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The study assessed the Cochin estuary and adjacent coastal Arabian Sea for their seasonal variation in nitrate (NO 3 −) and ammonium (NH 4 +) uptake rates by total and nano + picoplankton using the 15N tracer technique. The results suggested that the NO 3 − and NH 4 + uptake rates in the Cochin estuary are higher than those in the adjacent coastal Arabian Sea. NO 3 − and NH 4 + uptake rates in the nearshore stations in the off Cochin station were high, indicating the influence of the eutrophic estuary. NO 3 − and NH 4 + 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. • Nano + pico phytoplankton contributes up to 77-98 % of the total DIN uptake rate in the Cochin estuary. • Evidence of the influence of eutrophication in the estuary of DIN uptake rates in the coastal Arabian Sea. • DIN uptake potential of phytoplankton depends on TN:TP ratio rather than the individual nutrient concentrations. [ABSTRACT FROM AUTHOR]

The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003–2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore, we recommended the non-monsoon period (January–May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested for accurate assessment of the environmental status. [ABSTRACT FROM AUTHOR]

Abstract: Bacterial productivity (BP) and respiration (BR) were examined in relation to primary productivity (PP) for the first time in a shallow tropical ecosystem (Cochin Estuary), India. The degree of dependence of BP (6.3–199.7μg CL−1 d−1) and BR (6.6–430.4μg CL−1 d−1) on PP (2.1–608.0μg CL−1 d−1) was found to be extremely weak. The BP/PP (0.05–8.5) and PP/BR (0.02–7.9) ratios widely varied in the estuary depending on the season and location. There was a seasonal shift in net pelagic production from autotrophy to heterotrophy due to terrestrial organic matter input through rivers which enhanced the bacterial heterotrophic activity and very high pCO2 (106–6001μatm) levels. The heterotrophic zones were characterized by low PP but high bacterial production and respiration leading to oxygen undersaturation and exceptionally high pCO2. We propose that the CO2 supersaturation caused by increased bacterial respiration (in excess of PP) was a result of bacterial degradation of allochthonous organic matter. This indicates that sources other than planktonic compartment need to be explored to understand the C-cycling in this estuary. These results are of particular relevance to tropical ecosystems in general, where the bulk of world''s river discharges occur. [Copyright &y& Elsevier]