Environmental conditions and bio‐optical signature of a coccolithophorid bloom in the Patagonian shelf

Abstract

In January 2008, a patch of high reflectance detected by ocean color satellite images was sampled during a cruise over the southern Argentinean continental shelf. High calcite concentrations (particulate inorganic carbon (PIC)) found at the patch were associated with dominance of the coccolithophorid Emiliania huxleyi. Relatively low chlorophyll concentrations (0.29 to 1.48 mg m−3) were found, but both particulate attenuation (0.27 to 1.15 m−1) and backscattering coefficients at 660 nm (0.003 to 0.042 m−1) were noticeably high. Particulate inorganic to organic carbon (POC) ratio (PIC:POC) was highly variable (0.02 to 1.1), but mostly high, showing a significant correlation with particulate backscattering coefficient at 660 nm (r = 0.83, p < 0.005). The spectral dependency of the backscattering coefficient followed Gordon et al. (2009). Both the time evolution analyses of normalized water leaving radiance at 551 nm (nLw551) and the high PIC:POC pratios suggested an advanced stage of the coccolithophorid bloom, therefore with high detached coccoliths:cell ratios. Moreover, this was supported by a strong correlation between PIC and both particulate backscattering (r = 0.81, p < 0.005) and particulate beam attenuation coefficient (r = 0.7, p < 0.05). Remote sensing reflectance data were strongly related to particle backscattering and backscattering ratio, but not to absorption. NASA operational algorithms overestimated chlorophyll by a factor of ∼2 and estimated PIC with a relatively high root‐mean‐square (RMS) error (RMS = 97.9 mg PIC L−1). Better estimates of PIC values (RMS = 81.5 mg PIC L−1) were achieved when we used the original PIC‐specific backscattering coefficient (Balch et al., 2005).