Zooplankton community structure across an eddy-generated upwelling band close to a tropical bay-mangrove ecosystem

Abstract

Mesoscale eddies in the world's oceans are ubiquitous and bring about episodic pulses of nutrients into the photic zone. Transient in nature, the role of eddy pump- ing in coastal enrichment via plankton production, and sub- sequent organic Xux is not yet fully realised. In the context of a cyclonic cold-core eddy that propagates annually under the inXuence of the East India Coastal Current and enriches coastal waters in the western Bay of Bengal north of 16°N, a detailed study on zooplankton community structure along with phytoplankton composition and associated water quality was undertaken during April-May 2002 coinciding with the spring intermonsoon. Zooplankton samples were collected at 32 hydrographically diVerent (salinity 24.5-35.6 PSU) GPS Wxed locations representing bay-mangrove areas and nearshore waters (30 m) close to the River Godavari, which is one of India's largest estuarine systems. During the study, the bay-coastal waters were typiWed by elevated nutrient levels (nitrate 10.73-22.04 M), high salinity (27.98-35.52 PSU), and relatively low temperatures (30.63- 31.40°C). Altogether, 95 zooplankton taxa were encountered with copepods forming the predominant population. Agglom- erative Hierarchical Cluster Analysis (AHCA) and Non-metric Multidimensional Scaling (NMDS) based on Bray-Curtis similarity (PRIMER) analysis revealed appreciable alterations in zooplankton structure across bay-mangrove locations and coastal waters (Stress 0.11; ANOSIM test Global R: 0.94, P = 0.1%). Similarity Percentage (SIMPER) analysis revealed zooplankton associations through "discriminating species" for each location (Kakinada Bay, Cluster I, 27.9 § 3.0 PSU; upwelling band, cluster II, 35.5 § 0.2 PSU; oVshore waters, cluster III, 34.2 § 0.4 PSU; mangrove outlets, cluster IV, 32.7 § 1.3 PSU and mangrove creeks, cluster V, 33.5 § 0.6 PSU). The index of multivariate dispersion (IMD) illustrated high variability in zooplankton standing stock for mangrove/sea locations relative to the bay. Concurrent observations on phytoplankton revealed the importance of diatoms (r: 0.640, P · 0.05). Within the eddy-generated band of upwelled water, a signiWcant top- down control of diatoms by herbivorous zooplankton resulted in a comparative increase in abundance of dino- phyceans. Based on zooplankton abundance data and spe- cies association patterns, it was possible to distinguish diVerent zooplankton/copepod communities in accordance with mesoscale variability in physical, chemical and biological processes under tropical conditions. This was conWrmed through canonical correspondence analysis (CCA) that represented coastal-oVshore waters and the Bay environment in this area.