Effect of volcanic ash deposits on sockeye salmon lakes

Mathisen, OA; Poe, P

Eruption of the Mount St. Augustine volcano (Cook Inlet, Alaska) in January-February 1976 and deposition of volcanic ash over the Kvichak watershed, including Iliamna Lake, provided an opportunity to study effects of enrichment from the ash on the food chain which includes fry of sockeye salmon (Oncorhynchus nerka). These salmon spawn in many of the major oligotrophic lakes adjoining the North Pacific Ocean. Their food consists of such small zooplankters as Cyclops scutifer and Bosmina coregoni. Zooplankton production is limited by the lakes ' normally low dissolved nutrient contents, in turn limiting salmon production. A previous study showed that volcanic eruption caused an immediate increase in diatoms, followed three years later by an eight-fold increase of Cyclops scutifer over the next three years. The Mount St. Augustine 1976 eruptions deposited 6.6-14.9 million metric tons of ash over about 13,000 sq km of the Kvichak watershed, with density in the eastern end ranging 1.1-2.1 kg/sq m. Ash composition included high silica content (63.3% by wt), absence of nitrogen, and higher total phosphorus content (654 ppm) than from carcasses of parent salmon, though only .09 ppm was water-soluble orthophosphate. There was an almost immediate increase of phytoplankton in Iliamna Lake, especially Melosira, but as yet (1976) no change has occurred in total secondary production, or in size or abundance of juvenile sockeye salmon.

Effect of volcanic ash deposits on sockeye salmon lakes

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