Many of the ecological consequences of human activities cannot be fully understood in isolation. The influence of practices such as industrial agriculture, increasing urbanization, international trade, and fossil fuel combustion have resulted in changes to the climate, biogeochemical cycles, land use, and mobility of organisms. My dissertation investigates the interactions between changing nitrogen (N) biogeochemistry and the presence of a prevalent invasive species, zebra mussels. The effects of zebra mussels on biogeochemical processes such as N cycling have not been extensively examined, but many characteristics of zebra mussels suggest that they have the potential to influence controls on N cycling in aquatic systems. The activities of zebra mussels may directly or indirectly affect substrate availability and oxidationÌ¢âÂ"reduction (redox) potential, but this remains an understudied area linking invasion biology with ecosystem ecology.
I have examined the relationship between N cycling and zebra mussels on multiple scales, incorporating seasonality and spatial variability across several different ecosystems. I have also included multiple lines of study, ranging from small-scale mesocosm experiments in the laboratory to large-scale experimental manipulations and field surveys of invaded ecosystems. I have extended the concept of ‘benthification’ (by which zebra mussels increase deposition and retention of particles in the littoral zone, thereby focusing productivity and nutrient cycling in littoral benthic habitats) to include the effects of zebra mussels on N cycling. In general, I have shown that zebra mussels strongly increase N availability to benthic sediments. This may result in significantly higher nitrification and denitrification rates in the presence of zebra mussels. However, seasonal and spatial variation of invaded ecosystems will amplify or diminish the effects of zebra mussels on N cycling. The increased N availabile to the benthos resulting from zebra mussel activity may have alternate fates during different times of the year. We know from previous work that denitrification rates can be high during the winter, and increased N availability to denitrifiers from zebra mussel activity may magnify this seasonal pattern in both lakes and rivers.
I have also shown that seasonal and spatial variation of biogeochemical cycles is an important characteristic of aquatic ecosystems, though both are often overlooked. My dissertation research highlights the importance of zebra mussels in altering N dynamics in freshwater ecosystems, and emphasizes the need for more research at the disciplinary intersection between biogeochemistry and invasion biology.