Seasonal variation in phytoplankton in the southern Benguela: pigment indices and ocean colour

Abstract

Pigment indices and satellite imagery were used to investigate the seasonal variation in phytoplankton in the southern Benguela, with the aid of monthly monitoring on two transect lines. Sea surface temperature images revealed the similarity in thermal conditions during autumn, winter and spring, with a broad zone of 11-14°C water extending northwards from Cape Columbine and a narrow zone extending south along the coast to the Cape Peninsula. In summer, warm water (18-20°C) intruded from the South Coast into the West Coast offshore region and cold water (10-12°C) prevailed inshore at particular upwelling centres. The phytoplankton response to these thermal conditions resulted in elevated chlorophyll a (2-15mg m -3 ) in the 11-14°C waters during all four seasons. The zones of elevated chlorophyll were generally broader from Cape Columbine northwards and narrower south of Cape Columbine to the Cape Peninsula. Pigment indices indicated that phytoplankton biomass was high in summer, but also quite extensive in the autumn and spring, with diatoms generally dominating inshore and small flagellates offshore. Diagnostic indices indicated that flagellates were prominent at inshore localities during all seasons, however, suggesting that this group contributed significantly to phytoplankton communities on the shelf.

Primary production in the Benguela ecosystem, 1999–2002

“…These rates are comparable to those estimated in this study and affirm the similarity between the Benguela and Humboldt ecosystems. Phytoplankton communities in the Benguela are composed of diatoms and small flagellates, with diatoms generally being dominant in the inshore zones and flagellates being more important offshore (Barlow et al 2001(Barlow et al , 2005(Barlow et al , 2006. In this study, it is likely that the high productivity at inshore stations was on account of the dominance of diatoms and lower productivity offshore was associated with nanoflagellates.…”

Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

“…While surface Chla measurements are considered to be reasonably representative of phytoplankton biomass in the euphotic zone of the NB and SB shelf regions (Barlow et al, , ; Brown et al, ; Lamont, Brewin, et al, ; Louw et al, ), subsurface Chla maxima commonly occur on the AB shelf (Brown & Cochrane, ; Brown et al, ; Probyn, ; Probyn et al, ), as well as in the open ocean domains of each of these regions (Lamont, Brewin, et al, ). Thus, the variations of satellite Chla and size structure presented in this study are considered to represent the surface layer only.…”

Plankton productivity of the Benguela Current Large Marine Ecosystem (BCLME)

“…the common Chaetoceros, Nitzschia, Thalassiosira and Rhizosolenia) are found in both the northern and southern sub-systems, while small flagellates are prevalent in warmer, less saline waters offshore (e.g. Barlow et al, 2005Barlow et al, , 2006. Exceptions to this general pattern include the abundant diatoms Delphineis karstenii, restricted to the north, and Skeletonema costatum, particularly abundant in the south, while the large Coscinodiscus spp.…”