MEPS 581:103-117 (2017)  -  DOI: https://doi.org/10.3354/meps12313

ABSTRACT: The presence of kelp canopies in the surface oceanic layer modifies the vertical distribution of solar radiation in the water column and thus the sea surface temperature (SST). We examined the modification of SST due to kelp using observational data and a 1-D mixed layer model. The observational data included SST and the normalized difference vegetation index (NDVI), derived from Landsat 8 images from 2013 and 2014. Data analysis showed that SST in kelp forests is consistently higher than that in the ambient kelp-free waters and that the magnitude of SST increases with the increasing mean NDVI with a median value of (mean ± SD) +2.7 ± 1.4°C and a maximum asymptotic value of +5.4°C for an NDVI of 0.80. Using the mixed layer model, the effect of kelp on the vertical attenuation of solar radiation was simulated with an extinction depth derived from published field data. The model successfully captured the observed trend of increasing SST with NDVI. Analysis of the model result indicated that the increased absorption of solar heat caused by kelp changes the distribution of solar heating in the water column which enhances SST but leaves deeper temperatures relatively unaffected. Increased SST leads to a shallower upper mixed layer depth (MLD) and enhances the heat flux from ocean to atmosphere. However, SST in kelp areas may significantly drop below values in kelp-free waters due to strong vertical mixing events. Our study emphasizes the importance of kelp beds in regulating heat flux in coastal areas. While the model in this preliminary study captured some of the dynamics, the scatter of the SST observations for all NDVI values remains problematic. More in situ data—kelp concentrations, ocean current, and mixing strengths—and contemporaneous observations of NDVI would be valuable to determine if the scatter of SST about the empirical fit and model predictions could be reduced.

KEY WORDS: Solar radiation · Extinction depth · Surface heating · Mixed layer model · Heat flux · Landsat · Remote sensing