Waning of plankton food web in the upstream region of the Cochin backwaters during the Southwest Monsoon

Abstract

Heterotrophic bacteria (0.2-2 µm), pico-phytoplankton (0.2-2 µm), nano-phytoplankton (2-20 µm), micro-phytoplankton (>20 µm), heterotrophic nanoflagellates (2-20 µm), and micro-zooplankton (20 – 200 µm) present in the Cochin backwaters were quantified during the Southwest Monsoon. High fresh water influx during the period transformed the Cochin backwaters into an extensive fresh water zone with high levels of nutrients (nitrate av. 20 µM; phosphate av. 3 µM), which favoured high phytoplankton abundance (av. 31 ± 9 x 104 No. L-1) and chlorophyll a (av. 5.2 ± 3.2 mg m-3). Autotrophic pico-plankton and nano-plankton contributed the majority (av. 69 ± 16%) of the total chlorophyll a while heterotrophic bacteria was found to be high throughout the study area (av. 3 ± 0.6 x 108 No. L-1). Abundance of heterotrophic nano-flagellates and micro-zooplankton were markedly lower in the freshwater dominant region (av. 0.5 ± 0.3 x 106 No. L-1 and av. 3 ± 1 No. L-1) as compared to the downstream region (av. 6 ± 3 x 106 No. L-1 and av. 3222 ± 3619 No. L-1). This clearly indicated a weak microbial food web in a major part of the Cochin backwaters during the Southwest Monsoon. The ecological implication is that it leads to inefficient transfer of bacterial and pico/nano-phytoplankton biomass to higher trophic levels. This study provides further evidences to our earlier observation that most of the heterotrophic bacteria and phytoplankton carbon in monsoonal estuaries gets transported to the coastal regions during the Southwest Monsoon.