Where production is seasonal, estimates of production can be based on measurements of the decrease of nutrients in the water during the growing period. In temperate areas, concentrations of nitrate and phosphate in the surface layers reach a maximum during the winter months when photosynthesis is minimal and convectional mixing is occurring. The concentrations fall during the spring and summer due to the absorption of these nutrients by the phytoplankton. The Nor P-content of the phytoplankton being known, measurements of the reduction of nitrate and phosphate in the water enable estimates to be made of the quantity of new plant tissue formed. Allowances are necessary for the regeneration of nutrients within the photosynthetic zone by decomposition of tissues, replenishment from deeper water, and utilization of other sources of nitrogen, for example nitrite, ammonium or organic nitrogen. Production estimates can also be based on measurements of change of oxygen, carbon dioxide or silicate content of the water.
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