The extent to which wind-driven seasonal upwelling cycles manifest in surface ocean temperature and nutrient variability along a monitoring line in the Southern Benguela upwelling system was investigated. Monitoring conducted monthly over a six-year period 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, 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. The results suggest that remote sensing techniques would be inadequate tools to monitor upwelling events in the Southern Benguela. Also, the incidence of phytoplankton blooms is more likely triggered by stratified conditions associated with surface heating than relaxation of upwelling winds.
The Benguela upwelling system stretches from South Africa to Angola and is one of the ocean’s four large and most productive eastern boundary upwelling areas [
St. Helena Bay, located at the southern end of the Benguela Upwelling system, is the best studied and one of the most productive areas in this system [
This study applies data collected monthly over a 6-year period along the almost 200 km long St. Helena Bay Monitoring Line (SHBML) to investigate covariation 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 was investigated in the context of implications for the validity of remote sensing [
The study area is the St. Helena Bay Monitoring Line (SHBML) in the Southern Benguela, along the west coast of South Africa (Figure
St. Helena Bay Monitoring Line sampling station details.
Station number | Latitude | Longitude | Bottom depth (m) | Distance from coast (km) |
---|---|---|---|---|
1 | −32.299 | 18.302 | 27 | 3 |
2 | −32.310 | 18.276 | 30 | 7 |
3 | −32.330 | 18.177 | 76 | 17 |
4 | −32.373 | 17.991 | 104 | 35 |
5 | −32.413 | 17.808 | 150 | 53 |
6 | −32.461 | 17.609 | 189 | 73 |
7 | −32.505 | 17.422 | 235 | 92 |
8 | −32.570 | 17.204 | 283 | 115 |
9 | −32.604 | 16.986 | 311 | 134 |
10 | −32.653 | 16.808 | 386 | 153 |
11 | −32.699 | 16.620 | 564 | 172 |
12 | −32.745 | 16.435 | 1396 | 191 |
The SHBML station positions superimposed on bathymetry. Shallow depths are shown in green and deeper depths in blue tones.
Water samples were collected at the near surface (approximately 3 to 5 m depth) and deeper depths to within 5 m from the ocean seabed. Physical parameters were measured
Water samples for the study of dissolved inorganic nutrients were stored in acid-washed polyethylene bottles with pressure caps and kept frozen at −80°C until analysis ashore. No samples were kept frozen for longer than 3 months before analysis. Dissolved nitrate (
Samples for Chl-
Temperature and salinity variations at the near-surface (Figures
Inshore-offshore near-surface (a and c) and 20 m depth (b and d) variability over time for temperature (a and b) and salinity (c and d). The thick black line represents the upwelling index (UI). Temperature (°C) and salinity (PSU) values are indicated as labels on respective isobars. The temperature ranged between <11°C (dark blue) and >18°C (red). Salinity ranged between <34.7 PSU (dark blue) and >35.4 (red).
Temperature (°C) near surface
Temperature (°C) at 20 m depth
Salinity (PSU) near surface
Salinity (PSU) at 20 m depth
On a shelf-wide scale, clear gradients in temperature and salinity are evident, with both parameters increasing with distance offshore. Temperature increases from as low as 9.8°C at the coast (3 km offshore) to almost 22°C offshore (191 km from coast) at the near surface and from 9.34 to 20.61°C at 20 m depth (Figures
Temporal changes in (a) temperature and (b) salinity at station 1, near surface and at 20 m depth.
Linear regression coefficients between temperature and the upwelling index, which has a very well defined seasonal profile (black line in Figures
Linear regression coefficients (slopes) for physical and chemical variables as a function of the upwelling index at the surface (3 to 5 m) and at 20 m sampling depths, for stations 1 to 12. Bold values indicate slopes that are significant at
TEMP | SALINITY | O2 |
|
|
Si | Chl-A | |
---|---|---|---|---|---|---|---|
3–5 m @ St # 1 | −0.0031 | −0.0002 | −0.0072 | 0.0215 | 0.0039 | 0.0457 | 0.0328 |
|
−0.0012 | −0.0004 | −0.0028 | 0.0134 | |
0.0632 | 0.0139 |
|
0.0008 |
|
0.0017 | −0.0106 | 0.0034 | 0.0353 | 0.0308 |
|
−0.0002 |
|
0.0028 | −0.0098 | 0.0014 | −0.0108 | 0.0301 |
|
−0.0003 |
|
0.0031 | −0.0098 | 0.0007 | −0.0153 | |
|
0.0013 |
|
0.0038 | −0.0104 | −0.0002 | −0.0149 | |
|
0.0029 |
|
0.0024 | −0.0148 | −0.0011 | −0.0245 | 0.0285 |
|
0.0029 |
|
0.0024 | −0.0094 | −0.0022 |
|
|
|
0.0064 | −0.0009 | −0.0009 | −0.0016 | −0.0011 | −0.0139 | 0.0116 |
|
0.0068 | −0.0011 | −0.0004 | −0.0056 | −0.0011 |
|
|
|
|
0.0000 | −0.0019 |
|
|
−0.0151 | 0.0057 |
|
|
0.0002 | −0.0039 | −0.0064 | −0.0017 | −0.0097 | −0.0016 |
|
|||||||
20 m @ St # 1 |
|
|
|
|
|
0.0624 | 0.0233 |
|
|
|
|
|
|
|
0.0306 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
0.0099 | 0.0454 |
|
|
|
|
0.0379 | 0.0038 | −0.0007 | 0.0309 |
|
−0.0071 | −0.0013 | −0.0033 | 0.0273 | 0.0021 | 0.0069 | |
|
−0.0040 |
|
−0.0031 | 0.0155 | 0.0016 | −0.0079 | |
|
−0.0063 |
|
−0.0032 | −0.0007 | −0.0001 | −0.0168 | |
|
0.0011 | −0.0009 | −0.0020 | −0.0082 | 0.0004 | −0.0120 | 0.0317 |
|
−0.0016 |
|
−0.0015 | 0.0105 | 0.0001 | −0.0135 | 0.0142 |
|
|
0.0002 | −0.0028 | −0.0093 | 0.0004 | −0.0213 | 0.0029 |
|
0.0129 | 0.0000 | −0.0032 | −0.0097 |
|
−0.0072 | 0.0022 |
There is a more pronounced relationship between the upwelling index and near-surface salinity at the inshore stations than observed for temperature (Table
Near-surface ocean distribution patterns for nutrients exhibit across-shelf gradients, from high values along the coast (up to 26.5
Inshore-offshore near-surface (a and c) and 20 m depth (b and d) variability over time for dissolved phosphate (a and b) and Chl-
Dissolved phosphate (
Dissolved phosphate (
Chl-
Chl-
Temporal changes in (a) dissolved phosphate (
Nutrient and Chl-
The most pronounced low-salinity event observed occurred in the winter (June–August) of 2007 (Figures
Evaluation of variability in the upper ocean temperature gradient (shown for station 1 in Figure
Variability in the upwelling index (dashed line) versus the temperature difference between 30 and 10 m depths (solid line), at station 1, for the time period 2004 to 2009.
The balance between surface heating and vertical mixing is clearly in favor of surface heating in St. Helena Bay. This has implications not only for the application of remote sensing methods to track upwelling events, but also for biological processes and spatial and temporal variability therein. Planktonic organisms tend to be distributed throughout the water column when it is well mixed, which is winter-time along the SHBML. When the water column is stratified, that is, spring-summer along the SHBML, it favors the development of algal blooms. Also, this concentration of food particles in a stratified water column is believed to be advantageous to the growth of larval fish [
It is clear from the results that all physical and chemical parameters exhibit pronounced across-shelf variability along the SHBML in the surface ocean, but very poor seasonality and no significant relationship with the calculated upwelling index. This does not mean that the observed weak relationships do not provide valuable insight into upwelling processes in St. Helena Bay. It does mean, however, that the weak relationship between wind-induced upwelling and inner shelf temperature cast doubt on the usefulness of satellite observations in detecting upwelling events in this part of the southern Benguela Upwelling system, as reflected either in decreased sea-surface temperatures or in surface Chl-
The observation that surface heating and water column stratification resulting from it outbalance wind-induced vertical mixing during the upwelling season in St. Helena Bay has important implications. First of all, it explains the poor correlation between sea surface temperature variability and the upwelling index, as well as the poor correlation between the UI and nutrient parameters. Secondly, it implies that remote sensing techniques will be poor indicators of upwelling-related variability in surface temperature and chlorophyll distributions in St. Helena Bay. It is also a critical observation in regard to understanding the factors that contribute to harmful algal blooms and the increasing incidence of such events.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the Branch: Oceans and Coasts, Department of Environmental Affairs (DEA) for making use of the excellent laboratory facilities and the support of funding in this study. This study could not have been done without the data collection efforts of S. Jones, F. Frantz, G. Kiviets, G. Tutt, E. Wright and M. Worship during this 6-year period. Special thanks to CM Illert for assistance with data management and archiving. The officers and crew of the research vessels