Rates of photosynthesis, whether net or gross, can be expressed per unit biomass (specific photosynthetic rate) or per unit area or volume of the water. Typical units of biomass-specific photosynthetic rate (P*) for a phytoplankton population are mmoles CO2 (or O2) or mg C (carbon), per mg chlorophyll a per h. It is convenient to indicate it as P* (CO2), P* (O2) or P* (C), in accordance with the units being used. If the rate per m3 of water, Pv, is summed for every 1 m depth interval from the surface to the bottom of the euphotic zone, then the areal (or integral) photosynthetic rate, PA, i.e. the rate of photosynthesis in the whole water column beneath 1 m2 of surface, is obtained: its units are moles (or mmoles) CO2 (or O2), or g (or mg) C, m~2
It may be useful to indicate areal photosynthetic rate as PA(CO2), PA(O2) or PA(C), in accordance with the units used. Rates of photosynthesis of benthic macrophytes may be expressed per g dry mass of tissue, or per m2 of the substrate on which they are growing. Rates of photosynthesis of the microbial benthos - diatoms, cyanobacteria etc. on the surface, or within the interstices, of the sediments - are commonly expressed per m2 of the substrate.
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