Showing papers in "Aquatic Ecology in 2003"

The structuring role of free-floating versus submerged plants in a subtropical shallow lake

Abstract: In shallow temperate lakes many ecological processes depend on submerged macrophytes. In subtropical and tropical lakes, free-floating macrophytes may be equally or more important. We tested the hypothesis that different macrophyte growth forms would be linked with different bottom-up and top-down mechanisms in out-competing phytoplankton. We compared experimentally the effects of submerged and free-floating plants on water chemistry, phytoplankton biomass, zooplankton and fish community structure in a shallow hypertrophic lake (Lake Rodo, 34°55′S 56°10′W, Uruguay). Except for the retention of suspended solids, we found no other significant bottom-up process connected with either Eichhornia crassipes or Potamogeton pectinatus. Free-floating plants had a lower abundance of medium-sized zooplankton than any other microhabitat and submerged plants were apparently preferred by microcrustaceans. Fish showed a differential habitat use according to species, size-class and feeding habits. Dominant omnivore-planktivores, particularly the smallest size classes, preferred submerged plants. In contrast, omnivore-piscivores were significantly associated with free-floating plants. The density of omnivorous-planktivorous fish, by size class, significantly explained the distribution of medium-sized zooplankton, the high number of size 0 fish being the main factor. The abiotic environment and the structure of the zooplankton community explained little of the fish distribution pattern. Our results suggest that bottom-up effects of free-floating plants are weak when cover is low or intermediate. Top-down effects are complex, as effects on zooplankton and fish communities seem contradictory. The low piscivores:planktivores ratio in all microhabitats suggests, however, that cascading effects on phytoplankton through free-floating plant impacts on piscivorous fish are unlikely to be strong.

Exotic and indigenous freshwater gammarid species: physiological tolerance to water temperature in relation to ionic content of the water

Abstract: Bioinvasions by closely related species often lead to niche competition between exotic and indigenous species. The outcome of this competition is partly determined by differences in physiological tolerance of the competing species to the environmental conditions of the colonised habitat. Physiological tolerance of the invading gammarid species Gammarus tigrinus, Echinogammarus ischnus and Dikerogammarus villosus and the indigenous gammarid species Gammarus pulex, Gammarus roeseli and Gammarus fossarum from Dutch waters was studied in the laboratory by comparing their pleopod beats at rest at different water temperatures, which reflect the gammarid’s oxygen consumption. Pleopod beat frequencies increased from a minimum ventilatory activity of 0 beats per minute at 1 °C to maximum activity of up to 300 beats per minute at temperatures between 25 °C and 35 °C. At the state of maximum activity, a further increase in temperature was followed by a strong decrease in pleopod beat frequency, indicating acute stress, and subsequently mortality. Frequency response patterns of invading and indigenous gammarids were found to be highly similar, indicating a wide tolerance to temperature for all species. The tolerance of D. villosus, however, was reduced in brook water, indicating a lower competitive ability in relatively ion-poor water. G. tigrinus survived at higher temperatures in the more ion-rich, polluted waters than the indigenous gammarids, indicating a wider physiological tolerance and thus a higher competitive ability in these waters.

Extracellular enzyme-clay mineral complexes: Enzyme adsorption, alteration of enzyme activity, and protection from photodegradation

Abstract: Enzymes released extracellularly by micro-organisms have major functions in nutrient acquisition and organic matter degradation. Clay particles, common in many surface waters, can modify enzyme activity. Clay minerals are known to form aggregates with organic molecules, and the formation of enzyme-clay complexes could alter the level of activity. Montmorillonite clay and clay extracted from Elledge Lake (Tuscaloosa, Alabama) basin soil were combined with alkaline phosphatase, glucosidase, protease, and xylosidase solutions to assess adsorption and the effect of this adsorption on enzyme activity. Adsorption to Elledge Lake basin clay decreased alkaline phosphatase activity, and adsorption to montmorillonite was observed for all four enzymes with reductions in enzyme activities. Adsorption of substrate onto clay surfaces resulted in a concentration effect and increased enzyme activity associated with the particles. When enzyme-clay complexes were exposed to natural sunlight there was a decrease in enzyme activity, but this decrease was usually not significantly different from the adsorption only treatment. The formation of enzyme-clay complexes may serve to protect the enzymes from natural in situ photodegradation. The results indicate the complex interactive effects adsorption of enzymes to clay particles can have on the availability and capability of hydrolysis – reduction of enzyme reactivity, storage attached to clay particles with changes in transport and distribution, and protection from photodegradation.

Benthos as the basis for arctic lake food webs

Abstract: Plankton have traditionally been viewed as the basis for limnetic food webs, with zooplankton acting as a gateway for energy passing between phytoplanktonic primary producers and fish. Often, benthic production has been considered to be important primarily in shallow systems or as a subsidy to planktonic food web pathways. Stable isotope food web analyses of two arctic lakes (NE14 and I minus) in the Toolik Lake region of Alaska indicate that benthos are the primary source of carbon for adults of all species of benthic and pelagic fish present. We found no effect of turbidity, which may suppress benthic algae by shading, on food web structure. Even though Secchi transparency varied from 10.2 m in NE14 to 0.55–2.6 m in I minus, food webs in both lakes were based upon benthos, had four trophic levels, and culminated with omnivorous lake trout. We suggest that the importance of benthos in the food webs of these lakes is due to their extreme oligotrophy, resulting in planktonic resources that are insufficient for the support of planktivorous consumers.

Effects of filamentous algae and deposited matter on the survival of Fucus vesiculosus L. germlings in the Baltic Sea

Abstract: As a result of increased nutrient levels in the Baltic Sea during thepast 50 years, mass developments of filamentous algae have become a commonfeature along the Swedish east coast and deposition of organic matter has alsoincreased. To test whether these two factors have any effects on the early lifestages of Fucus vesiculosus a number of laboratory andfield studies were conducted. The amount of epilithic and epiphytic filamentousalgae on F. vesiculosus and the amount of deposited matterin the littoral zone were quantified during the two reproductive periods ofF. vesiculosus, early summer (May–June) and lateautumn (September–October). Both filamentous algae (Cladophoraglomerata) and deposited matter (introduced either before or aftersettlement of fertilized eggs) were shown to significantly decrease the numberof surviving germlings. The survival of germlings seeded on stones withfilamentous algae, or seeded on culture dishes concurrently with the lowestconcentration of deposited matter (0.1 g dm−2),was 5% or less. In the field, the amount of filamentous algae was significantlyhigher during F. vesiculosus summer reproduction, whereasthe amount of deposited matter collected in traps was significantly higherduring the period of autumn reproduction. The greatest biomass of filamentousalgae was observed at sheltered sites. Based on the negative effects offilamentous algae and deposited matter on Fucusrecruitmentand the observation of local and seasonal differences in abundance offilamentous algae and deposition, we suggest that the prerequisites for thesurvival of either summer or autumn-reproducing populations of F.vesiculosus in the Baltic Sea may differ locally.

Effects of unicellular and colonial forms of toxic Microcystis aeruginosa from laboratory cultures and natural populations on tropical cladocerans

Abstract: Three life-table experiments, two growth experiments and one feedinginhibition experiment, were performed to study the effects of the toxiccyanobacterium Microcystis aeruginosa on the cladoceransofa tropical lagoon (Jacarepagua Lagoon, Rio de Janeiro, Brazil).Differentexperimental designs were used to estimate toxic effects of both field samplesand laboratory cultures of Microcystis aeruginosa oncladoceran life history parameters and juvenile growth rates. Effects ofnutritional deficiency could be distinguished from toxic effects in experimentswhere green algae in high carbon concentration were mixed withMicrocystis. Our results show that natural assemblages ofMicrocystis caused much less pronounced toxic effects thanlaboratory cultures and that unicellular forms were more toxic than colonialforms, even though both contained high concentrations of toxins. One possibleexplanation is that colonies were too large to be ingested by the smallMoina micrura and Ceriodaphniacornuta. Feeding inhibition by single cells and small colonies seemsto be another mechanism that contributes to the harmful effects ofMicrocystis on cladocerans, both in the laboratory and inthe field. Thus, caution is needed in extrapolating results from the laboratoryto the field. We did find, however, that toxic algae in natural seston caninhibit growth and reproduction of native cladocerans populations.

Stable carbon and nitrogen signatures of decomposing tropical macrophytes

Abstract: The Pantanal of Mato Grosso, Brazil, is a large, seasonal wetland, which exhibits high macrophyte productivity at the beginning of the rainy season, when the floodplain becomes flooded. During inundation, from December through May, there is rapid turnover of decomposing macrophyte litter, which is subsequently colonized and consumed by various organisms. In this paper, the variation in the carbon and nitrogen isotope signatures of decomposing macrophytes and detritus was determined to provide an isotopic baseline for the elucidation of higher trophic levels. Seven abundant macrophyte species, Cyperaceae sp., Pontederia lanceolata, Cabomba furcata, Salvinia auriculata, Eichhornia crassipes, Nymphaea amazonum and Paspalum repens, were exposed in mesocosm decomposition experiments lasting 21 or 100 days. Stable isotope ratios of carbon and nitrogen and the atomic C/N ratios were determined for decomposing plant material, particulate organic matter (POM), the microbial film, and aquatic invertebrate larvae. The δ13C values for the macrophytes did not change during decomposition. However, the variability of δ15N was high (range of ± 6 ‰) due to microbial activity. There was no consistent difference in the isotopic signatures of macrophytes and POM. C/N ratios decreased from 17 to 50 in macrophytes, to 7 to 12 in POM. The isotopic signatures and C/N ratios of the microbial film were the same as those of POM. We concluded that heterotrophic processes did not fractionate stable carbon isotopes but caused an increase in the variability of stable nitrogen ratios and a change in the C/N ratios in our experimental system. Therefore, it was not possible to distinguish fresh and senescent material or even POM when used as a food source. The δ13C values of the aquatic larvae were closely coupled to those of the carbon source provided.

Spectral gradients of downwelling light in a fluvial lake (Lake Saint-Pierre, St-Lawrence River)

Abstract: Large fluvial lakes are understudied with respect to their underwaterlight climates. Fluvial lakes pose unique challenges for photobiologistsinterested in the interactions amongst light climate, nutrients and microbialcommunity structure and biodiversity. This is because fluvial lakes are typifiedby highly dynamic flow regimes often incorporating different inflows anddischarges each characterized by their own unique physico-chemical composition.These compositional characteristics include the concentrations of chromophoricdissolved organic matter (CDOM), suspended solids, and pigments such aschlorophyll. Together these factors contribute to the distribution andcomposition of the water masses that make up fluvial lakes. These water masses,in turn, flow over lakebeds that are typically complex in their morphometry andfeature extensive macrophyte beds, further enhancing the habitat heterogeneityof these ecosystems. We here report on the spectral attenuation of ultravioletradiation (UVR = 280–400 nm) and photosyntheticallyactive radiation (PAR = 400–700 nm) in the three mainwater masses of Lake Saint-Pierre and evaluate the relative contribution ofCDOM, and particulate organic material to UVR attenuation. We demonstrate thatUVR penetrates 18 to 30% of the water column (1% penetration depth) in the LakeSaint-Pierre ecosystem, and show how the underwater spectral UVR varies withinthe three water masses.

Growth and potential photosynthesis of cyanobacteria are stimulated by viable gut passage in crucian carp

Abstract: Growth and potential photosynthetic activity of phytoplankton passed through intestine of crucian carp (Carassius auratus) from a small Siberian reservoir were compared with those of phytoplankton taken the directly from the reservoir. The dominant phytoplankton species in the reservoir, Microcystis aeruginosa, showed a significant increase of growth after the passage. Subdominant Planktothrix agardhii also showed an increase in growth rate, while subdominants Anabaena flos-aquae and Aphanizomenon flos-aquae were not stimulated by the gut passage.

Microphytobenthos activity and fluxes at the sediment-water interface: interactions and spatial variability

Abstract: In this study oxygen and nutrient fluxes and denitrification rates across the sediment-water interface were measured via intact core incubations with a twofold aim: show whether microphytobenthos activity affects these processes and analyse the dispersion of replicate measurements. Eighteen intact sediment cores (i.d. 8 cm) were randomly sampled from a shallow microtidal brackish pond at Tjarno, on the west coast of Sweden, and were incubated in light and in darkness simulating in situ conditions. During incubation O2, inorganic N and SiO2 fluxes and denitrification rates (isotope pairing) were measured. Assuming mean values of 18 cores as best estimate of true average (BEA), the accuracy of O2, NH4 +, NO3 - and SiO2 fluxes calculated for an increasing number of subsamples was tested. At the investigated site, microalgae strongly influenced benthic O2, inorganic N and SiO2 fluxes and coupled (Dn) and uncoupled (Dw) denitrification through their photosynthetic activity. In the shift between dark and light conditions NH4 + and SiO2 effluxes (60 and 110 µmol m-2h-1) and Dn (5 µmol m-2 h-1) dropped to zero, NO3 - uptake (70 µmol m-2 h-1) showed a 30% increase, while Dw (20 µmol m-2 h-1) showed an 80% decrease. For O2 and NO3 - dark fluxes, 4 core replicates were sufficient to obtain averages within 5-10% of the best estimated mean, while 10-20% accuracy was obtained with 4-12 replicates for SiO2 and >10 replicates for NH4 + dark fluxes. Mean accuracy was considerably lower for all light incubations, probably due to the patchy distribution of the benthic microalgal community.

Sensitivity of US Gulf of Mexico coastal marsh vegetation to crude oil: Comparison of greenhouse and field responses

Abstract: Greenhouse and field studies were conducted to evaluate the effect of crude oil on selected US Gulf of Mexico coastal marsh species. Species showed different levels of sensitivity to oiling between greenhouse and field conditions. In greenhouse studies, two crude oils were used: South Louisiana crude oil (SLC) and Arabian Medium crude oil (AMC). The majority of Spartina patens plants died within one month following oiling with little or no recovery after three months. Panicum hemitomon and Spartina alterniflora were also adversely affected by oiling under greenhouse conditions but to a lesser extent than S. patens. The SLC or AMC oiling led to biomass reductions in S. alterniflora and S. patens. The dry biomass was not affected by oiling in P. hemitomon, Sagittaria lancifolia, Typha latifolia, and Scirpus olneyi. Results showed that S. patens plants were more sensitive to SLC as compared to AMC oil. Gross CO2-C fixation data collected in the greenhouse indicated no differences in recovery among species across oiling treatments for S. lancifolia, S. olneyi, and T. latifolia. Field studies with S. alterniflora, S. patens and S. lancifolia demonstrated initial sensitivity of these species to oiling, and recovery following oiling with SLC. Our data also showed that caution must be employed whenever results from greenhouse studies are extrapolated to predict oil impact on vegetation under field conditions. Development of any sensitivity index of plant responses to oiling should not be based on greenhouse experiments only. Field evaluations should be included which best depict plant responses to oiling. Thus, restoration measures of US Gulf of Mexico coastal marshes following oiling should rely primarily on field studies. The field research suggests that the US Gulf of Mexico coastal marsh vegetation are likely to recover from oil spills naturally without the need for remediation procedures.

Long-term development in a Baltic fish community exposed to bleached pulp mill effluent

Abstract: Observations near a Swedish pulp mill at the Bothnian Sea during 1982-1992 revealed low fish abundance and a disturbed community structure. The presence of endocrine disrupting or toxic substances in effluent water was indicated by biomarker responses, impaired reproduction and stimulated growth in perch (Perca fluviatilis). In 1992 the bleaching process was altered and a secondary treatment system was installed, improving the quality and decreasing the amount of effluent. This paper presents the results of follow-up studies during 1995-1998 with the objective to analyse any recovery of the fish community due to changes in mill operations. Although abundance and diversity of the fish community did recover, an effect of enrichment, indicated by high catches of cyprinids, was still visible in the vicinity of the mill outlet. Sexual maturation was delayed, gonad development was retarded, average growth rate was faster, and the condition factor was higher in perch caught in the vicinity of the effluent outlet, than in perch captured elsewhere. It is concluded that substances inhibiting reproduction and causing growth disturbance were still present in the effluents in effective concentrations.

Zooplankton, phytoplankton and the microbial food web in two turbid and two clearwater shallow lakes in Belgium

Abstract: Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium ('Blankaart' and 'De Maten') were monitored during the course of 1998-1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass - an indicator of zooplankton grazing pressure on phytoplankton was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.

Spatial patterns and population dynamics of plant-associated microcrustacea (Cladocera) in an English shallow lake (Little Mere, Cheshire)

Abstract: Little Mere (Cheshire) is a small (2.7 ha) and shallow (average depth 0.7 m) fertile lake in Cheshire, UK. Nymphaeids cover almost 40 % of its entire surface during the growing season (April to October) and practically all the rest is covered by a mixed community of submerged plants. The lake was intensively sampled for plant-associated Cladocera and zooplankters from April 1998-April 2000. Samples were collected at five sites across the lake, three of them located within lily beds, the other two over submerged plant beds of mixed composition. Specific sampling techniques were developed for floating lily leaves, petioles, submerged plants and water. Significant horizontal differences were identified for most cladoceran species, both open-water and plant-associated, for chydorid periphyton scrapers and for filter-feeders. Daphnia hyalina (L.) and Ceriodaphnia sp were significantly more abundant in lily beds than in more open water in both growing seasons, suggesting lily beds are an effective refuge against fish predation. Size-structure and egg-ratio data support this contention. Egg-ratio models were examined for Daphnia hyalina and Simocephalus vetulus (O.F. Muller), a plant-associated cladoceran. The fertility of S. vetulus in lily beds was generally high throughout growing seasons. The construction of egg-ratio models for this species was hampered by their generally very patchy distributions.

Physical-Biological Coupling in Southern Lake Michigan: Influence of Episodic Sediment Resuspension on Phytoplankton

Abstract: The influence of episodic, sediment resuspension on phytoplankton abundance/volume and composition, the photosynthetic maximum rate P B and efficiency B , and chlorophyll-specific growth Chl was evaluated during the spring isothermal period in southern Lake Michigan Laurentian Great Lakes, USA. Resuspension altered the nutrient and light climate of nearshore waters; light attenuation Kd and phosphorus concentrations corresponded p 0.0001 and p 0.001, respectively with concentrations of suspended particulate matter SPM. Phytoplankton cell volume and diatom cell abundance and volume were not associated with SPM concentrations p 0.05. Diatom composition displayed spatial dissimilarities corresponding with resuspension p 0.001; small centric diatoms exhibiting meroplanktonic life histories and pennate diatoms considered benthic in origin were most abundant within SPM-impacted, nearshore waters whereas taxa typically comprising assemblages in optically-clear, offshore waters and the basin-wide, spring bloom were not. Values of P B and B corresponded p 0.0001 with both Kd coefficients and SPM concentrations, potentially reflecting increased light harvesting/utilization within impacted assemblages. However, integral production was inversely associated with Kd coefficients and SPM concentrations p 0.0001 and photosynthesis was light-limited or nearly so for most assemblages. Although Chl values corresponded with Kd coefficients p 0.05, values were quite low x S.E., 0.10 0.004 d ‐1 throughout the study. Most likely, distinct rate processes between SPM- and nonimpacted assemblages reflected short-term compositional and corresponding physiological variations due to infusion of meroplankton and/or tributary-derived phytoplankton. Overall, resuspension appears to have little, if any, long-term impact upon the structure and function of the lake’s phytoplankton.

Response of coastal phytoplankton to ammonium and nitrate pulses: seasonal variations of nitrogen uptake and regeneration

Abstract: Seasonal variation in uptake and regeneration of ammonium and nitrate in a coastal lagoon was studied using 15N incorporation in particulate matter and by measuring changes in particulate nitrogen. Uptake and regeneration rates were two orders of magnitude lower in winter than in summer. Summer uptake values were 2.8 and 2.2 μmol N.l−1.d−1 for ammonium and nitrate, respectively. Regeneration rates were 2.9 and 2.1 μmol N.l−1.d−1 for ammonium and nitrate respectively. Regeneration/uptake ratios were often below one, indicating that water column processes were not sufficient to satisfy the phytoplankton nitrogen demand. This implies a role of other sources of nitrogen, such as macrofauna (oysters and epibionts) and sediment. Phytoplankton was well adapted to the seasonal variations in resources, with mixotrophic dinoflagellates dominant in winter, and fast growing diatoms in summer. In winter and spring, ammonium was clearly preferred to nitrate as a nitrogen source, but nitrate was an important nitrogen source in summer because of high nitrification rates. Despite low nutrient levels, the high rates of nitrogen regeneration in summer as well as the simultaneous uptake of nitrate and ammonium allow high phytoplankton growth rates which in turn enable high oyster production.

Effects of algal diets and temperature on the growth and fatty acid content of the cichlid fish Oreochromis niloticus L. – A laboratory study

Abstract: Effects of algal diets and temperature on the growth and fatty acid content of the cichlid fish Oreochromis niloticus L. - A laboratory study.

A comparison of the fatty acid composition of Gammarus lacustris and its food sources from a freshwater reservoir, Bugach, and the saline Lake Shira in Siberia, Russia

Abstract: We studied fatty acid (FA) composition in samples from bodies and intestinal contents of the littoral amphipod Gammarus lacustris Sars, from the Bugach freshwater reservoir. Simultaneously, samples of seston and bottom sediments were also collected from the reservoir during early August. There were no differences in FA composition of gut contents, seston and sediments of pebbly bottom. Seston was the main food source of Gammarus but some FAs Gammarus got from sediments. The FA composition of G. lacustris and seston from the Bugach freshwater reservoir were compared with those of the animals from the saltwater Lake Shira (Siberia). While FA composition of the two Gammarus populations differed significantly, those of seston were practically similar: the composition of long-chain unsaturated fatty acids, 20:5ω3, 22:6ω3 and 20:4ω6, were significantly higher in animals from saline Shira Lake, whereas 16:1 and 16:0 were higher in the freshwater populations of amphipods from the Bugach freshwater reservoir. Taking into account the relevant literature data, we hypothesise that this difference in C16 acid might be a distinguishing characteristic of FA composition of freshwater and saltwater crustaceans.

Response of an estuarine ecosystem to reduced organic waste discharge

Abstract: Since the mid-19th century large amounts of organic wastewere sluiced out into the Ems Estuary on the border between Germany and TheNetherlands. This waste originated from the regional potato flour and cardboardindustries making the inland waterways completely anoxic. In the estuaryseriousoxygen depletion occurred, especially during autumn. Most of the organic wasteentered the estuary in the Dollard, a brackish embayment of the Ems Estuary. Anintensive sanitation scheme was started in the 1970s, leading to a stepwisereduction of the organic waste load on the estuary. In this paper, a review isgiven of the response of the benthos living at intertidal mudflats,viz. microphytobenthos (diatoms), meiofauna (nematodes)andmacrozoobenthos. The benthos response is described mainly on the basis of dataobtained under conditions of high (ca. 1980), intermediate (1987) and largelyreduced (1993) organic waste loading.Reduction of organic loading caused significant changes in abundance, speciescomposition and standing stock of diatoms and nematodes. Macrobenthicpopulations recovered from being severely reduced regularly during the autumnalwaste discharges towards a more stable situation. Altogether, the intertidalmudflat benthos changed from organic waste loading stress towards a normal,estuarine environmental stress. This was especially the case at high and muddyflats in the southeast of the Dollard. At mixed sand/mud flats in the centre ofthe Dollard, a response of macrozoobenthos may have been obscured by theinvasion of the polychaete Marenzelleria cf.wireni.

The nature value of the ditch vegetation in peat areas in relation to farm management

Abstract: Farmers can play an important role in enhancing the diversity of plant species in ditches, since they carry out most of the management Little is known about the measures farmers can take to enhance aquatic biodiversity in ditches Therefore, a field study was set up to determine how dredging, ditch cleaning, and nutrient supply in the adjacent fields affect the floristic values of the ditch vegetation in peat areas A total of 240 ditches on 84 dairy farms were selected, to establish the effect of ditch cleaning, dredging, and nutrient supply Generalised linear modelling was used to determine the most relevant factors, and to obtain quantitative relations between those factors and two indices of floristic value Ditch management, field management, and water management were found to affect the floristic values of the ditch vegetation Increasing the water depth proved to be the most cost-effective measure Further improvements can be obtained by choosing certain machines or specific management periods, or reducing the N supply The effect of the P supply is not yet sufficiently clear The water boards can enhance floristic values by raising the ditch water table and by lowering the Cl− concentration in the ditches

Seston food quality and Daphnia production efficiencies in an oligo-mesotrophic Subalpine Lake

Abstract: Because of major biochemical imbalances between plants and animals, ecological efficiency at this interface may have a major impact on overall energy flow in ecosystems. In order to study relationships between seston food quality and energy transfer between primary producers and herbivores, we conducted five microcosm experiments in Castle Lake, California, USA during the summer of 1996. We simultaneously performed life table experiments to determine the effects of highly unsaturated fatty acids (HUFA) on Daphnia rosea growth, reproduction and survival. The results of these experiments suggest strong energy limitation of D. rosea growth in Castle lake during the study. D. rosea production was coupled with primary production in Castle Lake and in the microcosm experiments. D. rosea production efficiencies, i.e., the ratios of D. rosea productivity to primary productivity, decreased towards the end of the summer. A food quality index based on phytoplankton species composition and seston carbon to phosphorus (C:P) ratio were good predictors of D. rosea production efficiencies. The predicted D. rosea production pattern based on phytoplankton composition and primary productivity matched the zooplankton biomass dynamics in Castle Lake during 1991. Life table experiments showed HUFA effects on D. rosea population growth rates, reproduction and survival in support of the HUFA limitation hypothesis.

The North-American amphipods, Melita nitida Smith, 1873 and Incisocalliope aestuarius (Watling and Maurer, 1973) (Crustacea: Amphipoda: Gammaridea), introduced to the Western Scheldt estuary (The Netherlands)

Abstract: The American amphipod species Melita nitida andIncisocalliope aestuarius have been found in the WesternScheldt estuary (the Netherlands). This is the first record of these species inthe north-east Atlantic. Shipping is the most likely vector of introduction.Thedistribution of both species is investigated and compared with the distributionand the microhabitat of co-occurring amphipod species. Melitanitida is known from both the east and west coast of North Americaand I. aestuarius originates from the east coast of NorthAmerica. Until now neither has been reported from other parts of the world. Inthe Netherlands both species are restricted to the mesohaline part of theWestern Scheldt. Melita nitida occurs predominantly underPacific oysters at the underside of boulders, mainly sublittorally.Incisocalliope aestuarius is associated to hydrozoans.Bothmicrohabitats are hardly utilized by other amphipod species. Therefore, thetheory that the existence of many empty niches in north-western Europeanbrackish waters make this environment particularly susceptible to invasions ofalien species is corroborated. The application of hard substrates in a regionoriginally predominated by soft bottoms moreover facilitates the introductionofexotic species. The species community on hard substrates in the mesohaline partof the Western Scheldt contains a high proportion of introduced species:approximately one third of the macrofauna species is of allochthonous origin.

Maternal effects, conspecific chemical cues, and switching from parthenogenesis to gametogenesis in the cladoceran Moina macrocopa

Abstract: The change in reproductive mode from parthenogenesis to gametogenesis in Cladocera is controlled by multiple environmental cues. Maternal effects are involved in the control of reproductive switching. In this study, we estimated the readiness of Moina macrocopa females to change reproduction mode under the effect of conspecific chemicals on maternal and offspring generations. The results demonstrated that information about the chemical environment was not transmitted between generations (none of the females produced diapausing eggs in the control medium irrespective of their mothers' environment). Differences in maternal energetic investments were not significant, hence the maternal environment did not affect the fecundity. However, tested animals adapted to the effect of the diapause inducing factor. When offspring of mothers cultured in crowded water were also cultured in crowded water (the constant effect of conspecific chemicals), they switched less readily to gametogenesis than offsprings of mothers cultured in fresh medium.

Seasonal changes in the abundance of autotrophic picoplankton and some environmental factors in hypereutrophic Furuike Pond

Abstract: Seasonal change in the abundance of autotrophic picoplankton (APP) was investigated once or twice a week in relation to some environmental variables in a hypereutrophic pond, from July 1999 to June 2000 Cell density of APP ranged between 03×105 and 101×105 cells ml−1, overlapping the lower range of APP abundances given in the literature for hypereutrophic systems The pattern of seasonal change in concentration of dissolved inorganic phosphorus (03–203 μ mol P l−1) was similar to that of cell density of APP, suggesting that phosphorus limitation on APP abundance By contrast, nitrogen limitation seemed unlikely since the pattern of seasonal change in concentration of dissolved inorganic nitrogen was different from that of APP cell density We could not find any coupled oscillations between APP abundance and heterotrophic nanoflagellates, or between that of APP and ciliates The dominant ciliate taxa, based on their cell densities, were Cinetochilum margaritaceum, Cyclidium glaucoma, Halteria grandinella, Strobilidium sp and Urotricha spp The relative contribution of the < 2 μm fraction to total chlorophyll concentration was seasonally high (up to 162%), indicating seasonal importance of APP abundance as food for heterotrophs

Fluxes of bacterioplankton between a tidal estuary and the sea: returning to the "Outwelling Hypothesis"

Abstract: The tidal dynamics of bacterioplankton communities at the outer part ofa shallow estuary (Ria de Aveiro, Portugal) were studied during 6 tidal cyclesat a fixed sampling site. Bacterial numbers (0.2–8.1 ×109 cells l−1), aminopeptidase activity(189–1662 nmol l−1 h−1),β-glucosidase activity (1.7–67.0 nmoll−1 h−1) and potential glucoseincorporation (0.48–3.99 nmoll−1 h−1) followed a consistent patternof increase during ebb and decrease during flood.Fluxes of bacterioplankton populations and associated heterotrophic activitiesbetween the estuary and the coastal area during a tidal cycle were estimatedfrom the water flux as derived from a two-dimensional vertically-integratednumerical model. The net fluxes estimated for a tidal cycle ranged from−26.0 to −2.5 to × 1016 bacterial cells. The nettidal fluxes of potential heterotrophic activities ranged from −10 to−80 mol h−1 for aminopeptidase, −0.33to −1.10 mol h−1 for β-glucosidase and−0.18 to +0.03 mol h−1 for glucoseincorporation. Net fluxes were generally negative in sign indicating thetransfer of phyto- and bacterioplankton, as well as potential capacities for thedegradation and recycling of organic matter, from the outer estuarinecompartment to the sea.

Population growth of herbivorous rotifers and their predator ( Asplanchna ) on urban wastewaters

Abstract: We studied the population growth of three Brachionus species ( B calyciflorus , B patulus and B rubens ) using domestic (Mexico City) wastewater at different phases of treatment (treated: type C, partially treated: type B and crude: type A) We also evaluated growth characteristics of a predatory rotifer ( Asplanchna sieboldi ) fed B rubens raised on wastewater and compared the growth rates of animals fed cultured algae All the three tested brachionid species grew well in controls (ie fed on the alga Chlorella ) However, populations of B calyciflorus and B patulus did not grow on the fully treated wastewater (type C) All three brachionid species grew poorly on par tially treated wastewater (type B) In crude wastewater, B rubens reached densities as high as 200 ind ml –1 Regardless of Brachionus species, the rates of population increase per day (r) varied between 029 and 04 in the controls B rubens in crude wastewater experienced steep mortality in the first two days but stabilized thereafter It had generation times, which varied from 3 to 5 days depending on the treatment Prey ( B rubens ) raised on wastewater supported better population growth of A sieboldi , the highest being from crude wastes The growth rate of A sieboldi fed B rubens , raised on crude wastewater, was the highest (∽ 08 d –1 ) Thus, the present study showed that culturing of certain species of brachionid rotifers on crude wastewater or partially treated is feasible without addition of alga

Swimming behaviour and dispersal patterns of headstarted loggerhead turtles Caretta caretta

Abstract: Swimming behaviour and dispersal patterns were studied in headstarted loggerhead turtles Caretta caretta which were released at three different sites on the Caribbean island of Curacao (Netherlands Antilles) and at one site on the neighbouring island of Klein Curacao, after 1–25 yrs of captivity Turtles were tagged and followed up to a distance of 6125 m offshore, using a boat with a Global Positioning Unit The released turtles reverted to typical hatchling behaviour and showed an offshore migration almost perpendicular to the coastline No significant differences were found in directional swimming among the four sites The turtles swam almost continuously about 30 cm under the water surface; their mean overall swimming speed was higher than in adult wild loggerheads suggesting a 'frenzy'-like swimming stage The turtles exhibited diving behaviour, and the dive frequency and duration was comparable to that of similar-sized (wild) turtles The present study demonstrates that upon release the headstarted loggerheads behave naturally and show dispersal patterns similar to wild hatchling turtles The fact that the released turtles were still able to show offshore directional swimming suggests that the headstarting did not affect their short-term orientation abilities

Microbial sulfate reduction in a brackish meromictic steppe lake

Abstract: Patterns of sulfate reduction were studied in water and sediments of Lake Shira, South Siberia, Russia. The lake was characterized by a high level of sulfate (91-116 mM). The concentration of hydrogen sulfide in the anoxic waters of the lake reached 0.6 mM. In summer the sulfate reduction rate in the water column, measured by radiometric technique, varied from 0.25 to 9.81 μmol sulfate l-1 d-1. There were two peaks of sulfate reduction activity: just below the chemocline and near the sediment surface. Sulfate reduction rate in the profundal silts ranged from 4.1 to 90.6 μmol l-1 d-1. The zone of the most active sulfate reduction was restricted to the surface sediment layers. The acceleration of sulfate reduction rate (up to 236 μmol l-1 d-1) and the increase of density of viable sulfate reducers (up to 2 x 105 cells ml-1) were recorded in the littoral sediments adjacent to the mouth of the Son River and sewage discharge. It was apparently caused by the input of allochthonous organic substrates and also by a high environmental temperature. On an areal basis, sulfate reduction rate in the water was approximately 8 times higher than that in the profundal sediments. Sulfate reduction was the most important process of anaerobic oxidation of organic carbon in Lake Shira. In summer in the profundal zone of the lake, sulfate reducers were able to mineralize about 67% of the daily integrated primary production of phototrophic and chemotrophic organisms.

The 3D physical-biological model study in the Egyptian Mediterranean coastal sea

Abstract: The ecosystem of the Egyptian Mediterranean coastal area between longitudes 29°45′ E and 33°45′ E was seasonally investigated. The obtained data sets for a variety of environmental variables were previously interpolated over the studied area, and used in the coupled physical-biological model Fin-Est. Here, the model has been implemented to solve the central problems in case of modeling local areas and the handling of open boundaries and initial conditions, which almost have significant influence on the results, of both the hydrodynamic and the ecosystem models calculation. In the present study, a multi-step calculations procedure was used in applying the 3 D coupled model at first for the entire Egyptian Mediterranean coastal area. Then, it runs for local areas obtaining the initial conditions and the open boundary ones from larger area calculations. In particular, the zoom-in approach was used for the detailed study of the Eastern Harbour of Alexandria city-Egypt (model grid step = 50 m.) selected from the larger Alexandria Sea (model grid step = 1.25 Km), which in turn was chosen from the Egyptian Mediterranean coastal area (model grid step = 12.5 km). The sensitivity of the model simulations to both physical and biological main factors of the studied areas is tested. Both climatic conditions and land sources effects are considered among the model forcing factors. The resultant simulations are compared with the actual measured values of the parameters. The model sensitivity test results are discussed in the context of the model's capabilities and limitations, and with reference to the available knowledge of the ecosystems of the study areas.

The influence of biomanipulations (fish removal) on the structure of lake foodwebs, case studies using the LakeWeb-model

Abstract: This paper presents results on how intensive fishing (fish removal) islikely to influence the structure of lake foodwebs. The work is based on acomprehensive dynamic lake ecosystem model, LakeWeb, which accounts forproduction, biomasses, predation and abiotic/biotic interactions of nine keyfunctional groups of organisms: phytoplankton, bacterioplankton, two types ofzooplankton (herbivorous and predatory), two types of fish (prey and predatory),zoobenthos, macrophytes and benthic algae. The model uses ordinary differentialequations, the ecosystem scale and gives seasonal variations (the calculationtime is 1 week). It is designed to account for all fundamental abiotic/bioticinteractions and feedbacks for lakes in general for the nine target groups. TheLakeWeb-model has been calibrated and critically tested using empirical data andregressions based on data from many lakes. It has been shown that the model canclosely capture typical functional and structural patterns in lakes, whichshould give credibility to the results presented in this work. Obtaining suchresults using traditional methods, i.e., extensive field studies in one or a fewlakes, would be very demanding (in terms of money, persons involved and time).In this paper, results are presented for two lakes, one Swedish and oneBelarussian. The intensive fishing operations carried out in LakeBlacksastjarn, Sweden, to reduce Hg-concentrations in fish did notsucceed. A typical cost of an intensive fishing is about 10,000–30,000 USDper lake of this size (≤ 0.25 km2). The costs toremove fish would be about 40–120 USD per kg ww fish removed! Intensivefishing simulated for Lake Batorino, Belarus, to reduce the fish biomass willlikely increase the prey fish biomass as long as the predation pressure on preyfish is lower than during the prefishing stage. The biomass of predatory fishwill recover only slowly. However, this operation is not likely to succeed inlowering the algal volume in lakes with a high biomass of predatory zooplankton.This is easy to state qualitatively and the LakeWeb-model offers a practicallyuseful tool to quantify such changes and identify lakes where biomanipulationsare likely to fail or succeed.