Patrik Stenroth

Stable isotopes as an indicator of diet in omnivorous crayfish (Pacifastacus leniusculus): the influence of tissue, sample treatment, and season

Stable isotopes have been used to analyse food webs and (or) trace movements of animals for about 30 years. There has been some debate on the use of different tissues and treatments before isotope analysis, as well as on seasonal effects. We found different crayfish (Pacifastacus leniusculus) tissues (muscle, hepatopancreas, exoskeleton, gill, and whole body) to have different isotope values. Lipid extraction made whole-body carbon isotope values higher but had no effect on nitrogen isotope values. Acidification made whole-body isotope values lower. For crayfish, there was no seasonal or interannual variation in isotope values. In contrast to studies based on gut content analysis, we found adult crayfish to be at least as carnivorous as young-of-the-year crayfish. Earlier studies often have assumed that each food source contributes both nitrogen and carbon in equal proportions. Omnivores do not fit easily into this view. We suggest that nitrogen and carbon in an organism could come from different sources. Adopting this view for a pond food web could render crayfish both predators and detritivores as crayfish prey on nitrogen sources (other invertebrates) and consume large amounts of detritus to satisfy their carbon demand. (Less)

The influence of productivity and width of littoral zone on the trophic position of a large-bodied omnivore.

Omnivory is common in many food webs. Omnivores in different habitats can potentially change their feeding behaviour and alter their trophic position and role according to habitat conditions. Here we examine the trophic level and diet of the omnivorous signal crayfish (Pacifastacus leniusculus) in gradients of trophic status and lake size, both of which have been previously suggested to affect trophic position of predators separately or combined as productive space. We found the trophic position of omnivorous crayfish to be positively correlated with lake trophic status, but found no evidence for any influence of lake size or productive space on crayfish trophic position. The higher trophic position of crayfish in eutrophic lakes was largely caused by a shift in crayfish diet and not by an increase in trophic links in basal parts of the food web. Hence, our results support the “productivity hypothesis,” suggesting that food chains can be longer in more productive systems. Furthermore, stable isotope data indicated that larger crayfish are more predatory than smaller crayfish in lakes with wider littoral zones. Wider littoral zones promoted the development of intrapopulation differences in trophic position whereas narrow littoral zones did not. Hence, differences in habitat quality between and within lakes seem to influence the trophic positions of omnivorous crayfish.

Effects of the filamentous cyanobacterium Nodularia on fitness and feeding behavior of young-of-the-year (YOY) Eurasian perch (Perca fluviatilis)

This study reveals that both cyanobacterial toxicity and turbidity have the potential to reduce the growth and energy storage of young-of-the-year (YOY) perch and thereby influence survival rates. During the 1990s a reduction in recruitment of YOY perch (Perca fluviatilis) occurred along the Swedish East coast. Concurrently, large blooms of filamentous cyanobacteria have increased in the Baltic Proper and in coastal waters. This study examined whether extended exposure to toxic and non-toxic filamentous cyanobacterium Nodularia affect YOY perch growth and feeding behavior under simulated bloom conditions (30 days at 50 μg Chl a L(-1)). Specific growth rate (SGR), the somatic condition index (SCI) and the lipid content of YOY perch (10-12 weeks old) were significantly lower in perch exposed to Nodularia compared to fed controls (no Nodularia). YOY perch exposed to non-toxic Nodularia displayed a higher attack rate than perch living in Nodularia free controls in 2 out of 3 trials. Reductions in growth and energy storage, mediated by cyanobacteria, increase the risk of starvation and predation and could locally influence recruitment of YOY perch.