Dissolved Inorganic Nutrient Fluxes from Two Seasonal Rivers into Gazi Bay, Kenya

Abstract

The riverine contribution to the dissolved inorganic nutrient budget of Gazi Bay, a little-studied pristine system, has been investigated. River Mkurumuji had estimated nutrient discharge rates equivalent to 12·21±9·98kgNday−1in the form of NH4+, 37·45±30·59kgNday−1in the form of NO3−+NO2−and 31·66±25·86kgPday−1in the form of PO43−during the wet months of the study period. During the dry months, the corresponding values were 0·28±0·17, 0·27±0·16 and 0·93±0·57, respectively. River Kidogoweni discharged nutrients at rates equivalent to 5·37±4·69kgNday−1in the form of NH4+, 9·83±8·59kgNday−1as NO3−+NO2−and 9·00±7·87kgPday−1in the form of PO43−during the wet months. Corresponding mean discharge rates during dry months were 0·013±0·009, 0·0057±0·0038 and 0·035±0·023, respectively.From the salinity distribution patterns, it is evident that during wet seasons, River Mkurumuji influences the nutrient distribution patterns of coral reef and seagrass zones, while River Kidogoweni influences the nutrient distribution pattern of the mangrove zone of the Bay. There seems to be minimal riverine influence during thedry seasons. The mean gross primary productivity of the Bay during the two seasons is a manifestation of thisobservation. Mean values for the mangrove, seagrass and coral reef biotopes, respectively, were: 540·41±222·63, 552·22±291·36, 388·88±247·12mgCm−3day−1(wet seasons); and 377·67±159·7, 230·84±84·75, 240·27±115·29mgCm−3day−1(dry seasons).The levels of dissolved inorganic nutrient concentrations between the waters of both rivers are not significantly different (P=0·35), but are higher than for the Bay waters, which are characteristically oceanic. Gazi Bay nutrient levels are lower than those reported elsewhere within the region, therefore suggesting that the Bay is a relatively pristine environment.