Will Releasing Treated Wastewater Stimulate Algal Blooms in Southern California Estuaries?
Quarterly monitoring of Upper Newport Bay, a highly eutrophic southern California estuary, has provided conflicting indicators of nutrient limitation for the seasonal macroalgal blooms in this system. Water column N:P ratios were high, up to 370:1, suggesting phosphorous limitation, while sediment N:P ratios were low, (<4:1), suggesting nitrogen limitation. A microcosm experiment was conducted to test whether macroalgal biomass was nitrogen or phosphorous limited in this system. Results indicate that even at high nutrient levels (300uM N/30uM P) macroalgal growth was nitrogen limited. Despite high N loading over the course of the experiment, water column N remained low as it was removed by the increasing algal biomass. The data also indicate that macroalgae used N fluxing from sediments for growth. Nitrogen accumulated in algal tissue while decreasing in microcosm sediments.
These results have important management implications. Winter releases of treated wastewater have been permitted to begin next year in Upper Newport under the assumption that they will not worsen blooms because macroalgae are not present at that time to utilize the increased nutrients. Our study suggests that this assumption is incorrect. This microcosm experiment shows that macroalgae are capable of utilizing sediment nutrient reserves to fuel growth. Our field monitoring data support this finding. Sediment nutrient levels in the bay peak after the spring rainy season, then decline, reaching a minimum in early winter. We hypothesized that this decline was due to utilization of sediment nutrients by the substantial macroalgal blooms (max. of 1.1 kg wet weight/m2) that occur throughout summer and fall. The microcosm experiment supports this hypothesis and demonstrates that macroalgae are capable of using sediment stores of nutrients to fuel bloom events. These results indicate that winter releases of treated wastewater are likely to exacerbate bloom conditions in Upper Newport Bay by loading sediments and providing a larger source of nutrients to fuel the summer and fall macroalgal blooms.