Fluxes of nitrogen in Chaliyar River Estuary, India

Abstract

The Chaliyar river estuary is a positive coastal plain estuary with total drainage area of 2923 km2. We investigated the estuary's functions as sink and transformer of nutrients (uptake and release, retaining and recycling of nutrients) and relative contribution of external nutrient supply, nutrient budget, geo-chemical and biochemical pathways . The major source of nitrogen in the estuary is the river discharge. During pre-monsoon period, the percentage of inorganic nitrogen to total nitrogen pool is at the minimum and the major form is organic nitrogen. During the postmonsoon period, the contributions from both these components are almost equal, with a predominance of inorganic form. The dissolved nitrogen components are present in one form or other through out the estuary in all seasons and so nitrogen is not a limiting nutrient for primary production. The urea-N fraction remains < 7% with the lowest levels in pre monsoon. The contribution of Ammonia-N to total nitrogen pool is < 10% during monsoon and pre monsoon seasons. The maximum contribution > 25% is in the post monsoon season. When the monsoon discharges are maximum, 80 -90% contribution to the total nitrogen pool is by nitrate-N alone and in the premonsoon months, 75-85% of total nitrogen pool is from the organic -N. The net fluxes of inorganic nutrients in monsoon and post monsoon season are very large- with very small positive or negative fluxes for the pre-monsoon months. The average net fluxes are 4.97 to 9.61 mg m-2 s-1 for the monsoon months and 7.22 to 8.01 mg m-2 s-1 for the post monsoon months. During pre monsoon period, ammonia is transported towards the river (-0.47 to 0.03 mg m-2 s-1). The net fluxes of nitrite-N are very small and directed upstream during the pre monsoon months. The average flux of nitrate is of the order of 81.09 to 134 mg m-2 s-1 for the monsoon months and 4.87 to 33.23 mg m-2 s-1 in the post monsoon months. During the pre monsoon months, negative net flux is towards the river and the magnitude decreases upstream. The net flux of phosphate increases from marine end towards riverine end with the monsoon month's net flux of 3.52 to 4.63 mg m-2 s-1. During the post monsoon months, fluxes vary from 1.87 to 2.82 mg m-2 s-1. The net fluxes are very small and directed towards the river in pre monsoon months**. Except in the pre monsoon, there is net transfer of nutrients to sea. The mean annual fluxes for nitrate-N, Ammonia- N and phosphate-P are 31.01 mg m-2 s-1, +3.9 mg m-2 s-1 and +2.39 mg m-2 s-1 respectively. The average peak southwest monsoon transport of nitrate, ammonia and phosphate to sea are 13.7 Mg d-2 , 1.2 Mg d-1 and 0.61 Mg d-1 respectively.