Twenty sites along the St. Lawrence River were sampled to determine if the distribution and abundance of invasive mussels (zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis)) are explained by physicochemical variables. Calcium concentration, substrate size, and depth independently explained significant proportions of variation in biomass for both species. Zebra mussel populations occurred at calcium levels as low as 8 mg Ca·L1, but quagga mussels were absent below 12 mg Ca·L1, suggesting that they have higher calcium requirements. Both species increased in biomass with increasing substrate size but displayed contrasting patterns with depth. Using combinations of these environmental variables, we developed stepwise multiple regression models to predict zebra mussel biomass and quagga mussel biomass. The zebra mussel model included calcium concentration, substrate size, and depth (r2 = 0.36, P < 0.0001), while the quagga mussel model included only substrate size and depth (r2 = 0.32, P < 0.0001). These results suggest that dreissenid mussel abundance (and correlated impacts) will vary predictably across environmental gradients, but the same predictive model will not be accurate for both species.
We examined the role of solar radiation, and particularly the role of ultraviolet radiation (UVR), in regulating small-scale settlement patterns and early post-settlement mortality in the barnacle Balanus glandula, as well as community development of sessile organisms colonizing the upper mid-intertidal zone. Settlement of B. glandula cyprids was similar in treatments with and without UVR, suggesting that UVR does not directly influence site selection by cyprids. Once attached, mortality during the 1 to 2 d period from attachment to metamorphosis ranged from 60 to 100%, and half of the settlers that did metamorphose died during the following 5 d. Mortality during the period from attachment to metamorphosis was significantly lower in locations protected from UVR than in locations exposed to the full spectrum of solar radiation, but only by ca. 10%. Furthermore, UVR exposure had no detectable effect on the mortality rate of metamorphosed early juveniles. Ambient UVR levels also appear to have had little effect on the colonization of mid-intertidal habitats by sessile organisms: our study locations were colonized by 2 sessile invertebrate species and 2 algal species over a period of 2.5 mo in mid-summer, and densities of these species were similar in treatments with and without UVR. Nevertheless, solar radiation dose explained 43 to 65% of the variation in mortality among daily cohorts of B. glandula, and daily cohort mortality was often 100% during periods with the highest doses. This relationship between solar radiation and survival to metamorphosis was likely due to the effect of solar radiation on desiccation and heat stress. The high sensitivity of early postsettlement mortality rates to solar radiation suggests global climate change may significantly alter patterns of survivorship through this critical stage of life.
Summary 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. 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. 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. 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. 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. 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.
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