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Macrophyte Community Response to Nitrogen Loading and Thermal Stressors: Water Residence Time Ameliorates Eutrophication Symptom Expression
Citation:
Kaldy, Jim, C. Brown, Walt Nelson, AND M. Frazier. Macrophyte Community Response to Nitrogen Loading and Thermal Stressors: Water Residence Time Ameliorates Eutrophication Symptom Expression. 46th Benthic Ecology Meeting, Myrtle Beach, South Carolina, April 12 - 16, 2017.
Impact/Purpose:
Estuarine systems characterized by cool temperatures, high nutrient loads and rapid flushing may be resilient to some symptoms of eutrophication. Expression of eutrophication symptoms in estuaries can be variable between systems due to a multitude of interacting factors. We used experimental tanks to simulate nutrient loading and thermal stressors in Pacific Northwest estuaries. Our results suggest that some indicators of eutrophication such as the seagrass Nutrient Pollution Indicator may not be robust under warm temperatures while other indicators such as phytoplankton blooms, low dissolved oxygen or build-up of dissolved organic carbon may be removed by rapid flushing conditions. However, plant community shifts predicted under high nutrient loads were observed. Furthermore, under warming temperatures seagrass may be more susceptible to wasting disease. This abstract contributes to SSWR 4.02B.
Description:
Empirical determination of nutrient loading thresholds that negatively impact seagrass communities have been elusive due to the multitude of factors involved. Using a mesocosm system that simulated Pacific Northwest estuaries, we evaluated macrophyte metrics across gradients of NO3 loading (0, 1.5, 3 and 6x ambient) and temperature (10 and 20 °C). Macroalgal growth, biomass, and C:N responded positively to increased NO3 load and floating algal mats developed at 20 ºC. Zostera japonica metrics, including C:N, responded more to temperature than to NO3 loading. Z. marina biomass exhibited a negative temperature effect and in some cases a negative NO3 effect, while growth rate increased with temperature. Shoot survival decreased at 20 ºC but was not influenced by NO3 loading. Wasting disease index exhibited a significant temperature by NO3 interaction consistent with increased disease susceptibility. Community shifts observed were consistent with the nutrient loading hypothesis at 20 ºC, but there was no evidence of other eutrophication symptoms due to the short residence time. The Nutrient Pollution Index tracked the NO3 gradient at 10 ºC but exhibited no response at 20 ºC. We suggest that systems characterized by cool temperatures, high NO3 loads, and short residence time may be resilient to many symptoms of eutrophication.