Distribution of nutrients and major elements in riverine, estuarine and adjoining coastal waters of Godavari, Bay of Bengal

Abstract

Studies on the hydrographical parameters (salinity, temperature and dissolved oxygen) indicated distinct spatial and temporal variations. Nutrients (NO₃-N, NH₄-N, PO₄-P and SiO₄-Si) in general, exhibited a decreasing trend from riverine to estuarine and coastal region indicating their dominant occurrence with river water. This was also supported by relatively higher levels in September (monsoon) when compared with January/February (postmonsoon) and May (premonsoon). Nutrients showed in general, an increasing trend from surface to the bottom particularly at farthest stations in the coastal region, which was due to their uptake at surface and remineralisation in the water column. All the nutrients exhibited distinct non-conservative behaviour with a net removal of 26-36% for silicon, 19-24% for ammonium,14-16% for nitrate and 4-18% for phosphate. This was attributed to the combined effects of biological uptake by phytoplankton and desorption from the suspended particulates. Major elements (F, B, Ca and Mg) indicated an increasing trend from riverine to estuarine and shelf regions. They exhibited higher concentration in May followed by January/February and September and were predominanHn seawater. Fluoride showed non-conservative behaviour with a net removal of 13 and 10% during September and May respectively and conservative behaviour in January/February. While the former was attributed to its adsorption on

suspended solids in September and biological uptake in May, the latter was explained on the basis of simple physical mixing of seawater with fresh water. Boron showed non-conservative behaviour with a net removal of 12 to 19% which was attributed to its adsorption on to the suspended matter containing river borne clay minerals. Calcium and magnesium showed conservative behaviour in May and semi-conservative behaviour during September and January/February with a small net addition and removal respectively, in the latter. While the net addition in September may be due to their exchange

<span style="font-size:14.0pt;line-height:115%;font-family:" times="" new="" roman";="" mso-fareast-font-family:"times="" roman";color:black;mso-ansi-language:en-in;="" mso-fareast-language:en-in;mso-bidi-language:hi"="" lang="EN-IN">with other cations during early stages of mixing, the removal during January/February is believed to be due to their involvement in the biological and/or geochemical processes. Estimation of nutrient and major elemental fluxes from Godavari river into the Bay of Bengal indicated their predominant input in monsoon (June-November) than non-monsoonal (December-May) period.</span>