The importance of marine resources is globally recognized. Coastal fisheries, seagrass meadows, and mangroves play a major role in the livelihoods of many coastal communities, with mangrove ecosystem services alone valued at roughly US$16,500 per hectare (UNEP, 2006). Nutrients such as ammonium, nitrate, and phosphate are essential in the marine environment because they are required by phytoplankton for growth, and phytoplankton in turn form the base of the food web supporting fish and other marine organisms. However, when nutrient concentrations rise beyond natural background levels, they can drive eutrophication, altering algal biomass and species composition and, in severe cases, causing hypoxia and mortality of marine organisms (Smith, 2006).

This study investigated the influence of tidal state on nutrient concentrations in the coastal waters of Tudor Creek, a mangrove-fringed tidal creek in Mombasa, Kenya, within the Western Indian Ocean (WIO) region, which is subject to sewage pollution and other anthropogenic pressures (Kamau et al., 2015; Kamau, 2011). Tides in the ocean arise primarily from the gravitational influence of the sun and moon on the earth, producing spring and neap tidal cycles. Dissolved nitrogen (as nitrate and ammonium) and phosphate were analysed in duplicate samples collected hourly over 24-hour periods during both neap and spring tides, following standard nutrient sampling protocols (GESAMP, 1990).

Ammonium concentrations ranged from 0.005–0.031 mg/L (mean 0.015 mg/L) during neap tide and 0.004–0.020 mg/L (mean 0.012 mg/L) during spring tide. Nitrate concentrations ranged from 0.002–0.015 mg/L (mean 0.005 mg/L) during neap tide and 0.001–0.007 mg/L (mean 0.003 mg/L) during spring tide. Phosphate concentrations ranged from 0.003–0.0423 mg/L (mean 0.012 mg/L) during neap tide and 0.001–0.016 mg/L (mean 0.005 mg/L) during spring tide. All three nutrients were consistently higher during neap tide than spring tide, likely reflecting reduced flushing and dilution under weaker tidal exchange. Based on the trophic classification scheme of Siokou and Pagou (2002), Tudor Creek waters fall within the lower mesotrophic range, with phosphate concentrations exceeding those of nitrate and ammonium — a pattern consistent with more recent surveys identifying Tudor Creek as one of the more nutrient-enriched, sewage-impacted creeks along the Mombasa coastline (Okuku et al., 2011; Tuda et al., 2019).