Prolonged diapause (extended dormancy) is thought to greatly influence evolution in freshwater invertebrates by lengthening generation time, promoting higher levels of dispersal among populations by wind or animal vectors, and increasing effective population size. However, empirical tests of these predictions are relatively rare. Comparative studies can be informative in this regard, if the comparisons involve sympatric, closely related species that differ only in the presence or absence of a dormant life history stage. We built upon a previous study by ZELLER et al. (2006), which used this approach to study patterns of microsatellite variation in Eudiaptomus copepods from northern Europe. E. graciloides possesses diapausing eggs and adults, whereas the closely related species E. gracilis is ecologically and trophically similar but lacks diapause. To separate further historical processes from recent anthropogenic influences, we examined mitochondrial DNA sequence variation in these species from three lakes in northern Germany where they are sympatric. Biotic and abiotic variation among contemporary and historical populations was minimized by focusing on hydro-logically linked lakes separated by less than 10km. E. gracilis and E. graciloides possess very different patterns of mtDNA variation. Both species possess significant population structure on small spatial scales, but E. graciloides has far fewer alleles despite a larger average divergence. Phylogeographic analyses and Bayesian skyline plots revealed evidence for historic population expansions in both species, with the growth phase beginning thousands of generations earlier in E. gracilis than in E. graciloides. Levels of genetic diversity suggest that effective population size may be an order of magnitude larger in E. gracilis than in E. graciloides, even though E. graciloides possesses a diapausing egg bank. Although this would seem to be an ideal system for quantifying the genetic role of diapause, we found no support for the assumption that freshwater invertebrates with diapause categorically possess larger effective population sizes and higher rates of gene flow than closely related species that lack diapause.
The importance of top-down and bottom-up mechanisms in lentic systems has largely been explored in stable systems; however, their influence is likely to change along a habitat duration gradient. Using a case study of four intermittent ponds, we test the generality of these paradigms in systems nearing the extreme short end of this gradient. Intermittent ponds fill up with water on a seasonally predictable basis, and are considered more regulated by physicochemical constraints than by biological factors. We found that natural spatial (i.e., pond-to-pond) and temporal variations (i.e., seasonal and annual) were more important than food-web manipulations (i. e., exclusion of aerial colonization, addition of top-predators [beetle larvae and odonate nymphs], and addition versus removal of allochthonous resources [leaf litter] in field enclosures) in shaping the physicochemical environment. Spatial variation included mainly depth-related variables (e. g., conductivity and pH), whereas annual variation included variables associated with hydroperiod length and productivity (e. g., weeks after flooding, pH, nutrients, and chlorophyll-a). Seasonal variation was pronounced but lower in ponds with longer hydroperiod and in mid-season. Enhanced invertebrate predation pressure increased levels of chlorophyll-a. Exclusion of aerial colonizers increased levels of chlorophyll-a and phosphorus, and affected levels of dissolved oxygen (increasing in one pond and decreasing in another). Addition of leaf litter resulted in lower concentrations of dissolved oxygen, higher phosphorus, and lower pH. Leaf litter removal led to increased levels of dissolved oxygen, changed chlorophyll-a (increasing in one pond and decreasing in another), and higher pH (in one pond). We conclude that although pond characteristics, seasonal development, and annual differences were of greater importance than biological factors for shaping the physicochemical characteristics of these intermittent ponds, bottom-up and top-down effects were also influential.
Non-point source pollution by nitrates (NO 3 ) from fertilizers and animal wastes has potential effects on human health and eutrophication of surface waters. Until now one problem in determining sources of NO 3 has been the difficulty of identifying origin. Stable isotopes of nitrogen can be used as a signature of NO 3 to identify origin from animal wastes. NO 3 derived from animal waste has a δ 15 N signature of +10 +20‰, which is uniquely high compared to δ 15 NO 3 from other sources. The purpose of this research was to describe the distribution of δ 15 NO 3 , NO 3 and Cl in wells, springs, seeps and lakes in the vicinity of a concentrated animal feeding operation (CAFO), which was the suspected source of contamination. Nitrate concentrations and δ 15 NO 3 were higher in wells just below the waste spray area of the CAFO than above it. Chloride ion concentrations in wells confirmed a contaminated area below the waste spray area. Surface water samples had a wide range of NO 3 concentrations and were uncontaminated, except for samples from one seep and one spring. However, the mean δ 15 NO 3 in samples from springs were +3.9 to +5.0‰, values that are in a range reported for soil NO 3 . Thus, although data are not available on groundwater movement, both stable isotope signatures and chloride concentrations indicate that animal wastes were the source of NO 3 contamination.
The ecological significance of microbial communities in aquatic systems has been documented to varying degrees in marine and lentic systems; however much less is known about their relative abundance and distribution among streams with different environmental conditions. In this study, the relative abundance and distribution of different bacterial taxa, virus-like-particles (VLP) and protozoa in planktonic communities from nine streams in different regions of the USA were examined. There were clear differences in the abundance and distribution of the microorganisms examined among sites within a stream and among the streams. Also, there were strong relationships between the abundance of some of the microbiological variables measured and the environmental conditions of the streams. Most of the bacterial taxa examined were correlated with chlorophyll-a concentrations, while no significant relationships were observed between VLP numbers and any of the environmental or microbiological variables measured in the streams. In contrast, protozoa numbers were correlated with nitrate and dissolved organic carbon (DOC) concentrations. Our results suggest that differences in environmental conditions influence microbial numbers and distribution among the streams examined.
