Invertebrate predation and trophic cascades in a pe- lagic food web – The multiple roles of Chaoborus flavi- cans (Meigen) in a clay-turbid lake

Invertebrate predators often have dramatic effects on their prey communities. Unlike visually foraging planktivorous fish, invertebrate predators detect their prey by mechanoor chemoreception. Thus, fish and invertebrate predators affect zooplankton communities differently. Fish predation typically selects large cladocerans while invertebrate predators prefer prey of smaller size. Since invertebrate predators are the preferred food items for fish, their occurrence at high densities is often connected with the absence or low number of fish. It is generally believed that invertebrate predators can play a significant role only if the density of planktivorous fish is low. However, in eutrophic clay-turbid Lake Hiidenvesi (southern Finland), a dense population of predatory phantom midge (Chaoborus flavicans) larvae coexists with an abundant fish population. The population covers the stratifying area of the lake and attains a maximum population density of 23000 ind. m. This thesis aims to clarify the effects of Chaoborus flavicans on the zooplankton community and the environmental factors facilitating the coexistence of fish and invertebrate predators. Lake monitoring, enclosure study and laboratory experiments were conducted to examine the role of Chaoborus flavicans in the pelagic food web. In the stratifying area of Lake Hiidenvesi, the seasonal succession of cladocerans was exceptional. The spring biomass peak of cladocerans was missing and the highest biomass occurred in midsummer. In early summer, the consumption rate by chaoborids clearly exceeded the production rate of cladocerans and each year the biomass peak of cladocerans coincided with the minimum chaoborid density. The strong predation pressure by Chaoborus prevented cladocerans from attaining body lengths long enough to defend them against Chaoborus. In contrast, consumption by fish was very low and each study year cladocerans attained maximum biomass simultaneously with the highest consumption by smelt (Osmerus eperlanus). The results indicated that Chaoborus flavicans was the main predator of cladocerans in the stratifying area of Lake Hiidenvesi. The clay turbidity strongly contributed to the coexistence of chaoborids and smelt at high densities. Turbidity exceeding 30 NTU combined with light intensity below 0.1 E m sprovides an efficient daytime refuge for chaoborids, but turbidity alone is not an adequate refuge unless combined with low light intensity, since in aquarium experiments fish succeeded in preying effectively on Chaoborus at turbidities of 40–50 NTU, if light intensity were above 0.1 μE m s. In the non-stratifying shallow basins of Lake Hiidenvesi, light intensity exceeds this level during summer days at the bottom of the lake, preventing Chaoborus forming a dense population in the shallow parts of the lake. Chaoborus can be successful particularly in deep, clay-turbid lakes where they can remain high in the water column close to their epilimnetic prey without having to diurnally migrate long distances. Suspended clay alters the trophic interactions by weakening the link between fish and Chaoborus, which in turn strengthens the effect of Chaoborus predation on crustacean zooplankton. The aquarium experiments also support the conclusion that clay-turbidity disrupts visual predation by smelt. Since food web management largely relies on manipulations of fish stocks and the cascading effects of such actions, the validity of the method in deep clay-turbid lakes may be questioned.