Seasonal variation in remotely-sensed phytoplankton size structure around southern Africa

Abstract

The three-component model of Brewin et al. (2010) computes fractional contributions of three phytoplankton size classes (micro- (> 20 μm), nano- (2–20 μm), picophytoplankton (< 2 μm)) to the overall chlorophyll a concentration (Chla). Using in situ pigment data, model coefficients were adjusted for application to the southern African marine region. The refined model was applied to seasonal and monthly climatologies of MODIS Aqua Chla around Southern Africa. Chla > 1 mg m − 3 was limited to shelf regions along the coasts of Southern Africa and Madagascar, while values < 0.1 mg − 3 were found over most of the open ocean between the equator and 38°S during austral summer and autumn. In winter and spring, low values (< 0.1 mg m − 3 ) were restricted to smaller regions within subtropical gyres, while values up to 0.7 mg m − 3 extended over a much greater area of the open ocean. Shelf regions in the northern (NB) and southern Benguela (SB), Agulhas Bank (AB), Agulhas region (AR), and Mozambique Channel (MC) all showed similar seasonal cycles of size structure. On average, microphytoplankton comprised > 50% of the total Chla in these regions with little change throughout the year. The AR shelf differed, with picophytoplankton dominating in summer, and micro- and nanophytoplankton the rest of the year. In the open ocean domains of the NB, SB, and AB regions, nanophytoplankton dominated for most of the year, with picophytoplankton being more prevalent during summer and autumn. In contrast, in the AR open ocean, nanophytoplankton were dominant only during winter and early spring, whereas picophytoplankton dominated throughout the year in the MC open ocean. The refined model characterised previously unknown spatial and temporal changes in size structure in various ecosystems around Southern Africa.