Biomass production in marine ecosystems.

Biomass production in marine ecosystems.

In marine ecosystems, phytoplankton is the main source of primary production. The nutrient availability cycles make it important to distinguish between “new“ and “regenerated” primary production ; the former identifies the annually renewed nitrogen, while the second measures the rapid cycling of nitrogen through ammonia availability. All in all, this distinction acts as an indicator of the efficiency with which the upper layers of the ocean take up CO2 through photosynthesis (and thus to what level it compensates for rising atmospheric CO2 levels) [1-2].

Biomass / Production :

the new biomass produced by growth of organisms. Expressed as annual production. Production relative to biomass is an expression of the turnover of organisms or populations. Expressed in tons.
Most accurate measures rely on the concentration of chlorophyll a (mg Chl a m-3 or m-2), as compared to organic carbon concentration (measured via 14C labelling methods).
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Organic biological production based on photosynthesis by phytoplankton is function of several factors, including nutrient cycles, photosynthesis or temperature. The first two are outlined below.
Nutrient availability, essentially nitrates, ammonia, phosphate, silicate, are present at 1-12 mM levels, and contribute directly or indirectly at a constancy of C:N:P:O2 proportions in sea water. The so called Redfield ratio (120:16:1:180) is used to estimate carbon production and export, in turn essential in evaluating the process known as the “biological carbon pump”. Depending on ocean regions, the contribution of planktonic photosynthesis to carbon sequestration varies and can be as low as 10%, such as in northern Atlantic.
The type of nutrients that become limiting depends on the species composition, season, depth. For example, the atomic ratio of N and Si (usually 2) is impacted differently by diatoms or flagellates.
Photosynthetic rates depend on a series of factors, among which pigment types and composition, light availability, temperature, turbulence, stratification.
In terms of pigment composition and content, chlorophylls are the main energy captors, with chlorophyll a being shared by all species. Carotenoids, xanthophylls, phycobilins complete the spectrum as photoprotective pigments and, together, the relative absorption in marine phytoplankton ranges from 400-700 nm wavelength (blue-green-yellow bands).
Irradiance levels ranging from high, moderate to low light energy inputs can vary approximately 10-fold and result in photosynthetic rates estimated at 0.75-5 mg C (mg Chl a)-1 (µmol M-2 s-1)-1.
Light absorption can further be optimized by varying the pigment concentration. The ratio Chlorophyl a / organic carbon (Chl/C) concentrations is used to assess this adaptation mechanism : “shade adaptation” implies high ratios (> 0.025), while “high light adaptation” gives ratios of 0.01-0.025. “Chromatic adaptation” results when the concentration of different pigments is adjusted in order to respond to harmful strong light.

Production (yield) :

is a matter of developmental stage, body size, age at sexual maturity and number of individuals per harvest.

Harvesting :

induced mortality in natural populations and communities by systematic human activities [3].

Fisheries management :

a process where government agencies implement measures designed to meet specific objectives set for the fishery (seasonal/area closure, stock size etc) [4].

Maximum Sustainable Yield :

surplus production that can be harvested without altering the stock level ; is used to set quotas for worldwide fisheries [5].

1. Rey F : Phytoplankton : the grass of the sea. The Norwegian Sea Ecosystem 2004:97–136.
2. Skjoldal HR, Dalpadado P, Dommasnes A : Food webs and trophic interactions. The Norwegain Sea ecosystem Tapir Academic Press, Trondheim 2004, 447:506.
3. Blanchard JL, Law R, Castle MD, Jennings S : Coupled energy pathways and the resilience of size-structured food webs. Theoretical Ecology 2010:1-12.
4. Røttingen I : Management of pelagic fisheries in the Norwegian Sea. The Norwegain Sea ecosystem Tapir Academic Press, Trondheim 2004:535 - 547.
5. The Inter-American tropical tuna commission. 1954.
Crédits photo : Gordon T. Taylor NOAA At The Ends of the Earth Collection

Article publié ou modifié le

14 juin 2012