Site exposure, substrate, depth, and the thermocline affect the growth of native unionid mussels in a stratified lake

Abstract

The distribution of the native mussel Elliptio complanata (Lightfoot, 1786) in lakes is limited to nearshore areas above the thermocline. Their distribution appears to be constrained by wind-driven physical forces, and if so, we expect: 1) lower growth at the edges of their depth range, at shallow exposed sites, and at deep sites close to the thermocline; and 2) long-term changes in growth with lake warming and changing thermocline depth. This study compares the growth of juvenile and adult E. complanata over their depth range in a stratified lake and across sites with different exposures and sediment types. I used thin shell sections covering as long of a period as possible (7–54 y) to measure growth of 195 mussels. The average growth, measured as the von Bertalanffy growth rate coefficient, k, of juveniles ranged more than 3-fold (k = 0.06–0.27) around a single lake basin and that of adults 2-fold (0.3–0.6 mm/y). Mussel growth decreased with increasing depth and with increasing site exposure, but a positive relationship with sediment organic content suggests that sediments provide a refuge for adult mussels. Coherence between growth series was strongest at the deepest sites (6 m; expressed population signal = 0.82), where mussel growth responded to thermocline depth. Longer stratification periods and shallower thermoclines are expected to compress the range of these mussels in lakes and may similarly affect the wider littoral benthic communities.