Factors influencing the distribution of diatoms and other algae in the Ross Sea

Abstract

Quantitative microscopic analyses of sediment trap samples collected in the Ross Sea between January 1990 and February 1992 reveal striking temporal and spatial differences in algal bloom composition and size. Trap samples from the southwestern Ross Sea (site A, 76°30′S, 167°30′E) were dominated by the diatom Fragilariopsis curia (maximum of 92%), a species associated with both sea ice and the retreating ice edge in the Ross Sea. This species was probably seeded by melting congelation ice. Highest flux of diatom valves to both upper and lower traps in the 1991–1992 season occurred as a distinct event in mid‐February 1991, after which flux decreased by 1 to 3 orders of magnitude. As overall flux decreased, an increase in relative abundance of Chaetoceros resting spores and the open water species Thalassiosira antarctica was observed in the lower trap, suggesting some localized lateral advection. In the south central Ross Sea (site B, 76°30′, S 175°W), algal diversity was much higher, with a greater contribution of nondiatom material. The prymnesiophyte Phaeocystis antarctica, dinoflagellates, and cysts of unknown affinity were much more common than at site A, as were the diatoms Fragilariopsis cylindrus, Fragilariopsis kerguelensis, and Thalassiosira gracilis. Highest diatom flux occurred later (February 20 to March 6,1991) at this site and was an order of magnitude lower as compared to maximum flux at site A. Distinct differences in bloom size and composition between sites A and B may be a function of upper water column structure, differences in the amount of sea ice melting, and the type of sea ice present at the time of ice breakout (congelation versus pack ice). Despite rich surface productivities, lower silica flux to the seafloor at site B results from the higher proportion of nonsiliceous algae. At site C (72°30′S, 172°30′E), in the northwestern Ross Sea, diatoms again dominated algal flux; however, assemblage composition differed from that observed at both sites A and B. Relative abundance of F. curta was lower, averaging 60–70%, while the remainder of the assemblage was made up of F. cylindrus, other more robust species of Fragilariopsis, and Chaetoceros resting spores. Algal flux at site C is intermediate in style between sites A and B. Significantly, spatial differences in diatom assemblages noted above appear to be reflected in seafloor surface sediments, suggesting that downcore diatom data provide an interpretable record of paleoproductivity in the Ross Sea.