Clay-Turbid Interactions May Not Cascade—A Reminder for Lake Managers Jukka Horppila 1,2 and Anne Liljendahl-Nurminen 1,2 Abstract Food web management is a frequently used lake restora- tion method, which aims to reduce phytoplankton biomass by strengthening herbivorous zooplankton through reduc- tion of planktivorous fish. However, in clay-turbid lakes several factors may reduce the effectivity of food web management. Increasing turbidity reduces the effectivity of fish predation and weakens the link between zooplank- ton and phytoplankton. Therefore, the effects of fish stock manipulations may not cascade to lower trophic levels as expected. Additionally, in clay-turbid conditions inverte- brate predators may coexist in high densities with plank- tivorous fish and negate the effects of fish reductions. For instance, in the stratifying regions of the clay-turbid Lake Hiidenvesi, Chaoborus flavicans is the main regulator of cladocerans and occupies the water column throughout the day, although planktivorous Osmerus eperlanus is very abundant. The coexistence of chaoborids and fish is facili- tated by a metalimnetic turbidity peak, which prevents efficient predation by fish. In the shallow parts of the lake, chaoborids are absent despite high water turbidity. We suggest that, generally, the importance of invertebrate predators in relation to vertebrate predators may change along turbidity and depth gradients. The importance of fish predation is highest in shallow waters with low turbid- ity. When water depth increases, the importance of fish in the top-down regulation of zooplankton declines, whereas that of chaoborids increases, the change along the depth gradient being moderate in clear-water lakes and steep in highly turbid lakes. Thus, especially deep clay-turbid lakes may be problematic for implementing food web manage- ment as a restoration tool. Key words: clay turbidity, food web management, inverte- brate predators, top-down regulation. Introduction The potential productivity of lake ecosystems is deter- mined by bottom-up forces such as the availability of nutrients and light, whereas the actual productivity is determined by food web structure (Carpenter et al. 1987). Food web management (biomanipulation, Shapiro et al. 1975), which aims to control the actual productivity by changing the structure of the food web, has become a pop- ular restoration method for eutrophicated lakes. The method is based on the food chain theory (Hairston et al. 1960), according to which top carnivores are resource lim- ited, whereas primary producers are controlled by resour- ces if there is an odd number of trophic levels in the food chain and by grazers if there is an even number of trophic levels. In food web management, the theory is applied by depressing planktivorous fish through selective fishing and/or by strengthening the level of piscivorous fish through stockings or catch restrictions, which should result in a lower phytoplankton biomass in a given nutrient con- centration. The original definition of biomanipulation included manipulations of both top-down and bottom-up forces, but nowadays the term is usually understood as manipulation of top-down forces. The trophic cascade hypothesis, a derivative of the food chain theory, states that changes in the abundance of piscivores are mediated through planktivorous fish and herbivorous zooplankton to phytoplankton (Carpenter et al. 1985). Several attempts have been made to define the circum- stances facilitating successful lake restoration through food web management. Threshold values for lake mean depth, external nutrient loading, as well as total phospho- rus concentration have been presented (Jeppesen et al. 1990; Reynolds 1994). It has been widely accepted that bi- omanipulation possibilities are best in shallow, moderately eutrophic lakes (Reynolds 1994; Drenner & Hambright 1999; Benndorf et al. 2002), because macrophytes can col- onize large areas and improve the water quality in shallow lakes by reducing sediment resuspension and providing refuges for herbivorous zooplankton. In this article we pose a point of view that has been given too little space in the context of food web management— the role of inorganic turbidity. Cuker (1993) reminded that theories on cascading trophic interactions derived from clear-water lakes may not hold in mineral-turbid conditions. However, discussion on the subject has been absent, and in studies evaluating the usefulness of food web management, the role of mineral turbidity has been ignored. This is a shortcoming, because increased con- centrations of suspended material and enhanced turbidity 1 Department of Biological and Environmental Sciences/Limnology, Box 65, University of Helsinki, FIN-00014, Helsinki, Finland. 2 Address correspondence to J. Horppila, email jukka.horppila@helsinki.fi and A. Liljendahl-Nurminen, email anne.liljendahl@helsinki.fi Ó 2005 Society for Ecological Restoration International 242 Restoration Ecology Vol. 13, No. 2, pp. 242–246 JUNE 2005