Seasonal influences on size-fractionated chlorophyll α concentrations and primary production in the north-west Indian Ocean

Chlorophyll a (chl a) concentrations and primary production by the 0.2-2, 2-18 and > 18 mu m phytoplankton size-fractions were estimated along a transect in the NW Indian Ocean extending from the coast of Oman to 8 degrees N 68 degrees E during the late SW monsoon and autumn intermonsoonal seasons in 1994. Primary production was estimated using the C-14 technique with either in situ or simulated in situ incubations. During the late monsoon season, maximal chi a and production values were recorded in the coastal upwelling zone with values of 69 mg m(-2) and 3800 mg Cm-2 d(-1), respectively. The maxima, which were distributed patchily in this region, were dominated by the > 18 Irm size-fraction. Over the remainder of the transect chi a concentrations and production averaged 30 mg m(-2) and 1500 mg Cm-2 d(-1), respectively, with approximately equal contributions by the three size-fractions in the case of chi a at the majority of stations, but in general, with a maximum in production in the 0.2-2 mu m fraction. Immediately following cessation of the SW monsoon wind, chi a and production values over the northern part of the transect decreased to values similar to those over the southern part of the transect at the time of the SW monsoon, with the contributions by the three size-fractions being approximately equal. During the following intermonsoonal season, both chi a concentrations and production across the section were dominated by the 0.2-2 mu m size-fraction, with average chi a and production values of the order of 20 mg m(-2) and 750 mg Cm-2 d(-1), respectively. Considerable variation in production values, however, was exhibited across the transect. A clearly defined subsurface chi a maximum was only recorded at the southernmost stations of the transect in oligotrophic waters: the feature did not develop universally across the transect during the intermonsoon. (C) 1999 Elsevier Science Ltd. All rights reserved.