Seasonal and tidal variations in suspended particulate matter dynamics of two microtidal rivers of Karnataka, central west-coast of India

Abstract

The concentrations of suspended particulate matter (SPM) and particulate organic carbon (POC), particulate total nitrogen (PTN) of SPM, and sediments were studied in two microtidal rivers of Karnataka (Nethravati and Gurupur), India, to understand the influence of SPM on the coastal environment. Water sampling involved seasonal (wet, dry) and tidal (spring, neap) conditions, including 24-h time-series measurements. The results showed that estuarine turbidity maxima (ETM, high SPM zone) existed seasonally in both the rivers and formed due to several processes. In the Gurupur River, flocculation in the wet season (monsoon) and river meandering effect (postmonsoon) and resuspension of bottom sediments (premonsoon) in the dry season produced the respective ETMs. On the other hand, in the Nethravati R., the ETM was not distinct during the wet season, while flocculation (postmonsoon) and low to moderate material supply enhanced by strong tidal currents (premonsoon), resulted in the ETMs. The POC and PTN were high during the dry season as compared to the wet season in both the rivers. In general, the SPM balance in the estuary was maintained by its seaward export in the wet season (Nethravati R.) and its retention during the dry season, with a major part of the particulate organic matter produced and trapped in the estuaries during the dry season. The upstream tidal transport of SPM (dry season) can affect the coastal ecosystem management and maintenance of its ecological functions in the future.