Annual variations in bio-optical properties at the ‘Estación Permanente de Estudios Ambientales (EPEA)’ coastal station, Argentina

Variations in optical properties at a coastal station (EPEA) off the North coast of Argentina (38° 28′ S 57° 41′ W) were studied in 2000–2001. Changes observed in the absorption by three components of seawater (phytoplankton, detritus, and chromophoric-dissolved-organic-matter or CDOM) were analysed in relation to changes in environmental conditions (temperature, stability of the water column, irradiance) and changes in the phytoplankton community structure. An annual cycle typical of temperate seas was observed in the stability of the water column, with a strong thermocline in summer and a vertically homogeneous regime in winter. The proportion of detritus absorption at the surface was related to these changes in stability of the water column, being larger in winter due probably to re-suspension from the bottom. Absorption by phytoplankton and CDOM were not related to temperature or the stability of the water column and there was no covariation in absorption by the three seawater components. On the other hand, absorption by phytoplankton was significantly related to the predominant cell-size. The percentage contribution of ultraphytoplankton (<5 μm) to total chlorophyll-a concentration varied between 6% and 46% throughout the year, being the highest during summer. Accordingly, the specific absorption coefficient of phytoplankton at 440 nm (absorption/chlorophyll-a) varied between 0.01 in July (when there was a bloom of the large diatom Coscinodiscus wailesii) and 0.09 m2 mg Chla−1 in February (when Synechococcus spp. was predominant). The relationship between in situ and 1 km daily satellite estimated OC4V4 chlorophyll-a concentration showed a good correlation ( r 2 = 0.94 for 9 out of the 19 data points where an exact match up could be made). Marked variations were observed when comparing 8 day-9 km binned data with the 19 points. While some of the differences are due to the highly dynamic hydrography of this region, variations in phytoplankton composition also contribute to the difference between in situ and satellite-derived values.