Phosphorus has a major role in determining
productivity of plankton communities.
The pioneer experiments on the role of phosphorus in aquatic communities were
carried out by introducing radiophosphorus (32P), which was then followed as a
community-level tracer.
Such experiments can be done in the laboratory,
using either a plankton-and-water
sample from a natural aquatic community or the small-scale community that
develops through some weeks in an aquarium provided with water and nutrients
and an initial seeding with pond organisms. Such systems are called microcosms.
For example, Into a 200-liter aquarium
microcosm of the latter sort 100 microc of
32P-labeled phosphate (as phosphoric acid) were introduced. After the tracer
introduction the movements pattern showed the following sequence of four phases
1. There was initial, very rapid movement
from water into plankton organisms and
turnover from these back to water. In the experiment illustrated one half of the 32P
had moved into the plankton (predominantly single-celled green algae) within two
hours, and by twelve hours the distribution of 32P was in a steady-state balance
between plankton and water. Other experiments, using millipore filters for fuller
separation of small plankton cells and particles with bacteria from the water, have
shown even faster uptake (up to half the amount in the water in three minutes) and
balance (93 per cent in the plankton and particles after twenty minutes). Turnover
rates (in fractions of the 32P in the plankton returning to the water per unit time)
were 0.27/hr and 0.013/min for the two experiments. These rapid movements of
32P are affected by adsorption onto the surfaces of cells as well as absorption
through those surfaces. The mass of the plankton is very small in relation to the
water in which it is suspended. With the greater part of the phosphate in this small
mass, the concentration of the 32P is many thousand times as high in the plankton
as in the water. Concentration ratios (expressed as 32P content per unit dry mass
of organisms divided by 32P content of an equal mass of water) are in some
cases, with low phosphate content in the water, of the order of one to two million-
fold.
2 Somewhat more slowly through the first
few hours, the 32P moved into the
filamentous algae growing on the sides and on the mud-containing bottom trays of
the aquarium. Maximum 32P content per unit mass and approximate equilibrium
with the water were reached for attached algae within twenty-four hours. (In general,
the larger the organism, the larger the mass or pool of phosphate it contains, the
lower the turnover rate for this pool, the slower the uptake per unit mass, the later
the equilibrium is reached, and the slower the decline from that equilibrium if the
experiment is continued long enough for such decline to occur.) As a substantial
part of the 32P moved into the attached algae, the 32P in the water-and-plankton
together declined. Even though equilibrium of 32P content per unit mass of algae
and water was reached during the first day, the total content of 32P in attached
algae continued to increase, because of the growth in mass of the algae, until the
sixth day. Thereafter, because of the turnover of 32P between algae and water,
there was net movement of 32P out of the algae as 32P levels in the water declined.
3. The tracer moved into animals (water
fleas grazing the plankton algae, snails
grazing the sidewall algae) more slowly than into their food, at rates that varied with
size, food habits, and other characteristics of the animals. From the grazing
animals the 32P reached the carnivorous animals (fish feeding on water fleas).
Although the rate of uptake decreases along a food chain, the concentration ratios
in animals may be very high.
4. Through the latter part of the experiment
an increasing fraction of the tracer
moved into the bottom mud, the sediment (of recently settled plankton and animal
feces), and the film of microorganisms on the aquarium walls. Some turnover of
32P between these three parts of the ecosystem and the water continued. There
was, however, net movement of 32P "downward"—out of the water and active
circulation between water and organisms, and into the less active or bound forms of
the sediment, mud, and surface films. By gradual accumulation three quarters of the
32P had moved into these pools of less active turnover by the end of the
experiment, forty-five days after tracer introduction.
The tracer approach can be applied with
isotopes other than 32P, but such work is
limited as yet. The 32P transfer patterns are representative in broad features of the
ways other substances circulate, but details for other isotopes and compounds
necessarily vary. It is of interest to see whether the patterns of 32P movement in
lakes are similar to those in aquaria; in general they are. 32P introduced into the
surface waters of a lake is rapidly taken up by plankton, particles, and bacteria,
while only a small amount remains in the water. There is a slower, longer-term
movement of 32P into rooted plants along the shores of the lake, into animals, and
downward into the sediments. The plants are analogous to the side-wall algae of
the aquarium, but phosphorus movement through these higher plants appears to
involve two rates: a faster uptake and release by the diatoms and other
microorganisms forming the surface film on the plants, and a slower turnover
through the tissues of the plants themselves.
