Ecological risk assessment on nutrient over-enrichment in water quality: A case study of the Kenyan Coral Reef ecosystems

Abstract

The natural (seasonality) and anthropogenic drivers (e.g., coastal developments and change in land-use practices), continue to threaten the lives of organisms in shallow marine environments by compromising water quality. A case study was conducted to assess the impact of these drivers on nutrient over-enrichments using water and sediment samples that were collected from healthy live Porites spp. colonies from three Kenyan coral reef ecosystems: Tana Delta, Watamu, and Shimoni Mupunguti Reef. The evaluated nutrient over-enrichment levels were above the universal reference background concentrations of 0.01 and 0.003 mg/L for (N-(NO3- + NO2-)) and (P-PO43-) respectively. Ecological risk exposure to nutrient over-enrichments was evaluated using Renfield (N/P) ratio and nitrogen-phosphorous pollution index. Results on N/P ratios were < the Renfield ratio (N/P = 16 ratios), implying that phosphorous was in surplus in water columns in all the three reefs investigated. The nitrogen pollution indices were between > 0.85 to ˂ 2.45, while that of phosphorous, the pollution indices were < 0.5. The nutrient over-enrichment followed the order; Watamu > Tana Delta > Shimoni Mupunguti coral reef ecosystem. To identify the possible sources of the nutrient influx in the investigated coral reef ecosystems, a principal component analysis was carried out, and results revealed that anthropogenic activities e.g., tourism, coastal cities urbanization and change in land use practices accounted for 36.26%, while seasonality accounted for 17.82% of the total nutrients influx in the investigated coral reefs.