve as a food source for various macro-invertebrate taxa.     

Production of microcystins in calcareous Mediterranean streams: The Alharabe River,Segura River basin in south-east Spain

The development of epilithic cyanobacteria communities in a Mediterranean calcareous stream in the province of Murcia (SE Spain) was studied during the course of one year in an attempt to clarify the environmental variables that influence the production of microcystins. The predominant cyanobacteria were species of Rivularia, which formed conspicuous colonies throughout the year. Seasonally, other species were abundant: Schizothrix fasciculata, Tolypothrix distorta and Phormidium splendidum. All the species collected produced microcystins to a varying degree (up to five varieties), while the benthic community as a whole produced concentrations as high as 20.45 mg m⁻². At the same time, the presence of microcystins dissolved in water was confirmed. Among environmental variables, air temperature and silicate content were positively and strongly correlated with total microcystins, while nitrite, nitrate, orthophosphate, calcium and flow were negatively correlated with them. Dissolved microcystins were negatively correlated with microcystin LR, P.A.R. and total phosphorus and positively with rainfall. The production of microcystin YR seems to be regulated by different factors from those regulating the other main varieties (microcystin LR and microcystin RR). The data obtained indicate that all the tested benthic cyanobacteria produced microcystins in this shallow calcareous stream, which may contribute to their predominance in the prevailing conditions. The accumulation of microcystins in mucilaginous colonies of other groups of algae poses new questions concerning the possible ecological function of these compounds and needs further study.     

The predatory Chironomidae of an iron-rich stream: feeding ecology and food web structure

Abstract. 1. Three species of Tanypodinae (Chironomidae) were found in an acid and iron-rich stream in southern England. Maximum abundance was achieved in summer and they were sparse at other times. Individuals were aggregated on the stream bed and were overrepresented in accumulations of leaf litter.
2. The diets of all three species consisted of a mixture of prey (prominently detritivorous chironomid larvae) and detritus. More detritus and fewer prey were taken in winter than in summer.
3. When comparing large tanypod species with small and, intraspecifically, late instars with early, the proportion of guts containing prey increased with increasing body size.
4. Stonefly larvae were more prominent in the diet of Zavrelimyia barbatipes (Kieffer) in summer than in winter but for the other two species the reverse was true. A bigger proportion of Trissopelopia longimana (Staeger) guts contained prey in early summer than in August whereas more Macropelopia goetghebueri (Kieffer) guts contained prey in August. This was apparently a consequence of seasonal differences in the distribution of body size among the populations of these two species.
5. The stream contains two further common predators, Plectrocnemia conspersa (Curtis) and Sialis fuliginosa Pict. These are important predators of tanypod larvae but might also compete with them since they severely deplete populations of prey taken in common.
6. Analysis of the food-web in Broadstone Stream reveals remarkably high values of connectance (C and C_max) and of species richness times connectance (SC_max). Such characteristics are theoretically associated with fragile and dynamically unstable food webs, and may be found in 'constant' environments. There is also an apparently unusual prevalence of omnivory in the community.     

The ecology of the River Endrick: present status and changes since 1960

Long-term changes in water quality, sources of pollution, and the fish and benthic macroinvertebrate communities in the River Endrick catchment are described. The conservation status and management of the river are discussed and the need for research on nutrient loadings and the impact of recent fish introductions is emphasized.     

Zooplankton biomass and community structure in a Danube River floodplain system: effects of hydrology

1. Zooplankton density and biomass was examined in a Danube River floodplain section with highly variable hydrological dynamics. Temporal patterns were analysed to assess the effects of hydrological conditions on zooplankton community structure and the differential response of the two major zooplankton taxa, rotifers and crustaceans.
2. Calculated floodplain water age was used as an integrated parameter describing hydrological conditions and connectivity.
3. Total zooplankton biomass, crustacean biomass and crustacean species number were significantly positively related to water age. Rotifer biomass followed a hump-shaped relationship with water age, and rotifer species number decreased with increasing water age.
4. Rotifers dominated the community in periods of low to medium water ages. In periods of higher water ages the community was dominated by crustaceans.
5. We propose that the hydrological regime of floodplains is crucial for zooplankton biomass patterns and succession, through the alternation of washing-out effects, taxon-specific potential of reproduction and biological interactions. Flood events and high water levels reset the community to an early successional phase.     

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.     

Influence on phytoplankton biomass in lakes of different trophy by phosphorus in lake water and its regeneration by zooplankton

In 49 unpolluted lakes of north-eastern Poland the biomass of algae in summer is significantly related to the concentration of total phosphorus and to the rate of phosphorus regeneration by zooplankton. Using a model with equations describing these relationships, the biomass of blue-green algae and other phytoplankton groups was predicted for 14 polluted lakes. A good approximation of actual values was obtained only for the biomass of blue-green algae calculated from the estimated rate of P regeneration by zooplankton in these lakes. It is hypothesized that more-or-less edible algae of other classes did not show dependence on the rate of input of regenerated P because their biomass was heavily reduced by grazing of zooplankton.     

Physical variables driving epiphytic algal biomass in a dense macrophyte bed of the St. Lawrence River (Quebec, Canada)

The variables affecting epiphyton biomass were examined in a sheltered, multispecies macrophyte bed in the St. Lawrence River. Alteration of light penetration, resulting from the presence of dense macrophytes forming a thick subsurface canopy, primarily determined epiphyton biomass. Seasonal decrease of water levels also coincided with major increases in biomass. Plant morphology was the next important variable influencing epiphytic biomass, whereas the contribution of other variables (sampling depth, macrophyte species, relative abundance of macrophytes, and temperature) was low. Groups of lowest epiphyte biomass (0.1–0.6 mg Chla g^–1 DW) were defined by the combination of a low percentage of incident light (<13% surface light) and simple macrophyte stem types found below the macrophyte canopy. Highest epiphyte biomass (0.7–1.8 mg Chla g^–1 DW) corresponded to samples collected in mid-July and August, under high irradiance (>20% surface light) and supported by ramified stems. Our results suggest that epiphyton sampling should be stratified according to the fraction of surface light intensity, macrophyte architecture, and seasonal water level variations, in decreasing order of influence.     

Factors affecting the structure of epiphytic gastropod communities in the St. Lawrence River (Quebec,Canada)

A study of epiphytic gastropods associated with two submerged macrophytes (Myriophyllum spicatum and Vallisneria americana) was conducted at a soft and at a hard water site in the St. Lawrence River during two non-consecutive years in order to compare effects of macrophyte species and biomass, site and year in gastropod community structure. There was no effect of macrophyte species on total gastropod abundance nor on gastropod diversity, and few gastropod species showed a marked preference for either macrophyte species. Inter-site and inter-annual variations in diversity, total gastropod abundance and gastropod community structure were greater than variations among macrophytes. However, analysis of variations of epiphytic gastropod communities in one of the two sites shows that abiotic factors are important in explaining epiphytic gastropod distribution. Our results and results of other studies on gastropod population dynamics in the St. Lawrence River indicate that intra- and interspecific competition between gastropods is important on macrophytes and that they must be carefully considered in order to understand epiphytic community structure and population dynamics.     

Biota of the St. Marys River: habitat evaluation and environmental assessment

The St. Marys River provides vital habitat for many species of plants, invertebrates, fish, and birds. It is also subject to many at times conflicting uses, including recreational boating, sport and subsistence fishing, municipal and industrial withdrawals and inputs, as well as commercial ship traffic and hydroelectric power generation.In 1984, the United States and Canada jointly initiated the Upper Great Lakes Connecting Channels Study to identify and quantify the impacts of contaminants on these channels and their biota and to develop recommendations for more effective pollution control and surveillance programs. Results of the study in the St. Marys River showed that water entering the river from Lake Superior is of excellent quality. Industrial and municipal discharges in the Sault Ste. Marie, Ontario area have resulted in heavily contaminated sediments and a severely impaired benthic invertebrate community in this area and downstream; however, no major impacts on fish have been demonstrated. Nevertheless, major impacts of man on fish spawning and rearing habitats and on benthic and fish productivity have resulted from the alteration of channels, construction of navigation locks, and regulation of flow in the St. Marys Rapids. Increases in the productivity of such raptors as osprey during the 1980s suggest a reduction in organochlorine contaminants levels in their diet; however, the increasing numbers of gulls with low concentrations of PCBs, p,p-DDE and other organochlorines in their eggs may adversely affect gull young or their predators.