Migrants and Active Flux in the Atlantic Ocean

During the last decade, the plankton outburst during the so-called late winter bloom in subtropical waters was studied in relation to lunar illumination in the Canary Island waters (Lucifer project CTM2008-03538). Two scenarios were observed during the productive period. The first was the increase in mesozooplankton as the effect of higher primary production due to vertical mixing. The second scenario was the decrease in mesozooplankton due to predation by diel vertical migrants (DVMs). Mesozooplankton abundance and biomass were observed to change with the lunar cycle in the oceanic waters of the Canary Current (Hernández-León, 1998; Hernández-León et al., 2001, 2002, 2004, 2010). The most important conclusion of this decade of studies in the waters off the Canary Islands was to show that estimated consumption and subsequent transport of epipelagic zooplankton biomass by DVMs is on the order of the mean gravitational export and is an unaccounted flux of carbon to the mesopelagic zone that may play a pivotal role in the efficiency of the biological pump. 

Lunar cycle of biomass in the Canary Island waters. Observe the increase in biomass in the epipelagic zone during the illuminated phase of the moon due to the absence of diel vertical migrants in the upper layers.

Arístegui et al. (2003) observed a sharp increase of DOC and an increase of the electron transfer system activity (ETS) of microplankton coinciding to the deep scattering layer (DSL) in the Canary Current, suggesting an active transport of organic carbon by migrants.

Acoustic profile showing the depth of the Deep Scattering Layer (DSL) around the Canary Islands (Bordes et al., 2010)

Dissolved organic carbon (DOC) along a transect south of the Canary Islands. Observe the increase in DOC at the depth of the DSL (Arístegui et al., 2003)

Recently, during the eruption of the submarine volcano in El Hierro Island (Canary Islands), we observed very low values of DOC in the mesopelagic zone. Zooplankton biomass at the shallower layers around the island were also very low (about 5-10% of normal values) due to the lethal effect of emissions and >90% of organisms were small (<500 µm). The quite low zooplankton biomass and the barrier imposed by the low oxygen and pH of the volcano plume in the upper layers prevented DVMs to feed in the shallower layers (unpublished data). Thus, the most plausible explanation for the low DOC at depth is related to the fact that organisms living at the DSL were not able to feed and therefore to fuel the mesopelagic zone with particulate and dissolved organic carbon. This is another fingerprint to argue the importance of diel migrants in the functioning of the biological pump in the ocean.

Satellite picture of the plume produced during the eruption of the El Hierro volcano

Depth of the DSL below and outside the volcano plume. Observe the shallower depth of the DSL below the plume due to lower light penetration. Also observe the weak diel migration below the plume and the absence of migrants into the plume due to the anoxic conditions there. Normal diel migration and DSL depth were observed outside de volcano plume (Ariza et al., unpublished).

DOC vertical distribution during the eruption of the El Hierro volcano. Compare with normal conditions to the south (black) and north (red) of the Canary Islands. Observe the increase in DOC in the mesopelagic zone coinciding with the DSL (Arístegui et al., unpublished).

During the Malaspina cruise we measured the effect of the Atlantic and Pacific equatorial upwelling systems on the vertical distribution of acoustic backscatter from the surface to bathypelagic depths using the rosette-mounted L-ADCP. This acoustic device is able to reach the lower bathypelagic and to average the backscatter every 10 m. The enhancement of the acoustic signal below the upwelling zone was observed to reach 4000 m depth (Figure 1), coinciding with high abundances and activity of bacteria at those depths (Hernández-León et al., 2012). No enhancement of transparent exopolimeric particles (TEPs) were observed during the same survey (Pérez-Mazuecos, 2012), so we hypothesize about the increase of zooplankton and micronekton biomass at those depths. The results suggest an active carbon transport from the epipelagic zone driven by zooplankton and micronekton, enhancing the ladder of migration, the efficiency of the biological pump and giving an insight about the fate of an increased productivity at the shallower layers of the ocean.

Atlanctic transect during the Malaspina Cruise in 2011

Acoustic signal of the LADCP of the rosette sampler along the Atlanctic transect. Observe the epipelagic and mesopelagic layers as well as the increase in backscatter at the bathypelagic zone below the equatorial upwelling (Hernández-León et al., unpublished).

As above but during night  (Hernández-León et al., unpublished)

Pacific transect during the Malaspina Cruise in 2011

Acoustic signal of the LADCP of the rosette sampler in the Pacific transect from New Zealand (left) to Hawaii (right). Observe the epipelagic and mesopelagic layers as well as the increase in backscatter at the bathypelagic zone below the equatorial upwelling (Hernández-León et al., unpublished) .

As above but during night  (Hernández-León et al., unpublished).

We wonder whether geochemical estimates of new production which are in the range of 6.8-14.6 mmol C m^-2 d^-1 (Maiti et al. 2009), much higher than sediment trap measurements (2-6 mmol C m^-2 d^-1), could be conciliated as their values are near the addition of gravitational and conservative estimates of active fluxes assessed by Hernández-León et al. (2010). The estimation of export flux in oceanic waters taking into account gravitational and total active fluxes seems of paramount importance to understand the biological pump in the ocean. Measurements of active flux following the concept of the ladder of migration could help to understand the transport of organic matter, the organisms engaged and their effect on bacterial biomass and growth, the final consumers of this transport in the water column.

Conceptual model developed by Vinogradov during the mid twentieth century showing the ladder of migration. To our knowledge, our observation at the bathypelagic depths of the equatorial Atlantic and Pacific Oceans using the LADCP acoustics are the first fingerprints of the ladder of migrations. The present project seeks to unveil the existence of this phenomenon in the ocean of evident consequences for the knowledge of the biological pump. 

The project seeks to quantify active flux at the large-scale. To our knowledge this will be the first initiative to evaluate active flux in the ocean in order to envisage the importance of this flux in the warm ocean at a global perspective. To pursue this important objective we propose the sampling of 15 oceanographic stations  (see Figure below) covering the subtropical oligotrophic gyres and the upwelling region off Northwest Africa and the equatorial divergence where the development of a ladder of migration is expected.

Proposed transect along the Atlantic Ocean covering the oligotrophic gyres as well as the upwelling off Northwest Africa and the equatorial divergence.