Nutrient biogeochemistry in the Cross River estuary system and adjacent Gulf of Guinea, South East Nigeria (West Africa)

The Cross River, Calabar River, and Great Kwa River together form a network of an estuary system otherwise known as the Cross River Estuary system (CRES). Studies of dissolved nutrients and water mass dynamics were conducted mainly in February 2017. Nutrients measured included NO3-, NO2-, NH4+, dissolved inorganic phosphate (DIP), total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP) and dissolved silicate (DSi). The rivers were enriched with DSi and depleted of DIP. Dissolved nutrients were more elevated in the Great Kwa River than Cross River and Calabar River. The levels of DSi and DIN were less than average global concentrations, and that of river estuaries affected by anthropogenic activities. DON was the major form of TDN in the Calabar River estuary, and averagely accounted for 68-76% of TDN pool, while DOP accounted for similar to 54-75% of TDP pool. Similarly, DON was the dominant form of TDN, and represented 75-84% of TDN pool in the Cross River estuary. In contrast, TDN was dominated by DIN, which accounted for 76-84% of TDN pool, while DIP accounted for 56-61% of TDP pool in the Great Kwa River estuary. There was a switch from DOP (67% of TDP) dominated river water to DIP (67% of TDP) dominated estuarine water in the Cross River estuary, due to enrichment of DIP in the lower Cross River estuary. The adjacent shelf was dominated by DON (58% of TDN) and DIP (70% of TDP). Based on nutrient molar ratios, primary production was strongly limited by DIP, especially in the riverine regions, and switched to DIN limitation in the adjacent shelf. Nutrients in the CRES behaved non-conservatively, and were affected by biogeochemical and physical processes. Nutrient budgets were estimated using the classical LOICZ steady-state box model. Atmospheric deposition of dissolved nutrients was very limited compared to riverine supply. The exchange time of water between the ORES and adjacent sea was similar to 3.5 days, and may have affected biotic processes and nutrient transport. The CRES was an important source of DIP (137 x 10(3) moL day(-1)), DOP (92 x 10(3) moL day(-1)) and DON (4849 x 10(3) moL day(-1)) to the adjacent shelf, which was respectively 7, 4 and 2 times higher than the total riverine input. The ecosystem metabolism of the CRES, inferred from the Delta DIPsystem and Delta DINsystem indicated that respiration of organic matter was higher than primary production with respiration rate of -15.7 x 10(6) mol C day(-1) (-58 mmol C m(-2) day(-1)), while nitrogen metabolism was -3.8 x 106 mol N day(-1) (-14 mmol N m(-2) day(-1)), suggesting that the CRES was heterotrophic, and denitrifying more than nitrogen fixation during this study.