Trophic efficiency of plankton food webs: Observations from the Gulf of Mannar and the Palk Bay, Southeast Coast of India

Abstract

This paper introduces the structure and trophic efficiency of plankton food webs in the Gulf of Mannar (GoM) and the Palk Bay (PB) — two least studied marine environments located between India and Sri Lanka. The study is based on the results obtained from a field sampling exercise carried out in the GoM and the PB in March 2010 (Spring Intermonsoon — SIM), September 2010 (Southwest Monsoon — SWM) and January 2011 (Northeast Monsoon — NEM). Based on multivariate analysis of major environmental parameters during different seasons, it was possible to clearly segregate the GoM and the PB into separate clusters, except during the SWM. This segregation of the GoM and the PB was closely linked with the seasonally reversing ocean currents in the region, as evident from the MIKE 21 flow model results. During the period of relatively low phytoplankton biomass (<23mgCm−3), the organic carbon contribution of the microbial loop was significantly high — both in the GoM and the PB. During the SIM, the carbon biomass available in the plankton food web was significantly higher in the PB (av. 122.8±47.60mgCm−3) than in the GoM (av. 81.89±35.50mgCm−3). This was due to a strong microbial loop in the former region. In the GoM, phytoplankton contributed a considerable portion (>50%) of the carbon biomass during the SWM and the NEM, whereas, microbial loop contributed significantly (80%) during the SIM. The microbial loop was predominant in the PB throughout the study period, being as high as 83% of the total plankton biomass during the SIM. As compared to the PB, the mesozooplankton biomass was higher in the GoM during the SWM and the NEM and lower during the SIM. The relatively high mesozooplankton stock in the PB during the SIM was closely linked with a strong microbial loop, which contributed the major share (av. 101.6±24.3mgCm−3) of the total organic carbon available in the food web (av. 126.6±24.3mgCm−3). However, when microbial loop contributed >65% of the total organic carbon available in the food web, the trophic efficiency was found to be low (~3%), which can be attributed to the wide dispersal of organic carbon in the microbial loop. Importantly, during the NEM, when the copepod Paracalanus parvus was predominant in the PB, the trophic efficiency of the microbial loop dominant food web increased by more than a fold (7.2%). The study provides evidences for the first time from the field that exceptionally high abundance of efficient microzooplankton-consuming zooplankton can significantly increase the trophic efficiency of the microbial loop dominant plankton food web.