Abstract
The southern Brazilian coast is the major fishery ground for the Brazilian sardine (Sardinella brasiliensis), a species responsible for up to 40% of marine fish catches in the region. Fish spawning and recruitment are locally influenced by seasonal advection of nutrient-rich waters from both inshore and offshore sources. Plankton communities are otherwise controlled by regenerative processes related to the oligotrophic nature of the Tropical Water from the Brazil Current. As recorded in other continental margins, zooplankton species diversity increases towards outer shelf and open ocean waters. Peaks of zooplankton biomass and ichthyoplankton abundance are frequent on the inner shelf, either at upwelling sites or off large estuarine systems. However, meandering features of the Brazil Current provide an additional mechanism of upward motion of the cold and nutrient-rich South Atlantic Central Water, increasing phyto- and zooplankton biomass and production on mid- and outer shelves. Cold neritic waters originating off Argentina, and subtropical waters from the Subtropical Convergence exert a strong seasonal influence on zooplankton and ichthyoplankton distribution towards more southern areas. This brief review highlights the need for further experimental studies on zooplankton life cycle strategies in order to understand the major processes controlling food web dynamics in this shelf ecosystem.
Highlights
The southern Brazil shelf ecosystem accounts for over half of the country’s marine fisheries yield (Matsuura, 1996; Odebrecht and Garcia, 1997)
A seasonal trend similar to that observed for sardines is the usual picture in the case of zooplankton (Fig. 5), but in addition to biomass agglomerations associated with low-temperature inshore areas during summer, patches occurred on the intermediate and outer shelves and close to the shelf break, following an apparently recurrent pattern observed since early quantitative studies (Jacob et al, 1966)
These maxima probably result from cumulative growth of both coastal- and South Atlantic Central Water (SACW)-derived zooplankton taxa preying upon large phytoplankton aggregates that develop within upwelling systems associated with vortices and eddies of the Brazil Current (Gaeta, 1999)
Summary
The southern Brazil shelf ecosystem accounts for over half of the country’s marine fisheries yield (Matsuura, 1996; Odebrecht and Garcia, 1997). Fishery yields for this species have decreased dramatically in the last 20 years – not surpassing 120,000 tons per year – and catches below or around 35,000 tons per year have been the norm since 1999 (Cergole et al, 2005; FAO, 2003) This is due to over-exploitation of stocks (Cergole et al, 2005), and because of climate change and oceanographic anomalies that have led to recruitment failures (Matsuura, 1999). Fish spawning and recruitment in this oligotrophic region depend strongly on seasonal advection of nutrient-rich waters from both inshore and offshore sources, which affects the availability of planktonic food for the larvae (Matsuura et al, 1992) Physical processes such as oceanic fronts and eddies have been intensively studied because of their implications to biological processes, including the variability of zooplankton and ichthyoplankton stocks (Iles and Sinclair, 1982; Nakata, 1989; McGowen, 1993; Sabatés and Olivar, 1996; Grioche et al, 1999). This paper intends to fill part of this gap by presenting a brief overview of the effects of major circulation processes on the distribution of zooplankton and ichthyoplankton assemblages of the area
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