Impact of nutrients on phytoplankton productivity on coastal marine waters of Mtwapa, Mida, and Kilifi Creeks, Kenya

Abstract

Abstract. Pole TM, Tole MP, Otor SCJ. 2020. Impact of nutrients on phytoplankton productivity on coastal marine waters of Mtwapa, Mida, and Kilifi Creeks, Kenya. Ocean Life 4: 82-93. Some of the major sources of coastal and marine pollution affecting coastal conditions vary worldwide. The objective of this study was to determine the levels of three nutrients (PO4-, NO3-, and NH3) and whether they are within acceptable limits, their relationship with ongoing human activities, and marine primary productivity. Furthermore, to achieve these objectives, marine water samples were sampled and analyzed from identified point and area pollution sources in Mtwapa, Mida, and Kilifi Creeks, Kilifi County, Kenya. A quantitative one-way ANOVA was used to determine the variations between treatments. The Spearman correlation test was computed between nutrients and carbon and nutrients and rainfall. These studies revealed that the levels of nutrients fluctuated throughout the entire study period along the three creeks and the sampling stations. Moreover, the levels of nitrates were within the oligotrophic range in all three creeks. Mtwapa Creek recorded the highest levels of nitrates at ? = 0.05 from June to November 2011. The phosphate levels in the three creek waters were not significantly different throughout the sampling period. The ammonia levels in Mtwapa were in the higher mesotrophic levels of up to 0.009 mg/L, while Kilifi and Mida were within the oligotrophic levels. Alongside Mtwapa Creek, there was a positive correlation coefficient between phosphate and carbon fixation (r = 0.869) in the outer creek; on the other hand, the correlation coefficient for nitrates and ammonia was negative (r = -0.624 and -0.295). All the nutrients had a negative correlation coefficient with rainfall in outer Mtwapa Creek (r = -0.76, -0.37, and -0.336 for nitrates, phosphates, and ammonia, respectively. Kilifi was positively correlated with carbon fixation and all three nutrients in the inner creek. There was a positive correlation between nutrients and rainfall in all the sampling stations. At Mida there was positive correlations between rainfall and phosphates in all the six stations (r = 0.78, 0.3, 0.22, 0.78, 0.23 respectively). In Mtwapa, the high levels of nutrients in the outer creek stations and the negative correlation coefficient between the nutrients and rainfall suggests that runoff did not contribute to increased levels of nutrients but rather the waster roadside canal and sewage outfall from urban and tourist development contributed to increased levels. The higher levels of nutrients in the inner creek waters of Kilifi can be attributed to the destruction of vegetation close to the creek for farming upstream of the creek. Mida Creek had the lowest recorded nutrient levels apart from phosphates in all the sampling stations. These phosphate levels most likely were contributed by wastewater from some tourist establishments, especially at the Temple point hotel, which crept to the shore waters. Furthermore, to reduce the impacts of land-based human activities, it is recommended to analyze existing land-based activities that negatively impact coastal marine ecosystems and livelihoods to design mitigation measures.