There are several ways in which estimates of the amount of primary production in the sea have been attempted, and certain of these methods are outlined below (UNESCO, 1973; Vollenweider, 1969). All these techniques are inherently inaccurate and estimates of production will always be just that - estimates. Production rates are usually expressed as the weight of carbon fixed in organic compounds beneath unit area of sea surface in unit time; for example grams of carbon per square metre per day or year ( gC/m2/day; gC/m2/year) or sometimes as weight of carbon fixed in unit volume of water in unit time, for example gC/m3/day. Estimates of net primary production by phytoplankton mostly fall within the range 0.05-0.5 gC/m2/day with values as high as 5 gC/m2/day in the most productive sea areas. Differences in productivity between various ocean regions are discussed in Section 5.6 where comparisons are also made between production on land and in the sea.
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