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.
Previous analyses of larval chironomid δ13C have suggested that methanotrophic contribution to biomass is site-specific within lakes. We determined larval biomass, larval and methane gas stable carbon isotopes, and potential methane production from the sediments underlying different water column depths in Esthwaite Water, UK. Methane production increased whereas larval δ13C values typically became lighter with increasing lake depth. Reduced methane production at 15 m depth and correspondingly less13C-depleted larvae in the second year of study suggests that the rate of methane production influenced larval assimilation of methane-derived biomass. Larval distribution and other site-specific parameters, combined with two-source mixing models, were used to estimate potential methane-mediation via the abundant chironomid biomass to higher predators.
Cladocerans in the genus Eubosmina exhibit spectacular and confusing morphological diversification in the Baltic region. The paleolimnological record suggests that lineages in regional lakes diverged recently from morphologically conservative E. longispina/coregoni-like ancestors and developed strong seasonal elongation of antennules, mucrones, or dorsal humps (cyclomorphosis). Allozyme studies on Schleswig-Holstein populations by DE MELO agree with this interpretation by demonstrating: 1) genetic difference between geographically distant E. longispina populations, 2) clustering of E. coregoni forms with local E. longispina lineages, and 3) great genetic similarity among regional E. coregoni forms, despite major morphological differences. Retention of distinguishing traits (shape of dorsal humps, antennule length) in common-garden experiments emphasizes that micro-evolutionary (genetic) differences underlie "form" phenotypes. However, regression of extreme summer forms towards a more conservative morphology in laboratory culture indicates that part of the seasonal variability is attributable to developmental plasticity. Split-clone laboratory experiments demonstrate that longer antennule lengths of an exuberant form, E. coregoni kessleri from the Grosser Ploner See, are increased (induced) in the presence of predatory cyclopoids. Induction responses strengthen the notion that long-term transformations involve evolutionary responses to invertebrate predators (cyclopoid copepods, Leptodora) which colonized large lakes during post-glacial periods, with the particulars of responses showing lineage and lake specificity.
The seasonal change in helmet size in Daphnia cucullata has been studied for over one century. Recently it has been shown that helmets in D. cucullata, which have been found to reduce predator caused mortality, can be induced by chemical cues released by several predatory invertebrates. However, it also has been shown that tur- bulence induces this trait. The relation and interplay of both inducing cues is not known. Here we present results from lab experiments showing that predator cues and turbulence can act synergistically. Both factors in combination induced significantly larger responses, compared to each factor alone, and helmets reached the maximum sizes found in natural lakes. This result might help to explain the observation of large helmets in this species in nature. The ultimate reason behind the turbulence induction is yet unknown. We link both induction factors to predation, as the ultimate reason, by testing the hypothesis that D. cucullata can respond to turbulence produced by swimming invertebrates. We found that helmet growth increased significantly in direct contact to both the heterospecific Daphnia magna and the predator Cyclops sp. Treat- ments which accounted for chemical cues alone did not increase helmet growth in re- sponse to cues from D. magna ,o rCyclops under these conditions. Together, these re- sults indicate that D. cucullata is able to respond to mechanical cues produced by swimming invertebrates. Thus, our study suggests that both chemical cues and turbu- lence generated by predatory invertebrates might act synergistically to induce helmets as effective protection against invertebrate predation.
, Abstract: A stoichiometric approach is applied to model nutrient element content and population growth kinetics in phagotrophic flagell~tes. Available evidence is limited, but suggests that the nutrient composition of flagellates is not strictly homeostatic, but instead varies with the nutrient element composition of their food resources. A mathe- matical model is constructed that couples the C, N, and P contents of flagellates to their population growth rate and the nutrient fluxes assimilated from food resources. Variants of the model are explored to examine the effects of saturating ingestion, main- tenance respiration, and selective feeding from food mixtures. In agreement with observations, the models predict non-homeostatic variation in the nutrient content of flagellates. Population growth rate is predicted to vary with both food quantity and quality (in terms of nutrient element content). It is proposed that lack of homeostasis and selective feeding on prey with high nutrient content enhance fitness of pha- gotrophic flagellates under some conditions.
Similar to Daphnia, many planktonic ciliates are algivores that occur in vir- tually every natural lake and reproduce primarily asexually. Due to their larger popula- tion size and shorter generation time, their significance as algal consumers and second- ary producers may exceed that of Daphnia during algal blooms and when averaged over the season. The high reproduction rate, the ease of culturing, the accessibility to experimental manipulation, and the potential to apply sophisticated measuring techni- ques such as flow cytometry render some ciliate species ideal candidates for ecophy- siological laboratory experiments. This paper summarizes recent research in which ciliates have been used as model organisms for investigating the effect of environ- mental key parameters on planktonic organisms. Special attention is given to the (com- bined) effect of temperature, food, pH and predators. Niche partitioning has been stud- ied at the level of genus, species and clone. Open questions and emerging perspectives of ciliate research for issues of general ecological relevance will be discussed at the end of each section.
This investigation reports the community composition and biomass size-distribution of crustacean zooplankton communities in five small fishless lakes of the Precambrian Shield in Algonquin Provincial Park across one or two growing seasons. Contrary to predictions based on the size efficiency hypothesis and invertebrate predation models, zooplankton in these lakes were predominantly small. In most of the lakes surveyed, zooplankton communities were dominated by small calanoid copepods (mean adult body length < 1.0 mm). In only one lake did large zooplankton (mean adult body length > 1.5 mm) make up more than 25 % of the zooplankton biomass. Chaoborus abundance among these fishless lakes was an order of magnitude greater than in other lakes in the region with fish. We suggest the relatively small biomass of large bodied zooplankton compared with other fishless lakes is due to heavy Chaoborus predation combined with slowed zooplankton productivity in low pH waters. Despite the absence of large Cladocera, the relationship between zooplankton and phytoplankton biomass indicates strong "top-down" effects on phytoplankton. Grazing experiments in one of the lakes, and in a nearby lake with fish, indicate high grazing on some phytoplankton species, especially small dinoflagellates, and on protozoa.
We studied the leaf budget in two forest stream sites: site D was located in a stream that flows through deciduous forest, and site E was located in a stream that flows through a eucalyptus plantation. Leaf inputs, transport, benthic storage and breakdown rates at both sites at different periods were measured over 5 years. Data have been combined to calculate leaf budgets with the assumption that both streams were in steady state. The total leaf input was 20 % lower at site E, but the mean benthic stock of leaves and the amount of leaves that was processed were about 1.5 times higher than at site D. Inputs of nitrogen and phosphorus associated with leaves were 50 and 20 % lower at site E, but small differences in the benthic storage of nutrients were observed between the two sites. The streams processed 31-57% of the nitrogen, and 20-57% of the phosphorus contained in the leaf input. Afforestation with eucalyptus has a low impact on leaf litter processing as the longer residence time of eucalyptus leaves in the stream balances their lower processing rate. However, it modifies nutrient fluxes associated with leaf litter due to the low nutrient content of eucalyptus leaves and to their role as nutrient sources during leaf decay. At both streams, alder litter was processed more efficiently than other species and was the main source of nitrogen and phosphorus for stream decomposers. We hypothesize that dependence on this fast decaying species is an indication of the disturbance of CPOM dynamics due to historical changes in our streams and that it may be difficult to isolate the effects of former forest disturbances from the impact of eucalyptus plantations.
We investigated the dynamics of nutrient and light limitation of phytoplankton in a reservoir ecosystem in relation to storm-mediated variation in stream discharge, and how dynamics differed at a shallow site near stream inflows versus one in deep water near the lake outflow. Storm-mediated discharge events reduced the severity of nutrient limitation and increased the severity of light limitation, as predicted by a model of reservoir resource limitation developed by KIMMEL et al. (1990). The severity of nutrient limitation was negatively correlated with discharge to the lake; the correlation was strongest with discharge over the preceding 10-14 day period and weaker at shorter and longer time scales. However, discharge events also flushed phytoplankton from the lake and enhanced light limitation, so it is not clear by which mechanism(s) discharge events mediate phytoplankton resource limitation. Phytoplankton near stream inflows were less nutrient limited than phytoplankton at the lake outflow, consistent with predictions of the KIMMEL et al. (1990) model. However, this was true even when streamflow was negligible, suggesting alternative mechanisms for reduced nutrient limitation near stream inflows. In contrast to predictions of the model, phytoplankton were not more light limited near stream inflows than at the outflow; shallower depth near inflows compensated for higher turbidity, in terms of the light climate experienced by phytoplankton. Our results show that the mechanisms by which discharge events mediate phytoplankton resource limitation are complex and require further study in reservoirs as well as other aquatic systems subject to a high degree of temporal variation in discharge.
An approach is described here that has been designed to determine the fac- tors that control growth and standing crop in natural populations of phytoplankton. This approach involves the use of 1) small volume bioassays of natural phytoplankton samples, incubated in semi-natural conditions, 2) indirect determination of phyto- plankton biomass during growth by in vivo chlorophyll-a fluorescence measurements, and 3) computer-designed factorial plans. A commercial computer program was used to achieve optimal planning of the assays, statistically sound and rapid data analysis, and forecasting of phytoplankton responses to inputs. An example of field application of the protocol is described, in which a phytoplankton population of a Western African Lake was studied. This approach can be used to rapidly identify the main bottom-up control factors and their interactions, and further refining of analysis with optimization of sample number. Modelled responses also make it possible to predict how phyto- plankton biomass would change in reaction to changes in environmental conditions. The pros and cons of the approach are discussed taking into account the use of frac- tional design and the reduction of sample numbers.
Simuliid silk pads and their effect on tufa deposition and biofilm development with respect to current velocity and time of exposure were studied during the winter on tufa barriers in Plitvice Lakes (Croatia). Analysis of silk pad types on artificial substrates showed that moderate velocity substrates were slightly more favourable for simuliid larvae as landing attachment site, while they strongly preferred high velocity substrates as wandering attachment sites and exclusively as long term attachment sites for feeding. Although silk covered only 0.78 % of surface area on glass slides, it accumulated 17 % of diatoms and 56 % of individual calcite crystals under moderate velocity conditions and 13 % of diatoms and 41 % of individual calcite crystals under high velocity conditions. This strongly suggests that simuliid silk pads influence periphyton development and serve as biomediator in early stages of tufa formation. SEM analysis has revealed diatoms on the silk with micrite on their mucus strands, indicating diatom synergistic role in tufa deposition. Diatom and calcite densities were mainly functions of time, with higher densities of both under moderate velocity conditions. Maximum dimension of calcite crystals was not affected by the two-month time of exposure, with larger crystals found under moderate velocity. We conclude that tufa deposition during the winter season was controlled primarily by trapping and binding microcrystalline calcite. Simuliid silk changes surface properties, enhancing initial tufa deposits and biofilm development, thereby generating aggregated distribution of initial biofilm-micrite coating on glass slides.
The genetic structure of cyclic parthenogenetic zooplankton populations is strongly determined by the consequences of combining sexual and asexual reproduction in the same life cycle. Since the pioneering population genetic studies on freshwater zooplankton in the 1970's, a distinction has been made between the genetic structure of permanent and intermittent populations. However, the results of many studies do not fit the expectations of this dichotomous model, for example when large lake populations are considered. In this paper, we present a unifying framework for understanding the genetic structure of cyclic parthenogenetic zooplankton populations, focusing on three factors that determine their degree of clonality and within-population genetic diversity as well as their among-population genetic differentiation: the size of the dormant egg bank, length of the growing season, and strength of clonal selection. We illustrate the importance of each of these factors, and show that our broader concept better explains the variation in genetic structure observed in natural populations of cyclic parthenogens than the earlier implicitly dichotomous model.
Rock pools on granite outcrops occur worldwide and are poorly studied, despite their intrinsic biological interest. In semi-arid Botswana, such habitats occur mainly on the granite outcrops in the southwestern Hardveld zone. To date, studies on these systems have focused mainly on individual species or particular interactions. By means of frequent sampling (every other day) during an entire wet phase (hydrocycle), we attempted to get a time integrated overview of invertebrate species composition in a set of 18 rock pools from two clusters (meta-communities). A faunal list is presented and described. Rock pool species were separated in permanent and ephemeral inhabitants, based on their strategy to survive or escape the frequent dry phases of their habitat, respectively. A new chydorid species, four new turbellarian taxa and two new ostracod species were discovered. All new taxa were permanent inhabitants, illustrating the need for more intense and time-integrated studies of these ephemeral systems and especially the permanent residents with specific adaptations to the vagaries of their variable habitat. The best sampling strategy to assess species richness in these rock pool systems is to randomly sample three to four pools in a cluster, each in the final phase of their hydrocycle.
We examined how community measures (density, biomass, and richness) and community composition were related to microhabitat variables (including hydraulic parameters, substrate, and food resources) in La Tordera stream in Catalonia (NE Spain). We collected macroinvertebrate samples on six dates between November 2001 and September 2002 upstream and downstream of a point source input. Macro-invertebrate density and biomass were positively correlated with food resources and complexity of habitat architecture (benthic organic matter, chlorophyll-a, vascular plants, and mosses) while taxa richness was negatively correlated with conventional (water velocity and depth) and complex hydraulic parameters (Froude number, Reynolds number, roughness shear velocity). Inorganic substrate exerted a minor influence on macroinvertebrate distribution. Ordination analysis revealed that the microhabitat variables of major significance at the two reaches were CPOM, chlorophyll-a, filamentous algae, and maximum velocity. Sand coverage was only relevant for macro invertebrates at the upstream reach, and moss at the downstream reach. The number of significant correlations between macroinvertebrates and microhabitat variables was higher at the upstream reach than at the downstream reach mainly because of higher taxa richness.
To examine the interactive effects of prey density, prey mobility and habitat structure on prey selection by benthic feeding fish, feeding experiments were conducted with 95-105 mm (total length) pumpkinseed (Lepomis gibbosus), given four sizes (< 4 mm, 4-7 mm, 7-9 mm, > 9 mm) of active or immobilized amphipods (Gammarus pseudolimnaeus) as prey. The experiments involved the assessment of prey size selectivity, total number of prey consumed and size-specific indicators of foraging efficiency such as capture success and handling time. Pumpkinseeds were size selective only at medium and high prey densities, preferring large Gammarus when selectivity occurred. The proportion of mobile and immobile prey consumed increased with prey size in unstructured habitat, whereas no selectivity occurred with mobile prey in structured habitat. Both habitat and prey density significantly affected prey consumption, with more prey eaten at higher densities and in unstructured habitat. Prey mobility did not affect the number of prey consumed when mobile or immobile prey were offered in separate trials, but the consumption rate of immobile prey increased in structured habitat when they were offered with mobile prey in the same trial. We conclude that prey density and habitat structure affect pumpkinseed size selectivity and consumption rates in a similar manner, whereas the effect of prey mobility on these factors is more dependent on the combination of density and habitat and on whether mobile and immobile prey are present at the same time.
Few studies have documented temporal changes in bacterial communities in multiple habitats in streams. In this year long study in the West Branch of the Mahoning River in Northeast Ohio, USA, bacteria in water, leaves, and sediments were examined. Bacteria were enumerated using 4,6-diamidino-2-phenylindole (DAPI) and fluorescent in situ hybridization (FISH) using taxon-specific probes for the Domain Bacteria and Burkholderia cepacia. Physical and chemical variables were also monitored. Total bacterial abundance in water (based on DAPI staining) peaked during October 2000 and July 2001; while on leaves, total abundance peaked in January then declined through April with a second June peak. The peak in sediments was during October 2000 and numbers did not differ significantly between a pool and a riffle. Domain Bacteria numbers also exhibited significant temporal changes but the seasonal patterns differed from those based on DAPI staining. Abundance of B. cepacia varied temporally on leaves but not in water and sediments. Contrary to other studies, no significant correlations were seen between bacteriological and physical/chemical variables measured. However, spring run off seems to have been a factor in temporarily reduced numbers on leaves and sediments and increasing bacterioplankton numbers, likely due to allochthonous inputs. Based on prior studies, we expected the pattern of temporal change in bacterial numbers to vary among habitats. However, there were no differences between pool and riffle sediments and no significant correlations between bacteriological and abiotic variables. This may reflect the ability of bacteria to persist under varying temperature/nutrient conditions and flow regimes. The ability of B. cepacia to maintain fairly constant populations, in contrast to the overall assemblage, likely reflects the extreme versatility of this organism.
