AME 24:175-186 (2001)  -  doi:10.3354/ame024175

ABSTRACT: Thraustochytrids, a group of osmoheterotrophic chromistan protists are ubiquitous in the sea. However, little is known of their ecological role, particularly in oceanic waters. To obtain an insight into their dynamics in this realm, we investigated thraustochytrids and bacteria in the water column, up to 2000 m in the central Arabian Sea. Four seasons, namely the end of the southwest summer monsoon (September/October 1993), the summer pre-monsoon (April/May 1994), the end of the northeast winter monsoon (February/March 1995) and the peak period of the southwest monsoon (July/August 1995) were studied. Thraustochytrids were estimated using the acriflavine direct detection method (AfDD) and bacteria by the acridine orange direct counts (AODC). Thraustochytrids were present in substantial numbers throughout the 150 m water column in all the stations investigated during the end of the biologically productive summer and winter monsoons (0 to 1313 x 10³ and 3.7 to 183 x 10³ cells l^-1 water respectively). Their populations in the upper 150 m were positively related to bacteria and particulate organic matter during these 2 periods, the relationship being significant during the end of the winter monsoon. They were infrequent during the low productive summer pre-monsoon period (present in < 29% of the total 41 samples), a period of high bacterial abundance, as well as in the peak-productive southwest summer monsoon (present in 10 of 19 samples). Vertical distribution was seasonally variable. Thraustochytrids were regularly detected in the Arabian Sea oxygen minimum zone (OMZ) at 200 to 1000 m. Similar to the upper 150 m, thraustochytrids below 200 m were least frequent in the summer pre-monsoon (present in 49% of samples), although high numbers, up to 266 x 10³ l^-1 were found at 1000 m in 1 station. Their numbers in the OMZ ranged from 0 to 38 x 10³ l^-1 during the end of the summer and winter monsoons. Both thraustochytrids and bacteria showed a distinct peak at 250 to 500 m in the OMZ during these 2 seasons. Dense populations of thraustochytrids were detected even at depths up to 2000 m. We hypothesise that seasonal increases of thraustochytrid populations are related to particles following phytoplankton degradation at the end of the biologically productive seasons. Our estimates suggest that during the 3 seasons, maximum thraustochytrid C biomass reached values ranging from 36 to 217% of bacterial C biomass in the upper 150 m and 34.5 to 56% of bacterial C biomass between 200 and 2000 m.

KEY WORDS: Thraustochytrids · Abundance · Arabian Sea · Seasonal distribution