Factors Affecting Phosphorus Uptake in Karstic Rivers of the Edwards Plateau, Central Texas
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Phosphorus (P) is a limiting nutrient for microbial primary producers in many aquatic systems and an overabundance of it via urban and agricultural runoff has led to eutrophication of waterways across the globe. There are a variety of ways to study nutrient dynamics, but nutrient spiraling theory is often used as a measure of efficiency and limitation in lotic ecosystems. However, there is a relative dearth of knowledge on nutrient uptake in larger rivers (> 200 L s-1) due to methodological constraints. Recent improvements in methodology (pulse-tracer addition, Tank et al. 2008) have allowed for measurements of nutrient uptake to be made in larger systems. Furthermore, the relative contribution of various biotic and abiotic factors that affect nutrient uptake is less studied in lotic systems than in other aquatic ecosystems (i.e lakes and wetlands). Therefore, the purpose of this study was to quantify P uptake and examine the factors that influence P uptake in relatively larger discharge riverine ecosystems, specifically, in karstic, spring- fed rivers of the Edwards Plateau, in central Texas. I utilized a pulsed tracer addition method to measure P uptake in 6 rivers and one large creek of varying discharges, channel morphology, and biological assemblages. These uptake estimates were calculated according to the traditional nutrient spiraling metrics as well a method put forward here to express the maximal instantaneous capacity of these systems for P uptake. The abundance and chemical makeup of benthic biofilms present in these systems were also studied to determine the degree of influence these biofilms have on nutrient uptake.
I found that levels of chlorophyll a and particulate P in benthic biofilms were significant predictors of uptake rates. In general, there was a high degree of correlation between benthic chlorophyll-a, benthic particulate P, water column SRP and dissolved mineral load indicating that biological and physicochemical factors are highly interrelated and may work in concert to affect P cycling in these systems. My results indicate that P uptake rates for the rivers in this study are rapid when compared to similarly sized non- karst rivers due to (1) the low availability of dissolved phosphate in the river, (2) abundance of algae-dominated biofilms, and (3) interaction with dissolved minerals (especially Ca2+), resulting in precipitation of insoluble mineral forms of P onto benthic surfaces. We have also shown that pulsed tracer additions can be a simple and effective tool for studying nutrient dynamics in streams and rivers.