High export via small particles before the onset of the North Atlantic spring bloom

Abstract

Sinking organic matter in the North Atlantic Ocean transfers 1–3 Gt carbon yr−1 from the surface ocean to the interior. The majority of this exported material is thought to be in form of large, rapidly sinking particles that aggregate during or after the spring phytoplankton bloom. However, recent work has suggested that intermittent water column stratification resulting in the termination of deep convection can isolate phytoplankton from the euphotic zone, leading to export of small particles. We present depth profiles of large (>0.1 mm equivalent spherical diameter, ESD) and small (<0.1 mm ESD) sinking particle concentrations and fluxes prior to the spring bloom at two contrasting sites in the North Atlantic (61.30°N, 11.00°W and 62.50°N, 02.30°W) derived from the Marine Snow Catcher and the Video Plankton Recorder. The downward flux of organic carbon via small particles ranged from 23 to 186 mg C m−2 d−1, often constituting the bulk of the total particulate organic carbon flux. We propose that these rates were driven by two different mechanisms. In the Norwegian Basin, small sinking particles likely reached the upper mesopelagic by disaggregation of larger, faster sinking particles. In the Iceland Basin, a storm deepened the mixed layer to >300 m depth, leading to deep mixing of particles as deep as 600 m. Subsequent restratification could trap these particles at depth and lead to high particle fluxes at depth without the need for aggregation (“mixed‐layer pump”). Overall, we suggest that prebloom fluxes to the mesopelagic are significant, and the role of small sinking particles requires careful consideration.

Phytoplankton dynamics in contrasting early stage North Atlantic spring blooms: composition, succession, and potential drivers

“…A switch from negative to positive net heat flux has been linked to spring bloom formation (Taylor and Ferrari, 2011b;Smyth et al, 2014), but here the net heat flux was negative for the majority of the study at both sites (C. Lindemann, personal communication, 2014;Giering et al, 2015). Irradiance is a key driver of phytoplankton growth and bloom formation; the main spring bloom did not occur until daily PAR reached its seasonal maximum of 45 mol photons m −2 d −1 (Fig.…”

“…Instead, we observed periods of stability, characterised by increased stratification, Chl a, and PP, followed by periods of instability where increased mixing weakened the developing stratification. Increased mixing detrains phytoplankton out of the surface waters, reducing both Chl a biomass and PP, and exporting them to depth (Giering et al, 2015). One such mixing event occurred between 27 and 29 April (days 118 and 120), where minor stratification ( σ t = 0.019) disappeared ( σ t < 0.001) over the upper 25 m, surface Chl a halved from 1.18 to 0.62 mg m −3 , and the fluorescence profile became well-mixed (Fig.…”

“…Therefore, we suggest that the early stages of a spring bloom are characterised by periods of instability and net growth, and that rather than a single smooth transition into a bloom, for a period of weeks prior to the main spring bloom event, phytoplankton form temporary miniblooms during transient periods of stability. The export flux from these pre-bloom communities is a potentially significant food source to the mesopelagic (Giering et al, 2015).…”

Drivers of Carbon Export Efficiency in the Global Ocean

“…This mechanism assumes that the abundance of zooplankton is correlated with grazing pressure on phytoplankton, the rate of fecal pellet production and the contribution of fecal pellets to export flux, as demonstrated experimentally (e.g., Cavan et al, 2015;Ebersbach & Trull, 2008;Landry, 1995). LP-HE regimes may also be associated with transient, nonbiological processes which may contribute to high export efficiency, for example, the mixed layer pump (Dall'Olmo et al, 2016;Giering et al, 2016), in which variability in the mixed layer depth results in detrainment of particles. Unfortunately, there are insufficient temporally resolved observations in our data set to examine this potential seasonality.…”

The Intraseasonal Dynamics of the Mixed Layer Pump in the Subpolar North Atlantic Ocean: A Biogeochemical‐Argo Float Approach

“…When detrainment fluxes exceed entrainment fluxes, the intraseasonal ML pump can lead to a net export of carbon to the mesopelagic. In the northeast Iceland basin, Giering et al () have shown that the prebloom flux of small particles driven by the ML pump can be of similar magnitude to the total particle export rate by sedimentation observed during, and after, the spring bloom period. However, the analysis of long‐term sediment trap data from 3,000 m at the Porcupine Abyssal Plain (49°N, 16°W) revealed that prebloom deep fluxes are small (Lampitt et al, ).…”