Seasonal flux patterns and carbon transport from low-oxygen eddies at the Cape Verde Ocean Observatory: lessons learned from a time series sediment trap study (2009–2016)
Authors: Fischer Gerhard , Romero Oscar E. , Karstensen Johannes , Baumann Karl-Heinz , Moradi Nasrollah , Iversen Morten , Ruhland Götz , Klann Marco , Körtzinger Arne 
Published: December 2021

Abstract. Mesoscale eddies are abundant in the eastern tropical North
Atlantic and act as oases for phytoplankton growth due to local enrichment
of nutrients in otherwise oligotrophic waters. It is not clear whether
these eddies can efficiently transfer organic carbon and other flux
components to depth and if they are important for the marine carbon budget.
Due to their transient and regionally restricted nature, measurements of
eddies' contribution to bathypelagic particle flux are difficult to obtain.
Rare observations of export flux associated with low-oxygen eddies have
suggested efficient export from the surface to the deep ocean, indicating
that organic carbon flux attenuation might be low. Here we report on
particle flux dynamics north of the Cabo Verde islands at the oligotrophic
Cape Verde Ocean Observatory (CVOO; approx. 17∘35′ N, 24∘15′ W). The CVOO site is located in the preferred pathways of
highly productive eddies that ultimately originate from the Mauritanian
upwelling region. Between 2009 and 2016, we collected biogenic and
lithogenic particle fluxes with sediment traps moored at ca. 1 and 3 km water
depths at the CVOO site. From concurrent hydrography and oxygen observations,
we confirm earlier findings that highly productive eddies are characterized by
colder and less saline waters and a low-oxygen signal as well. Overall, we
observed quite consistent seasonal flux patterns during the passage of highly productive eddies in the winters of 2010, 2012 and 2016. We found flux
increases at 3 km depth during October–November when the eddies approached
CVOO and distinct flux peaks during February–March, clearly exceeding low
oligotrophic background fluxes during winter 2011 and showing an enhanced
particle flux seasonality. During spring, we observed a stepwise flux
decrease leading to summer flux minima. The flux pattern of biogenic
silicate (BSi) showed a stronger seasonality compared to organic carbon.
Additionally, the deep fluxes of total mass showed an unusually higher
seasonality compared to the 1 km traps. We assume that BSi and organic
carbon/lithogenic material had different sources within the eddies. BSi-rich
particles may originate at the eddy boundaries where large diatom aggregates
are formed due to strong shear and turbulence, resulting in gravitational
settling and, additionally, in an active local downward transport. Organic
carbon associated with lithogenic material is assumed to originate from the
interior of eddies or from mixed sources, both constituting smaller,
dust-ballasted particles. Our findings suggest that the regularly passing
highly productive eddies at CVOO repeatedly release characteristic flux
signals to the bathypelagic zone during winter–spring seasons that are far
above the oligotrophic background fluxes and sequester higher organic
carbon than during oligotrophic settings. However, the reasons for a lower
carbon flux attenuation below eddies remain elusive.