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Organic Carbon Moved from the Ocean Surface to Depth by Daily Vertical Migration of Small Marine Animals

According to the authors1 it has been found by acoustic data assessment that the depth to which this migration moves is tied closely to the levels of subsurface oxygen throughout much of the global ocean. Many small marine animals feed near the surface at night and move to deeper, darker depths during the day. These daily migrating animals include zooplankton from almost all invertebrate phyla, and micronekton, small animals that swim actively, such as shrimp and small fish. This movement to depth during the day is called diel vertical migration, is believed to reduce the risk of predation by sight (Lampart, 1989; Hays, 2003). It has been difficult to determine the biogeochemical consequences of this phenomenon, that occurs throughout the world ocean, partially as a result of field data on the migration zooplankton and micronekton being of limited availability.  Bianchi et al., (Bianchi et al., 2013) have used acoustic and model data to demonstrate that diel vertical migration exacerbates the depletion of oxygen in environments of the global ocean that are already at low oxygen levels.

Phytoplankton take in carbon dioxide and  nutrients in the brightly lit ocean surface waters and generate organic matter, some of this organic matter being transported down to the mesopelagic zone, between depths of 100-1,000 m. At this depth the process is reversed to release nutrients and carbon dioxide while consuming dissolved oxygen. The result of this is that the vertical oxygen profiles exhibit a minimum in the mesopelagic zone in much of the global ocean (Sarmiento et al., 2006).

It is believed that most of the organic material being transported downwards results from the sinking under the influence of gravity of dead phytoplankton, faecal pellets, and other particles of detritus (Michaels & Silver, 1988). There is a rapid decline with depth of this particle flux through the mesopelagic ocean, only a small fraction continuing on to the deep sea. Deep water convection (Carlson, Ducklow & Michaels, 1994) may also physically mix dissolved organic carbon from the surface into the mesopelagic zone. It is indicated by most carbon budget estimates that the metabolic demands of the biological community of the mesopelagic zone cannot be met by these sources of carbon alone (Steinberg et al.,, 2008; Burd et al., 2008). It is suggested1 that the diel vertical migrators may aid the meeting of this demand as they excrete dissolved and particulate organic carbon at depth that was originally consumed at the surface, as well as being food for the mesopelagic carnivores. biogeochemical fluxes have been under sampled in much of the mesopelagic zone relative to the surface, and to some extent, relative to depths below 1,000 m.

Bianchi et al. (Bianchi et al., 2013) has constructed a global inventory of diel migration depths of small marine animals, specifically zooplankton , micronekton, and small fish, by the use of measurements from shipboard acoustic Doppler current profilers (ADCPs), that were originally deployed to measure ocean currents. Their results indicated that the depth of diel migration was positively correlated with the depth of the subsurface oxygen minimum, the migrators diving deepest in regions where the subsurface oxygen minimum is deeper in the water column. The migrators dive only to the upper bounds of the low-oxygen waters in the vicinity of oxygen minimum zones, the autors1 suggesting this may be to shelter from predators that are less tolerant to low oxygen levels. The data-derived relationships between the depth of migration and the levels of oxygen in the subsurface waters have been incorporated into a numerical model by Bianchi et al., it is shown by the results that by clustering in the upper margins of the oxygen minimum zones, organic matter breakdown (respiration) is accentuated by the vertical migrators, which strengthens the oxygen deficit.

The authors1 say there are limitations associated with the application of single frequency ADCP data to biological problems, the most significant being that little if any information is provided on the taxa of zooplankton, micronekton or larger fish constituting the diel vertical migrators. It is necessary to 'ground-truth' acoustic data such as these with other methods, such as the use of nets to determine which animals undergo diel vertical migration and contribute to the consumption of oxygen at depth. The use of multi-frequency acoustic methods that allow the identification of size classes and even major taxa (Lawson et al., 2008), and by the use of towed camera systems, progress has been made. The authors1 suggest the further use of such technologies would lead to a better understanding of which organisms are involved in this diel vertical migration as well as their abundance, now that a clearer picture of the biogeochemical significance of these vertically migrating organisms is being gained.

It is believed the geographic range, productivity and composition of marine ecosystems are likely to change as a result of climate change, which means such questions are becoming increasingly urgent (Doney et al., 2012). As the solubility of oxygen in water is lower in warmer water in the warmer oceans it is expected that the oxygen levels in the subsurface water will decrease and the circulation of the ocean will slow down (Keeling, Kortzinger & Gruber, 2010). It has been suggested that already oxygen minimum zones in the tropics and subtropics may be expanding horizontally and vertically, and these zones limit the habitat of many marine species and support atypical biogeochemical processes, such as denitrification, in the water column (Stramma et al., 2012). The authors1 suggest a systematic monitoring of the mesopelagic zone will be required, as well as studies that are targeted at resolving the distribution, diversity and biogeochemical function of the various organisms in these waters where there is little light penetration, to determine the response of migrating zooplankton, micronekton and fish populations to such environmental perturbations on a large scale (Robinson et al., 2010; Wishner et al., 2008). It has been shown by Bianchi et al. (Bianchi et al., 2013) that oxygen deficiencies in these waters are accentuated by the diel vertical migration of small marine animals, as this migration focuses the breakdown of organic matter at the upper margins of existing oxygen minima. The need for ocean biogeochemical modelling to move beyond the representation, that is relatively simplistic, of the biological carbon pump, in which the majority of carbon is transported to depth by the sinking of particles, to one that is truly mechanistic incorporating sub-models of mesopelagic fluxes and transformations.

Sources & Further reading

  1. Doney, Scott C., and Deborah K. Steinberg. "Marine Biogeochemistry: The Ups and Downs of Ocean Oxygen." Nature Geosci 6, no. 7 (07//print 2013): 515-16.


Author: M. H. Monroe
Last Updated 06/07/2013
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