Abstract
One predicted result of climate change on upwelling systems is that increasing pressure gradients between land and sea will result in greater equatorward wind stress and therefore increased upwelling intensity. Notable ecosystem changes have been documented in eastern boundary upwelling regions in recent decades, including the southern Benguela, where striking changes in zooplankton, pelagic, demersal, line, squid and rock lobster fisheries and seabird abundances have been shown. Nutrient availability has undeniable bottom- up influences on trophic structure and food-web pathways by mediating phytoplankton competition and succession. In this first long-term study of nutrient variability in the Benguela system, we compiled and interrogated historical nutrient, oxygen and hydrographic data from a defined geographic area incorporating St Helena Bay. General linear models (GLMs) were applied to archived nutrient data to establish longterm concentration trends where seasonal effects are excluded. Surface concentrations were also excluded as these are modified by phytoplankton productivity. Nitrate and phosphate concentrations both increased between 1983 and 2004 by ~40% and ~50% respectively, while silicate showed no trend over this period. Oxygen concentrations declined by ~30% between 1960 and 2004. The prominent increases in nitrate and phosphate, together with decreasing oxygen concentrations, suggest a parallel increase in primary (new) productivity, leading to elevated POC/N flux and later decomposition processes that utilize oxygen below the pycnocline. Both processes have important future implications for biogeochemical cycles, ecosystems and commercial fisheries that we discuss here. Furthermore, due to the retention of upwelled water and biogenic material in St Helena Bay, the region could provide early indicators of change.