Implications of coastal upwelling and mud banks on the structuring of phytoplankton communities in the nearshore waters of Alappuzha, southwest coast of India

Abstract

A comprehensive study on phytoplankton dynamics, encompassing chlorophyll a biomass, primary productivity, and community composition and abundance, was conducted in the Alappuzha mud bank region (< 15 m depth) along the southwest coast of India from April to November 2017. This region, recognized as a potential fishing ground for sardines, mackerels and anchovies, is significantly influenced by seasonal hydrographic processes, including thermal stratification, coastal upwelling, and mud bank formation, particularly during the southwest monsoon (SWM). During the pre-SWM period, the region exhibited low chlorophyll a (0.45–2.2 mg m⁻³) and primary productivity (208.3–365.3 mgC m⁻³ d⁻¹), primarily driven by nanophytoplankton (2–20 μm), consisting mainly of smaller diatoms (e.g., Trieres mobiliensis, Thalassiosira sp., Pleurosigma sp. etc.) and dinoflagellates (Tripos furca, Prorocentrum micans etc.). In contrast, the peak SWM period exhibited a significant increase in chlorophyll a (3.4–16.9 mg m⁻³) and primary production (436.3–908.1 mgC m⁻³ d⁻¹), predominantly contributed by microphytoplankton (20–200 μm), particularly Thalassiosira sp., fuelled by nutrient-rich upwelled waters and calm sea conditions, mainly within mud bank region. Despite the persistent dominance of diatoms (Thalassiosira sp., Skeletonema costatum, Trieres mobiliensis etc.) and certain autotrophic dinoflagellates, phytoplankton chlorophyll a and primary productivity declined markedly during the waning and post-SWM periods due to nutrient exhaustion. Flow cytometry-derived picophytoplankton data revealed a clear dominance of picoeukaryotes (PEUK) and nanophytoplankton across the study region during the Pr. SWM period. However, during the SWM period, a pronounced increase in Synechococcus abundance was observed throughout the study region. Although the mud bank region exhibited higher surface layer turbidity (> 8 NTU) during the SWM, the overall phytoplankton growth, as indicated by chlorophyll a and primary productivity remained unaffected, highlighting the overriding influence of nutrient enrichment over light attenuation.