Re-evaluation of ‘paradox of mesozooplankton’ in the eastern Arabian Sea based on ship and satellite observations

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 is based on a very limited seasonal data (from 5 to 7 locations). Therefore, we reconsidered the paradox based on intensive in-situ observations at 37–40 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 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 region (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. 1 mg m − 3 ) in the northern region during most of the year through winter convection, open ocean upwelling and lateral advection from the Arabian coast. On the other hand, chlorophyll a was low (av. 0.2 mg m − 3 ) 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.