Dissolved Nutrients in the western side of Pemba Channel
Hydrographic data collected over the past three decades has provided a relatively coherent view of mean seasonal dynamics in the pelagic environment on the Pemba Channel. Surface waters are cold and nutrient-rich during NE months. Prevailing winds are from the north, resulting in general upwelling and causing the Columbia River effluent to lie to the south and along the Pemba Channel coast.
Distributional features of dissolved nutrients (nitrate and phosphate) are displayed in figure 1 and 2. Monthly means are not supported by data as the sampling was done based on monsoon tendency; therefore, more attention should be focused on spatial patterns in nutrient concentrations rather on the apparent differences in mean concentrations between months.
On a channel scale, surface nutrient concentrations are generally high everywhere during SE monsoon season but are higher near the coast during NE monsoon season. High nutrient water from the upwelling events is evident in the northwest corner of the region during NE. The Pangani River is a sources of high nitrate (> 5 pM), phosphate (> 1 pM) and silicate (> 20 pM) input to the nearshore pelagic environment in both NE and SE monsoon seasons.
The effects of NE upwelling are more apparent for surface nitrate and phosphate distributions, particularly in the northwest part of the channel. However, nearshore, upwelling-related features for all nutrients are not clearly resolved in these channel maps due to the coarse scale of averaging and smoothing.
Monthly surface distributions of nutrients in the coastal waters of the Pemba Channel are affected by two seasonal phenomena, northeast monsoon season, southeast monsoon season and the timing of long rainfall. The Pangani River influence is particularly apparent throughout the year but its more apparent during the long rain season that occurs in the late season of NE and early SE monsoon season in figure 1.
Figure 1: Phospate distribution in the Pemba Channel during a) southeast and b) northeast monsoon season. The arrows indicate the pathways of the East Africa Coastal Currents.
Phosphate decline during the bloom but not to the same extent as nitrate. Consequently, the reintroduction of nutrients to the surface waters via upwelling is more pronounced for the nearshore gradient in summer nitrate than for the phosphate gradients although upwelling clearly influences the latter two nutrients as well. The peak in nearshore nutrient concentrations due to upwelling occurs during NE while upwelling leads to enhanced nutrient levels (e.g., 5-10 pM nitrate) in the channel throughout the NE monsoon season.
Figure 2: Nitrite distribution in the Pemba Channel during a) southeast and b) northeast monsoon seasons. The arrows indicate the pathways of the East Africa Coastal Current.
