Experimental effects of grazers on autotrophic species assemblages across a nitrate gradient in Florida springs

Authors: Nifong, Rachel

Submitted to: Aquatic Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2017
Publication Date: 3/1/2017
Citation: Nifong, R.L. 2017. Experimental effects of grazers on autotrophic species assemblages across a nitrate gradient in Florida springs. Aquatic Botany. doi:10.1016/j.aquabot.2017.02.010.

Interpretive Summary: Many presume that increasing nitrate pollution is responsible for promoting nuisance algal growth and impacting aquatic plant communities that support spring ecosystems. However, there is mounting evidence that reducing nitrate concentrations may not fully restore aquatic plant communities in natural spring-fed ecosystems. This study investigated the role of snails in maintaining healthy aquatic plant communities across a range of nitrate conditions. Results show an absence of snails significantly increased nuisance algae, but the presence of snails resulted in a measurable increase in aquatic plant success. This study highlights the need to maintain grazer populations, not just nitrate concentration reductions alone, in to the management of threatened spring habitats.

Technical Abstract: Springs face accelerated degradation of ecosystem structure, namely in the form of autotrophic species assemblage shifts from submerged vascular macrophytes to benthic filamentous algae. Increasing nitrate concentrations have been cited as a primary driver of this shift and numeric nutrient criteria have been developed to address rising nitrate concentrations and restore macrophyte dominance. However, the mechanistic basis of competitive outcomes between algae and macrophytes in these systems remains unresolved. This study sought to assess how grazing by gastropods influences both algae and macrophyte standing stocks and net production by manipulating gastropod densities in in-situ mesocosms across three levels of nitrate concentrations and autotrophic assemblage composition. The overall effect of grazing was assessed across the range of nitrate concentrations and effect sizes on algae and macrophytes were compared separately within nitrate concentration levels. Results suggest macrophyte success was significantly diminished in the absence of grazers, while the absence of grazers more than tripled the relative standing stock of algal taxa. No consistent pattern of grazer influence was found for net production of either macrophytes or algae. The effect of grazer absence resulted in the largest relative algal standing stocks in low nitrate conditions whereas grazer absence resulted in negative effect sizes on macrophytes standing stocks in low and intermediate nitrate environments. Grazer presence had positive effects on macrophyte standing stocks across all nitrate levels. These results suggest that both nitrate loading reductions and the fostering of healthy populations of grazers would be most effective in the management of these flowing water systems.