Abstract:
During the summers of 1956 and 1957 an investigation was made of
certain ecological relations involving light in Duck Pond, Wellfleet,
Massachusetts, because of the unusually high clarity of the water. The
maximum transparency observed (extinction coefficient, k = 0.11) was far
greater than most ponds and about equal to that in the slope water
beyond the continental shelf off the Atlantic coast. The illumination
reaching the bottom of the pond at 18 m was 11% of the surface light and
made possible a thick growth of Sphagnum at that depth. Phytoplankton
was scarce and consisted mostly of minute forms. The zooplankton, which
ranged in abundance up to 78 organisms per liter, consisted almost
entirely of one species of copepod, Diaptomus minutus Lillj. Quantitative
sampling of this population by means of a pump at a series of depths and
at various hours of the day revealed a partial migration of this species
from near the bottom to the surface at sunset on one occasion, but no
large fraction of the population carried out a vertical migration on 4
other sunset periods or 2 sunrise periods subsequently studied. On
certain of these occasions, however, there was a slight but detectib1e
movement of the animals toward the surface at sunset followed by a
redistribution to deeper levels. At sunrise the animals showed a tendency
to move at first toward the surface and then away from it, although
on one occasion the population remained quite evenly distributed at all
levels. It is pointed out that because of the high transparency those
zooplankters living in the pond are able to withstand high illumination
at all depths. Relations between the extreme water clarity and the activities
of the zooplankton, as well as other unusual features of the pond
are discussed.
