Do freshwater gastropods avoid the benthic cyanobacterium Lyngbya wollei?

We experimentally assessed the mechanisms underlying the reduction in gastropod abundance in areas dominated by the filamentous, toxin-producing cyanobacterium Lyngbya wollei, which has replaced the large beds of Vallisneria americana in some areas of the St. Lawrence River. We hypothesized that the reduction in gastropod abundance was due to adult avoidance of cyanobacterial mats in favour of filamentous chlorophytes or vascular macrophytes. Of the 4 gastropod taxa offered a choice between L. wollei and filamentous chlorophytes, 3 (Pleurocera acuta, Amnicola limosa, and Gyraulus parvus) were either indifferent or even attracted to the cyanobacterium, and only Viviparus sp. preferred the chlorophytes. Lyngbya wollei exhibited higher nitrogen (N) contents (5-6%) and a lower carbon to nitrogen (C: N) ratio (6.1-7.6) than filamentous chlorophytes (3-5% N, C:N ratio 8.2-11.6). When offered a choice between L. wollei and the ribbon-leaved V. americana supporting either natural or partially removed epiphyte cover, P. acuta and A. limosa preferred the macrophyte with its natural epiphyte cover. Epiphytes on V. americana were twice as abundant and had a lower C:N ratio than natural epiphytes on L. wollei. Additional experiments exposing juvenile Bithynia tentaculata and Physa gyrina to different filamentous substrata showed similar growth but lower survival when gastropods were reared with L. wollei rather than with chlorophytes. Overall, our results showed that direct avoidance of L. wollei by adult gastropods did not explain their decline in areas dominated by cyanobacterial mats. Instead, gastropod decline likely resulted from habitat degradation coincident with reduced macrophyte abundance combined with decreased survival of juveniles exposed to L. wollei.