First report on the occurrence of Gonyaulax polygramma bloom during the onset of Noctiluca scintillans bloom along the Tuticorin coast, southeast coast of India.

Dense and green-coloured patches were encountered on the sea surface waters of the Tuticorin coast on 22nd October 2022. Microscopic investigation revealed that the discoloration is caused by plankton, green Noctiluca scintillans. In order to find out the causes that trigger the bloom of N. scintillans, plankton samples were collected for 5 days in fourteen days duration from 22nd October to 4th November. During the peak bloom period, the abundance and biovolume of N. scintillans reached 1.56 × 104 cells/L and 21.8 × 1010µm3/L, respectively. The highest concentration (73.65 mg/m3) of chlorophyll-a was recorded during blooming period that was caused by Gonyaulax polygramma and endosymbiont, Pedinomonas noctilucae in N. scintillans. Formation of G. polygramma bloom is being reported for the first time in Tuticorin, southeast coast of India, with a species abundance of 36.9 × 104 cells/L. Present study concluded that besides the optimum hydrological conditions and eutrophic nature of the system, abundant prey (G. polygramma) facilitated the N. scintillans bloom.

Role of salinity on zooplankton assemblages in the tropical Indian estuaries during post monsoon.

The estuary is the transition zone between the riverine and marine environments, in which the zooplankton act as a trophic connector in the energy transfers from primary producers to secondary consumers. Zooplankton biovolume and species assemblages with reference to physical, chemical and biological properties in the Indian estuaries are rarely studied. To examine the zooplankton variability in abundance and diversity, we therefore, investigated seventeen Indian estuaries during the post monsoon of the year 2012. Based on salinity conditions, estuaries were classified into oligohaline, mesohaline and polyhaline. A marked spatial gradient in salinity was observed between the upstream and downstream estuaries. Relatively, salinity was high in downstream areas, resulting in high zooplankton biovolume and diversity perceived in downstream areas. In contrast, nutrient concentrations were higher in the upstream than the downstream estuaries, resulting in high phytoplankton biomass (in terms of chlorophyll-a) perceived in the upstream estuaries. Zooplankton abundance was numerically dominated by Copepoda, constituting approximately 76 % of the total zooplankton count. Zooplankton population was highly similar between upstream and downstream in the oligohaline estuaries. In contrast, heterogeneous assemblages were noticed between upstream and downstream in the mesohaline and polyhaline estuaries. Under oligohaline conditions, surface waters dominated by Acartia clausi, A. dane, A. plumosa, Cyclopina longicornis, Oithona rigida and Tigriopus sp. In contrast, under mesohaline and polyhaline conditions, Acartia tonsa, Acartia southwelli, Acartia spinicauda, Paracalanus spp. Centropages typicus, Temora turbinate, Oithona spinirostris and O. brevicornis become the key dominant species. Eucalanus spp., and Corycaeus spp. were indicator species in the downstream estuaries. Our findings suggest that zooplankton diversity and numerical abundance were chiefly governed by salinity rather than phytoplankton biomass (chlorophyll-a) in the Indian estuaries during the post monsoon.

Seasonal variability in plankton food web composition in Tuticorin coastal waters, south east coast of India.

To understand the impact of seasonal variability on plankton food web composition in tropical coastal waters, samples were collected from three locations along Tuticorin coastal waters during postmonsoon, summer, and northeast (NE) monsoon seasons. During the NE monsoon, the total suspended matter (TSM) and nutrient concentrations were relatively higher, whereas salinity and plankton (phytoplankton and zooplankton) abundances were lower. Cluster analysis also revealed that the NE monsoon formed into a separate cluster because of the lower phytoplankton abundance caused by higher loads of TSM; this arrests light penetration, thereby resulting in a decrease in plankton abundance. The increase in zooplankton biomass coincided with the decrease in diatoms (p < 0.05), and the increase in cyanobacteria may reflect that grazers food choice has a significant impact on the base of the food web composition. The present study states that the phytoplankton biomass was greatly influenced by seasonality and associated changes rather than the huge supply of nutrient loads.

Influence of river discharge on abundance and composition of phytoplankton in the western coastal Bay of Bengal during peak discharge period.

To understand the influence of river discharge on phytoplankton composition along western coastal Bay of Bengal (BoB), surface water samples were collected during peak discharge period. River discharge from the Ganges influences northwest (NW) coastal BoB whereas peninsular rivers (Godavari and Krishna) discharge to the southwest (SW) coastal Bay. River discharge from the Ganges is an order of magnitude higher than peninsular river resulting in low saline, less suspended matter and lower nutrients concentrations in the NW and contrasting to that was observed in the SW. ~50%of the phytoplankton were composed of Thalassiosira spp., Nitzschia spp., Microcystis spp., Amphiprora spp. and Thalassionema spp. in the SW whereas Thalassiosira spp., Nitzschia spp., Chaetoceros spp., Merismopedia spp. and Peridinium spp. in the NW. Significant variability in phytoplankton composition was observed from coast to offshore. Our study revealed that river discharge and associated physico-chemical characteristics governed the phytoplankton community along western coastal BoB.

Intra-annual variations in phytoplankton biomass and its composition in the tropical estuary: Influence of river discharge.

To examine the influence of river discharge on phytoplankton composition, time-series observations were conducted at upper, middle and lower Godavari estuary during 2009. The salinity variations in the estuary were depended on freshwater discharge and tidal exchange. River discharge brought a significant amount of nutrients, however it did not induce phytoplankton bloom due to severe light limitation driven by high turbidity. Enhanced phytoplankton biomass and abundance were associated with water column stratification during moderate discharge period, suggesting that water column stability is more important than nutrients to promote phytoplankton blooms. The contribution of diatoms to the total phytoplankton abundance increased with decrease in salinity and vice versa for blue-green algae. The relationship of phytoplankton abundance with salinity and nutrients suggests that low salinity and high N:P ratio favored growth of blue-green algae, whereas high salinity and low N:P/N:Si favored diatoms. This study suggested that discharge brought modification in phytoplankton composition.

Elucidation of seasonal variations of physicochemical and biological parameters with statistical analysis methods in Puducherry coastal waters.

The present investigation aimed to study the effect of monsoonal and anthropogenic influences on the water quality parameters of Puducherry coastal waters. Surface water sampling was performed at three fixed stations in four distinct seasons during 2011. Physical water quality parameters such as salinity and TSM showed strong seasonal and spatial variability. Evaporation and monsoonal runoff seem to be the major controlling forces for these parameters in the coastal waters. Seasonal distribution of the parameters showed a random pattern for nitrate and a well-defined pattern for silicate. Chl-a was minimum during monsoon when high TSM was encountered in the system. Moreover, factors that regulated the phytoplankton biomass varied with seasons. Moreover, TSM was strongly correlated with silicate. The relationship between Chl-a and nutrients were more consistent throughout the year, and much weaker correlations were noticed between Chl-a and TSM. Cluster analysis depicted the existence of a marked seasonal heterogeneity.