Encyclopedia of Environmental Science and Engineering, Volume I and II

113

Primary productivity in aquatic systems, like the same

process in terrestrial environments, provides the base of

the food web upon which all higher levels of an ecosystem

depend. Biological productivity is the increase in organic

material per unit of area or volume with time. This addi-

tion of organic matter is the material from which the various

plant and animal communities of an ecosystem are made,

and is dependent on the conversion of inorganic matter

into organic matter. Conversion is accomplished by plants

through the photosynthetic process. Plants are therefore

considered to be the primary producers , and in an aquatic

ecosystem these plants include algae, bacteria, and some-

times higher plants such as water grasses and water lil-

lies. Primary productivity , the first level of productivity in

a system, can be measured as the rate of photosynthesis,

addition of biomass per unit of time (yield), or indirectly

by nutrient loss or a measure of respiration of the aquatic

community.

METHODS OF STUDY

Standing crop refers to the part of biological production

per unit area or per unit volume that is physically present

as biomass and that is not lost in respiration. Standing crop

measurements over a period of time give an indirect mea-

sure of productivity in terms of yield. Plankton, microscopic

floating plants and animals, can be collected in a plankton net

and may be counted under a microscope or weighed. Aquatic

biologists have used standing crop measurements to estimate

productivity longer than any other method (e.g. Lohman,

1908). This method is still also used for periphyton (attached

algae) or rooted plants.

Only within the past few decades have biologists pro-

gressed from merely counting numbers of organisms to

calculating biomass, and more recently, to expressing bio-

mass yield. Fishery biologists, like farmers, for many years

have measured fish productivity in terms of tons produced

per acre of water surface per year. Calculating biomass and

biomass yield is an important step forward since changes in

standing crop reflect the net effect of many biological and

physical events and therefore are not directly proportional

to productivity. For example, the standing crop of a phyto-

plankton community may be greatly diminished by preda-

tion and water movement, while photosynthetic rates of the

survivors may remain high.

The measurement of plant pigments such as chlorophyll a

is also a standing crop measurement that is frequently used

and may now be done through remote sensing by aircraft or

satellites.

UPTAKE OF NUTRIENTS

Another early attempt at measuring the rate of production in

aquatic ecosystems was to measure the inorganic nutrients

taken up in a given system and to calculate the amount of

biological production required to absorb this amount. Atkins

(1922, 1923) studied the decrease in carbon dioxide and

phosphate in measuring production in the North Sea, and

Steel (1956), also working in the North Sea, estimated the

annual cycle of plant production by considering changes in

the inorganic phosphate in relation to vertical mixing of the

water mass. Many biologists consider phosphorus to be a

difficult element to study in this respect because organisms

often store it in excess of their requirements for optimum

growth.

Measuring nutrient uptake in an indirect method of deter-

mining the rate of productivity in an aquatic ecosystem and is

influenced by various other biological activities. Nevertheless,

it has been important in the development toward more precise

measurements of the dynamic aquatic ecosystem.

MEASUREMENTS OF OXYGEN AND CARBON

DIOXIDE

The net rate at which the phytoplankton community of a

given ecosystem incorporates carbon dioxide may be esti-

mated in moderately to highly productive aquatic environ-

ments by direct measurement of the short-term fluctuations

in the dissolved oxygen it produces. The calculations are

based on the assumption that a mole of oxygen is released

into the environment for each mole of carbon dioxide reduced

in photosynthesis. This method precludes the necessity of

enclosing the phytoplankton in a bottle. If measurements are

made at regular hourly intervals over a 24-hour period, the

average hourly decrease in oxygen during times of darkness

when no photosynthesis is occurring can be determined. It is

assumed that respiration removes this amount of oxygen each

hour throughout the day thus giving a measure of the gross

rate at which the community incorporates carbon dioxide.

AQUATIC PRIMARY PRODUCTION

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