Impact of ionic aluminium on extracellular phosphatases in acidified lakes

We studied direct inhibiting effects of aluminium (Al) on extracellular phosphatases produced by the plankton of acidified lakes in the Bohemian Forest. In laboratory experiments we tested the effect of different Al concentrations (0–1000 µg l⁻¹) on kinetic parameters of acid phosphatases (pH optimum ∼5.0) at pH between 4.5 and 5.2. We observed a significant reduction of an apparent substrate affinity at Al concentrations between 300 and 1000 µg l⁻¹ at pH 4.5 and 4.8 (but not at 5.2). In contrast, maximum acid phosphatase activity (AcPA) remained unchanged. Such behaviour of saturation kinetics is compatible with the assumption that ionic Al acts as a competitive inhibitor of acid phosphatases. To decide whether the observed Al effects could be explained alternatively by complexation of Al with substrate, we tested statistically the best fits of data with both possible models (competitive versus complexation). Experimental results supported the competitive hypothesis rather than the complexation model suggested originally by some authors. Furthermore, we tested the Al effect within a wide range of pH from 4.0 to 6.0. For pH values < 5.2, the results of an Al–pH matrix experiment gave a more detailed picture: the higher the Al concentration, the wider the pH range in which Al could negatively affect AcPA. The ecological ramifications of this effect were evaluated in the context of field AcPA data on three strongly acidified lakes.     

Total phosphorus and piscivore mass as drivers of food web characteristics in shallow lakes

We assessed the relative influence of total phosphorus and piscivore biomass on the abundance of benthivores, soft‐rayed planktivores, spiny‐rayed planktivores, zooplankton and phytoplankton in 69 shallow lakes in the prairie and parkland areas of Minnesota, USA. Piscivore biomass was the best predictor for three of these response variables, exhibiting a negative relationship with soft‐rayed planktivores, a positive relationship with benthivores, and a weaker positive relationship with large‐bodied cladocerans. Total phosphorus and piscivores comprised the best model for predicting spiny‐rayed planktivores, while neither variable showed any strong relationship to small‐bodied cladocerans. Total phosphorus was positively related to phytoplankton abundance, and was the best predictor among all candidate models. Moreover, contrary to predictions of trophic cascade theory, the relationship between chlorophyll a and total phosphorus did not differ between lakes with and without piscivores. Our results indicated top‐down influences of piscivores extended through parts of two trophic levels, but failed to influence zooplankton – phytoplankton interactions, leaving phytoplankton abundance constrained largely by total phosphorus. Lack of a relationship between piscivores and phytoplankton was likely due to high densities of larval planktivores less susceptible to piscivory, as well as positive influences of spiny‐rayed planktivores and benthivores on algal abundance. These results support the idea that top–down influences of piscivores on phytoplankton abundance may be reduced in more diverse fish communities where some prey species are less susceptible to piscivory.     

Influences of fish on food web structure and function in mountain lakes

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Sulfate control of phosphorus availability in lakes

During summer stratification large amounts of phosphorus (P) accumulate in anoxic bottom waters of many lakes due to release of P from underlying sediments. The availability to phytoplankton of this P is inversely related to the Fe:P ratio in bottom waters. Using data from 51 lakes, we tested the hypothesis that sulfate concentration in lake water may be critical in controlling the Fe:P ratio in anoxic bottom waters. Results showed that Fe:P ratios in bottom waters of lakes were significantly (p<0.001) related to surface water sulfate concentrations. The higher Fe:P ratios in low sulfate systems is due not only to higher iron concentrations in anoxic bottom waters but also to lower P concentrations in anoxic waters. Thus, our results suggest that anthropogenically induced increases in sulfate concentrations of waters (e.g. from fossil fuel burning) may have a double effect on P cycling in lakes. Higher sulfate concentrations can both increase the magnitude of P release from sediments as well as increase the availability of P released from sediments into anoxic bottom waters.     

Duckling response to changes in the trophic web of acidified lakes

We reared American Black Duck (Anas rubripes Brewster) and Common Goldeneye (Bucephala clangula Linnaeus) ducklings on two Quebec laurentian lakes in which we manipulated brook trout populations (Salvelinus fontinalis Mitchill), lake acidity and lake productivity to relate waterfowl foraging to trophic status of lakes. We developed a preliminary model to assess the effects of lake acidity and productivity, fish predation and interspecific fish/duck competition in relation to available food (aquatic invertebrates). We then validated the model using a factorial analysis of the relationships between the variables pertaining to the diet of the fish and ducklings, and the environmental characteristics of the lakes (acidity, biological production and fish predation).The first factorial axis can be interpreted in terms of biological productivity, while the second axis illustrates the effect that fish have on the quantity and type of food available to ducklings. Two different trends appear to occur depending on whether the carrying capacity of the lake is reduced by acidification of the water or increase through liming or fertilization. In the first case, fish predation appears to have a marked effect on available food, whereas in the second case, interspecific fish/duck competition is apparently to blame for changes in the diet of ducklings. In both instances, but to a lesser extent, fish compete increasingly (exploitation and/or interference) with the ducklings, forcing them to feed to a greater extent in riparian sites that are less accessible to fish.     

Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem processes to variation in the vertical and horizontal structure of decomposer food web under nitrogen poor and N-enriched conditions. Microcosms with humus and litter layer of boreal forest floor, birch seedlings infected with mycorrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growing period for the birch, we irrigated half of the microcosms with urea solution, and the other half with de-ionised water to create two levels of N concentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microcosms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PLFA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the biomass of their microbivorous and microbi-detritivorous prey, and microbi-detritivores affected the biomass and community structure of microbes (indicated by PLFA-analysis). Moreover, predatory mites and nematodes increased microbial biomass and changed the microbial community structure. The decomposer food web structure affected also N uptake and growth of plants. Microbi-detritivorous fauna had a positive effect, whereas predators of microbial and detritus feeding fauna exerted a negative influence on plant N uptake and biomass production. The impact of a trophic group on the microbes and plant was also strongly dependent on species composition within the group. Nitrogen addition magnified the influence of food web structure on microbial biomass and plant N uptake. We suggest that addition of urea-N to the soil modified the animal-microbe interaction by increasing microbial growth and altering community structure of microbes. The presence of microbi-detritivores and predators reduced loss of carbon from the microcosms, and the food web structure influenced also water holding capacity of the materials. The changes in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of stochastic population dynamics on food web structure

We develop a stochastic, individual-based model for food web simulation which in the large-population limit reduces to the well-studied Webworld model, which has been used to successfully construct model food webs with several realistic features. We demonstrate that an almost exact match is found between the population dynamics in fixed food webs, and that the demographic fluctuations have systematic effects when the new model is used to construct food webs due to the presence of species with small populations.     

Effects of resource pulse magnitude on nutrient availability,productivity, stability,and food web dynamics in experimental aquatic ecosystems

Oceanographic processes play a key role in influencing the structure of the marine planktonic ecosystems. Taking advantage of the quasi-simultaneous collection of a large ichthyoplanktonic dataset in different regions of the Central Mediterranean Sea (Italian/Maltese, Tunisian and Libyan waters), this study aimed at the identification of the main environmental drivers that control the structure of the larval fish assemblages. Spatial distribution and taxa composition were related to physical forcings (geostrophic currents and wind stress) and environmental conditions (bottom depth, temperature, salinity, chlorophyll-a concentration). ANOSIM and SIMPER identified contribution of fish taxa to the average Bray–Curtis dissimilarity among regions. In Italian and Libyan waters, two assemblages (neritic and oceanic) were identified, while a mixed assemblage characterized only some stations. Two neritic and one oceanic assemblages were discriminated in Tunisian waters. Random Forest classification model highlighted the essential role of the bathymetry, while Lagrangian simulations evidenced the action of the hydrodynamics in mixing neritic and oceanic assemblages in the Italian/Maltese and partially in Libyan waters. These findings highlighted the importance of the multidisciplinary approach and shed light on the potential value of the ichthyoplanktonic surveys for the assessment of the state of the marine ecosystem and the conservation of the fishery resources.     

Effects of disturbance on phosphorus turnover and plankton dynamics

In coastal lagoons, one common type of anthropogenic disturbanceis opening of the sand barriers that isolate these lagoons fromthe sea. We investigated how a disturbance caused by an artificialopening of the sand barrier affects phytoplankton and bacterioplanktondynamics and alkaline phosphatase activities (APA) in a Braziliancoastal lagoon. We propose an index of potential phosphorusturnover time (PPTT) calculated as the ratio of total phosphorus(TP) and APA in an attempt to quantify the effect of disturbanceson phosphorus recycling within the plankton community. Openingthe barrier caused increased phytoplankton biomass, TP concentrations,APA and PPTT, and decreased bacteria to phytoplankton biomassratio. PPTT is easily calculated and may be used to detect planktonresponses to anthropogenic disturbances.     

Characterization of bacterial communities in four freshwater lakes differing in nutrient load and food web structure

The phylogenetic composition of bacterioplankton communities in the water column of four shallow eutrophic lakes was analyzed by partially sequencing cloned 16S rRNA genes and by PCR-DGGE analysis. The four lakes differed in nutrient load and food web structure: two were in a clearwater state and had dense stands of submerged macrophytes, while two others were in a turbid state characterized by the occurrence of phytoplankton blooms. One turbid and one clearwater lake had very high nutrient levels (total phosphorus > 100 microg/l), while the other lakes were less nutrient rich (total phosphorus < 100 microg/l). Cluster analysis, multidimensional scaling and ANOSIM (analysis of similarity) were used to investigate differences among the bacterial community composition in the four lakes. Our results show that each lake has its own distinct bacterioplankton community. The samples of lake Blankaart differed substantially from those of the other lakes; this pattern was consistent throughout the year of study. The bacterioplankton community composition in lake Blankaart seems to be less diverse and less stable than in the other three lakes. Clone library results reveal that Actinobacteria strongly dominated the bacterial community in lake Blankaart. The relative abundance of Betaproteobacteria was low, whereas this group was dominant in the other three lakes. Turbid lakes had a higher representation of Cyanobacteria, while clearwater lakes were characterized by more representatives of the Bacteroidetes. Correlating our DGGE data with environmental parameters, using the BIOENV procedure, suggests that differences are partly related to the equilibrium state of the lake.     

Photosynthesis,mixotrophy and microbial plankton dynamics in two high Arctic lakes during summer

Photosynthesis and microbial plankton dynamics of two lakes in the Kongsfjorden catchment of Spitzbergen (Svalbard Archipelago, 78°N) were investigated during the summer of 2000. One of the lakes (Tvillingvatnet) served as the water supply for the village of Ny Ålesund. The other was a deeper, larger unnamed lake in the Ossian Sarsfjella reserve—named Lake OS in this study. Both lakes can be classified as oligotrophic on the basis of chlorophyll a and inorganic nutrient concentrations. Chlorophyll a concentrations ranged between 0.33 and 1.65 µg l ^-1 in Tvillingvatnet and 0.2 and 0.3 µg l ^-1 in Lake OS. The phytoplankton was dominated by chrysophytes and cryptophytes, with the diatom Rhizosolenia and a number of dinoflagellate species. Rates of photosynthesis were typically low, between 24.5 µg and 1.0 mg l ^-1 day ^-1 in Tvillingvatnet (photosynthetic efficiency 0.004–0.26), and between 3.1 and 29.5 µg l ^-1 day ^–1 in Lake OS (photosynthetic efficiency 0.0019–0.0085). Among the phytoflagellates (PNAN) there were a number of mixotrophs which reached their peaks of abundance before other PNAN. Mixotrophy appeared to provide a competitive advantage. Grazing rates for the mixotroph Dinobryon ranged between 0.063 and 1.12 pg C cell ^-1 day ^-1 in Lake OS and between 0.22 and 1.11 pg cell ^-1 day ^-1 in Tvillingvatnet, with rates increasing between July and August. However, Dinobryon removed less than 1% of bacterial biomass day ^-1, while the heterotrophic nanoflagellates (HNAN) removed up to 28% of bacterial biomass day ^-1. Bacterial concentrations were low, with a maximum of 28.8×10 ⁸ l ^-1 in Tvillingvatnet and 23.6×10 ⁸ l ^-1 in Lake OS. Ciliated protozoan and rotifer diversity in the plankton was low.     

Integrating food web diversity, structure and stability

Given the unprecedented rate of species extinctions facing the planet, understanding the causes and consequences of species diversity in ecosystems is of paramount importance. Ecologists have investigated both the influence of environmental variables on species diversity and the influence of species diversity on ecosystem function and stability. These investigations have largely been carried out without taking into account the overarching stabilizing structures of food webs that arise from evolutionary and successional processes and that are maintained through species interactions. Here, we argue that the same large-scale structures that have been purported to convey stability to food webs can also help to understand both the distribution of species diversity in nature and the relationship between species diversity and food web stability. Specifically, the allocation of species diversity to slow energy channels within food webs results in the skewed distribution of interactions strengths that has been shown to confer stability to complex food webs. We end by discussing the processes that might generate and maintain the structured, stable and diverse food webs observed in nature.     

Ciliate community structure and interactions within the planktonic food web in two alpine lakes of contrasting transparency

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Fish communities and food web interactions in some shallow Mediterranean lakes

Hydrobiologia - Fish communities of five shallow Spanish Mediterranean lakes were studied and stomach content analysed to determine the foraging pattern of each species and the influence on the...     

Genetic variation, predator-prey interactions and food web structure

Food webs are networks of species that feed on each other. The role that within-population phenotypic and genetic variation plays in food web structure is largely unknown. Here, I show via simulation how variation in two key traits, growth rates and phenology, by influencing the variability of body sizes present through time, can potentially affect several structural parameters in the direction of enhancing food web persistence: increased connectance, decreased interaction strengths, increased variation among interaction strengths and increased degree of omnivory. I discuss other relevant traits whose variation could affect the structure of food webs, such as morphological and additional life-history traits, as well as animal personalities. Furthermore, trait variation could also contribute to the stability of food web modules through metacommunity dynamics. I propose future research to help establish a link between within-population variation and food web structure. If appropriately established, such a link could have important consequences for biological conservation, as it would imply that preserving (functional) genetic variation within populations could ensure the preservation of entire communities.     

Composition,dynamics and production of Rotatoria in the plankton of some lakes of the Danube Delta

The Rosu, Puiu and Porcu lakes from the Danube Delta are lacustrine ecosystems characterized by a particularly great variation of the biotic and abiotic factors. This variation causes the development of a zooplankton reduced from the point of view of number and biomass, but rich from the taxonomic point of view. Its monthly and annual fluctuations can hardly be correlated to the rest of the plankton fauna and microflora. Rotifer production is low, turnover at the level of plankton rotifers being relatively uniform and dependent on water temperature.     

Elemental stoichiometry of lower food web components in arctic and temperate lakes

Lakes were surveyed to assess the potential patterns of latitudinalvariation in carbon:nitrogen:phosphorus (C:N:P) stoichiometryof lower food web components. Thirty-four lakes were surveyedat an arctic latitude (68°38'N, 149°38'W) and 10 lakesat a temperate latitude (46°13'N, 89°32'W) during 1997.The temperate data set was augmented with earlier survey resultsemploying similar methods. It was hypothesized that differencesin environmental variables across latitude would cause differencesin community C:N:P ratios, leading to differences in trophicinteractions. Physical measurements (light, temperature), sestonand zooplankton were collected from each lake. Seston and zooplanktonwere analyzed for C, N and P content, and zooplankton were countedand measured for biomass estimates. The degree of trophic interactionbetween seston and zooplankton was assessed by (i) measuringelemental imbalances between seston and zooplankton and (ii)calculating the potential recycling ratio by the zooplanktoncommunity available for seston. Seston C:nutrient, but not N:P,ratios were higher in temperate than arctic lakes. Conversely,arctic zooplankton had higher C:nutrient, but not N:P, ratiosthan zooplankton in temperate lakes. Elemental imbalances weregreater in temperate than in arctic lakes, but N:P stoichiometryof potential zooplankton recycling was nearly identical betweenthe two latitudes. Zooplankton community C:N:P ratios were notrelated to either latitude or seston C:N:P. In accordance withstoichiometric theory, relative abundances of calanoid copepodswere positively correlated with seston C:N in temperate lakes.Additionally, relative abundances of Daphnia were negativelycorrelated with seston C:N ratios in temperate and arctic lakes,and positively correlated with N:P ratios in the arctic. Ingeneral, these results suggest that seston and zooplankton communitystoichiometry differ across latitude, and these differenceshave the potential to affect trophic interactions.     

Effects of mixing on the pelagic food web in shallow lakes

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Seasonal variations in food web dynamics of floodplain lakes with contrasting hydrological connectivity in the Southern Gulf of Mexico

Hydrobiologia - Floodplains are some of the most productive and diverse ecosystems on Earth. The Usumacinta River Basin, in the Southern Gulf of Mexico, hosts several floodplain lakes, whose...