A review of past and present summer primary production processes in the Ross Sea in relation to changing ecosystems

We analyse primary productionprocesses during austral summer 1996 and 2001 in different environmental conditions such as ice-free waters and extensive ice-covered areas. Spatio-temporal distribution of phytoplanktonbiomass and functional groups along with photosynthetic parameters are presented. Production vs irradiance (P vs E) experiments were performed using 14 C incubations at several stations and three or four different depths to define the eco-physiology of phytoplanktoncommunities.The results of the oceanographic campaign conducted in ice-free waters of the Ross Sea (summer 1996) emphasize that these ecosystems are characterized by high nutrient low chlorophyll(HNLC) conditions due to limiting factors (eg Fe). Conversely, the results of the oceanographic cruise in extensively packice-covered areas (summer 2001) indicate that the average phytoplanktonbiomass (estimated from Chl a ) was about three times the values recorded in ice-free conditions, but the primary productionwas relatively lower. In fact, in situ primary productionand P vs E experiments over few days show that high primary productionvalues occurred in most of the area, but only within the first five meters of the water columnand the melting pack ice. Notwithstanding some high values of phytoplanktonbiomass during the 2001 campaign, water columnstability, similar irradiance levels along the water column, photosynthetic capacitywas suppressed in deeper layers, indicating a low carrying capacity of the pelagic ecosystem due to iron limitation, as shown by low values of the photo-chemical efficiency of photosystem II (PSII), variable fluorescence and maximal fluorescence ratio (Fv/Fm). In contrast with a very high variability in phytoplanktonbiomass at several temporal and spatial scales, photosynthetic parameters (P B max , a, E k ) varied within narrow ranges. Relevant changes in phytoplanktonabundance and species composition are reported in this study although the environmental factors that drive these changes in primary productionprocesses and prevalence of principal functional groups of phytoplanktoncommunities compared to the past (1996 and 2001) are still unknown. The effect of these changes on the carrying capacity of Ross Sea ecosystems, carbon export and the potentially new asset of the food web will need to be determined.