Floods and dissolved oxygen drive autotrophic community structure in Suwannee River (Florida, USA) springs

Abstract

Dramatic loss of submerged aquatic vegetation (SAV) and proliferation of nuisance algae in the karst springs of the Suwannee River basin, Florida, USA, have been principally attributed to press disturbances such as nutrient pollution. The role of pulse disturbances, specifically river intrusion events (RIEs), in driving this shift has been hypothesized but remains poorly documented. During RIEs, acidic floodwaters high in dissolved organic matter from adjacent blackwater rivers displace the clear, alkaline groundwater within a spring, reducing light availability and altering dissolved O2 (DO) concentrations, with potentially important consequences for autotroph communities. We asked, to what extent do RIEs, alone and in combination with other environmental gradients such as DO, influence autotrophic community structure in Florida springs? To address this question, we surveyed autotrophic community structure at 62 springs and quantified flood disturbance frequency using water-quality observations and geomorphic predictors. RIE frequency showed a relationship with both algae and SAV cover. The relationship between RIE frequency and algae cover was mediated by baseline DO concentrations, with higher RIE frequency associated with increased algae cover in high-DO springs but reduced algae cover in low-DO springs. High-DO springs were more likely to support SAV, suggesting that disturbances promote algal proliferation by reducing the competitive advantage of SAV. Variables that predicted flood disturbance frequency included the spring-pool elevation relative to the receiving river and local stage variance. These findings suggest that interacting effects of press (low DO) and pulse (RIE) disturbance regimes shape autotrophic communities, informing approaches for successful SAV restoration.