Using Eddy Correlation to Understand the Impacts of Hydrodynamics , Irradiance, and Surface Area on Ecosystem Metabolism Dynamics in Coral Reefs, Seagrass Meadows, and Ice Sheets

Quantitative studies of primary production in complex environments such as coral reefs, sea-ice and seagrasses are difficult due to the spatial variability and the fast temporal dynamics of their metabolism. The non-invasive eddy correlation technique represents the best available tool for the estimation of ecosystem metabolism through benthic oxygen (O 2 ) flux rates because it does not disturb the sediment, natural hydrodynamics, or irradiance, and integrates over a large area. I used the eddy correlation technique to examine metabolic rates over reef crests, reef slopes, and seagrasses in the Florida Keys, USA and sea-ice sheets in the Nuup-Kangerlua Fjord, Greenland. The resulting high-resolution O 2 fluxes showed dynamic responses to hydrodynamics, light, nutrients, and the benthic community composition. On the shallow reef crests, numerous fluxes rates were as high as 4500 mmol O 2 m -2 d -1 , which can only be explained by the efficient light utilization of the phototropic community and the large surface area of the complex canopy structure. This efficient light utilization also led to linear ecosystemwide photosynthesis-irradiance curves for the reef crests and the seagrass meadows. In contrast, ice sheet ice algal communities were well-adapted to the low-light conditions below ice, and showed maximal production rates of ~2 mmol O 2 m -2 d -1 . Further, due to the importance of irradiance measurements in determining and evaluating primary production, a method was developed to utilize simple light loggers to mimic sophisticated photosynthetically active radiation sensors across a range of environments and conditions. ii The respiration rates in all environments were the highest directly after sunset, suggesting that the highly labile photosynthates produced during the day fueled earlynight respiration. The flow velocity was also correlated with respiration rates likely due to the enhanced ventilation of seagrass canopies, sediments, and the reef framework. Although seagrasses had relatively low net ecosystem metabolism compared to reefs, their ubiquitous nature in south Florida enables them to exchange 1750 kg carbon d -1 , compared to only 2 kg carbon d -1 for coral reefs during the summer. These direct measurements of complex reef, sea-ice, and seagrass systems revealed that in situ, ecosystem-scale measurements are important to the accurate determination of carbon cycling.

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