Understanding the processes regulating the distribution of populations in space and time is fundamental to predicting their ecological roles in communities and ecosystems. For decades, the distribution and dynamics of sessile species have been studied at various scales using areal and satellite imagery in both marine and terrestrial systems, including arid ecosystems, coral reefs, and marine mussel beds. In contrast, such methods had limited use in studies of freshwater benthos due to the general lack of large epifauna. The introduction of dreissenid bivalves (zebra and quagga mussels) into North American freshwaters in late 1980s created extensive visible aggregations of mussels on lake bottoms and made it possible to employ underwater imagery to survey their populations. But to date, traditional small-scale (0.05m2) PONAR sampling has limited the precision of spatiotemporal population dynamics afforded even by extensive monitoring studies. To supplement traditional benthic studies, we developed a sampling method, which analyses footage recorded from a GoPro video camera on a towed benthic sled, and compared mussel cover, density, and biomass estimates based on replicate PONAR grabs vs. 500 m-long benthic video transects across 43 sites sampled in Lake Michigan in 2015.
Results/Conclusions
Video transects dramatically increased the extent of spatial coverage sampled compared to traditional PONAR grabs, and improved the precision of density and biomass estimates. By substantially increasing the ability to detect interannual population changes in densities, video transects could be a useful and cost-effective method for monitoring Dreissena populations. Video analysis also revealed dramatic differences in Dreissena distribution patterns with depths, with the highest heterogeneity in distribution, large mussel aggregations but low overall coverage at shallow (<30 m) depths, and the highest coverage and sizes of Dreissena druses but the lowest heterogeneity in distribution at intermediate depths (31 – 100 m). In contrast, at depths over 100 m Dreissena coverage was the lowest with the smallest druse sizes. Interaction between environmental factors (e.g. hydrodynamics and food supply) and self-organization likely govern the distribution and patch size of Dreissena. Because Dreissena is an ecosystem engineer affecting benthic communities, the dense mussel beds on previously soft substrates are changing the pattern of spatial distribution and increasing abundance of benthos with cascading effects to higher trophic levels. The continuing offshore expansion of quagga mussel reefs may intensify the effects of these powerful filter feeders on ecosystem structure and functioning already observed in the Great Lakes.