Advances in the Aquatic Eddy Covariance Technique and the Unique Challenges of Aquatic Boundary Layers: Waves, Ebullition, and Biogeochemical Mass-Balance Closure

Abstract:

The aquatic eddy covariance (AEC) technique was derived from boundary layer theory that is commonly applied in the atmospheric eddy covariance technique. Over the past two decades, the AEC technique has been applied primarily in shallow coastal ecosystems, resulting in over 60 published papers, mostly using benthic oxygen flux as an analog for carbon exchange. The AEC community has described a number of factors that are unique to aquatic boundary layers including; the contamination of turbulence and chemical sensor measurements by surface waves, the presence of ebullition that produces a direct flux of gas to the atmosphere not captured by dissolved chemical sensors, and the need to address the previously identified roles of pelagic metabolism, lateral transport, and air-sea exchange that complicate biogeochemical mass-balance closure in coastal ecosystems. This research presents new instrument and analyses framework guidelines to enable eddy covariance analysis in the presence of waves through spectral separation of turbulence and waves. The rates of gas transport by ebullition in highly productive seagrass meadows highlights that the important rates of net ecosystem metabolism (e.g. net carbon sinks or sources) can be biased by highly spatiotemporally specific rates of ebullition. Combining new technological advances for measuring ebullition through time, systems that automate pelagic metabolism measurements, and applying parameterizations appropriate for coastal ecosystems to estimate air-sea exchange provide the ability to close biogeochemical budgets in highly dynamic coastal ecosystems. Notable advancements in sensor technology (e.g. optodes, pH sensors) and spectral analyses are also providing for the applications of new sensors and tracers that were previously incompatible with the AEC technique. These advancements of the AEC technique position it to become an important tool for the analyses of biogeochemical fluxes in coastal ecosystems, much like the common application of eddy covariance analysis to measure terrestrial-atmosphere exchange.