Mussels partition resources from natural waters under flowing conditions

Abstract

The feeding of three co-occurring freshwater mussel species (Ptychobranchus fasciolaris, Quadrula quadrula and Potamilus alatus) of differing conservation status on river water from their collection site was examined under static and ecologically relevant flow conditions using three characteristics of the suspended river seston from a turbid river (Sydenham River, Ontario): (1) Chlorophyll a fluorescence; (2) size of fluorescent particles determined under flow cytometry; and (3) individual algal taxa identified under flow cytometry. Differences in the clearance rate (CR; water volume cleared of material per mussel and time) based on changes in chlorophyll a concentration were observed among species (P. alatus > Q. quadrula). Mussels had higher CR under flowing conditions and higher CR were observed on four of the nine algal taxa under flowing conditions. Feeding electivity analysis indicated that all mussel species selected for larger particles (28–35 μm size fraction) while rejecting smaller ones (12–19 μm). Whereas mussels did not appear to partition resources by size, mussel species exhibited resource partitioning of algal taxa (i.e., selectively feeding on different algal species; under flowing conditions: small centric diatoms were preferred by P. fasciolaris and Q. quadrula and avoided by P. alatus; Cryptomonas were preferred by P. fasciolaris and avoided by Q. quadrula and P. alatus; Chloromonas were preferred by P. alatus and avoided by P. fasciolaris). This study provides a novel mechanism, hydrodynamically mediated resource partitioning, in which the partitioning of resources occurs from within a flowing fluid rather than through the spatial or temporal partitioning of resources by organisms exploiting different microhabitats, to explain the existence of high mussel species richness within the same river reach (i.e., 24 species in this study). Unfortunately the species specific relationships noted above are likely vulnerable to climate change and changes in land use practice due to agriculture and urbanization.