A coherent status of summer monsoon phytoplankton communities (2017-2018) along the Western Indian continental shelf: Implications for fisheries.

The western Indian continental shelf is characterized by contrasting biogeochemical features from south to north mostly governed by monsoon wind forcing. Here we present the first detailed study on the phytoplankton community (analyzed by marker pigments and microscopy) during the summer monsoon addressing the interannual variability (Aug 2017 and 2018) along the Western Indian Continental Shelf waters (8° N-21° N, at 200 m isobaths). A distinct interannual variability in monsoon impacted the hydrography and nutrient stoichiometry that was further cascaded to the phytoplankton community structure. The timing of the monsoon onset, wind speeds, and the strength of the alongshore wind component were the major factors that governed the interannual variability in the development and progression of the upwelling as well as phytoplankton bloom. The upwelling-dominated southwestern shelf was characterized by cold, nutrient-rich waters dominated by the marker pigment of diatoms, fucoxanthin, and microscopy also revealed the presence of large, chain-forming diatoms (Chaetoceros sp.; Dactyliosolen fragilissimus). In contrast, low nutrient warm waters in the northern shelf housed nanophytoplankton and picophytoplankton along with small diatoms (Thalassiosira sp.) and dinoflagellates (Gymnodinium sp.). Interestingly, in a few stations in the southern shelf, the surface waters were low saline that was intensified in 2018 compared to 2017 inhibiting upwelled water from reaching the surface. Consequently, due to low nutrient levels caused by reduced upwelling and low salinity, picoplanktonic cyanobacteria dominated replacing the larger diatoms that were found plenty in 2017. Likely, such a niche shift adversely impacted the planktivorous fish catch (Indian Oil Sardine) that was seen to be less in 2018 relative to 2017. Any further changes in monsoon variability and freshwater flow to this region may have direct consequences on the phytoplankton community as well as fisheries.

Phytoplankton community shift in response to experimental Cu addition at the elevated CO2 levels (Arabian Sea, winter monsoon).

Understanding phytoplankton community shifts under multiple stressors is becoming increasingly important. Among other combinations of stressors, the impact of trace metal toxicity on marine phytoplankton under the ocean acidification scenario is an important aspect to address. Such multiple stressor studies are rare from the Arabian Sea, one of the highest productive oceanic provinces within the North Indian Ocean. We studied the interactive impacts of copper (Cu) and CO2 enrichment on two natural phytoplankton communities from the eastern and central Arabian Sea. Low dissolved silicate (DSi < 2 µM) favoured smaller diatoms (e.g. Nitzschia sp.) and non-diatom (Phaeocystis). CO2 enrichment caused both positive (Nitzschia sp. and Phaeocystis sp.) and negative (Cylindrotheca closterium, Navicula sp., Pseudo-nitzschia sp., Alexandrium sp., and Gymnodinium sp.) growth impacts. The addition of Cu under the ambient CO2 level (A-CO2) hindered cell division in most of the species, whereas Chla contents were nearly unaffected. Interestingly, CO2 enrichment seemed to alleviate Cu toxicity in some species (Nitzschia sp., Cylindrotheca closterium, Guinardia flaccida, and Phaeocystis) and increased their growth rates. This could be related to the cellular Cu demand and energy budget at elevated CO2 levels. Dinoflagellates were more sensitive to Cu supply compared to diatoms and prymnesiophytes and could be related to the unavailability of prey. Such community shifts in response to the projected ocean acidification, oligotrophy, and Cu pollution may impact trophic transfer and carbon cycling in this region.

Interlinking diatom frustule diversity from the abyss of the central Arabian Sea to surface processes: physical forcing and oxygen minimum zone.

This study analyzed the diversity and abundance of diatom frustules including the ancillary parameters using the core top sediments from five locations (21, 19, 15, 13, and 11°N) along the central Arabian Sea (64°E), an area profoundly influenced by atmospheric forcing (monsoons) and oxygen minimum zone (OMZ) with high spatial variability. Significantly higher organic carbon (0.97 ± 0.05%) and diatom frustules (5.92 ± 0.57 × 104 valves g-1) were noticed in the north (21, 19, 15°N) where natural nutrient enrichment via open-ocean upwelling, winter convection, and lateral advection support large diatom-dominated phytoplankton blooms and intense OMZ. Conversely, the south (13, 11°N) depicted significantly lower organic carbon (0.74 ± 0.08%) as well as frustules (4.02 ± 0.87 × 104 valves g-1) as this area mostly remains nutrient-poor dominated by small-medium-sized phytoplankton. The north was dominated by large-sized diatoms like Coscinodiscus that could escape grazing and sink consequently due to higher ballasting. Furthermore, the presence of the intense OMZ in the north might reduce grazing pressure (low zooplankton stock) and mineralization speed facilitating higher phytodetritus transport. Relatively smaller chain-forming centric (Thalassiosira) and pennate diatoms (Pseudo-nitzschia, Fragilaria, Nitzschia, etc.) were found throughout the transect with higher abundance in the south. The euphotic diatom diversity from the existing literature was compared with the frustule diversity from the sediments suggesting not all diatoms make their way to the abyss. Such distinct spatial north-south variability in diatom frustule size as well as abundance could be attributed to cell size, grazing, and water column mineralization rates related to OMZ.

Interannual variability in particulate organic matter distribution and its carbon stable isotope signatures from the western Indian shelf waters.

Particulate organic carbon and nitrogen (POC, PN, collectively particulate organic matter, POM) and the stable isotopic signature of POC (δ13CPOC) are important to delineate its sources and recycling in shelf water. The present study provides insights into the factors responsible for spatial and interannual variability in POM and δ13CPOC values along the western Indian shelf waters (8° N -21° N) during the southwest (SW) monsoon (August) 2017 and 2018. The dominance of phytoplankton-derived POM with a negligible terrestrial influence was evident from the positive correlation between POC and TChla contents, ratios of C: N, and δ13CPOC signatures. Prominent upwelling signatures [cold nutrient-rich water, higher POM, total Chlorophylla (TChla), and δ13CPOC values] were noted in the south (8-12° N), whereas low nutrient warm waters (lower values of POM, TChla, and δ13CPOC) were prevalent in the north (13-21° N). Phytoplankton biomass was significantly higher and matured in 2017 due to an early and stronger upwelling in the south. In 2018, delayed and weak upwelling (evident from Ekman offshore transport and pumping velocity) resulted in the late development of phytoplankton bloom and lower POM. Furthermore, considerably lower nutrient supply within the mixed layers in 2018 compared to 2017 was partially attributed to the enhanced spatial expansion of low salinity waters closer to the surface. In the north, in 2018, higher wind speeds enhanced vertical mixing resulting in increased nutrient supply and TChla compared to 2017. We conclude that monsoon wind speed in the northern shelf and strength as well as the timing of the upwelling, including freshwater flux in the south, can be the key factors in modulating the interannual variability in POM distribution and δ13CPOC signature in the western Indian Shelf waters.

Seasonal distribution of phytoplankton and its association with physico-chemical parameters in coastal waters of Malvan, west coast of India.

Malvan coast is one of the Marine Protected Area (MPA) of Maharashtra because of its rich coral reef and biodiversity. The study investigated on phytoplankton assemblage and their diversity with respect to physico-chemical parameters covering protected and unprotected area of Malvan coast. Physico-chemical parameters such as salinity, nitrite, nitrate, and ammonia did not display seasonality due to inadequate fresh water influx and allochthonous nutrient input. Positive correlation of phosphate (r = 0.96, p < 0.0001) and silicate (r = 0.91, p < 0.0001) with Total Suspended Solids (TSS) confirmed their autochthonous origin as a result of resuspension of bottom sediments during monsoon. A total of 57 phytoplankton species were identified mostly dominated by Bascillariophyceae (40 species), followed by Dinophyceae (9 species), Chlorophyceae (5 species), Cyanophyceae (2 species), and Dictyochophyceae (1 species) from Malvan coast. Canonical correspondence analysis (CCA) revealed that water temperature and TSS were the most significant parameters influencing the distribution and seasonal shift in phytoplankton species such as Skeletonema costatum and Chaetoceros sp. during pre-monsoon and Psedo-nitzschia sp., Streptotheca thamensis, Eucampia zodiacus, and Lithodesmium undulatum during post-monsoon. Silicate and phosphate had minor influence on phytoplankton distribution. Shannon-Wiener diversity index as a pollution index suggested that the study area was incipiently polluted except at bay stations. Despite of various human interventions the water quality and phytoplankton assemblage of this area has not reached to an alarming situation. The current study provides a valuable baseline data on phytoplankton assemblage from Malvan coast.