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Unexpected Responses of autotrophs to nutrient loading: Influence of water residence time on eutrophication expression
Citation:
Kaldy, Jim, C. Brown, S. Pacella, C. Tenison, AND S. Stryffeler. Unexpected Responses of autotrophs to nutrient loading: Influence of water residence time on eutrophication expression. CERF 2017, Providence, Rhode Island, November 05 - 09, 2017.
Impact/Purpose:
Estuarine systems characterized by cool temperatures, high nutrient loads and rapid flushing may be resilient to some symptoms of eutrophication. Water residence time appears to control expression of eutrophication symptoms in estuaries. At longer residence time, carbon limitation appears to be an important factor controlling community response to nutrient loading. We used experimental tanks to simulate nutrient loading, thermal stress and a gradient of residence times in Pacific Northwest estuaries. Our results suggest that under some conditions, estuarine primary producer communities can be impacted by carbon limitation as indicated by high pH and low pCO2 despite being nutrient enriched. Short term phytoplankton blooms, a common symptom of eutrophication, were observed in 3 and 10 d residence time systems but not in 1 d residence time systems. Warm temperatures also appeared to exacerbate the effects of water residence time and nutrient loading. Previous work has shown that seagrass communities are more susceptible to wasting disease infection at warm water temperatures. Taken together these results suggest that eutrophication expression is a complex response to water residence time, nutrient loading and warming. And that these factors interact to control the structure and function of estuarine primary producer communities in North America. This abstract contributes to SSWR 4.02B.
Description:
Water residence time may critically influence the expression of estuarine eutrophication symptoms such as phytoplankton blooms, anoxia/hypoxia, build-up of organic matter, and altered community composition. While a conceptual model was developed in the late 1990’s; pioneering empirical studies were conducted with long RT and warm temperatures. Few studies have evaluated response to a range of RT treatments. We used mesocosm experiments to evaluate autotroph community response to nutrient loading at 3 different residence times (1, 3 and 10 d respectively). Nutrient loads to all tanks were 20 mM N d-1 and 1.2 mM P d-1, representing inputs to Yaquina Estuary, OR during upwelling conditions. Biomass and growth responses of Z. marina, Enteromorpha spp., phytoplankton and microalgal flora were assessed along with biogeochemical factors in experiments conducted at both 10 and 20 °C. Phytoplankton were present throughout the experiments and responded to system RT treatments, exhibiting short term blooms in the 3 and 10 d RT treatments. Nutrients accumulated in tanks with long residence times, while DO rapidly increased in all tanks and became continuously supersaturated. Likewise, pH increased above 8.5 and pCO2 measurements suggested substantial carbon limitation, particularly in long RT treatments. Z. marina plant mortality only occurred at 20 °C. Seagrass growth, Enteromorpha spp biomass and growth and microalgal flora exhibited patterns consistent with C limitation especially in the 20 °C, long RT treatments. Preliminary results suggest that carbon limitation may have been a factor influencing alterations in autotroph community structure and dynamics under high nutrient loads.