Growth rate as a measure of primary productivity in benthic algae

Primary productivity is considered to be the production of organic matter from inorganic matter and the rate of primary production is the amount of organic matter produced per unit time. Gross production is the total amount of organic matter produced and net production is the total production less the amount lost to respiration (Odum and Odum, 1959). Many techniques have been evolved to estimate indirectly rates of primary production. Some of these methods are growth rate and harvest of standing crop, gas exchange rates, radioactive isotope uptake, and chlorophyll concentration. It is the purpose of this paper to consider measurements of growth rates as approximations of primary production in benthic marine algae. The parameter of growth is defined in many ways, and some that have been used are irreversible increases in linear dimensions, dry weight, surface area increase, and cell numbers. The usual estimates of growth in laboratory cultures of unicellular algae arc cell counts, optical density of cultures, total dry weight, and total cell volume (Boalch, 1961). Total dry weight is the only parameter of growth that can ea.~ily be used for multicellular algae. Thus, Boalch (1961) used an increase in dry weight to measure growth in laboratory cultures of Ectocarpus. Dry weight estimates require the destruction of the organism, and thus they are really harvest techniques. The nondestructive techniques for estimating continuous growth rates have evolved around measurements of changes in plant size and they can be used in situ as well as in the laboratory. Neu.-hul and Haxo (1963) have estimated the growth rate of young Macrocystis plants by measuring the change in the blade surface area as a function of time. Growth of Halimeda in laboratory culture was measured by linear increase iu thallus length (Colinvaux et al., 1965). as was growth of Gracilaria cultured on rafts (Jones 1959a, b). Chapman (1961) measured growth in the fronds of Egregia laevigata by measuring the increasing distance between two marks on the stipe. Increases in dry weight per unit time is an estimate of growth. It is also a measure of primary production if the increase in dry weight is organic matter. Since an increase in weight is not always a result of an increase in organic matter, an improvement might be made by taking both dry weight and ash-free measurements (Clendenning, 1962). Harvest techniques for measuring primary production have other drawbacks. A harvest sample for dry weight determination is taken from the standing crop and represents the organic matter that has neither been respired nor consumed. Thus, it represents something less than net production. If independent measurements of respiratory rates are made, better estimates of primary production are obtained but one still has little hope of getting reliable estimates of gross primary production unless consumption by grazers can be estimated (Odum and Odum, 1959). Leighton, et al. (1966) have reported on the effects of grazers in giant kelp beds. All techniques that depend on harvest of standing crops suffer from this difficulty. They are only useful in situations where herbivores are not present and a steady state condition is never reached. This is not the case in corn-