The influence of pre-settlement and early post-settlement processes on the adult distribution and relative dominance of two invasive mussel species

1. The structure of a community is governed by a complex combination of processes whose relative importance varies over time and space. Larval dynamics, settlement and recruitment are thought to be important processes limiting adult abundance and distribution of benthic invertebrates with planktonic larvae. 2. Two invasive molluscs with similar morphology and resource needs, the Eurasian zebra mussel Dreissena polymorpha and the quagga mussel Dreissena rostriformis bugensis, co-occur in several North American lakes and rivers but often differ in their adult distribution over depth. Following establishment, the quagga mussel typically replaces the zebra mussel in abundance, particularly in deeper waters. A field sampling programme conducted over 3 years in a lotic system (the Soulanges Canal connected to the St. Lawrence River) examined the extent to which adult distribution and the differential dominance of these two species are determined by larval supply (i.e. larval abundance near settlement sites), settlement and recruitment. 3. Total dreissenid larval abundance in the water column at two depths was determined weekly, and larval competence (size) and species-specific larval composition was estimated, during the main settlement period over three consecutive years. The pattern of total dreissenid settlement over the depth gradient was determined by deployment of settlement plates at both depths. Total abundance and proportional abundance of zebra mussel and quagga mussel juveniles and adults in each depth zone were determined monthly from July to September each year. 4. Mean dreissenid larval size did not differ between depths and the supply of late-stage larvae was generally low, but total larval abundance was consistently greater in deeper water. This differential larval abundance established settlement and recruitment patterns in the canal but contrasted predictions based on total adult dreissenid abundance – which was higher in the shallow zone. Therefore, the significant factor dictating the abundance of adult mussels in these two depth zones must be post-recruitment mortality, rather than larval supply, settlement or recruitment. 5. Despite a strong species-specific adult depth zonation, larvae and juveniles showed no consistent differences in species proportions over the depth gradient. In fact, zebra mussels dominated larval abundance at both depths for c. 50% of the sampling dates and dominated juvenile abundance at both depths throughout most of the sampling period. In contrast, the proportional abundance of zebra mussels in the adult dreissenid community was consistently 4–5 times higher in the shallow zone. 6. These results indicate that larval supply, settlement and recruitment processes are not responsible for determining total adult dreissenid distribution or species dominance. Rather, these patterns appear to be governed by post-recruitment factors that manifest themselves in later stages of mussel development and growth.