Identifying key links between biogiochemical processes and marine foodwebstructure

Voir le sujet précédent Voir le sujet suivant Aller en bas

Identifying key links between biogiochemical processes and marine foodwebstructure

Message  Paul Tréguer le Lun 8 Avr - 7:05

Un flagship(2010-2013) a été financé par le Consortium EUR-OCEANS (piloté par AWI et BAS, participants français : Paul Tréguer PT, IUEM-Brest, et Bernard Quéguiner, MIO-Marseille) sur le thème des liens clefs entre les processus biogéochimiques et la structure des réseaux trophiques marins. Ci-après quelques extraits choisis, adaptés par PT, pour contribuer au débat dans le cadre du forum Arctique.

Messages : 1-les études à mener sur l’océan Arctique peuvent grandement bénéficier des concepts formulés et des méthodologies développées pour l’océan Austral,
2-S’agissant des impacts du changement climatique ou de la biodiversité de garder la vision bi-polaire est particulièrement fertile.


Identifying key links between biogeochemical processes and marine foodweb structure
PECS Group (EUR-OCEANS Flagship for Polar Change and Synthesis)
(with inputs from PT)


Understanding and predicting how polar regions respond to change is a globally relevant issue that requires circumpolar scale analyses. It is widely accepted that the risk posed by climate change is higher and more imminent for the polar oceans than almost any other large marine ecosystem. Assessment of the likely responses of polar ecosystems to change is required to support the management and protection of the ecosystem services they provide and to predict feedbacks and effects on the Earth System. Furthermore, because of their sensitivity, contrasts and relatively simple structure, polar ecosystems serve as a model system for developing methods for global application and as an early indication of the effects of change.

1-Sea-ice extent and changes:
The Southern Ocean is characterized by a large seasonal variation in sea ice extent and a large fraction of young (first year) ice. Until the beginning of the 1990s large regions of the Central Arctic Ocean were ice covered even in summer and multi-year ice was quite abundant. During the last two decades Arctic Ocean summer sea-ice extent decreased dramatically and a large proportion of multi-year ice was lost; thus the Central Arctic Ocean sea ice variations show more and more similarities to those observed in the Southern Ocean’s Weddell Sea (both roughly 2.8 million km2 in size). The dramatic change in sea ice extent in the Arctic Ocean has far reaching consequences for marine ecosystems and cycling of elements (Figure 1). The Antarctic has areas of both ice reduction and expansion, with the overall sea-ice extent changing little (even slightly increasing). However, the warming and sea-ice loss observed around the Antarctic Peninsula is comparable in speed to changes in the Arctic Ocean.

2-Physical dynamics of water masses
In the two polar ocean meso and sub-mesoscale processes have a major role in the transfer of mass and energy between deep waters and surface waters, thus impacting biogeochemical fluxes and the structure of ecosystems. In nutrient-limited systems mesoscale eddies have been described as oases for higher trophic marine life (e.g. Godo et al. 2011).
A peculiar feature in the Southern Ocean is the increase of westerly winds over the last two decades driven by the destruction of stratospheric ozone (‘ozone hole’) and increasing levels of greenhouse gases. This can be measured by a positive trend of the Southern Annular Mode (SAM) index. The impact of stronger wind forcing on Southern Ocean circulation and mixing is still controversially discussed by physical oceanographers. There are indications that changes in ocean circulation might have severe consequences for the cycling of CO2 (e.g. Le Quéré et al., 2007) probably involving variations of the physical as well as the biological carbon pumps.

3-Nutrient limitations
Strong nutrient limitations play an essential role both in the Southern Ocean and the Arctic Ocean. The Southern Ocean includes the largest of the world’s high-nutrient low-chlorophyll (HNLC) regions due to iron limitation, whereas nitrate and phosphate are in ample supply. Silicic acid concentrations are very high in upwelling waters south of the Antarctic Circumpolar Current (ACC) but decrease also to limiting levels towards the Antarctic Polar Front area. In the Arctic Ocean, nitrate is most often the limiting nutrient while surface dissolved iron can be well above 1 nmol L-1 and thus 5 to 10 times higher than in most open ocean areas of the Southern Ocean. Differences in mixed layer depths (stratification) and vertical nutrient distributions can explain the occurrence of deep chlorophyll maxima in the Arctic Ocean and the high values of vertically integrated biomass in the Southern Ocean. Rapid climatic driven shifts of diatoms at high latitudes have been recently shown (Alvain et al. 2013), and silicic acid limitation can be expected for the Arctic ocean.

4-Primary production: nutrient or light limitation?
The type of limitation (nitrate and/or light) will influence the development of primary production (PP) in the Arctic Ocean. Satellite-based estimates point to an increase of PP with decreasing summer sea ice. An extrapolation to an ice-free Arctic Ocean would lead to a doubling of PP compared to current values; this prediction is, however, not very robust as state-of-the art biogeochemical ocean general circulation model simulations give either different results or results similar to each other but for different reasons (nutrient versus light limitation).





Figure 1 :Arctic ocean : basis for understanding the pelagic marine ecosystem (cf. ARCTOS Network, http://www.arctosresearch.net)

5-Sea ice as habitat
Sea ice, in both polar oceans, provides an important habitat for ice algal communities, heterotrophic protists, bacteria, and small zooplankton. Organisms in sea ice find high nutrient concentrations in high-salinity brine channels and can escape predation by larger zooplankton. Extremely high concentrations of small organisms in sea ice may be an important factor in evolution. The contribution of ice algae to PP in the Arctic Ocean (Figure 1) is not well known but increases with increasing latitude. Estimates vary by one order of magnitude but are much lower than depth-integrated PP in the water column. Sea ice assemblages in both polar oceans are generally quite similar, although dominated by different pennate diatom species (e.g. Nitzschia frigida in the Arctic and N. stellata in the Southern Ocean). The centric diatom Melosira arctica which grows in extensive, long mats attached to the underside of sea ice plays an important role in the Central Arctic Ocean.

6-Food web structure
Phytoplankton blooms in polar oceans are dominated (in terms of biomass) by relatively few diatom species and the colony-forming haptophytes of the genus Phaeocystis. The Southern Ocean harbours a high degree of endemic species, in particular heavily silicified diatoms of the iron-limited ACC that make up the siliceous sediments surrounding Antarctica (opal belt). The sediments north of the seasonal ice zone consist mainly of frustules of the silica-sinking diatom Fragilariopsis kerguelensis while Chaetoceros spores contribute a large part to the surface sediment in the Scotia Sea. Although ubiquitous from the ACC to the coastal current blooms of Phaeocystis antarctica occur regularly only in the Ross Sea. Phytoplankton assemblages in the Arctic Ocean are similar to those in boreal and temperate oceans due to its connectivity with the North Atlantic and North Pacific. Coastal blooms that form over the extensive Arctic shelves are dominated by weakly silicified diatoms and Phaeocystis pouchetii. The low productive, picophytoplankton dominated Central Arctic Ocean is determined by perennial ice cover and strong haline stratification which limit light and nutrient availability. Reduction in summer sea ice extent and increased freshening in a warming Arctic will lead to changes in phytoplankton community composition, as already shown for the Canadian Basin, with cascading ramifications for carbon flux and food web dynamics. Coccolithophores can be found in both polar oceans, but never grow to bloom proportions except in adjacent subpolar regions (for example, Patagonian shelf). Nitrogen-fixing cyanobacteria generally play a minor role in polar oceans, however, a recent study has shown that cyanobacteria introduced by river plumes along Arctic shelves may fix nitrogen in the Arctic Ocean; nitrogen fixation was also observed near the Kerguelen Islands.



Figure 2 : Southern ocean : possible links between Si and Fe limitation and the structure of marine ecosystem (courtesy of C. Klaas, AWI)

The classical view of the Southern Ocean food chain ‘diatoms-krill-whales’ (Figure 2) is based largely on the observations dating from the earlier days of polar research carried out under British leadership in the Scotia Sea and around South Georgia. Recent compilations of zooplankton observations carried out by the international community (including large data sets from the former Soviet Union) have shown that (1) the largest biomass of krill occurs in the Scotia Sea, (2) krill abundance shows very large interannual variation, and (3) krill biomass decreased dramatically between 1970 and 2000 and has not recovered thereafter (Atkinson et al., 2004).

The lush life around and on the island of South Georgia (large colonies of elephant and fur seals and penguins) is in part supported by the advection of krill and is largely dependent on recruitment further south and west (Antarctic Peninsula, ice covered Weddell Sea). This situation is similar to the Arctic Ocean ecosystem that profits from the advection of large amounts of the copepod Calanus finmarchicus from the North Atlantic.

7-Modelling
The simulation of polar marine ecosystems and the cycling of elements are challenging for both hemispheres. The Arctic Ocean is strongly forced by inflow of water masses from the Atlantic and the Pacific and the topography (many islands, shallow shelves) requires a fine resolution. Freshwater inflow by large rivers plays a significant role. Physical models are unable to properly simulate the strong stratification and shallow mixed layers of the central Arctic Ocean. This in turn affects the simulation of primary production. Further, models have to take into account denitrification in shelf regions. The flow in the Southern Ocean is dominated by upwelling in the south, northward Ekman transport partially compensated by southward transport via mesoscale eddies that are generated by instabilities of the ACC, and by subduction/mode water formation at the Antarctic Polar Front. Models able to resolve mesoscale eddies and to simulate biogeochemical processes with some complexity (including iron) are still computationally expensive, hence, only short-term integrations (years to decades) are currently feasible.


References

Alvain et al. (2013). Rapid climatic driven shifts of diatoms at high latitudes. Remote Sensing Environment, 132: 195-201

Atkinson et al. (2004). Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature, 432, 100-103
Godo et al. (2011) Mesoscale Eddies Are Oases for Higher Trophic Marine Life. PlosOne, 7: e30161

Le Quéré C. et al. (2007). Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change. Science, 316 : 1735-1738

Paul Tréguer

Messages : 1
Date d'inscription : 07/03/2013

Voir le profil de l'utilisateur

Revenir en haut Aller en bas

Voir le sujet précédent Voir le sujet suivant Revenir en haut

- Sujets similaires

 
Permission de ce forum:
Vous ne pouvez pas répondre aux sujets dans ce forum