FATTY-ACID COMPOSITION AND COMMUNITY STRUCTURE OF PLANKTON FROM THE SAN-LORENZO CHANNEL, GULF-OF-CALIFORNIA

The structure of the plankton community and fatty acid composition of nano-, micro-and zooplankton are described during four seasons of 1994 from the San Lorenzo Channel. During August, the warmest temperature in the surface water was observed and a thermocline developed between 20 and 30 m. In the remaining months, a well-mixed layer occurred in t he upper 30 m. The chlorophyll a content of the nanoplankton fraction (<38 mu m) was higher than the microplanktonic fraction (38-200 mu m) year round. Maximal chlorophyll values (1.5-3 mu l(-1)) occurred in Ja nuary, which may be associated with organic matter, since phytoplankto n was lower than at other seasons. The relative abundance of diatoms i ncreased from January (57% of phytoplankton) to November (99 %). The i ncrement was mainly due to Nitzschia and Chaetoceros. Dinoflagellates were always low (0.03-1.36 cells ml(-1)). Copepods (mainly Eucalanus) dominated the zooplankton in winter and fall, while in spring and summ er, the abundance of doliolids was similar to the copepods (Nannocalan us minor dominated). Four fatty acids (16:0, 16:1, 18:0, 18:1) were th e most conspicuous in the plankton, representing usually between 40 an d 80% of the total fatty acids throughout the water column. In winter, higher fatty acid content and higher relative amounts of 16:0 and 16: 1 were observed than in the warm months. Stearic acid (18:0) peaked du ring fall. The major seasonal differences occurred in the nanoplankton , which had peaks of 20:5 during January, and 16:4 in April. A strong decrease in polyunsaturated fatty acids (PUFA) occurred during the war m months. The fatty acid composition of microplankton and larger zoopl ankton was similar in winter-spring. Individual copepods of selected s pecies (Eucalanus sewelli, Rhincalanus nasutus, Centropages furcatus a nd Labidocera acuta) showed fatty acid profiles similar to the mixed z ooplankton, with some differences in content of PUFA. (C) 1997 Academi c Press Limited.