Local-scale spatial patterns of freshwater mussels in the Upper Mississippi River

Abstract

Mussel distributions are often described as clumped or patchy. However, few surveys of mussel populations have been designed to quantify these spatial patterns. We used indicators of spatial autocorrelation to quantify spatial patterns of adult and juvenile (≤5 y old) freshwater mussels at local (fine) spatial scales (i.e., <300 m) within 14 sites along ~700 km of the Upper Mississippi River, USA. Juveniles were patchily distributed in 43% of the sites, whereas adults were patchily distributed in 50% of the sites. Juveniles and adults displayed the same spatial pattern in 64% of the sites. Hotspots, areas with locally-high densities, for adults overlapped with juvenile hotspots at ½ of the sites, suggesting that the spatial and temporal mechanisms that structure mussel distributions persist over long periods of time. A comparison of our results with a similar study focused at a coarser spatial scale suggests that patchiness in mussel distributions can occur at multiple scales and that the detection of such patchiness is a function of sampling extent and spatial grain. Mussels play an important ecological role in rivers, so information on their spatial patterns aids our understanding of the spatial structure and function of riverine ecosystems as well as the ecological consequences of their population declines.