Episodic Flow Reversals as a Driver of Ecosystem Change in Florida’s Springs

Abstract:

The spring ecosystems of north Florida have become markedly degraded, with widespread proliferation of filamentous algal mats replacing vascular macrophytes. Because of the relative temporal stability of springs, the focus has been entirely on drivers such as nutrient enrichment and flow reduction operating over long time scales. However many of these systems are subject to extreme events in the form of episodic flow reversals. During flood events on adjacent blackwater rivers, river stage exceeds aquifer head and tannic, high DOC water travels up spring runs, even penetrating into the aquifer itself. In addition to directly altering the light environment, DOC respiration depletes dissolved oxygen, inducing hypoxic conditions and creating a trophic cascade where impacts to algal consumers enable algal proliferation. Synthesis of river and aquifer stages from numerous springs in the Suwannee River basin indicates that these reversal events are becoming far more frequent with time in response to both changes in rainfall patterns and groundwater pumping. Events which previously occured only once or twice per decade are now occuring intra-annually. This increase in reversal frequency and duration is remarkably concurrent with the emergence of nuisance algal mats. Furthermore, re-analysis of existing algal abundance data suggests that proximity to major rivers, and hence potential for flow reversal is a significant predictor of algal abundance. Our results suggest that despite general temporal stability, episodic flow reversals may play a key role in regulating the ecosystem state. Focusing on these extreme events may be critical to springs protection and restoration.