Short-term variability in halocarbons in relation to phytoplankton pigments in coastal waters of the central eastern Arabian Sea

Halocarbons released from oceans are known to be important in the atmospheric processes. Their abundance in seawater is influenced by water column conditions and phytoplankton community. In the present study halocarbon abundances together with pigment composition were monitored in the coastal waters of the central eastern Arabian Sea. Short-term variability for the period December 2005–March 2007 was studied for chlorinated hydrocarbons (CHCl3, CCl4) and bromocarbons (CH2Br2 and CHBr3) for the first time in the Arabian Sea. Variability in pigments was used to understand the influence of phytoplankton community composition on halocarbon concentrations in this region. Halocarbons and phytoplankton pigments showed strong short-term variability, with maximal values generally associated with southwest (summer) monsoon high productivity. Decreased chlorocarbon abundances in sub-surface layers indicate their possible consumption under oxygen deficient conditions, particularly during the summer monsoon and Fall-Intermonsoon periods. Halocarbon abundances were in the order: CHCl3, CH2Br2, CHBr3 and CCl4. The CHCl3 showed significant positive relationship with fucoxanthin (r = 0.93, p = 0.001, n = 155) suggesting the importance of diatoms and prymnesiophytes in the Arabian Sea. Among the phytoplankton pigments chlorophyll-a showed a positive relationship with marker pigment fucoxanthin. Of the four phytoplankton groups studied, the percent dominance was in the order: diatoms > nanoflagellates > cyanobacteria > dinoflagellates. Our study shows that the utilization of halocarbons during microbial oxidation of organic matter could be a significant sink of these compounds although they might be produced in significant quantities by biochemical processes during high productivity periods, as can be seen from our observations in summer monsoon in the Arabian Sea.