The dynamics of phosphorus retention during an eight-year P-addition in a Neotropical headwater stream

Understanding the capacity of stream ecosystems to retain nutrients through physical-chemical processes and biotic assimilation has been a central goal of stream ecologists for decades. Currently, most of our understanding of nutrient saturation is based on short-term (<1 day) nutrient addition experiments, while predicting total stream ecosystem response to long-term anthropogenic nutrient loading requires considering the stream’s capacity to remove nutrients over extended periods. Dissolved phosphorus (P) retention results from a combination of biotic and abiotic mechanisms, which could follow different trajectories through time. Short-term biotic P uptake by algae and heterotrophic microbes typically involves direct assimilation from the water column and is saturated at low background soluble reactive phosphorus (SRP) levels (MULHOLLAND et al. 1990). However, during long-term P-loading, the biotic community could also respond by increasing biomass (PETERSON et al. 1985, SLAVIK et al. 2004, but see GREENWOOD & ROSEMOND 2005), temporarily increasing the community’s P-retention capacity. Abiotic P-sorption is an equilibrium process controlled by the relative concentrations of sorbed-P and dissolved-P, although it also depends on sediment size, iron, aluminum, organic content, and pH (MEYER 1979). During long-term P-loading, sediment should become increasingly P-saturated, decreasing abiotic retention. Because biotic and abiotic P-uptake mechanisms could have opposite responses to long-term Ploading, the relative importance of each mechanism will control a stream’s retention capacity over time. Specifically, where P-uptake is dominated by biotic pathways, streams could become temporarily more efficient at removing dissolved-P, whereas where P-uptake is primarily abiotic, streams potentially become less efficient. Here we present data from an 8-year experimental P-addition in a first-order stream, the Carapa, at La Selva Biological Station, Costa Rica. Previous experiments of P-dynamics in the Rio Salto at La Selva indicate that most uptake of P due to the input of naturally P-enriched regional groundwater is due to abiotic sorption (TRISKA et al. 2006). Biweekly measurements of dissolved P concentration upstream and at 3 downstream stations during the long-term P-injection in the Carapa allowed us to calculate P-uptake rates over the 8-year study. Assuming a dominance of abiotic control, we predicted that sediments would become saturated over time, decreasing Puptake efficiency.