THE ROLE OF NEARSHORE MIXING ON PHYTOPLANKTON SIZE STRUCTURE OFF CAPESANTA-MARTA-GRANDE, SOUTHERN BRAZIL (SPRING 1989)

Nearshore mixing and the distribution of chlorophyll a (Chi a), its si ze fractions (< 1 mu m, 1 to 20 mu m, > 20 mu m) and the abundance of phytoplankton and phycoerythrin containing chroococcoid cyanobacteria (PECC) were investigated in November 1989 off Cape Santa Marta Grande in southern Brazil (29 degrees S; 50 degrees W). The subsurface Chl a maximum layer (SCM) at coastal and mid-shelf stations was up to 85 % m icroplankton (> 20 mu m), while nanoplankton (1 to 20 mu m) and picopl ankton (< 1 mu m) comprised the main Chi a fractions in the surface la yer and at oceanic stations. Highest Chi a in the SCM was due to the p resence of large (40 to 150 mu m diameter) centric diatoms (Coscinodis cus, Thalassiosira) with high cellular Chi a content. PECC abundance v aried between 3 . 10(5) and 1.4 . 10(8) cells . l(-1), with maximum ab undances slightly lower depths than microplankton Chi a. The productio n of a mixed nutrient-rich water mass seems to be the most important f actor controlling the phytoplankton distribution during the study. Nut rient injection into the euphotic zone occurred through bottom driven turbulence both during upwelling and downwelling conditions, fueling t he subsurface growth of large diatoms in the SCM, and contrasting with the otherwise small organisms in the upper layer and offshore. The gr owth of large cells in the SCM was a food source for most filter feede rs, and a simple and short trophic system, with low losses, is predict ed. On the other hand, a more complex food web may be found in the upp er layer and offshore stations, due to the occurrence of low Chi a ass ociated with predominantly small phytoplankton cells.