The relative importance of suspended versus benthic food resources to freshwater mussels in central Texas, U.S.A.

Abstract

Abstract As unionid mussels continue to decline in North America much remains unknown regarding their primary food resources and feeding relationships. Our objective was to investigate spatial, temporal, intra‐ and interspecific variation in food resource use. In this study, we quantified stable C (δ13C) and stable N (δ15N) isotopic signatures for five mussel species and their potential food resources across four catchments and three seasons in central Texas, U.S.A. Across all species and sites, Bayesian mixing models suggest C derived from coarse particulate organic matter (CPOM; bulk detrital leaf packs) contributed on average 51% to the mussel diet and C derived from suspended particulate organic matter contributed 41% on average to the mussel diet. Mussel stable isotope signatures exhibited minimal variation across species within a site, but significant differences across sites and seasons. Although significant differences in food source contributions were observed between sites and species, differences were relatively small and either CPOM or suspended particulate organic matter were consistently the primary food resource. Mussels were potentially consuming microbial biofilms associated with CPOM pools, but preferential assimilation of detrital biofilms remains to be quantified. Carbon:nitrogen ratios of mussel soft tissue varied seasonally with summer ratios approximately 3 times higher on average than spring and autumn ratios, which is suggestive of poor food quality and thermal stress during the summer. Some species had significant positive relationships between isotopic ratios and shell length, which indicates changes in food resource incorporation with increasing mussel size. Together these data suggest the C in mussel tissues was of benthic and suspended origin. These findings provide further evidence that mussels effectively exploit benthic in addition to planktonic food resources which emphasises the need to maintain riparian habitats and allochthonous inputs.