DISTRIBUTION OF BIOGENIC SILICA AND PARTICULATE ORGANIC-MATTER IN COASTAL AND OCEANIC SURFACE WATERS OFF THE SOUTH SHETLAND ISLANDS IN SUMMER

Biogenic silica (BSi), lithogenic silica (LSi), particulate organic ca rbon (POC) and nitrogen (PON), and chlorophyll a (Chl a) concentration levels were measured in the surface waters (< 100 m) off the northern coast of the South Shetland Islands in summer 1991. High concentratio n levels of BSi and LSi were recorded in the oceanic area and the coas tal area, respectively. However, marked regional differences were not observed for POC, PON and Chl a concentrations. The mean BSi/POC atomi c ratio (+/- SD) in the oceanic area (0.27 +/- 0.17) was 6 times that in the coastal area (0.045 +/- 0.020), except for the bloom situation (0.19 +/- 0.029). In contrast, the mean POC/PON atomic ratio was not s ignificantly different in the coastal area (5.9 +/- 1.4) and the ocean ic area (5.2 +/- 1.7). Nitzschia spp. were the dominant diatoms in the oceanic area but not in the coastal area. High BSi/POC ratios have be en reported for blooms dominated by Nitzschia spp. even in the coastal regions of the Antarctic Ocean. The area difference in the BSi/POC ra tios was probably related to the difference in species composition of phytoplankton and not to regional contrast. This species contributes s ignificantly to high BSi/POC ratios in the Antarctic Ocean.