Seasonal variability and the relationship between dissolved inorganic nutrients and selected environmental parameters inshore and offshore of St. Helena Bay

Abstract

The purpose of the present study was to apply data collected monthly over a 6-year period along the almost 200 km long St. Helena Bay Monitoring Line in the Southern Benguela upwelling system to investigate co-variation between an upwelling index calculated from nearby wind records and physical and chemical properties along the transect. The extent to which the well-documented seasonal upwelling cycles is manifested at the surface along an inshore-offshore variation was investigated in the context of implications for the validity of remote sensing as a monitoring tool in this ocean area and to improve understanding of the physical drivers of biological processes in St. Helena Bay. This study shows that surface temperature and nutrient concentrations exhibit very poor seasonality and weak correlation with the upwelling index. This is, despite clear evidence for spatial inshore-offshore gradients in temperature, nutrients, and chlorophyll-a, consistent with an upwelling regime. The upper ocean temperature gradient shows a much better correspondence to the upwelling index but at the same time demonstrates that surface heating, and not vertical mixing related to upwelling, controls the upper ocean temperature gradient. In this study linear lagged correlations were also examined and discussed to gain insight into the effect upwelling has on the surface waters in St. Helena Bay with the view of determining the following: (1) Does upwelling lead to an increase or decrease of the water properties? (2) What is the characteristic lag between an upwelling event and its effect on these water properties? (3) Is the effect and/or lag different for the different seasons? and (4) Is the effect and/or lag different for stations inside the bay and those outside the bay?. A combination of surface turbulent cooling through upwelling occurred after a lag of 8 to 10 days in winter and early summer, but less than half in late summer, similar to results obtained with salinity. However, the rest of the salinity results fit in poorly with the temperature results. The only significant correlation obtained with the inshore stations during late summer is the inexplicable positive correlation at a lag of 7 days. For all three seasons virtually none of the oxygen results fit the expected pattern. All three nutrients showed a more positive correlation coefficient and significance than the negative ones. Significant negative correlations occurred mainly during late summer at lags of 7 to 9 days caused by planktonic depletion of nutrients. Also, in this season, significant positive correlations between south-north wind and nutrients only occurred at short lags. This observation supports the earlier temperature-based conclusion that the influence of upwelling develops most rapidly at this time of the year. A rapid increase in chlorophyll-a levels followed by nutrient enrichment of the surface layers are evident. iii The results suggest that remote sensing techniques would be inadequate tools to monitor upwelling events in the Southern Benguela. Secondly, the incidence of phytoplankton blooms is more likely triggered by stratified conditions associated with surface heating than relaxation of upwelling winds. Finally, these results also emphasise the importance of validating lagged outputs against real-time measurements in supporting a simpler hydrological model in narrowing down these significant uncertainties.