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Macrophyte Community Response to Nitrogen Loading and Thermal Stressors in Rapidly Flushed Mesocosm Systems
Citation:
Kaldy, Jim, C. Brown, Walt Nelson, AND M. Frazier. Macrophyte Community Response to Nitrogen Loading and Thermal Stressors in Rapidly Flushed Mesocosm Systems. JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY. Elsevier Science Ltd, New York, NY, 497:107-119, (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 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. Taken together these results suggest than nutrient loading and warming may cause continued declines in seagrass populations along in North America. This manuscript contributes to SSWR 4.02B.
Description:
Increased nitrogen loading has been directly linked to the proliferation of planktonic and macroalgal blooms at a global scale with negative impacts on estuarine ecology and human health. Under excessive anthropogenic nutrient loads, seagrass systems can be replaced by either macroalgae or phytoplankton dominated systems. Although this process is conceptually simple, empirical determination of nutrient loading thresholds that negatively impact seagrass communities has been elusive due to the multitude of factors involved. We developed a mesocosm system that simulated estuarine conditions characteristic of ecosystems along the Pacific Coast of North America and evaluated multiple macrophyte metrics to assess community response across gradients of NO3 loading and temperature. Despite NO3 loading up to 6 x ambient, the development of phytoplankton blooms or the accumulation of dissolved organic carbon was precluded by rapid system turn-over rates (~200% d-1), mimicking the large semi-diurnal tidal exchange. Green macroalgae growth, biomass, and thallus C:N exhibited a positive response to increased nitrate load and floating algal mats developed only in tanks at 20 ºC. Zostera japonica metrics, including tissue C:N, were generally more responsive to temperature than to NO3 loading, suggesting that temperature is the dominant driver for this species. Z. marina leaf biomass exhibited both a temperature and NO3 effect, while growth rates only exhibited a temperature response. Shoot survival exhibited a negative effect at 20 ºC but was not influenced by NO3 loading. Wasting disease index exhibited both temperature and NO3 effects supporting previous suggestions that Z. marina may be more susceptible to disease under warmer conditions and increased nutrient levels. Experimental estuarine macrophyte communities exhibited community shifts consistent with the nutrient loading hypothesis at 20 ºC, but there was no evidence of other eutrophication symptoms (phytoplankton blooms or hypoxia) due to the short system residence time. The Z. marina Nutrient Pollution Index (NPI) tracked the NO3 gradient at 10 ºC but exhibited no response at 20 ºC, which may limit the utility of this metric in areas with marked thermal seasonality. Our work suggests that systems subjected to cool temperatures, high NO3 loads, and short residence time may be resilient to many symptoms of eutrophication.
