Nitrogen stable isotope composition (δ15N) of particulate organic matter (POM) has been used to infer dominant nitrogen cycling processes in lakes. However, very few studies have compared the isotope variations in lakes along trophic state and other biogeochemical gradients. Here we report an analysis of δ15NPOM and selected environmental variables from 96 subtropical lakes to assess the patterns and controls of isotope variations. Results indicate that δ15NPOM values varied from −2.8 to 13.2‰ and were not significantly correlated with total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a). The 15N depletion in POM was found across the entire trophic gradient and likely reflected contributions from planktonic nitrogen fixation. The 15N enrichment was attributed to high primary production and the contributions of anthropogenic wastes in eutrophic lakes, and the presence of microzooplankton in the water samples of the oligotrophic lakes. The δ15NPOM was negatively correlated with water color and positively correlated with pH. Because water color is indicative of light availability which affects phytoplankton growth and pH is significantly correlated with Chl a concentration in the study lakes, the close relationship between water color, pH and δ15NPOM therefore suggests primary productivity-driven isotope fractionation. Results from this study did not reveal the importance of trophic state, nitrogen concentration and surface area to the variations of δ15NPOM and points to the complex interactions of nitrogen cycling processes in lakes with diverse biogeochemical features.

The biomass and composition of the phytoplankton was studied at seven sampling stations located at different water bodies (open water, inner pond and canal) of a large, shallow turbid lake (Lake Fertő, Neusiedlersee). The open water was characterized by picocyanobacteria and meroplanktonic diatoms, the artificial canal by epiphytic diatoms, cryptophytes and chlorophytes. The inner ponds, based on the phytoplankton composition, were positioned between these water bodies. High picoplankton abundance (>106 cells.mL−1) and predominance (up to 80% contribution to the total phytoplankton biomass) were detected in the open water and the inner ponds, which was hypothesized to be the result of the turbid environment by the suppression of the top down control and by light-limitation. Information on the diversity of the picoplankton in the open water of the lake has been presented first time in this study. Based on molecular analysis (16S rRNA gene and cpcBA-IGS region) the dominant group of picocyanobacteria belonged to the Cyanobium gracile cluster (group A) of the picophytoplankton clade in April. Members of two other picocyanobacterial groups (group B and C) were also detected.

Resource availability influences the flow of energy through food webs, thus affecting biomass and trophic structure of consumer communities. To assess the effect of availability of allochthonous (leaf litter) and autochthonous (periphyton) resources on the trophic structure of macroinvertebrates, we sampled monthly for a year two sites (upstream forested, downstream open canopy) of Challhuaco, a low order mountain stream in the Patagonian Andes. Leaf litter and periphyton dynamics showed the expected pattern, peaking in autumn and spring, respectively. Temporal variations of shredders and scrapers were synchronized, with the logic delay, with resource abundance. Although leaf litter mass was similar at both stream reaches, shredders were more abundant at the forest site. Periphyton biomass was higher at the forest, while scrapers were more abundant at the open site. The patterns of resource and consumer abundance, and community functional structure were mostly explained by the interaction of abiotic factors and trophic relationships.

Though much is known about freshwater snail ecology, their circadian rhythms remain poorly investigated. Well-fed, stress-free, mid-size adults of six species common in central Italian lakes were exposed to natural sunlight and photoperiod, and their activity status was recorded at 3-h intervals during a 9-d indoor experiment. All species exhibited evident diurnal habits despite high individual variability, with midday-to-early-afternoon activity peaks. Activity was correlated with diel light conditions but not with short-term changes in albedo. The prosobranch Bithynia (=Codiella) leachii and the pulmonates Physa (=Physella) acuta and Planorbis planorbis were the most active species and exhibited the longest-lasting response to daytime food addition. The prosobranch Valvata piscinalis exhibited long periods retracted in its shell with the operculum shut, and the remaining taxa (the pulmonates Galba (=Lymnaea) truncatula and Radix (=Lymnaea) auricularia) exhibited an intermediate degree of activity. P. acuta was the most active species at night and exhibited the quicker response to nighttime food addition. Alertedness to (diurnal) predators may be highest for the highly active P. acuta and P. planorbis, whose antipredator defenses are mainly behavioral. Diel activity patterns and other ecological characteristics suggest that P. acuta may be favored in food-rich habitats, while V. piscinalis may not be able to fully exploit food resources, especially if in limiting quantities. All snail species – and P. acuta in particular – may stimulate periphyton metabolism while keeping its biomass low by grazing mainly during the time of maximum photosynthesis.

Largemouth bass, Micropterus salmoides, have inhabited Lake Biwa in central Japan for over three decades as top predators. Lake Biwa consists of two basins – a large, deep north basin and a small, shallow south basin. Since the mid 1990s, vegetation and bluegill sunfish, Lepomis macrochirus, have thrived in the lake – especially in the south basin. Because dense vegetation can mediate the predator-prey relationship between largemouth bass and bluegill, the largemouth bass in the south basin are assumed to be under less favorable conditions than those in the north basin. The length-weight relationship of the largemouth bass in the two basins differed; the body weights of largemouth bass in the north basin significantly exceeded those in the south basin. Moreover, largemouth bass in the south basin appeared to have significantly larger gapes than those in the north basin. Such fish in the south basin were likely selected during and after the ontogenetic diet shift stage, probably because they are capable of handling bluegill more efficiently than those with normal-sized gapes.

Growth of colonial Microcystis aeruginosa was investigated by performing an incubation experiment. Colonies of M. aeruginosa were separated based on size (colony diameter <100 μm, 100–200 μm and >200 μm) by filtration. Additionally, the cells around the surface of the colonies were separated from those inside the colonies by short-term ultrasonic treatment followed by filtration. Experimental results indicate that M. aeruginosa grew continuously throughout a 35-day incubation period in a nutrient-rich medium at specific growth rates between 0.045 and 0.310 d−1. On day 14, larger colonies exhibited insignificantly higher specific growth rates. However, on day 35, the specific growth rates of colonies with diameters less than 100 μm insignificantly exceeded those of larger colonies. Internal cells of the colonies tended to grow faster than peripheral cells. Furthermore, the specific growth rates of the cells that comprised colonies with diameters of below 200 μm exceeded those of peripheral cells. These results suggest a potential growth strategy of M. aeruginosa in maintaining a high growth rate, eventually leading to the dominance of large colonies, which have notable ecological advantages over smaller ones.

Chironomid assemblages collected from seven different streams in South Korea were investigated. The subfamily composition of chironomids was clearly differentiated accross different levels of organic pollution. Species abundance distributions (SADs) of chironomid communities were compared with the total macroinvertebrate communities across different levels of pollution. The number of species with minimal range of abundance was lower in SADs for chironomid communities compared with total communities. The log normal distribution was accepted for both total and chironomid communities, while the geometric series was relatively more suitable for chironomids and the log series were more fitted to total communities. The a values in the log normal distribution increased in chironomid communities across different levels of pollution, while γ values increased at the polluted sites for both chironomid and total communities. In the Power law analysis, the parameter decreased in chironomid communities. The dominance decay model was more fitted to chironomid communities in clean conditions while random fraction and assortment models were more suitable for the polluted sites.