Bloom of the diatom, Biddulphia sp. and ecology of Pulicat lagoon, Southeast India in the aftermath of the 2015 north east monsoonal rainfall.

Pulicat lagoon in the south-east coast of India is recharged with the highest fresh water influx annually during north-east winter monsoon. An abrupt heavy rainfall in November-December 2015 was a flood calamity in the region that inundated the lagoon. We investigated the physico-chemical characteristics and palynological profile of the surface sediments from the lagoon at water depth between 0.5 and 2 m to understand the impact of the 2015 event. On the basis of a 'marine index', three zones were demarcated, the 'LC zone' (Araniar river-lagoon confluence, south), the 'LM zone' and the 'LL zone' (Kalangi river confluence, north) showing values of 4.55, 1.25 and 0.25, respectively, indicating the extent of tidal influence. The dissolved oxygen, pH and salinity ranged between 6.4 and 9.6 mg/L, 7.96-9.23 and average ~ 12.3 ppt at different sites, respectively. The highest salinity (31 and 31.8 ppt) was in the LC zone along with the highest dissolved inorganic nitrogen (DIN) concentrations (275-288 mmol/m3) too. The dissolved inorganic phosphates (DIP) ranged between 8.5 and 29.5 mmol/m3, which was relatively high. Sandy sediment, high sodium (Na+) and chloride (Cl-) ions too indicate the landward extension of seawater influx. Forsterite (Mg2SiO4) and beta-uranophane (Ca[(UO2)SiO3(OH)]2.(H2O) minerals show higher values with the dominance of olivine (MgFe)2SiO4) and quartz (SiO2) in this zone indicating magnitude of fresh water influx too through Araniar river. The highest calcium, magnesium and potassium ions were also observed in LC zone. Fe-rich goethite, calcite and aragonite were recorded at all sites but with dominance of illite in LL zone. Halite, an evaporite recorded from all sites shows inundation of the entire lagoon during flooding event. Results show a bloom of Biddulphia pulchella, B. biddulphiana and B. laevis in association with Cladophora in LC zone which serves as potential indicators of physico-chemical characteristics of the lagoon showing intense response to catastrophic events of floods due to above normal monsoon variability.