Nutrient loading and benthic native-invasive species dynamics

Background/Question/Methods

The effects of nutrient loading on the success of biological invasions is best understood in terrestrial systems, where studies have generally shown that increased nutrients facilitate the spread of invasive plants. To date, these relationships have remained largely unexamined in marine systems and in higher trophic levels. In addition to excess nutrients, small-scale disturbances caused by invasive species affect ecosystem processes in mudflat communities, potentially creating feedback loops that facilitate invasion. I conducted a suite of lab and field studies to examine the effects of interactions between nutrient loading and invasion-related disturbance by the invasive mud snail, Ilyanassa obsoleta, on the native mud snail Cerithidea californica, in San Francisco Bay, CA. Where both species coexist, I. obsoleta has been documented as competitively displacing the native snail through behavioral interactions. Using a complete factorial design, I experimentally manipulated densities of C. californica and I. obsoleta as well as nutrient levels to investigate the role of nutrients in invasion success in this system. Response variables included changes to microalgal community composition, using HPLC and snail growth and mortality.

Results/Conclusions

Results indicate that nutrient additions increased microalgal biomass on the open mudflat. Native snail growth was significantly positively affected by nutrients (p<0.0001), while invasive snail growth was not (p=0.73). Both species grew most in the low-density, single-species treatments and least in the 2-species (high density) treatments, confirming negative interspecific interactions. Both snails—but particularly I. obsoleta—lowered the diatom biomass at lower sediment depths, while maintaining high diatom biomass at the sediment surface, compared to the controls. Other studies have shown that nutrients can shift the composition of microalgae from edible diatoms to toxic cyanobacteria when cyanobacteria are present, resulting in C. californica mortality. The results presented here suggest that the native species responds to the effects of the additional nutrients on diatoms in the open mudflat, while the invasive species is unaffected by shifting nutrient conditions. Nutrient loading impacts may be habitat-dependent: at lower tidal elevations with high diatom abundance, nutrients enable C. californica growth. However, at higher elevations, with higher abundance of cyanobacteria, increased nutrients may facilitate an increase in toxic cyanobacteria and concomitant mortality. Thus, nutrients may play an important role in determining invasion success at higher trophic levels.