| |
While changes in stream gross primary production (GPP) and shifts in vegetative communities have traditionally been attributed to nutrient enrichment, other factors such as grazer presence and density may influence overall stream structure and function. Evidence shows that grazers are a top-down control in algae-dominated streams; however, the specific feedbacks between rates of total stream GPP, grazer effects, and nutrient cycling have been variable and little is known about these interactions at nutrient levels below ambient. To further our understanding of these linkages, a nutrient depletion chamber was created and paired with high-resolution in situ sensors to estimate stream metabolism and characterize nitrate uptake (UNO3) pathways (i.e. plant uptake and denitrification) at Gum Slough Springs, Florida. Paired chambers with and without the presence of snails (Elimia floridensis) were deployed across submerged aquatic vegetation (SAV; Vallisneria americana and Sagittaria kurziana), algae (Lyngbya), and combined SAV/algae substrates. Using these methods, we tested our hypotheses that 1) grazers alter stream ecosystem metabolism and therefore impact nitrogen cycling through multiple pathways, 2) rates of stream metabolism and UNO3 respond differentially to grazer presence by vegetative regime, and 3) the presence of grazers alleviates nutrient limitation if present in the system. While GPP and UNO3 were higher under SAV and there was a general lack of plant nutrient limitation even at low nitrate concentrations (i.e. zero-order uptake kinetics), initial results suggest grazer presence alleviates nitrate depletion (and therefore potential limitation) and elicits different responses by substrate type. Continued work includes estimating grazer effects on denitrification, quantifying snail nutrient excretion contributions, and scaling up estimates from the patch to reach level. Overall, this study will further our understanding of grazer-production-nutrient-microbial interactions within stream ecosystems, making it possible to predict changes in feedbacks when one part of the biotic or abiotic ecosystem is altered. |