Seasonal and interannual phytoplankton dynamics and forcing mechanisms in the Northern Benguela upwelling system

• A 12 year time series of Chl-a were analysed for the northern BUS.
• It was linked with temp, nitrate, silicate, Chl-a and wind.
• Inshore and offshore variability of Chl-a in northern BUS with no significant trends.
• Temperature, nitrate, silicate, wind and Chl-a had shown seasonality.
• Three Chl-a peaks in April, August and December have different forcing mechanisms.

Seasonal phytoplankton blooms are one of the key features of the productive northern Benguela upwelling system (nBUS), however they are not well described thus far. In this study twelve years (2000–2012) of in situ chlorophyll-a data from a monitoring transect off the Namibian coast were analysed to assess the long-term and seasonal variability in chlorophyll-a as a measure of phytoplankton biomass and the occurrence of phytoplankton blooms. On the shelf, low chlorophyll-a concentrations were identified in 2001/2002, 2005/2006, and 2011/2012. The concentrations on the shelf were highest in 2008/2009 and 2010/2011. Major phytoplankton blooms defined at chlorophyll-a concentrations > 18 mg m− 3 occurred in five of the 12 years (2002/2003, 2004/2005, 2008/2009, 2009/2010 and 2010/2011) while minor blooms (> 13 mg m− 3) occurred in almost every year. The calculated climatology of the chlorophyll-a time series revealed a clear seasonality. Three chlorophyll-a maxima typically develop inshore over the year: an austral winter peak (August), an early austral summer peak (December) and a late summer/autumn peak (April). The analysis of synoptic hydrographic, nutrient and wind data revealed three different forcing mechanisms that all initiate an influx of nutrients into the surface mixed layer.