Impacts of Eutrophication on the Carbonate Chemistry and Foundation Species in Estuaries

Globally, aquatic ecosystems are experiencing macroalgal blooms that have a wide range of impacts on ecosystem structure and functioning. Many of these impacts are relatively well studied in temperate estuaries. However, one potential impact of macroalgal blooms in estuaries that has not been fully explored is the effect these blooms may have on estuarine carbonate chemistry. These changes in carbonate chemistry and associated shifts in pH may have impacts to other aquatic plants that are also utilizing CO2 as a resource. There is some limited evidence for impacts on carbonate chemistry; for example, diel cycles of pH have been documented in nearshore areas due to increased photosynthesis during the day and respiration during the night. We hypothesize that if more macroalgae is added to an estuarine system, diel cycles may become more pronounced. We conducted a flow-through mesocosm experiment to assess the impacts of a gradient of macroalgal biomass additions on carbonate chemistry, dissolved oxygen (DO) concentration, and seagrass growth metrics. The experiment lasted 45 days and consisted of 5 levels of macroalgal biomass (0, 1, 1.5, 2 and 3 kg wet mass m-2) with two replicates for each level for a total of 10 mesocosms. Each mesocosm started with the same number of of Zostera marina shoots that were cut to similar size. We measured pH, temperature, and DO twice daily (during the ends of the light and dark cycle) and collected samples for total alkalinity weekly (TA). We also collected samples for carbonate chemistry analysis three times, (weeks 1, 3, and 6). Data are still being analyzed, but preliminary results revealed an overall reduction of pH and DO as algae decomposed, and increased amplitudes of diel cycles of pH and DO in tanks with higher macroalgae biomasses. We expect lower pH to be associated with increased respiration in mesocosms with more decomposing macroalgae. We expect our results will provide insight into critical thresholds where macroalgae produce negative effects on seagrasses through alterations in carbonate chemistry. We hope our results will inform conservation and management of critical seagrass habitat o the west coast of the US.

Impact/Purpose

Globally, estuaries are experiencing increases in nutrients which can lead to increased biomass (eutrophication) in the form of macroalgal blooms. These algal blooms can have negative impacts to the local ecology and important foundation species such as seagrasses. One of the less studied impacts of macroalgal blooms are the changes that can occur to the pH and carbonate chemistry. When algal biomass increases, the algae can take up more CO2 during photosynthesis. However, during the nighttime, and when algae decompose there can be an increase in CO2 through respiration. To determine how increased algae will impact estuarine water carbonate chemistry and pH and how these changes will impact seagrass, we conducted an experiment in flow-through tanks at the EPA facility located along the Yaquina Bay estuary.  We tested the effects of a gradient of macroalgae biomass on the carbonate chemistry of the water and seagrass growth. We found that in treatments with higher macroalgae, there was lower pH and greater changes in pH throughout the light and dark cycle. This suggests that algae can cause a decrease in pH when it decomposes. We are expecting one manuscript to be published with the results of this project. We are currently working on finishing the lab work and analyzing data.

Citation

Zweng, R., P. Fong, AND Jim Kaldy. Impacts of Eutrophication on the Carbonate Chemistry and Foundation Species in Estuaries. Fong Lab - Research Presentation, NA, OR, May 02, 2022.