Global primary production

In Table 5.1, comparisons are made between primary production on the land and in the sea. The concept that open ocean areas have a productivity comparable to that of deserts on the land and much lower than that of coastal areas and upwelling zones, is well established. However, techniques for measuring organic production are constantly being refined and some recent data suggest that there may be two to three times as much organic matter per unit of surface area in the open ocean than previously reported.

Month Month

Figure 5.17 (a) Curve of calculated values of phytoplankton standing crop on Georges Bank. Circles are observed values; (b) Curve of calculated values of zooplankton population on Georges Bank. Circles are observed values.

((a) From Riley (1946), by courtesy of Sears Foundation for Marine Research; (b) from Riley (1947), by courtesy of Sears Foundation for Marine Research.)

Month Month

Figure 5.17 (a) Curve of calculated values of phytoplankton standing crop on Georges Bank. Circles are observed values; (b) Curve of calculated values of zooplankton population on Georges Bank. Circles are observed values.

((a) From Riley (1946), by courtesy of Sears Foundation for Marine Research; (b) from Riley (1947), by courtesy of Sears Foundation for Marine Research.)

5.6.2 Ocean primary production

Most of the primary production in the oceans originates from the phytoplankton. The large seaweeds of shallow water can achieve very high production rates. Estimates in excess of 30 gC/m2/day are reported for Californian kelps whereas 5 gC/m2/day would be high for phytoplankton. However, being limited to a very narrow belt at the sea's margin, the seaweeds contribute probably only about 0.05 per cent of the total production of the sea.

Differences in the fertility of the seas in different localities, and at different times, depend upon the availability of plant nutrients in the surface layers. Certain areas are of consistently high fertility, others are ocean deserts, and in many regions the fertility fluctuates seasonally. The general distribution of primary production in the world's oceans is shown in Figure 5.18. Such maps are the result of many years' sampling. Satellite observations (see Section 5.2.1) can now provide monthly pictures of seasonal variations on a world-wide scale.

The areas of good fertility and high productivity include most of the seas overlying wide continental shelves. There are several reasons why this shallow water is relatively rich in nutrients. Waves erode the coastline and stir up the sediments, releasing nutrients into the water. Fresh water running off the land may carry additional nutrients, including trace elements such as iron and manganese which are often scarce in deep water due to precipitation. Where there are centres of human population, sewage is usually poured into the sea and provides nutrients after decomposition. The tidal flow of the water above the shelf may cause sufficient turbulence to keep the water column well mixed, ensuring that nutrients regenerated below the euphotic zone are quickly restored to the surface, and there

Table 5.1 Comparison of Gross Primary Production on Land and in the Ocean (Source: Odum (1971) modified by Duxbury and Duxbury (1996).)

Ocean and land area

Amount (gC/m2/yr)

Open ocean

Deserts

Grassland

Coastal ocean Forests

Common crops Pastures

Upwelling zones Deep estuaries Rain forests Moist crops Intensive agriculture

Shallow estuaries Sugarcane and sorghum

50 50 50

25-150 25-150 25-150 25-150

150-500 150-500 150-500 150-500 150-500

500-1250 500-1250

is unlikely to be much loss of nutrients to deep levels of the ocean unless shelf water is flowing down the continental slope, as occurs for example in cascading (see Section 5.6.3). Processes leading to enrichment of oceanic surface water near the continental slope have been mentioned earlier (see page 184), and much of this fertile water may be blown over the shelf. From time to time, some of the innumerable deep water pelagic creatures which ascend to the surface during darkness may also be carried over the shelf and augment the food supplies of neritic water. The very dense growth of benthic algae that exists on some coasts contributes greatly to the primary production of these areas.

The fertility of deep water depends largely upon the extent to which water from deep levels is brought to the surface. In temperate areas, the surface layers are well provided with nutrients by convection during the winter and early spring, but the supply of nutrients diminishes during the summer when the formation of a thermocline prevents replenishment by vertical mixing. Upwelling of deep water around Antarctica produces in the Southern Ocean the world's widest expanse of highly fertile open sea. Fertile areas are also produced by upwelling in the currents along the eastern part of the Atlantic, Indian and Pacific Oceans at low latitudes (see page 183 and Section 1.3.3 and Figure 1.6). In the north Indian Ocean, upwelling occurs in the northernmost part of the Arabian Sea during the winter north-east monsoon, and off the Arabian and Somali coasts in summer when the Monsoon Current develops.

Poor fertility occurs where vertical water mixing is minimal. Throughout the tropics, wherever a permanent thermocline is present, production rates are mainly

Moderate productivity (0.1- 0.2 g C/rr^/day) Very high productivity (>0.3 g C/m2/day)

Figure 5.18 The distribution of primary production in the world's oceans.

Moderate productivity (0.1- 0.2 g C/rr^/day) Very high productivity (>0.3 g C/m2/day)

Figure 5.18 The distribution of primary production in the world's oceans.

low despite rapid regeneration. However, because production continues through the year without much seasonal decline, it is probable that the total annual production in most tropical areas substantially exceeds that of temperate seas. The Sargasso is a semi-tropical marine desert area where even horizontal mixing by surface currents is slight. Here the production rate is very poor except during the brief winter period when some convectional mixing may take place.

In the Arctic there is relatively little upwelling compared with the Southern Ocean, and the Arctic is correspondingly less fertile. In the Mediterranean, fertility is low to moderate because nutrients are continually lost in the deep outflow which forms the bottom current through the Strait of Gibraltar. The inflowing surface current is derived from surface levels of the Atlantic which are relatively poor in nutrients.

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