Experimental evidence for grazer control of algal stable states in Florida’s spring-fed rivers

Declining aquatic grazer populations can impact algal biomass, potentially prompting nuisance accumulations that are resistant to grazer recovery. Gastropods appear to influence algal biomass accumulation in Florida’s spring-fed rivers, but their role in regulating nuisance algal proliferation remains contentious. Additionally, practical grazer-density thresholds, which may vary with environmental factors (e.g., light, oxygen, nutrients, flow), remain poorly quantified. Using in situ experimental grazer manipulations at four sites spanning environmental conditions in a spring-fed river, we tested three hypotheses: 1) grazers control algal biomass at high grazer abundance, 2) high initial algal biomass limits grazer controls, and (3) differences in dissolved oxygen (DO) mediate grazer–algal interactions. Our results support all three hypotheses. At low initial algal biomass, high-density grazer enclosures saw limited algal accumulation, while biomass reached nuisance levels in grazer exclosures. In contrast, high initial algal biomass resisted grazer control, indicating resilient bloom conditions. Grazer control thresholds varied, with weak grazer–algae relationships under low DO and high sedimentation conditions, and stronger grazer control and clear effect hysteresis under high DO conditions. Top-down controls mediated by DO emerge as a key regulator of algal proliferation in Florida’s springs, underscoring the importance of grazer conservation to maintain and restore these iconic ecosystems.

Duke Scholars

Author James Brendan Heffernan Environmental Natural Science