No abstract

For a short-lived organism, such as a fresh water zooplankter, the ways of coping with years of local recruitment failure are either to disperse between habitats and recolonise or to disperse in time through diapause. Diapause is common among freshwater zooplankton and is generally seen as a way to escape periods of harsh environmental conditions. The egg-bank or pool of diapausing copepodites in lake sediments resulting from the production of diapausing stages has several implications for zooplankton ecology, genetics, and evolution which we outline in this review. The presence of a benthic dormant stage also creates a coupling between the benthic habitat and the pelagic, and we argue that zooplankton phenology is a result of selective forces in both habitats. The spatial distribution of diapausing eggs appears to be governed by random resuspension dynamics coupled with higher hatching rates in shallow waters. For diapausing copepodites, however, an active choice of where Aquatic Sciencesand how deep to enter the sediment may affect their distribution. In a reanalysis of published data, we found a size-dependent bathymetric distribution and vertical distribution in the sediment of diapausing cyclopoid copepodites. Our review of published laboratory studies showed that predictors of seasonal change such as photoperiod and temperature were the only type of cues used for the termination of diapause. We also found a relation between generation length and the type of cue used for diapause induction: copepods mainly used seasonal cues from the abiota, rotifers mainly used cues from the biotic environment, and cladocerans used a mix of both types. We describe patterns in emergence timing and contribution to population dynamics from studies using in situ estimation of emergence, and conclude that hatching from dormant stages may qualitatively and quantitatively affect zooplankton population dynamics and seasonal succession.

Abstract. We investigated the effects of an exotic snail, the golden apple snail (Pomacea canaliculata) on biodiversity and ecosystem functioning in tropical wetland ecosystems. This large snail (up to 80-mm shell height) has invaded large parts of Southeast Asia during recent decades. A survey of natural wetlands in Thailand showed that high densities of the snail were associated with almost complete absence of aquatic plants, high nutrient concentrations, and high phytoplankton biomass, that is, a complete shift in both ecosystem state and function. A field experiment demonstrated that grazing by the snail can cause the loss of aquatic plants, a change toward dominance of planktonic algae, and thereby a shift toward turbid water. Estimates of biologically fixed nutrients released through snails grazing on aquatic plants revealed that phosphorus releases were sufficient to explain the recorded increase in phytoplankton biomass. Hence, our study demonstrates how an herbivore may trigger a shift from clear water and macrophyte dominance to a turbid state dominated by planktonic algae. This shift and the continuing aggressive invasion of this exotic species are detrimental to the integrity and functioning of wetland ecosystems, and to the services these provide in Southeast Asia.

The amount of plastics released to the environment in modern days has increased substantially since the development of modern plastics in the early 1900s. As a result, concerns have been raised by the public about the impact of plastics on nature and on, specifically, aquatic wildlife. Lately, much attention has been paid to macro- and micro-sized plastics and their impact on aquatic organisms. However, micro-sized plastics degrade subsequently into nano-sizes whereas nano-sized particles may be released directly into nature. Such particles have a different impact on aquatic organisms than larger pieces of plastic due to their small size, high surface curvature, and large surface area. This review describes the possible sources of nano-sized plastic, its distribution and behavior in nature, the impact of nano-sized plastic on the well-being of aquatic organisms, and the difference of impact between nano- and micro-sized particles. We also identify research areas which urgently need more attention and suggest experimental methods to obtain useful data.

To assess the principal factors regulating periphytic algal communities, I conducted a field study along a productivity gradient of Swedish and Antarctic lakes ranging from extremely low productivity meltwater to highly productive lakes. Because the Antarctic lakes lack fish, as well as molluscs anId insect larvae, the data set provides considerable variation in food web composition, offering a rare opportunity to evaluate the importance of these factors in determining the biomass of periphytic algae. Among the possible factors investigated, water temperature, substrate characteristics, and food web composition seem to be of minor importance. Instead, there was a curvlinear relationship between the biomass of sediment-associated periphytic algae and lake productivity, suggesting a shift from nutrient limitation to light limitation caused by phytoplankton. A temporal study in the littoral zone of one of the lakes included in the spatial study showed that the temporal variation in periphyton biomass within the lake was low compared to the variation between lakes. Th,e temporal study also strengthened suggestions from the spatial study by pointing to nutrient availability as a major factor determining periphytic algal biomass.

Abstract. Adaptations and counteradaptations are common in coevolving predatorprey systems, but little is known of the role of maternal transfer of adaptive traits in mediating species interactions. Here, we focused on tolerance against cyanobacterial toxins and asked whether this tolerance was an induced defense developed during Daphnia's lifetime, whether it was a trait that is constantly expressed, and whether such tolerance to the toxin can be transferred to the next generation through maternal effects. These questions were addressed by feeding a single clone of Daphnia magna a diet with and without algal toxin and recording changes in fitness (as intrinsic rate of population increase). Analysis of F1, F2, and F3 generations revealed that the increased tolerance to toxic Microcystis was an inducible defense developed during an individual's lifetime, and that this trait could be transferred from mother to offspring. This maternal effect was expressed in several fitness parameters, including shorter time to maturity and first reproduction, and higher numbers of offspring compared to inexperienced individuals. In some circumstances, such maternal effects may increase population production by up to 40% and may help to stabilize material and energy transfer to higher trophic levels.

The Biology of Lakes and Ponds focuses on the interactions between the abiotic frame, such as turbulence, temperature, pH and nutrients, and the organisms, including interactions with and among organisms at the individual, population and community level. The book fills this niche between traditional limnology and evolutionary ecology by focusing on physiological, morphological and behavioural adaptations among organisms to abiotic and biotic factors and how interactions between biotic processes and abiotic constraints determine the structure and dynamics of lake and pond systems. In addition, the book describes and analyses the causes and consequences of human activities on freshwater organisms and ecosystems and covers longstanding environmental threats, such as eutrophication and acidification, as well as novel threats, such as biodiversity loss, use of everyday chemicals and global climate change. However, also signs of improvement and the possibilities to restore degraded ecosystems are discussed and provide hope for future generations.

Abstract. In a variable and unpredictable environment, phenotypic plasticity in morphology or behavior may considerably improve an organism's protection against environmental threats and thereby its fitness. Here I demonstrate that common freshwater organisms, copepods (Crustacea), show a plastic response by adjusting pigmentation level in relation to two environmental threats: ultraviolet radiation (UV) and predation. The red pigment in copepods, astaxanthin, reduces damage caused by UV radiation, but makes the organism more conspicuous, thereby exposing it to higher predation pressure. In a field survey of six lakes sampled monthly for 16 mo, I quantified UV and predation threat, as well as copepod pigmentation level. The relative threat ratio (UV/predation) was generally lowest during summer and highest during spring; this pattern was paralleled by pigmentation level among copepods. Moreover, the level of pigmentation among copepods in lakes with high predation pressure was lower than among those copepods in lakes with lower risk of predation. In a complementary experimental study performed under constant UV threat, calanoid copepods in the absence of predation threat responded with almost three times higher pigment levels, compared to those with fish present (caged). Hence, the correlative field survey and the mechanistic experiment together suggest that the level of pigmentation in copepods is an inducible and adjustable defense, governed by the aim to improve individual protection against prevailing threats from both predation and UV radiation.