Lentic macroinvertebrate assemblage structure along gradients in spatial heterogeneity, habitat size and water chemistry

Littoral zones of small water bodies are spatially heterogeneous habitats, harbouring diverse biotic communities. Despite this apparent heterogeneity, many studies have stressed the importance of water chemistry in determining the structure of littoral macroinvertebrate assemblages. The purpose of this study was to consider the relative importance of several spatial and water chemistry variables in explaining the patterns in the structure of macroinvertebrate assemblages in 21 lentic water bodies in northeastern Finland. Water bodies were selected to represent various habitat conditions ranging from small permanent bog ponds to small forest lakes. According to canonical correspondence analysis (CCA), the most important environmental factors related to assemblage composition were water body area, moss cover, total nitrogen and water hardness. In general, species composition in small bog ponds tended to differ from that in larger lakes with forested shoreline. Total species richness was best explained by a composite variable (PCA) describing physical habitat heterogeneity, species richness being lowest in small bog lakes with simple bottom structure and low amount of aquatic plants. Species numbers in dominant functional feeding groups were related to different environmental factors. Shredder species richness was best explained by a regression model incorporating total nitrogen and the amount of organic matter, both of which were negatively related to the number of shredder species. The number of gatherer species increased with mean substratum particle size. Scraper species richness was negatively affected by the abundance of detritus and positively affected by depth, and a model including both variables explained most of the variation. Variation in the number of predatory species was best explained by a regression model including moss cover and lake area.     

Biology of <Emphasis Type="Italic">Cheirogaleus major</Emphasis> in a Littoral Rain Forest in Southeast Madagascar

Greater dwarf lemurs (Cheirogaleus major) are small nocturnal primates from the rain forests of eastern Madagascar. I investigated a population of Cheirogaleus major in a littoral rain forest of Southeast Madagascar during 2 rainy seasons to supplement the sparse information available for the species. I collected data on morphology, group composition, sleeping behavior, home range, and social organization via mark/recapture, radio telemetry, and focal individual observations. I identified 2 presumed family groups, and my data from radiotracking revealed a monogamous social organization. In each group, I found an adult pair and its presumed offspring sharing home ranges and sleeping sites together. I also observed gregarious behavior of group members during their nocturnal activity. I found no difference in body measurements between sexes, but body mass and tail circumference increased significantly from November to February, indicating a fatting period before hibernating.     

The littoral fish assemblage of the Desventuradas Islands (Chile) has zoogeographical affinities with the Western Pacific

A systematic account of fish species inhabiting waters surrounding the Desventuradas Islands, off Chile, is presented. The 61 species recognized belong to 33 families of Osteichthyes, and one of Chondrichthyes (Squalidae). It was found that the icthyofaunal links were stronger to the Western Pacific (90.7%) than to the Eastern Pacific (6.9%), despite the fact that these islands are much closer to the South American coast than to the coasts of Oceania. Hydrographic and physical barriers are postulated as explaining this pattern.     

Food-web indicators accounting for species interactions respond to multiple pressures

Food-web indicators for marine management are required to describe the functioning and structure of marine food-webs. In Europe, the Marine Strategy Framework Directive (MSFD), intended to lead to a ‘good environmental status’ of the marine waters, requires indicators of the status of the marine environment that also respond to manageable anthropogenic pressures. Identifying such relationships to pressures is particularly challenging for food-web indicators, as they need to be disentangled from linkages between indicators of different functional groups caused by species interactions. Still, such linkages have not been handled in the indicator development. Here we used multivariate autoregressive time series models to identify how fish indicators in an exploited food-web relate to fishing, climate and eutrophication, while accounting for the linkages between indicators caused by species interactions. We assembled 31-year long time series of indicators of key functional groups of fish in the Central Baltic Sea pelagic food-web, which is characterized by strong trophic links between cod (Gadus morhua) and its main fish prey sprat (Sprattus sprattus) and herring (Clupea harengus). These food-web indicators were either abundance-based indicators of key piscivores (cod) and zooplanktivores (sprat and herring) or size-based indicators of the corresponding trophic groups (biomass of large predatory fish (cod ≥ 38 cm) and biomass of small prey fish (sprat and herring <10 cm)). Comparative analyses of models with and without linkages among indicators showed that for both types of indicators, linkages corresponding to predator-prey feedbacks and intra-specific density-dependence were essential to explain temporal variation in the indicators. Thus, no indicator-pressure relationships could be found that explained the indicators’ variation unless such linkages were accounted for. When accounting for these, we found that the indicators overall respond to multiple pressures acting simultaneously rather than to single pressures, as no pressure alone could explain how the indicators developed over time. The manageable pressures fishing and eutrophication, as well as the prevailing hydrological conditions influenced by climate, were all needed to reproduce the inter-annual changes in these food-web indicators combined, although individual relationships differed between the indicators. We conclude that our innovative indicator-testing framework can therefore be used to identify responses of food-web indicators to manageable pressures while accounting for the biotic interactions in food-webs linking such indicators.     

Body size and substrate association of littoral insects in relation to vegetation structure

Variation in substrate association types and maximum size of aquatic insects were studied in a vegetated littoral zone of three lake basins. The basins differed from each other in trophic status, biomass of benthivorous fish, and abundance of macrophytes. Four types of substrate association – swimmers, crawlers, semisessiles and burrowers, respectively – were assumed to represent decreasing vulnerability to fish predators. Large-sized species were also hypothesised to be more vulnerable to fish predators. The distributions of species traits were examined in relation to vegetation density. Inferring from ``predation hypothesis' opposite selection pressures on the species traits were expected along the vegetation density. Dense macrophyte beds were thought to be dominated by invertebrate predators and open water by fish predators, since the predation efficiency of fish decreases in complex environments. In the case of invertebrate predator domination, large size and higher activity should be favoured traits among the prey species. Distribution patterns of modes of the two studied traits were explored separately for predatory and non-predatory insects. As expected, swimmers and large-sized crawlers were characteristic of the insect assemblages of dense macrophyte beds. The densities of Odonata, Corixidae, Dytiscidae, Ephemeroptera and Sialidae were higher among macrophytes than in open water, where these insect taxa were possibly depleted by fish. On the other hand, the small-sized and fairly immobile Chironomidae were the most abundant group in open water. These results support the existence of a predator transition zone among littoral vegetation, ranging from domination of invertebrate predation among the dense beds to that of fish predation in open water.     

Convective water exchanges during differential cooling and heating: implications for dissolved constituent transport

Convective water exchange patterns, determined from water temperature variations, were examined in the Minky Creek embayment of Guntersville Reservoir, Alabama (USA), during the month of September, 1990. During periods of differential cooling, cooler water originating from shallow stations moved as an underflow current toward the center of the embayment, while warmer water moved as an overflow current toward the shore. During periods of differential heating, convective exchange was much more shallow, occurring in the upper 3 m of the water column. As warmer water from shallow regions moved out as an overrflow current, this water was replaced by a return flow of cooler water originating from the pelagic epilimnion. Wind speed appeared to influence convective exchange patterns during differential heating phases by affecting the depth of the surface mixed layer. The potential for convective exchanges during periods of differential cooling and heating occurred on 54% and 74% of the days, respectively, in September. This high potential for horizontal water exchanges in Guntersville Reservoir has strong implications for the lateral transport of dissolved constituents.     

Reproductive consequences of the terrestrial habit in Orchestia (Crustacea: Amphipoda)

The reproductive bionomics of four species of British littoral Orchestia are compared and contrasted. Three of the marine species studied form a series which occupy successively higher positions on the shore and demonstrate increasing degrees of preadaptation to terrestrial life.

The fundamental adaptations of primitive terrestrial Orchestia are: adoption of a cryptozoic habit, and a reduced growth rate and adult body size. As a consequence of slow growth and small body size, the number of eggs/brood and number of broods/year is markedly reduced. Commonly associated adaptations are: female larger than the male, sex ratio bias to the female, and a tendency to endemism.     

The Life History and Occurrence of Sialis lutaria L. (Megaloptera) in the Littoral Zone of Lake Esrom,Denmark

The life history and distribution of Sialis lutaria L. in the exposed littoral zone of Lake Esrom, Denmark were examined during 1979/80. Sialis showed an annual life cycle with mean annual abundance of 20 and 40 ind m⁻² at 0.4 and 2.0 m depth, respectively. Growth peaked during July with rates of about 10 % d⁻¹. Net annual production averaged 0.2 g DW at 0.4 m depth and 0.8 g DW at 2.0 m depth. The annual P/B-ratios were estimated in the range of 4.6-7.1.     

Structure and dynamics of zooplankton communities in the littoral zone of some North Carolina lakes

Species composition and seasonal, dynamics of zooplankton in the littoral zone were studied in three piedmont North Carolina lakes for one year. Nygaard's compound index indicated oligotrophic conditions in Belews Lake and eutrophic conditions in Salem Lake and Lake 150. A total of 32 species of zooplankton were found in the samples. Significantly fewer species of truely littoral zooplankton were present in the oligotrophic lake regardless of season (P < 0.05). Eutrophication appears to favor increased diversity in zooplankton of the littoral zone. Habitat heterogeneity, provided by a well developed littoral zone containing aquatic macrophytes, may be the major factor contributing to the maintenance of more taxa of zooplankton in eutrophic systems.Distinct taxa of Cladocera and Copepoda are present in the littoral zone, exhibiting seasonal dynamics that are largely different from the limnetic fauna. Such dynamics may result from predation by characteristically limnetic species of cyclopoid copepods which seek prey in the littoral zone. The numerical and seasonal abundance of predators and prey are inversely related and the true littoral taxa, represented in the present study by the genera Alona, Chydorus, Pleuroxus, Sida, Simocephalus, and Eucyclops, contained no predaceous species. The seasonal dynamics of these genera are very similar even in widely separated geographical regions, indicating that the factor(s) responsible operate independently of climatic and chemical variables. Intrazooplankton predation appears to be a more plausable explanation than segregation along depth which, although consistent with data from studies of limnetic waters, cannot be related to populations of zooplankton in shallow littoral areas.     

Predictive Length–Dry Mass Regressions for Freshwater Invertebrates in a Pre‐Alpine Lake Littoral

To date, no regressions for invertebrates from lentic environments are available to facilitate biomass estimation. Predictive length–dry mass regressions for 20 freshwater invertebrate taxa from a pre‐alpine lake littoral in Central Europe are presented here. Regressions were calculated by fitting a power function to body‐length parameters. For Gammarus roeseli, the head height, an easily measurable length parameter, was used. Differences between regressions of different taxa were confirmed by analyses of covariance. Equations for invertebrates grouped at a higher taxonomic level had lower coefficients of determination; therefore, the use of regression equations at the lowest possible taxonomic level is indicated. A comparison of regressions with those published for related taxa in streams revealed significant differences, supporting the opinion that equations should be applied carefully and their use restricted to waterbodies with similar abiotic conditions.