No abstract

No abstract

In the central and eastern Arabian Sea (EAS), biomass and production of phytoplankton are known to vary spatially and seasonally whereas, biomass of mesozooplankton (MSP) is reported to be constant. This apparent contradiction has been called 'Arabian Sea Paradox'. However, it is important to note that the paradox of MSP is based on a very limited seasonal data (from 5-7 locations). Therefore, we reconsidered the paradox of MSP based on intensive in-situ observations at 39 -40 locations in the EAS. In agreement with the known seasonal difference in phytoplankton standing stock in the EAS, we analysed the MSP data in two ways using two-way nested ANOVA. In 'basin scale' analysis, MSP data were analysed from the EAS were pooled and seasonal and inshore -offshore variations were analysed for the entire region. In 'regional scale' analysis, MSP data were analysed separately for (a) northern region (north of 15°N) and (b) southern regions (15°N and south of it). Satellite data of chlorophyll a, SST and wind speed were also analysed to show the major differences in oceanographic features in the northern and southern EAS. The analyses showed prominently high chlorophyll a (av. 1mg m ) in the southern region during most of the year mainly due to thermohaline stratification. The MSP biomass was distributed almost in a similar way as that of phytoplankton stock with statistically significant spatial and seasonal variations in the northern and southern regions. In this paper, we review the 'paradox of MSP' and present clear and new evidences to show that this concept is not logically applicable for EAS.

The influence of environmental parameters on the spawning aggregation of Euphausia sibogae was investigated along the southwest coast of India during the peak phase of summer monsoon 2005. The prevailing ecological conditions between the aggregation period (peak phase) and non-aggregation period (early phase) were also compared. The aggregation was observed at station 1 (8 • N; 76.5• E, 480 ind·m −3 ) and 6 (10 • N; 75.5 • E, 839 ind.m −3 ) during the peak phase of the summer monsoon. Eggs (14769 eggs m −3 ) and different developmental stages were observed in higher abundance at station 6. The physicochemical conditions indicated that the aggregation coincided with the upwelling. The nutrient enrichment due to the upwelling triggered phytoplankton blooms, and this appeared to provide a conducive environment for spawning and development of E. sibogae.

No abstract

The seasonal ecological response of microzooplankton in the southeastern Arabian Sea is presented.During the spring intermonsoon period (March), stratification and depletion of nitrate in the surface waters (nitracline was at 60 m depth) caused low integrated chlorophyll a (av. 19 ± 11.3 mg m -2 ) and primary production (av. 164 ± 91 mgC m -2 d -1 ). On the other hand, nutrient enrichment associated with coastal upwelling and river influx during the onset and peak summer monsoon resulted in high integrated chlorophyll a (av. 21 ± 6 mg m -2 and av. 29 ± 21 mg m -3 respectively) and primary production (av. 255 ± 94 mg Cm -2 d -1 and av. 335 ± 278 mgC m -2 d -1 respectively). During all three periods, diazotropic cyanobacterium Trichodesmium erythraeum dominated in the nutrient depleted surface waters. A general increase in abundance of larger diatoms was evident in the surface waters of the inshore region during monsoon periods. The microzooplankton abundance was found to be significantly higher during the spring intermonsoon (av.241 ± 113 x10 3 ind.m -2 ) as compared to onset of summer monsoon (av. 105 ± 89 x10 3 ind.m -2 ) and peak summer monsoon (av.185 ± 175 x10 3 ind.m -2 ).Microzooplankton community during the spring intermonsoon was numerically dominated by ciliates while heterotrophic dinoflagellate was the dominant ones during the monsoon periods. The high abundance of ciliates during the spring intermonsoon could be attributed to the stratified environmental condition prevailed in the study area which favors high abundance of smaller phytoplankton and cyanobacteria, the most preferred food of ciliates. On the other hand, the dominance of heterotrophic dinoflagellates during the monsoon periods could be linked to their ability to graze larger diatoms which were abundant during the monsoon periods. The overall results show low abundance of microzooplankton in the eastern Arabian Sea during the monsoon periods mainly due to a decline in ciliates abundance. This decline during the monsoon periods could be the result of (a) low abundance of smaller phytoplankton and (b) high stock of mesozooplankton predators (av. 245 ml 100 m -3 ).

No abstract