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In many of Florida’s spring-fed rivers, cover of benthic and periphytic algae is increasing and in some cases are replacing submerged aquatic vegetation (SAV). A potential driver of these shifts in primary producer community structure is hydraulic control of algal abundance. Flow velocity drives the drag force on submerged algae and is a significant control on algal colonization and sloughing rates. This study investigated the relationship between flow velocity and algal and SAV abundance in spring-rivers, and consisted of three components: 1) Analysis of observational algal abundance and flow velocity data from multiple spring systems to determine a critical threshold velocity for algal presence; 2) A flow suppression experiment to determine the intrinsic algal growth rate and hysteretic behavior of algal establishment; 3) Development of an ecohydrological SAV growth model with feedbacks to reach-scale flow velocity. The analysis of observational algal abundance and flow velocity data was comprised of the application of a Bayesian statistical model to data from each of the multiple spring system datasets. This model determined if there was a critical threshold present in the data, and what its value was. Results from all the spring systems studied suggest a critical threshold velocity of 0.215 m/s with 95% CI (0.160 m/s-0.270 m/s). The flow suppression experiment was comprised of multiple deployments of flow blocking structures, which reduced flow velocity to near zero. Algal cover was monitored daily following the flow reduction. After substantial algal growth, flow was restored and the algal cover was monitored daily. Results from all deployments suggest an intrinsic algal growth rate of 1.14 1/day (95% CI: 0.70/day-1.60/day), and suggests that algal establishment is not strongly hysteretic. Results from the ecohydrological SAV growth model suggest that the model can reproduce the behavior observed in actual spring run stage, discharge, and velocity relationships. |
