Seasonal variability in satellite-measured surface chlorophyll in the Patagonian Shelf

Abstract

Six years (January 1998—December 2003) of SeaWiFS ocean color satellite data are used to estimate monthly climatological means and to present the near-surface chlorophyll-a seasonal evolution in the Patagonian Shelf. The southern part of the shelf presents elevated chlorophyll concentrations from spring through autumn, while the northern part shows three regions with particular characteristics. The external region, bordering the 200 m isobath, presents elevated concentrations from spring through autumn supported by the nutrient-rich waters from the Malvinas (Falkland) Current. The central region presents a typical pattern of temperate regions, characterized by two well-defined maxima, a stronger spring bloom and a weaker fall bloom, and low chlorophyll values throughout summer (scarce availability of nutrients) and winter (light being the limiting factor). Even though the displacement direction of the spring and fall blooms do not agree with previous information reported in the literature, they are interpreted based on the heat exchange in the air–sea interface that controls the development and erosion of the seasonal thermocline. Finally, the coastal region presents less-marked seasonal variability and isolated small areas with elevated concentrations associated with frontal areas are observed. The spatial mean chlorophyll evolution, averaged over the whole shelf (less than 200 m depth), shows a marked annual cycle with high values from spring to autumn, supporting the importance of frontal regions as a fertilization mechanism. An increasing trend in chlorophyll concentrations, within the 6 years analyzed here (in the order of 23%), is apparent based on an increasing of the maximum annual values. From the comparison with in situ data it can be concluded that satellite information reproduces the spatial patterns of chlorophyll fields obtained from more classical data, while differences exist in absolute values obtained from both methodologies.