The Effect of Eelgrass on the Air-Sea Heat Flux and Vertical Temperature Profile in a Shallow Embayment

If we are to protect and restore important coastal ecosystems, we must better understand the processes controlling water temperature in these systems. Here, we focus on the air-sea heat flux and vertical heating profile in Mumford Cove, Groton, CT, a shallow embayment supporting eelgrass (Zostera marina). To calculate air-sea heat flux, numerous investigators use the COARE bulk flux algorithm (Fairall et al. 1996; Edson et al. 2013). However, the algorithm was initially developed with data from the tropical Pacific Ocean and uses a constant albedo of 5.5%. In our system, incoming radiation interacts with components not important in open water: eelgrass canopy, bottom sediments, and higher concentrations of chlorophyll, CDOM, and suspended material. Each alters the absorption and scattering of light in the water column, affecting the total water column albedo. Using the radiative transfer model HydroLight (Mobley 1994), we find net shortwave heat flux into the water is reduced up to 25 W m^-2 as a result of increased albedo associated with eelgrass. We use measured albedo in Mumford Cove over eelgrass beds and eelgrass-free areas to test the model prediction. Our HydroLight model runs also indicate the distribution of thermal energy within the water column is altered by the eelgrass canopy, resulting in enhanced sea-surface temperature (SST). Observed vertical temperature profile time series illuminate the effect of eelgrass on radiant heating rates in the surface layer. Since longwave, latent, and sensible heat fluxes are functions of SST, eelgrass can affect all components of the air-sea heat flux. We compare the air-sea heat flux terms estimated by COARE using both default and observed albedo values as well as SST measured in and out of the eelgrass bed. This study indicates the importance of considering site-specific albedo for more accurate estimation of the air-sea heat flux and to improve understanding of the heat budget in shallow coastal waters.