Abstract:
Organic matter, nutrients and suspended particles from terrestrial sources are transported
to near-shore waters through rivers, and at the river mouth water meets the Ocean forming
a plume. River plumes play a major role in nutrient dynamics, maintaining the
biodiversity and, productivity of near-shore waters strongly influence the distribution of
phytoplankton, zooplankton, pelagic & benthic fishes. The objective of this study was to
determine the influence of river plume on the near-shore water quality of Nilwala river
mouth. Sampling was carried out on monthly basis at eleven random sampling points
within 1.5 km2 of study area covering four monsoon periods (from May 2016 to April
2017). Temperature, pH, salinity, conductivity, and dissolved oxygen (DO) were
measured onsite whereas total suspended solids (TSS), nitrate and phosphate
concentrations were measured in the laboratory. River discharge varied from 6.82 to
25.18 m3s-1 and direction and distribution of the river plume were interpreted using the
variation of surface water salinity. Salinity values of the study site varied between 4.30
and 33.05 ppt during the study period. The mean temperature and mean DO were within
the range of 26.48-29.30°C and 5.86-6.18 ppm respectively whereas mean pH values
were varied from 7.8 to 8.78. Nitrate and phosphate concentrations were fluctuated
between 0.28 to 2.23 mg/L and 0.00 to 0.04 mg/L respectively. Temperature (r = 0.517;
p< 0.01) and TSS (r = 0.710; p< 0.01) showed significant positive correlation whereas
phosphate (r = - 0.318; p< 0.05) showed a negative correlation with river discharge within
the study site. However, no significant relationship was recorded between DO and the
river discharge. The TSS value positively correlated to the sediment load carried by river
water. Further, negative correlation between salinity and phosphate concentration
suggests that most of the phosphorus in the river waters are adsorbed by suspended solids
and mixing process of both sea water and freshwater masses at the river mouth caused
abrupt changes to the chemical composition of near shore waters.
