Marine-derived nutrients, commercial fisheries, and production of salmon and lake algae in Alaska

For decades ecologists have recognized the potential importance of marine-derived nutrients (MDN) deposited in freshwater ecosystems by spawning anadromous salmon. Previous studies have shown that some MDN are retained in freshwater ecosystems. A popular hypothesis linking MDN to salmon population productivity posits that MDN provided by post-spawning mortality of salmon are critical for salmon population dynamics because they enhance prey populations in the freshwater ecosystems used as nursery habitats. We tested this hypothesis by reconstructing historical sockeye salmon populations for the last 300 years in Bristol Bay, Alaska. Stable nitrogen isotope chronologies in lake sediments and sockeye catch and escapement histories show that commercial fisheries intercepted about two-thirds of MDN bound for freshwater spawning grounds since about 1900. Reconstruction of lake algal production using, fossil pigments shows that this loss of MDN has reduced lake algal productivity to about one-third of its level before commercial fishing. However, contrary to expectation, recent sockeye population sizes (sum of spawning escapement and fishery catch) in the last century were equivalent to those before the advent of commercial fishing. These data demonstrate that the MDN subsidy is important for the productivity of coastal lakes but that some sockeye salmon populations are limited by other features of ecosystems such as the amount of suitable spawning habitat.