Extensive spatial analysis of the light environment in a subtropical shallow lake, Laguna Iberá, Argentina

Variations in the attenuation of photosynthetically available radiation were analysed using extensive spatial sampling in two seasons in a subtropical wetland lake. Simultaneously with the attenuation measurements, the principal absorption and scattering components of the water column were also measured. The elevated spatial resolution used in the study allowed the determination of spatially distinct optical water classes within the lake. Changes in dissolved organic matter, phytoplankton and tripton concentrations led to a wide variation in the vertical attenuation coefficients. These changes depended on local characteristics of the ecosystem and time of year. The spatial distribution of the attenuation coefficients was examined in relation to the hydrological and geomorphological characteristics of the littoral area of the lake. The impacts of two small rivers on the light environment and attenuation components are shown. Finally, the resulting model was used to examine the possible impacts of changes in light availability at the lake bottom in relation to recent changes in lake water level.     

Estimating Virus Production Rates in Aquatic Systems

Viruses are pervasive components of marine and freshwater systems, and are known to be significant agents of microbial mortality. Developing quantitative estimates of this process is critical as we can then develop better models of microbial community structure and function as well as advance our understanding of how viruses work to alter aquatic biogeochemical cycles. The virus reduction technique allows researchers to estimate the rate at which virus particles are released from the endemic microbial community. In brief, the abundance of free (extracellular) viruses is reduced in a sample while the microbial community is maintained at near ambient concentration. The microbial community is then incubated in the absence of free viruses and the rate at which viruses reoccur in the sample (through the lysis of already infected members of the community) can be quantified by epifluorescence microscopy or, in the case of specific viruses, quantitative PCR. These rates can then be used to estimate the rate of microbial mortality due to virus-mediated cell lysis.     

Interpretation of bryozoan microfossils in lacustrine sediment cores

Bryozoan statoblasts were isolated from the surficial sediments of 30 Florida lakes representing a gradient of trophic conditions. A prerequisite for lake selection was a good data base for water chemistry that was collected within the past 5–10 years. A novel technique of employing 210-Pb as a dilution tracer was used to estimate sedimentation rates for each lake. The net annual accumulation rate for bryozoan statoblasts was calculated for each lake and correlated with twelve physical, chemical, and biological variables. The results suggest that bryozoan distributions are strongly controlled by the extent of the littoral zone and phytoplankton biomass. It appears that bryozoan abundance in excessively weeded lakes may be limited by phytoplankton in spite of the richly developed habitat. Separation of the importance of food requirements from habitat requirements was not possible from this initial survey.     

Comparative studies of the population genetic structure of the Brachionus calyciflorus species complex from four inland lakes in Wuhu,China

To study how the population genetic structure in zooplankton respond to environmental conditions, using comparative limnology, the genetic diversity and genetic differentiation of the B. calyciflorus complex collected from four inland lakes in Wuhu City, China were investigated based on the 16S rRNA gene and nuDNA ITS sequences. The results displayed a high genetic diversity, and the nucleotide diversity of the 16S rRNA gene was higher than that of the ITS sequence. The phylogenetic analyses grouped the four populations into two cryptic species (Bc-JT and Bc-FL) with strong support. The two cryptic species were found in lakes with different trophic levels, demonstrating significant ecological specialization. The origins of clone TW12 were not consistent in the phylogenetic trees between two genetic markers, which might be attributed to the effects of male-mediated gene flow on the phylogenetic relationships of rotifers. The nucleotide diversity of the cryptic species Bc-JT was higher than that of Bc-FL, indicating that eutrophication might decrease the genetic diversity of cryptic species. The total phosphorus concentration in water bodies might be the most important factor affecting the genetic diversity of species.     

Application of <Superscript>13</Superscript>C mineral carbon for assessment of the primary production of organic matter in aquatic environments

The possibility of measuring the rates of light and dark CO₂ assimilation using ¹³C carbonate was demonstrated on Lake Kichier (Marii El). The application of methods utilizing the stable ¹³C and the radioactive ¹⁴C isotopes resulted in comparable values of the rates of light and dark CO₂ fixation. Due to its absolute environmental safety, the method with ¹³C mineral carbon can be recommended as an alternative to radioisotope methods for qualitative measurements of CO₂ fixation rates in aquatic ecosystems.     

A Biomonitor for Tracking Changes in the Availability of Lakewater Cadmium over Space and Time

Determining the exposure of organisms to contaminants is a key component of Ecological Risk Assessments (ERAs). Effective estimates of exposure consider not only the total concentrations of contaminants in an organism's surroundings but also the availability of the contaminants to organisms. Contaminant availability can be inferred from mechanistic models and verified by measurements of contaminant concentrations in organisms. We evaluated the widespread lake-dwelling insect Chaoborus as a potential biomonitor for use in exposure assessments for three metals: cadmium (Cd), copper (Cu), and zinc (Zn). We show that larvae of this midge maintain constant their concentrations of the essential metals Cu and Zn and thus cannot be used to monitor them. In contrast, larval Cd concentrations varied widely both among lakes and in a given lake over time. We were able to relate these variations in biomonitor Cd to changes in lakewater Cd and pH using the Free Ion Activity Model (FIAM). Our results suggest that Chaoborus larvae could be used as an effective tool for estimating the Cd exposure of organisms in lakes for the purposes of ERAs.     

SHOREBIRD HABITAT USE AND NEST-SITE SELECTION IN THE PLAYA LAKES REGION

Abstract: Wetlands in the Playa Lakes Region (PLR) provide important habitats for wintering waterfowl, cranes, and both migrant and breeding shorebirds. Playa Lakes Region wetlands experience naturally fluctuating hydroperiods but are exposed to anthropogenic stresses, which are exacerbated during summer and may influence PLR wetland occupancy and selection by breeding shorebirds. We examined wetland-scale habitat use and nest-site selection of the 4 dominant shorebirds (American avocets [Recurvirostra americana], black-necked stilts [Himantopus mexicanus], killdeer [Charadrius vociferus], snowy plovers [C. alexandrinus]) nesting in playas, saline lakes, and in both created and riparian wetlands in the PLR of Texas, USA. All 4 species nested in saline lakes. Only avocets and killdeer nested in playas, and snowy plovers nested in riparian wetlands. No nests were found in created wetlands. Wetland habitat changed (P < 0.001) during the breeding season, while water habitats generally decreased. Used (i.e., shorebirds found nesting) wetlands had more (P < 0.05) mudflats than non-used (i.e., shorebirds not found nesting) wetlands, which had more (P < 0.05) dry habitats. Used and non-used wetlands had similar (P > 0.05) amounts of water habitats. Nests were located close to vegetation on bare dry ground and dry ground with vegetation. Because water is ephemeral in PLR wetlands, shorebirds must select—in a somewhat predictive manner upon arrival—wetlands with suitable nest-site and brood-rearing habitat. Although surface water is necessary for nesting, its presence is not adequate for delineating suitable PLR wetland habitat for breeding shorebirds. Our findings that created wetlands cannot compensate for regional wetland losses in habitat or function highlights the need for conservation of natural PLR wetlands     

Artemia habitats: Ion concentrations tolerated by one superspecies

The geographic distribution, history, and ionic composition of habitats of Artemia franciscana are reviewed with emphasis on habitats with extreme values for ionic concentrations or ionic ratios: a) high-chloride waters (sea water salterns and Zuni and Great Salt Lakes); b) high-sulfate lakes in Saskatchewan (Chaplin and Little Manitou) and on the Okanogan plateau of Washington (Penley Lake complex); and c) high-carbonate habitats in Nevada (Fallon), in California (Mono Lake) and in the Nebraska sandhills (Jesse and Antioch).First-instar nauplii from populations representative of each of these three habitat clusters were tested for tolerance of potassium (0–5 g K l^-1), magnesium (0–1.3 g Mg l^-1), and calcium (0\2–0.6 g Ca l^-1). Viabilities were recorded until survivors reached adulthood in pairs of simple defined synthetic culture media which differed in only one parameter. Eight populations showed four levels of tolerance of high potassium. Of four populations tested, all had high viability and fertility in media lacking potassium (above the level in the yeast diet). Artemia from sea water salterns or from Zuni, Chaplin, or Great Salt Lakes could not tolerate low levels of calcium (<20 mg l^-1). This accounts for their inability to tolerate hypersaline high-carbonate waters. Mono and Fallon nauplii had high viability and fertility in media with low levels of calcium (0–10 mg l^-1) but lacking magnesium. They could not survive for seven days, however, in low-calcium (< 10 mg l^-1) media that contained moderate amounts of magnesium (1.3 g l^-1), indicating that magnesium interferes with utilization of low levels of calcium.For each of the three cations, the range of concentrations encountered by each population in the habitat is narrower than the range affording high viability in laboratory media. As expected, the midpoints of the two ranges are sometimes similar. In many cases, however, the narrower range of ionic concentrations reported for lake water is at the end of the range affording high viability in the laboratory.     

Phosphorus and Nitrogen Budgets of Barton Broad and Predicted Effects of a Reduction in Nutrient Loading on Phytoplankton Biomass in Barton,Sutton and Stalham Broads,Norfolk, United Kingdom

Barton, Sutton and Stalham Broads are shallow, man-made lakes formed in mediaeval times when peat excavations were flooded. Recently, a once diverse submerged macrophyte flora has, in most broads, been replaced by large phytoplankton populations. This change has been attributed to increased nutrient loadings. The seasonal cycles of chlorophyll α, nitrogen and phosphorus in the three broads are described and the inputs of nitrogen and phosphorus to Barton Broad are budgeted. A reduction in the phosphorus loading is recommended as the best method of restricting phytoplankton populations in these broads. An equation relating phosphorus loading, flushing rate, mean depth and retention coefficient to mean standing phosphorus concentration is used to predict the reduction in phosphorus loading required to limit phytoplankton populations and permit the re-establishment of submerged macrophytes.