Examining Metabolism in Seagrass Meadows: Trends in Respiration and Non-Dissolved Gas Ebullition

Seagrass meadows provide many valuable ecosystem services, including carbon sequestration and storage of ‘blue carbon’. Enhancing carbon cycling through seagrass restoration can thus act as a nature-based solution for climate change, as these ecosystems naturally remove carbon dioxide from the atmosphere through their high rates of productivity. Estimated metabolic values such as Gross Primary Productivity (GPP), Respiration (R), and Net Ecosystem Metabolism (NEM) are used in quantifying in situ carbon sequestration in seagrass meadows and other blue carbon ecosystems, as well as assessing ecosystem health. However, there are several assumptions within the methods and calculations that are conventionally used for estimating GPP, R, and NEM. In this thesis, I examine two of these assumptions: 1) respiration is constant and equal during day and night in calculations of GPP and R, and 2) dissolved oxygen (O2) fluxes, which are used to estimate metabolic values, represent the total O2 fluxes in the system and O2 ebullition is negligible. These assumptions were tested in the restored eelgrass (Zostera marina) meadow at the Virginia Coast Reserve Long Term Ecological Research site (VCR LTER), located on the Eastern Shore of Virginia. To assess the first assumption, I examined a long-term data set of seagrass meadow O2 fluxes collected using the aquatic eddy covariance technique, which allows for direct measurements of nighttime respiration. For the second assumption, I used a novel, automated bubble trap design to sample ebullition at a fine temporal scale and measured the content of O2 and other gases in the collected gas volume to calculate O2 flux in the form of bubbles.

In my first chapter, I found that respiration varied significantly over a 24-hour period in a seagrass meadow. Measurements showed a linear decrease in respiration of 29% through the night, from dusk to dawn. A corresponding linear increase in daytime respiration coupled with production described by a standard photosynthesis–irradiance curve accurately predicted measured daytime O2 fluxes. The results of this study show that the assumption in metabolic estimates that night- and daytime respiration are constant and equal is not upheld. However, if respiration can be approximated as we found here by linear relationships, standard means for calculating daily metabolic numbers remain valid if estimates are based on full 24-hour records of O2 flux data.

In my second chapter, I found that the O2 bubble flux makes up a small percentage of total O2 fluxes, only increasing daily GPP estimates by 2.4% in July, 3.6% in August. However, even though accounting for O2 ebullition would only increase total O2 flux estimates by a few percent, it may still provide some value to incorporate into estimates of metabolic numbers. Some temperate seagrass meadows, like the one studied here, have been shown to be on the threshold between being net autotrophic and net heterotrophic in the past. The results also showed the variability in ebullition of the greenhouse gases CO2 and CH4, with this study being the first to our knowledge to quantify CO2 ebullition in a seagrass meadow over a relatively long sampling period. More study is needed to understand the highly variable nature of CH4 ebullition, a very strong greenhouse gas. Using the novel, automated bubble trap technique, it was possible to examine bubble fluxes at a fine temporal scale (every 10 minutes), which allowed for comparison with quickly changing environmental variables. We found that high light availability, supersaturated O2 conditions, and low tides are the important drivers in stimulating non-dissolved gas flux.

In summary, I found that the assumption that respiration stays constant and equal between night and day was not supported, and that the assumption that non-dissolved O2 fluxes are not a significant component of total O2 fluxes is upheld. Improving the understanding of metabolism in a temperate seagrass meadow provides more insight into the role these ecosystems play in climate change mitigation through their in-situ carbon sequestration and storage potential.