| |
Many of Earth’s aquatic ecosystems have experienced shifts in ecosystem structure and function due to nutrient enrichment and subsequent algal proliferation. In many of Florida’s spring-fed rivers, benthic and periphytic algae are replacing submerged aquatic vegetation (SAV), however the cause of these shifts in primary producer community structure is unclear. An alternative and perhaps concurrent hypothesis to eutrophication-driven algal proliferation is hydraulic control of algal abundance. This work presents preliminary results of an ongoing three-year study to characterize local- and reach scale hydraulics and hydrodynamics in the Silver River (Ocala, FL). We investigate local-scale relationships between flow velocity and algal abundance by mapping periphytic algal cover distributions on SAV beds along with spatially and temporally corresponding flow velocity distributions. SAV beds were divided into standardized quadrants and photographed, and algal abundance was quantitatively computed from quadrant images by extracting average color pixel values using a photometric color system calibrated against field measurements. Field observations and preliminary results from the Silver River suggest a clear inverse relationship between flow velocity and benthic and periphytic algae cover. At the reach scale, tracers allow for the investigation of reach-scale physical properties of streams. Typically, a tracer is released upstream and its concentration is measured downstream, producing a break through curve (BTC). The BTC contains information about the physical properties of the reach through which the tracer has passed, such as transient storage and residence time distribution. These properties are important in dictating stream chemistry and biology. Here we examine how the Sliver River’s physical transport properties vary across flow conditions by comparing tracer studies in October 2009, March 2015, and October 2015. Observed BTCs were fitted to the OTIS model, a one dimensional transport and mixing model, using non-linear regression optimization and Bayesian inference. Results suggest that the Silver River’s hydraulic transport properties are substantially different under the three different stage/discharge conditions. For example, mean residence time of the spring run decreased and the transient storage increased with increasing discharge. In addition, tracer experiment results were used to calibrate and validate a hydrodynamic EFDC model, which will be used to further test relationships and feedbacks between flow, vegetation, and algal cover. |