Inter-seasonal variation in nitrogen uptake rates of the eutrophic Cochin estuary and adjacent coastal Arabian Sea.

The study assessed the Cochin estuary and adjacent coastal Arabian Sea for their seasonal variation in nitrate (NO3-) and ammonium (NH4+) uptake rates by total and nano + picoplankton using the 15N tracer technique. The results suggested that the NO3- and NH4+ uptake rates in the Cochin estuary are higher than those in the adjacent coastal Arabian Sea. NO3- and NH4+ uptake rates in the nearshore stations in the off Cochin station were high, indicating the influence of the eutrophic estuary. NO3- and NH4+ uptake rates conducted in off Mangalore transect were significantly lower than those of the off Cochin as it does not have an exchange with eutrophic systems. The nano + picoplankton's contribution to the total DIN uptake rates in the Cochin estuary was 77-98 %, indicating the relevance of nano + pico phytoplankton in the N cycling of the region.

Biogeochemical and physical drivers of hypoxia in a tropical embayment (Brunei Bay).

Dissolved oxygen is an ecologically critical variable with the prevalence of hypoxia one of the key global anthropogenic issues. A study was carried out to understand the causes of low dissolved oxygen in Brunei Bay, northwest Borneo. Hypoxia was widespread in bottom waters in the monsoonal dry season with dissolved oxygen < 2 mg/L throughout the coastal zone. This was a result of riverine nutrient input primarily from the Padas river driving excess primary production and its subsequent sinking into stratified bottom water where its decomposition consumed oxygen. Despite higher riverine nutrient input in the wet season hypoxia was less extensive due to the combination of turbidity reducing coastal primary production, the intrusion of oxygen-rich water from the South China Sea into offshore bottom layer waters and horizontal flushing increase advection of phytoplankton biomass out of the bay. Future investigation of hypoxia in shallow tropical regions therefore needs to consider the role of monsoonal season.

Hydrodynamic variability and nutrient status structuring the mesozooplankton community of the estuaries along the west coast of India.

The influence of distinct tidal characteristics and nutrient status on mesozooplankton community was studied in six major estuaries along the west coast of India during the late-monsoon (MS) and post-monsoon (PM) periods. The macro-tidal estuaries in the north (Amba and Thane) exhibited higher nutrient concentration compared to the micro- and meso-tidal estuaries located in the south (Cochin and Nethravati) and central (Zuari and Mandovi) west coast of India. The markedly higher nitrate and phosphate levels in the macro-tidal estuaries during PM indicated anthropogenic contributions from domestic and industrial effluents, which significantly impacted the mesozooplankton community structure. Nutrient enrichments favored higher phytoplankton standing stock leading to low DO levels. In the micro- and meso-tidal estuaries, meso- and euryhaline copepods dominated whereas in the macro-tidal estuaries, the copepod community was dominated by euryhaline and coastal species. Furthermore, the high-saline eutrophic environment of macro-tidal estuaries formed congenial for the increased jellyfish preponderance during PM. The predation pressure exerted by the jellyfish population on the crustacean zooplankton and ichthyoplankton exerted an adverse impact on the potential fishery stock in the macro-tidal estuaries. Thus, the study reveals that the nutrient enrichment favoring a shift in the mesozooplankton community structure from nutritionally superior crustacean plankton to less desirable jellyfishes, which in turn, may lead to a threat on the estuarine pelagic energy transfer and ecosystem deliverables.

Nutrient stoichiometry (N:P) controls nitrogen fixation and distribution of diazotrophs in a tropical eutrophic estuary.

Nitrogen fixation and its ecological regulation are poorly understood in the tropical estuaries, which are highly influenced by anthropogenic disturbances. In this study, we investigated the role of nutrient stoichiometry in the diversity, abundance and activity of N2-fixing bacterial community and their seasonal variations in the water column of a tropical eutrophic estuary (Cochin estuary). The N2 fixation rates in the estuary ranged from 0.1 to 2.0 nmol N2 l-1 h-1, with higher activity during post-monsoon and lower during monsoon. The rates are appeared to be primarily controlled by dissolved inorganic nitrogen and phosphorous (N:P) ratio. Clone library analysis of nitrogenase (nifH) gene revealed that the major N2 fixing phylotypes belong to Cluster I and Cluster III diazotrophs. The overall findings of this study suggest that monsoon induced seasonal changes in nutrient stoichiometry control the distribution and activity of diazotrophs in a tropical estuary.