Long-term zooplankton responses to nutrient and consumer subsidies arising from migratory sockeye salmon Oncorhynchus nerka

Migratory animals, such as Pacific salmon, can significantly shape communities in recipient habitats both by altering the flux of resources, and changing community composition and subsequent trophic interactions. Here we mainly used paleo-ecological records from natural sockeye salmon nursery lakes to quantify the response of plankton communities to the influx of salmon-derived nutrients and consumers (juvenile salmon). Our long-term data show that increases in the density of spawning salmon often elevated influx of nutrients, and, in turn, zooplankton production over the past few centuries. In contrast, significant correlations were not detected in two lakes with extremely low or high average spawner densities (i.e. 1.5 and 34.7 x 10(3) spawners km(-2) year(-1) respectively). With increasing spawner densities across lakes, analysis of the size structure of subfossils in sediments revealed a strong decrease in body size of a main juvenile salmon prey item (Eubosmina longispina; r(2) = 0.36, p < 0.001, n = 67), consistent with an overriding effect of predation in lakes with high salmon densities. These long-term data not only highlight the key role of salmon-derived nutrients in stimulating plankton communities, but also suggest that the relative effect of nutrient and consumer subsidies varies along gradients of lake production, despite a single ultimate causal mechanism (migrating fish).