Characterization of phytoplankton functional groups in a tropical shellfish harvesting estuary (Ashtamudi) and adjacent nearshore waters (southwest coast of India)

A pioneering study on phytoplankton marker pigments, by adopting the HPLC-CHEMTAX analytical approach, was carried out in one of the major shellfish harvesting estuaries (Ashtamudi estuary—AE) on the southwest coast of India and also its adjacent nearshore waters (< 20 m depth) to study the dynamics of phytoplankton functional groups (PFGs). The AE, in general, appeared to be warm (> 29 °C) during the non-monsoon seasons, along with the prevalence of higher salinity (> 25) and NH4-N (> 5 µM) levels. However, during the summer monsoon (SM), the prevailing substantial river influx converted the AE into a low salinity (< 10) dominated system, provided with enhanced levels of NO3-N (4.6–12.6 µM) and PO4-P (0.2–1.4 µM), specifically on its upper reaches. The dissolved inorganic nitrogen (DIN) in the AE was mainly comprised of ammonium (NH4-N), approximately up to ~ 81.8% of the DIN, regardless of seasons, which signifies the eutrophic state of the estuary, the upstream sampling locations, in particular. Concerning the phytoplankton community, a pronounced spatio-temporal variation in chlorophyll a biomass was discernible in the AE, with an exceptional increase (6.4–12.1 mg m−3) during the SIM period. The prevalence of a conspicuous increase in chlorophyll a (av. > 5 mg m−3) along with enhanced zeaxanthin (av. > 1.5 mg m−3) recorded in the AE during the non-monsoon (specifically SIM period) season apparently signified the characteristic governance of cyanobacterial community. During the SM period, the estuary sustained more or less similar concentrations of certain marker pigments, i.e. alloxanthin, zeaxanthin, fucoxanthin and chl b, which representing the co-occurrence of cryptophytes, cyanobacteria, diatoms and chlorophytes, respectively. In contrast, the nearshore waters, wherein enhanced nitrate (NO3-N) and phosphate (PO4-P) levels prevailed, irrespective of seasons, sustained dominance of fucoxanthin over other marker pigments, which indicated the numerical supremacy of diatoms. The CHEMTAX analysis, adopted for estimating the chlorophyll a equivalents of various PFGs, corroborated the supremacy of cyanobacterial derived chlorophyll a in the estuary, and it was conspicuous during the non-monsoonal seasonal periods. Even though the estuary has shown remarkable spatio-temporal hydrographic inconsistencies, that variability was not much operative in generating extreme changes in the nutrient components and subsequent phytoplankton community compositions. From the conspicuous increase in N:P and Si:P ratios, resulting mainly from the low levels of P, it can be concluded that the AE has been a P-limited system for phytoplankton growth (especially for large-sized phytoplankton, e.g. diatoms and dinoflagellates), mainly during the non-monsoonal seasonal periods. Perhaps, this P-limitation, along with the prevalence of warm water column and enhanced NH4-N levels, could be the potential causes of the preponderance of cyanobacterial populations in the AE.