Geographical variation of photosynthetic yield

Equations such as 11.4-11.8 predict the amount of phytoplankton photosynthesis per unit area of the ecosystem per day for a specified set of values of the crucial parameters of the system: concentration ([Chl ]) and photosynthetic characteristics (Pm, Ek) of the phytoplankton, penetration of light into the water (Kd), incident radiation (Ed(0+), daylength). All these parameters vary not only throughout the course of the year, but also, taken over the whole year, with geographical location - from one part of the world's oceans to another, from one inland water body to another - because of differences in the average values of the controlling physicochemical parameters of the system such as depth, insolation, nutrient concentration, temperature, optical properties of the water, stability of the water column etc. The consequent geographical variation in annual photosynthetic yield can conveniently be characterized in terms of the total amount of carbon fixed by photosynthesis per m2 of aquatic ecosystem per year. Data for phytoplankton primary production in

Table 11.2 Annual phytoplankton primary production in the oceans. Data from literature surveys by various authors.

Primary production (g C m 2 yr 1)

Continental shelf Deep ocean Reference

Ocean

Indian 259 84 1070

Atlantic 150 102 1070

Northeast Atlantic 203 390

Pacific 190 55 1070

Arctic 50-250 25-55 1164

Southern 184 130 31 Regions within Pacific Ocean

Tropical deep ocean 28 734

Tropical/temperate transition 49 734 zone

Temperate deep ocean 91 734

Continental shelf, temperate 102 734

Inshore coastal, temperate and 237 734 tropical

Peruvian upwelling 1350-1570 1435 Various shelf, coastal and estuarine waters

Irish Sea 101-140 499

Northern Ireland coastal 194 499

Northeast USA coastal 260-505 1210

Chesapeake Bay 347-662 527

Narragansett Bay 323 1026

North Sea 100-300 1504

German Wadden Sea 124-176 1358

Barents Sea 60-80 320

Southern Aegean Sea 25 608

45 estuaries 190 154

various parts of the sea are listed in Table 11.2. Detailed discussions of productivity in various parts of the ocean may be found in Raymont (1980), Cushing (1988) and in the proceedings of the workshop on Productivity of the Ocean: Present and Past, edited by Berger, Smetacek and Wefer (1989).

In addition to the rate of carbon fixation per unit area, the productivity of marine ecosystems can be characterized in terms of the total amount of carbon photosynthetically fixed per year in specific defined ocean regions. This is usually expressed in gigatonnes (Gt, 109 tonnes) C yr-1. Such calculations are carried out using data on the distribution of phytoplankton chlorophyll, sea-surface temperature and incident PAR,

Table 11.3 Total phytoplankton carbon fixation per year in different oceanic regions.

Ocean region

Total carbon fixation (Gt C yr-1)

Reference

Arctic

1.1

826

Atlantic

14.8

826

Pacific

19.7

826

Indian

6.5

826

Eastern tropical Atlantic

2.3

927

(5° N-10° S, 25° W-10° E)

Subtropical and tropical Northeast

0.82

564

Atlantic (5°-40° N, 6°-30° W)

Southern Ocean

Latitude >30° S

14.2

933

Latitude >50° S

1.95

32

Global ocean

CZCS data

45-50

826

SeaWiFS data

52-55

1261

obtained by remote sensing, over the area in question, using an algorithm of the type discussed in the previous section. Some of the published estimates are presented in Table 11.3. The total net oceanic primary production of 45 to 55 Gt yr-1 (Table 11.3) is carried out by an estimated phytoplankton biomass of Gt C, which is only ^0.2% of the photo-synthetically active C biomass on Earth.377 Agawin et al. (2000) estimate that 24% of this phytoplankton biomass consists of picophytoplankton (<2 mm), and that they contribute 39% of the global oceanic production.

The least productive waters are the deep oceans in tropical latitudes. Such waters have a permanent thermocline, which greatly impedes transport of nutrients up from the depths where they are regenerated by mineralization of sedimenting phytoplankton and zooplankton faecal pellets. In the temperate oceans, nutrient levels in the upper layer are restored each winter when thermal stabilization breaks down. The highest rates of production are achieved when the high insolation and temperatures of tropical regions are combined with a plentiful nutrient supply in the form of upwelling water from the depths: the Peruvian upwelling is probably the most productive region of the oceans. Phytoplankton productivity is higher in the shallower waters of the continental shelf, and higher still close inshore, than in the deep oceans, even in tropical latitudes. Nutrient supply from the land, and tidal mixing, are thought to be important factors. Close inshore, recycling of nutrients from the bottom and the impossibility of deep circulation of the phytoplankton (below the critical depth) are also likely to contribute to the enhanced productivity. In one of the world's oceans - the Arctic - total primary production appears to be increasing, because of the increased area of open water resulting from global warming.1029

In the case of inland waters, oligotrophic (nutrient-poor) lakes have phytoplankton primary productivities usually in the 4 to 25 g C m-2 yr-1 range whereas eutrophic (nutrient-rich) lakes typically fix 75 to 700 g C m-2 yr-1,520,1143 although, for the saline Red Rock Tarn in Australia, an annual production of 2200 g C m-2 has been reported.520 Hammer presents an extensive compilation of productivity data for inland waters in the 1980 IBP report.781 Brylinsky's survey of a large number of lakes all over the world for the IBP report indicated that phytoplankton productivity is negatively correlated with latitude. This can reasonably be explained in terms of the diminution of annual solar radiation input with increasing latitude.

We have noted earlier that, by bringing about the transfer of the greenhouse gas, carbon dioxide, from the atmosphere to the deep ocean (the biological pump), marine phytoplankton productivity plays a major role in the regulation of global climate. There is another important mechanism of climate control in which phytoplankton may play a role, namely, cloud formation. Many classes of unicellular marine algae - prymnesiophytes, dinophytes, prasinophytes, some diatoms and chrysophytes854 - produce large amounts of dimethylsulfoniopropionate (DMSP), which acts as an osmolyte. This is released into the sea, either passively by leakage, or actively by zooplankton grazing or viral lysis, and is then broken down to dimethyl sulfide (DMS) by microbial action. The DMS escapes from the ocean, and is the main biogenic source of reduced sulfur to the atmosphere,77 where it is oxidized to sulfate in the form of submicron aerosol particles, which act as condensation nuclei for water vapour, thus promoting cloud formation. It was suggested by Charlson et al. (1987) that the connection between DMSP-producing algae and clouds represents a climate-regulating mechanism.

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