Microbial oceanography of the dark ocean’s pelagic realm

New review in Limnology and Oceanography by Javier Arístegui, Josep Gasol, Carlos Duarte and Gerhard Herndl about the mesopelagic and bathypelagic food webs. They give a nice account of the current status of knowledge on sources and fates of organic matter in the dark ocean, phylogenetic and functional diversity and metabolism of dark-ocean microbes, and the contribution of dark-ocean microbes to carbon flux in the meso- and bathypelagic ocean.

Schematic simplification of (A) organic matter fluxes and (B) food webs in the mesopelagic and bathypelagic zones of the dark ocean (source: Limnology and Oceanography).

Abstract. The pelagic realm of the dark ocean represents a key site for remineralization of organic matter and long-term carbon storage and burial in the biosphere. It contains the largest pool of microbes in aquatic systems, harboring nearly 75% and 50% of the prokaryotic biomass and production, respectively, of the global ocean. Genomic approaches continue to uncover the enormous and dynamic genetic variability at phylogenetic and functional levels. Deep-sea prokaryotes have comparable or even higher cell-specific extracellular enzymatic activity than do microbes in surface waters, with a high fraction of freely released exoenzymes, probably indicative of a life mode reliant on surface attachment to particles or colloids. Additionally, evidence increases that chemoautotrophy might represent a significant CO2 sink and source of primary production in the dark ocean. Recent advances challenge the paradigm of stable microbial food web structure and function and slow organic-matter cycling. However, knowledge of deep-ocean food webs is still rudimentary. Dynamics of particle transformation and fate of the exported material in deep waters are still largely unknown. Discrepancies exist between estimates of carbon fluxes and remineralization rates. Recent assessments, however, suggest that integrated respiration in the dark ocean’s water column is comparable to that in the epipelagic zone, and that the dark ocean is a site of paramount importance for material cycling in the biosphere. The advent of new molecular tools and in situ sampling methodologies will improve knowledge of the dark ocean’s microbial ecosystem and resolve current discrepancies
between carbon sources and metabolic requirements of deep-sea microbes.

Marta Moyano

Congratulations to Dr. Marta Moyano! Last Thursday Marta Moyano defended her Ph.D. Thesis entitled "Temporal and spatial distribution of the ichthyoplankton in the Canary Islands". She obtained the European Ph.D. degree with the higher mark "Cum Laude". Marta has been working very hard during the last four years in the Biological Oceanography Group. She sampled ichtyoplankton every week during two and a half years, performed an impressive sampling during the ConAfrica 0603 cruise on board the RV "Hespérides" and visited Australia to work on the analysis of fish larvae otoliths. Thousands of ichtyoplankton samples were counted and analysed. This impressive work will be a reference for all those interested in the early stages of fish in the Canary Current. She was also a nice person and it was a pleasure to work with her and to have discussions about her work and in general about oceanography. Once more: Congratulations!

Dr. Marta Moyano

Cover of the Thesis manuscript by Marta Moyano

The main conclusions of her thesis were:

(1) The larval fish community off Gran Canaria Island is highly diverse. Neritic (Clupeidae, Sparidae & Gobidae) and oceanic (Myctophidae, Gonostomatidae & Photichthydae) larvae equally contribute to the larval assemblage.

(2) Two temperature-dependent seasonal LFAs off Gran Canaria:
•Winter-spring assemblage, which occurs during the mixing of the water column and the late winter bloom. Relatively high abundances of Sardinella aurita, Boops boops and Cyclothone braueri and presence of e.g. Pomacentridae sp1 and Lobianchia dofleini.
•Summer-autumn assemblage, which occurs during the stratification period of the water column. Relatively high abundances of larvae of Gobidae species and of Cyclothone braueri, Ceratoscopelus warmingii, Pomacentridae sp2, and Anthias anthias, and presence of Trachinus draco and Tetraodontidae sp1.

(3) Two stagnation regions up- and downstream of Gran Canaria are confirmed as accumulation areas of eggs and neritic fish larvae on a long-time scale. But the composition of the larval assemblage is not site-dependent.

(4) Strong relationship between mesoscale oceanographic processes and fish larvae, especially between upwelling filaments and sardine and anchovy.
•Filaments transport larvae of African neritic fish species into the oceanic region and towards the Canary Islands
•This larval transport is responsible for the high abundances of neritic larvae recorded in the oceanic realm during summer.
•Clupeoid larvae transported to Gran Canaria Island enhance local larval fish populations.
•Upwelling filaments may be trapped by the quasi-permanent cyclonic eddy, and this complex may act either as a retention or dispersal structure for the African neritic fish larvae.

(5) Metabolic activity of clupeoid larvae during an upwelling filament event revealed that grazing decreased offshore, while respiration increased, fish larvae might be affected by the transition of the planktonic community from the eutrophic to the oligotrophic regime.

The "Canary Eddy Corridor"

Today, the announcement of an accepted paper by Sangrà et al. in Deep Sea Research I. They report, from remote sensing and in situ observations, a new type of permanent structure in the eastern subtropical Atlantic Ocean, that they call the "Canary Eddy Corridor". The phenomenon is a zonal long-lived (> 3 months) mesoscale eddy corridor, whose source is the flow perturbation of the Canary Current and the Trade Winds at the Canary Islands. The latitudinal range of the corridor spans 22ºN-29ºN, and extends from the Canaries to at least 32ºW, near the mid-Atlantic Ridge. This is the main region of longlived westward propagating eddies in the subtropical northeast Atlantic. Below is the information appeared in the Aviso web page. The first figure was elected as the image of the month!

Eddies are seemingly sowed more or less haphazardly in the ocean. However, regular features exist, especially when there are dominant winds and/or currents, and islands or other topographic features. The "Canary Eddy Corridor" is a recurrent feature that can be detected in the more than 16 years of merged altimeter data. The phenomenon is an East-West corridor of eddies, born of the perturbation of the Canary Current flow and of the Trade Winds at Canary Islands. It extends from the Canaries to at least 32°W, close to the Mid Atlantic Central Ridge. Anticyclonic long-lived eddies (a few years life span) have been observed as far west as 50°W, well beyond this Ridge. Altimeter observations and drifter trajectories indicate that the eddies in this corridor first propagate South (following the Canary Current flow); then, at about (18°W, 25°N), they move westward (with a slight southward direction for anticyclonic eddies), on average at about one degree per month.

Paths of (at least) 6 month-old eddies in the North-East Atlantic. In red, anticyclonic eddies, in blue cyclonic ones, over the October 1992 - September 2006 period. A group of anticyclonic eddies can be seen moving from the South of the Canary islands westward to the mid-Atlantic. (Credits Universidad de Las Palmas de Gran Canaria/IMEDEA)



Geostrophic velocities computed from altimetry in September (left) and December 1998 (right), tracking the centre of an intense anticyclonic eddy South of Gran Canaria. The blue line corresponds to a drifter trajectory three days before and after the altimetry map. The buoy trajectory clearly matches the altimeter observations of this eddy. (Credits Universidad de Las Palmas de Gran Canaria/IMEDEA)

Left, depth-averaged (0 to 100 m) velocities as obtained from ADCP data (black arrows), superposed onto sea surface height as derived from merged altimeter data on September 2002. Both data sources show an intense anticyclonic eddy south of El Hierro island (South-Westernmost Canary island). Right, geostrophic velocities superposed onto a drifter trajectory (blue line). The trajectory corresponds to 15 days before and after the corresponding image, tracking the periphery of the same anticyclonic eddy observed left, one month later. The shape and eddy intensity from both data sources coincide well, with a size close to 100 km. (Credits Universidad de Las Palmas de Gran Canaria/IMEDEA)

Finally, a video of the relative vorticity from SSH. Observe the eddies (cyclonic and anticyclonic) shed by the Canary Islands and the drifting of those mesoscale structures (supplementary material of Deep Sea Research I).

Inma Herrera

Inma Herrera sent us some photographs from Hamburg. She is performing a stay with Dr. Myron Peck from the "Institut für Hydrobiologie und Fischereiwissenschaft". She is currently working on the relationship between growth and the activity of the enzyme AARS (amynoacil-tRNA sinthase) in copepods and fish larvae. During her stay in Hamburg she will compare growth in herring larvae with AARS and the RNA/DNA ratio. After Hamburg, a lot of work is waiting for her in the Canary Islands. In Autumn we will start the field sampling of the Lucifer project (see projects in this blog). Inma, thank you very much for the nice photographs and good luck with your experiments.

Inma in a nice office with the forest behind

The players (from left to right): Stephanie Borchardt, Myron Peck and Inma Herrera (nice photograph).

The victim: a herring larvae

The local fishermen providing the local herring for the local experiments (from left to right): Fritz, Jürgens, Ullrich and Eberhard.

Inma working very hard as observed from her tired face.

The feeding experiments running.

The copepod cultures made for the feeding of larvae.

The fathers of the victim.

Looking for sexual products to produce victims.

The killers (from left to right): Inma Herrera, Stephanie Borchardt and Lars Christiansen.