Short-term variation and long-term changes in the oceanographic environment and zooplankton community in the vicinity of a sub-Antarctic archipelago

Mesozooplankton community structure in the vicinity of the Prince Edward Is lands (PEIs) was investigated during six surveys conducted in late austral summer (April/May) from 1996 to 1999. Zooplankton samples were collected by oblique tows using a Bongo net fitted with 300-mum mesh. Surface temperatu re, average temperature and chlorophyll a were measured in conjunction with each net tow. The positions of the Sub-Antarctic Front (SAF) and the Antar ctic Polar Front (APF), in relation to the islands, were determined by CTD and/or XBT transects to the west of the islands (upstream). Both fronts wer e characterized by a high degree of latitudinal variation. Changes in posit ion of the fronts occurred rapidly, the SAF moving up to similar to 120 km in a 2-week period. Consequently, the oceanographic environment in the vici nity of the PEIs was subject to a high degree of intra- and inter-survey va riation. The positions of the SAF and APF appeared to have a significant im pact on phytoplankton biomass in the vicinity of the PEIs, possibly through the alteration of local oceanographic flow dynamics. Water retention over the island shelf in 1996, associated with location of the SAF far to the no rth of the PEIs, corresponded to enhanced chlorophyll-a concentrations (sim ilar to1.54 mg m(-3)). Conversely, when the fronts were close to the island s, as in 1997 and 1999, higher current velocity limited water retention and chlorophyll-a concentrations in the interisland region were relatively low (similar to0.4 mg m(-3)). Cluster analyses showed that, in many instances, there was greater similarity among zooplankton communities from different surveys than among communities within surveys, indicating that short-term v ariability exceeded inter-annual variability. The population structure of t he copepod Calanus simillimus indicated that there was inter-annual variati on in the timing of the biological season. Differences in the population st ructure of species, and consequently their contribution to abundance and bi omass, may therefore have been an important contributor to inter-annual var iation in community structure. Evidence is provided of a long-term southwar d shift in the position of the SAF. It is postulated that this may affect t he PEIs by increasing the proportion of allochthonous energy input, because the PEIs now lie in the path of the front, altering the trophodynamics of the island ecosystem. Lower mesozooplankton biomass associated with warmer sub-Antarctic water may have important negative consequences for higher tro phic levels that depend on mesozooplankton for food.