THE IMPORTANCE OF MARINE-DERIVED NUTRIENTS TO LAKE PRODUCTIVITY AND SALMON POPULATION DYNAMICS OVER THE LAST FIVE CENTURIES IN SOUTHWEST ALASKA

For decades ecologists have been fascinated by the importance of marine-derived nutrients deposited in freshwater ecosystems by spawning migrations of anadromous salmon. Stable isotopes and mass balance studies show that much of the nutrients deposited by salmon carcasses are retained in freshwater and riparian ecosystems. These observations led to the hypothesis that salmon migrations are critical for maintaining the productivity of coastal freshwater ecosystems, and thus, the freshwater phase of salmon populations as well. We tested this hypothesis by reconstructing historical sockeye salmon populations for the last five centuries in Bristol Bay, Alaska. Stable nitrogen isotope chronologies in sediments of nursery lakes and time series of fishery catches show that commercial fisheries have intercepted about 66% of the marine-derived nutrients bound for freshwater spawning grounds since 1900. Reconstruction of lake algal production using fossil photosynthetic pigments in sediments shows that this loss of nutrients has reduced current lake productivity to about one third of the level prior to the development of commercial fishing. However, contrary to expectation, the sizes of entire sockeye runs (i.e., spawning escapement and fishery catch) in the last century are approximately equivalent to those we have reconstructed for periods before the advent of commercial fishing at the end of the 19th century. These data suggest that the marine nutrient subsidy is important for the productivity of pristine coastal lakes, but that sockeye populations are limited by other factors such as suitable spawning habitat and climatic variation.