Hydroclimate mediates effects of a keystone species in a coldwater reservoir

Abstract

Many reservoirs exhibit interannual variations in water content resulting from climatic variability. Hydroclimatological factors may also affect physicochemical conditions such as thermal stratification and dissolved oxygen concentrations, which in turn have important implications for reservoir biota. We investigated the interplay among water budget, climate, and food web interactions involving a keystone planktivore, Mysis diluviana, occurring at Lake Granby, Colorado, during 1954–2009. A popular kokanee (Oncorhynchus nerka) fishery has been maintained at this 2938 ha montane reservoir, but survival and growth of kokanee has varied widely, with spawning adults ranging in size from 272 to 449 mm total length and population size varying by an order of magnitude (21–269 fish/ha). The kokanee population was strongly affected by Mysis, through competition for food (Daphnia) and because Mysis probably enhanced lake trout recruitment and predation on kokanee. Physicochemical conditions in the reservoir, driven by hydroclimatologic factors, mediated the effects of Mysis on Daphnia, kokanee, and presumably lake trout by controlling habitat suitability, distribution, and survival of Mysis. Effective fishery management in reservoirs requires an improved understanding of the complex interactions among hydroclimate, reservoir conditions, and food web dynamics.