The growth of natural populations of both animals and plants is extremely difficult to measure under natural conditions. For animals, there is no alternative but to estimate the growth rate — for one species at a time — from changes in numbers and weights of individuals in the field, allowing for losses resulting from predation and death. For photosynthetic plants, however, it is possible — and very much easier — to estimate the overall productivity of all of the species in an ecosystem by measuring the uptake or release of inorganic chemicals (e.g. CO2, O2, NO-3 + NH+4) in the environment, and most attention has been concentrated on this approach. These rates of exchange of inorganic chemicals between plants and their environment are correlated with the rates of individual processes that contribute to growth, such as photosynthesis and ion uptake, rather than with growth rate itself, and direct studies of plant growth in the sea have been largely restricted to a few species, mainly kelps, which are particularly convenient for such work (see p. 83). It is, therefore, important to examine the relationships between photosynthesis, nutrient uptake and growth, as revealed by laboratory experiments, before attempting to interpret the seasonal and geographical variations of plant biomass and primary productivity in the sea.
Requirements for growth
The algae are physiologically more diverse than vascular plants, and this diversity is reflected in their nutrient requirements.