Are upwelling systems an underestimated source of omega-3 in the ocean?
The case of the southern Benguela upwelling system
The Benguela Upwelling System (BUS) is one of the world’s most
productive ecosystems, supporting globally relevant pelagic fisheries.
BUS marine community can change as a function of nutrients and omega-3
long chain polyunsaturated fatty acids (hereafter, omega-3)
availability. Phytoplankton growth is supported by upwelled nitrate, a
new source of nitrogen (N), or by recycled N forms such as ammonium.
Preferential assimilation of one N form over another may lead to
differences in omega-3 production between high and low food-quality
species. This study evaluates how upwelling and the N source(s) used by
phytoplankton influence omega-3 production. Sampling was conducted in
the BUS at an anchor station sampled daily for 10 consecutive days. An
upwelling event on days 5-6-7 supplied high concentrations of nutrients
to surface waters, while pre- and post- upwelling the water column was
well-stratified with low nutrient concentrations. Omega-3 and
phytoplankton concentrations declined to ⁓zero during the upwelling
event. Nanoplankton (2.7-10µm) were responsible for most of the
productivity (30-95%) and relied on nitrate for their growth. Omega-3
concentrations at the surface reached peaks of 215.5 and 175.3µgL-1 pre-
and post-upwelling, which were up to 10-times higher than previous
measurements from the BUS. Pre-upwelling, non-diatom trophic markers
were dominant, with a rapid switch (over just two days) to diatom
trophic markers post-upwelling. This study defines the key role of
upwelling in promoting phytoplankton omega-3 production, which is
tightly coupled to the introduction of new-N during upwelling. The high
concentrations of omega-3 reported suggest that global omega-3
production is largely underestimated.