Scale-dependent effects of zebra mussels on benthic invertebrates in a large eutrophic lake

Abstract

We studied mechanisms by which zebra mussels may elevate densities of aquatic invertebrates at local and lake-wide spatial scales in a productive lake. Locally, zebra mussels provide structural refuge from predation and deposit organic material. An experiment demonstrated that the density of benthic invertebrates increased locally as a function of zebra mussel density in Oneida Lake, New York. On a lake-wide spatial scale, light penetration increased dramatically in response to zebra mussel invasion. Consequently, the area of lake bottom likely to support benthic primary production also increased. The maximum depth of light penetration sufficient to support benthic algal photosynthesis increased from 7.5 m in pre-invasion years (1975–1991) to 10.5 m in post-invasion years, and macrophytes extended to a greater depth in post-invasion years. Algae and macrophytes may provide food or structural refuge to invertebrates. An intensive survey of one shallow bay showed that benthic invertebrate densities were higher in 1995 than in 2 pre-invasion years (1916 and 1967), even on substrates not colonized by zebra mussels. Continuous data (1967–1997) showed that invertebrates on soft substrate in Oneida Lake have not significantly increased above peak historical levels. However, the abundance of 2 grazing taxa (Amphipoda and Gastropoda) has increased in shallow relative to deep areas, suggesting that grazing invertebrate populations have responded to modification of light penetration by zebra mussels. Phosphorus levels in the lake have declined in the past 30 y, possibly keeping the increase of benthic invertebrate populations below historical levels.