The different roles of crayfish in benthic food webs

Abstract: I show that crayfish consume detritus, algae, macrophytes, and an array of invertebrates. Crayfish were found to reduce invertebrate densities. However, prey mobility seemed to be a key factor regulating the strength of interactions between crayfish and prey. Since crayfish seek to maximize growth, they benefit from feeding on invertebrates, shown to promote high growth rates. I found crayfish to increase their intake of invertebrate prey in eutrophic lakes. This suggests that crayfish have different roles in different lakes, they seem to be predators in eutrophic lakes and more like primary consumers in oligotrophic lakes. In lakes where crayfish are predacious they elongate the food chain between basal resources (i.e. detritus and algae) and top predators like perch. This since perch are likely to prey heavily on crayfish when these were available. Moreover, I found crayfish to be larger in eutrophic lakes, this indicates that they grow faster in eutrophic waters. The rationale behind this conclusion is that the mean sizes of crayfish, possible to catch in baited traps, at the beginning of each growing season (summer) would be very similar between populations due to intensive harvest the previous autumn. Consequently, I interpreted the differences in mean size of crayfish at the end of the growing season (when harvest starts) to be a measure of growth differences between populations. Furthermore, the results of this thesis indicate that omnivores like crayfish could derive carbon from some food sources and simultaneously derive nitrogen from other sources. I found periphyton to be the chief ultimate nitrogen source for crayfish in streams. About half of the nitrogen seemed to come from direct consumption of algae and the other half via consumption of invertebrates that in turn relied on periphyton as their chief nitrogen source. In contrast, I found no clear trend regarding ultimate carbon sources for stream crayfish. In addition, these findings suggest that omnivorous organisms potentially could be at different trophic level with respect to different elements.