| Popis: |
Freshwater mussels represent a particularly imperiled group of aquatic organisms. To date, there has been a limited understanding of their dietary and nutritional subsidies, including those from both autochthonous (aquatic) and allochthonous (terrestrial) sources. Nutritional subsidies to freshwater mussels were assessed using the natural abundance isotopic ratios (d13C, d15N, d2H, and D14C) of mantle tissue for five species of freshwater mussels and of their potential nutritional organic matter (OM) dietary sources at four sampling times downstream of a low-head dam in a (7th order) river in Ohio (USA). The relative contributions of the identified potential nutritional resources to mussel biomass was evaluated for bulk benthic vs. pelagic (OM) pools, general autochthonous vs. allochthonous OM sources, and specific individual OM sources using a Bayesian mixing model (MixSIAR). Using the four isotopes, the combination of all individual potential nutritional resources sampled accounted for all mussel tissue nutritional subsidies. Mussels had similar d13C and d15N values to previous studies (all of which have used d13C and d15N exclusively) and the ranges for each of the four isotopes were similar across species and sampling date. In spite of the relatively small species and temporal differences in d13C, d15N, d2H, and D14C isotopic values of mussels, these differences were generally still significant. In particular, Quadrula pustulosa was isotopically differentiable from the other species as well as mussel tissue from samples collected in August and June vs. October. The pelagic bulk OM pool subsidized about twice as much mussel tissue (~68%) as the benthic OM pool (~33% of mussel tissue). Grouped autochthonous sources also assumed twice the importance (~65% of mussel tissue) than the grouped allochthonous sources (~35% of mussel tissue). No single potential individual nutritional OM source could explain the isotopic values of the freshwater mussel tissues, supporting the contention that mussels have an omnivorous multi-source diet. Phytoplankton and bacteria+fungi were quantitatively the most important individual OM subsidies to freshwater mussel nutrition at our study site. Temporal shifts in freshwater mussel tissue isotopic values were minimal, suggesting that i) that mussels may not be limited by individual nutritional resources and/or ii) that the long turnover times of freshwater mussel tissues made detection of temporal nutritional subsidy shifts difficult to discern. |
