Top-down and bottom-up regulations in a high nutrient-high herbivory macrotidal coastal ecosystem in Northern Patagonia, Argentina

MARTINETTO PAULINA; MIRTA TEICHBERG; IVAN VALIELA; DIANA MONTEMAYOR; OSCAR IRIBARNE

Daytona Beach, FL

Conferencia; CERF 2011: 21st Biennial Conference of the Coastal and Estuarine Research Federation. Societies, Estuaries & Coastas: Adapting to Change; 2011

Coastal and Estuarine Research Federation

One of the main assumptions in coastal eutrophication studies is that eutrophication causes a significant shift in benthic communities along with great losses of both species and functional diversity. In this study we use San Antonio Bay (40° 7’ S and 64° 9’ W, Río Negro, Argentina) to understand the relative importance of bottom-up and top-down controls on macroalgal blooms in a macrotidal system with high nutrient supply and high consumer abundance. Our results show that nutrients, pH, and O2 concentrations were higher during low tide. A field experiment showed that the biomass accumulation rate of Ulva lactuca ranged between 6 and 12% d-1 and was reduced by herbivory by 60%. The biomass accumulation rate did not differ in thalli with different initial internal nutrient pools. There was a negative relationship between the percentage of algae consumed and the N content in algae tissue suggesting compensatory feeding by herbivores. In a second esxperiment, herbivory reduced the biomass accumulation rate of U. lactuca when PO43- or no nutrients were added, but not when NO3- was added. In the absence of herbivory, the addition of nutrients did not increase U. lactuca biomass accumulation rate. Under these conditions, top-down and bottom-up forces interact in the regulation of macroalgae proliferation. Although the ultimate mechanism operating is unknown, the high herbivory pressure and the response to NO3- addition only when herbivores were present are evidence that both regulation forces act conjointly. Altogether, our results suggest that nutrients remain high enough for adequate time intervals to be assimilated by macroalgae and support blooms. Large water exchange during tides, however, can diminish the potential negative effects of macroalgal accumulation (oxygen depletion, high ammonium concentrations) on herbivores so that herbivores can have a large impact on macroalgae.