Bathypelagic particle flux signatures from a suboxic eddy in the oligotrophic tropical North Atlantic: production, sedimentation and preservation

Abstract

Abstract. Particle fluxes at the Cape Verde Ocean Observatory (CVOO) in the eastern tropical North Atlantic for the period December 2009 until May 2011 are discussed based on bathypelagic sediment trap time series data collected at 1290 and 3439 m water depth. The typically oligotrophic particle flux pattern with weak seasonality is modified by the appearance of a highly productive and low oxygen anticyclonic modewater eddy (ACME) in winter 2010. The eddy passage was accompanied by unusually high mass fluxes, lasting from December 2009 to May 2010. Distinct biogenic silica (BSi) and organic carbon flux peaks were observed in February–March 2010 when the eddy approached CVOO. The flux of the lithogenic component, mostly mineral dust, was well correlated to that of organic carbon in particular in the deep trap samples, suggesting a close coupling. The lithogenic ballasting obviously resulted in high particle settling rates and, thus, a fast transfer of epi-/mesopelagic signatures to the bathypelagic traps. Molar C : N ratios of organic matter during the ACME passage were around 18 and 25 for the upper and lower trap samples, respectively. This suggests that some production under nutrient (nitrate) limitation in the upper few tens of meters above the zone of suboxia might have occurred in the beginning of 2010. The δ15N record showed a decrease from January to March 2010 while the organic carbon and N fluxes increased. The causes of enhanced sedimentation from the eddy in February/March 2010 remain elusive but nutrient depletion and/or a high availability of dust as ballast mineral for organic-rich aggregates might have contributed to the elevated fluxes during the eddy passage. Remineralization of sinking organic-rich particles could have contributed to the formation of a suboxic zone at shallow depth. Although the eddy has been formed in the African coastal area in summer 2009, no indication of coastal flux signatures were found in the sediment traps, suggesting an alteration of the eddy since its formation. This confirms the assumption that suboxia developed within the eddy en-route. Screening of the biomarker fractions for the occurrence of ladderane fatty acids that could indicate the presence of anammox (anaeobic ammonia oxidation) bacteria, and isorenieratene derivatives, indicative for the presence of green sulfur bacteria and, thus for photic zone suboxia/anoxia was negative. This could indicate that suboxic conditions in the eddy had recently developed and the respective bacterial stocks had not yet reached detection thresholds. Another explanation is that the fast sinking organic-rich particles produced in the surface layer did not interact with bacteria from the suboxic zone below. Carbonate fluxes dropped considerably in February 2010, mainly due to reduced contribution of shallow dwelling planktonic foraminifera and pteropods. The deep-dwelling foraminifera Globorotalia menardii, however, showed a major flux peak in February 2010, most probably due to the suboxia/hypoxia. The low oxygen conditions forced at least some zooplankton to stop diel vertical migration. Reduced "flux feeding" by zooplankton in the epipelagic could have contributed to the enhanced fluxes of organic materials to the bathypelagic traps during the eddy passage.

Dead zone or oasis in the open ocean? Zooplankton distribution and migration in low-oxygen modewater eddies

“…The foraminifera, which are mostly too small to be quantified well with the UVP5, but in contrast to other rhizaria are well preserved in buffered formaldehyde in seawater solution, were highly abundant in the surface of the eddy core. Here, the distribution shift likely also includes a community shift, since a marked dominance change from surface-dwelling to subsurface-dwelling species was found in sediment trap data during the transit of the 2010 ACME (Fischer et al, 2015). In that ACME, also an export flux peak by foraminifera was observed.…”

“…The enhanced surface primary productivity of the eddy also resulted in an approximately 5-fold increase of large particles, well visible down to 600 m depth. This indicates a massive export flux by sinking marine snow (see also Fischer et al (2015) for sediment trap data of the 2010 ACME), which is thus made available to higher trophic levels at greater depths. As an example, phaeodaria (in supergroup rhizaria) are one of the few exclusively mesopelagic groups (only found deeper than approximately 200 m in UVP profiles).…”

“…This imaging tool allows for in situ quantification of particles > 60 µm and plankton > 500 µm with high vertical resolution (Picheral et al, 2010). Thumbnails of all objects > 500 µm were extracted using the ImageJ-based ZooProcess macro set (Gorsky et al, 2010) and sorted automatically into 41 categories using Plankton Identifier (Gasparini, 2007). Experts validated the automated image sorting.…”

“…The > 1000 µm fraction was scanned completely, whereas fractions comprising no more than approximately 1000 objects were scanned for the two other fractions. "Vignettes" and image characteristics of all objects were extracted with ZooProcess (Gorsky et al, 2010) and sorted into 39 categories using a Plankton Identifier (Gasparini, 2007). Automated image sorting was then manually validated by experts.…”

“…We used acoustic (shipboard ADCP) and optical (Underwater Vision Profiler, UVP) profiling methods as well as vertically stratified plankton net hauls to resolve the vertical and horizontal distribution of zooplankton. Moreover, we used acoustic and oxygen time series data from the CVOO mooring of one extreme low oxygen eddy observed in February 2010 (Karstensen et al, 2015;Fischer et al, 2015) to derive a more general picture about the zooplankton sensitivity to low oxygen concentrations.…”