3rd UF Water Institute Symposium Abstract

Submitter's Name	Dina Liebowitz
Session Name	Posters - Hydroecology
Category	Ecology
Poster Number	16
Author(s)	Dina  Liebowitz,  University of Florida (Presenting Author)
	Matthew Cohen,  University of Florida
	Thomas  Frazer, University of Florida
	James Heffernan, Florida International University
	Experimental Evidence of Environmentally Mediated Top-Down Control of Algal Proliferation in Florida’s Springs
	Many of Florida’s 700+ springs have undergone dramatic shifts in autotroph dominance (from submerged aquatic vegetation to benthic filamentous algae), yet the drivers of these changes remain ambiguous. While nitrogen enrichment has been implicated, new data suggest other drivers are at play in these unique spring-fed karst ecosystems. Recent surveys revealed a negative relationship between gastropod biomass and algal abundance across and within springs. In this study we expand on these observational findings, by conducting in situ grazing experiments to examine three linked hypotheses developed to help explain patterns of algal density in Florida’s springs. These are (1) that the dominant gastropod grazers can control algal accumulation; (2) that there is potential for hysteretic behavior, i.e. once an algal bloom forms, even abundant grazers cannot exert enough grazing pressure to induce a low-algae state; and (3) dissolved oxygen (DO) influences the distribution and grazing efficiency of gastropods. We conducted grazer enclosure experiments (in situ cages containing high grazer densities, zero grazers, or partially caged controls) in four connected springs with similar water chemistry, but varying DO regimes (steady high DO, steady low DO, and two fluctuating DO regimes).We ran the enclosure experiments in two directions: one with initial conditions of low algal biomass, and the second with initial conditions of high algal biomass, to explore potential hysteresis and density dependent removal efficiency. This allowed us to explore the mediating effects of DO regimes (levels and fluctuations) on grazing efficiency, and the ability for grazers to control algal accumulation under algal bloom and non-algal bloom conditions. We observed significant grazer impacts at all sites, with grazing rates and algal accumulation varying due to environmental factors and initial conditions. Additionally, we report preliminary results from laboratory mesocosm experiments conducted to closely examine the mechanism of DO stress as it affects grazing efficiency.