Adding Nutrients to Enhance the Growth of Endangered Sockeye Salmon: Trophic Transfer in an Oligotrophic Lake

Abstract

Snake River sockeye salmon Oncorhynchus nerka listed under U.S. law as endangered in 1991 in response to a decline in anadromous adult numbers, spend their first 1–2 years in Redfish Lake, Idaho, before migrating to the sea. To determine how nutrient enhancement might influence phytoplankton, zooplankton, and fish production, we performed fertilization experiments in large enclosures in this oligotrophic lake using juvenile kokanee (lacustrine O. nerka) as analogues for endangered sockeye salmon. Fertilization of the metalimnion substantially increased chlorophyll a (150%), phytoplankton biovolume (75%), primary productivity (250%), and zooplankton biomass (200%), and moderately increased fish growth (12%) over our control enclosures. Community composition of phytoplankton and zooplankton changed little, and water transparency declined less than 15% compared with controls. Thus, we concluded that metalimnetic fertilization could maintain the aesthetic value of these lakes while increasing zooplankton food resources for juvenile salmon. Our results suggest that whole‐lake fertilization would aid in the recovery of Snake River sockeye salmon.

Food web responses to low-level nutrient and 15 N-tracer additions in the littoral zone of an oligotrophic dune lake

“…This was evidenced both by the significant increase in periphyton Chl a concentrations in enriched sites over the relatively short duration of the experiment, as well as the rapid increase in ␦ 15 N signatures of periphyton following the initial addition of the tracer. Together, these responses indicate that periphyton can rapidly assimilate nutrients added to the water column in Lake McKenzie's littoral zone, and these findings support the assumption that primary producers (especially algae) in oligotrophic systems rapidly assimilate added nutrients (Budy et al 1998).…”

“…However, results from algal bioassays are often difficult to extrapolate, and it is impossible to predict the long-term consequences of nutrient additions without an understanding of trophic transfer of nutrients from producers to higher order consumers (Mazumder and Lean 1994;Budy et al 1998).…”

Effects of epilimnetic versus metalimnetic fertilization on the phytoplankton and periphyton of a mountain lake with a deep chlorophyll maxima

“…Wurtsbaugh, unpublished observation) where they may have been missed by our sampling procedure. In a similar limnocorral experiment done in a nearby oligotrophic lake, Budy et al (1997) demonstrated that epilimnetic nutrient enrichment increased both zooplankton and fish production. Because of conflicting evidence over the importance of algae in deep layers for zooplankton (cf.…”

“…The limnocorral experiment indicates that metalimnetic nutrient fertilizations may provide advantages over epilimnetic additions in programs designed to promote fish production (i.e., sockeye salmon; Budy et al 1997) especially when aesthetic impacts are important. The two strategies provided similar enhancement of primary production, but metalimnetic fertilization resulted in less periphyton and filamentous cyanobacteria growth, deeper Secchi depths, and greater phytoplankton biomass (as chlorophyll) than did the epilimnetic fertilization.…”

“…If loading occurs to the metalimnion, phytoplankton in the deep chlorophyll layer may be stimulated without increasing algal abundance in the epilimnion, which is most frequently sampled to track eutrophication. Secondly, the epilimnia of lakes are sometimes fertilized to increase fish production (e.g., Budy et al 1997). If algae living in the metalimnia are also nutrient limited, these layers could be fertilized to increase food-web production, perhaps without the negative effect of decreasing water transparency in the epilimnion.…”