- The distribution and abundance of the interstitial acoel turbellarian,
Diatomovora amoena Kozloff, 1965 was studied in an
estuarine intertidal sand flat in Yaquina Bay, Oregon, from May
1970 through May 1971.
Monthly measurements of biological (organics, sulfides, chlorophyll,
and carotenoids), pore water (salinity, pH,oxygen, and temperature),
and sediment (fine sediment percentage, grain size,
sorting, skewness, and kurtosis) factors were made along a transect
at four intertidal stations with elevations of -2.0, 0.0, 1. 6, and 3.0
feet, stratified by selection from a curve for tidal exposure, and at
two depths (0, 0 cm to 0. 5 cm and 0. 5 cm to 1. 0 cm) of the sediment.
Estuarine factors that were monitored continuously included temperature,
salinity, tide elevation, and insolation. The interrelationships among these environmental parameters, their roles in the interstitial
sediment system, and the hydrology of the groundwater in the beach,
were investigated in order to characterize the interstitial environment
of the sand flat and to determine the environmental factors limiting
the distribution and abundance of D. amoena.
Seasonality was indicated in most of the factors measured. The
sediment system was strongly reducing during summer and fall as
organic production increased. Particle size analyses showed that
transport and deposition of fine sediments contributed to the development
of reducing conditions.
The properties of the interstitial environment of D. amoena
were found to be controlled by the level of groundwater, rate of
percolation, and degree of mixing within the beach. Density of
D. amoena was highest during the fall and early winter, and lowest
throughout the winter.
Summer production of plant material in the lower intertidal lead
to reducing conditions at the sediment surface. Reduction in animal
density at the lower two stations was attributed to these reducing
conditions, and to the rafting of animals away from the intertidal with
the algal mat. Decrease in animal density in the upper two stations
was attributed to lethal low temperatures and salinities that occurred
during heavy precipitation in the winter and coincided with low tidal exposure. Exclusion of animals from depths greater than 0. 5 cm in
the sediment was attributed to lethal levels of sulfide.
Tolerance of D. amoena to temperature, salinity, and sulfide
was determined experimentally. The 25 combinations of temperature
and salinity, and the 12 combinations of temperature and sulfide that
were employed were selected on the basis of actual levels measured
in the study area. The temperature and salinity survival results were
fitted to a response surface which was used to evaluate these factors
in limiting animal distribution.
Survival of acoels was independent of temperature up to 6 hours
of exposure, and strongly temperature-dependent after 24 hours of
exposure. Initial mortality was attributed to osmotic stress.
Upon exposure to sulfide at 50 μgm S/ml, these animals did not
survive beyond 6 hours, demonstrating that sulfide in high concentrations
is toxic to D. amoena. At lower concentrations of sulfide
(10 μgm S/ml), the acoels were able to live for over 20 hours. Lowering
the temperature at both concentrations helped to prolong the
lives of the animals.
Levels of sulfide similar to those used in the experiment Levels of sulfide similar to those used in the experiment
(10 μgm S/0.5 cm³ = 50 μgm S/ml.) were found at the sediment surface
in the lower two stations during September, at which time animal
density was found to be decreasing. At the upper two stations in
September, where the level of sulfide was 3 μgm S/0.5 cm³, the acoel
population was found to be increasing, thus bearing out the assumption
that population density is, indeed, affected by sulfide.
The nature of the interstitial sediment system as a habitat for
meiofaunal organisms was explored and the role of the groundwater
hydrology, as a buffer against seasonal variation in the estuary, in
maintaining this system was examined. A portion of the littoral shore
considered in this investigation was conceptualized as a factor model,
the principal parts of which were selected for study. The seasonal
cycles of the major input factors were found to be relatively stable
from year to year, while the timing of these cycles varied. The
numerous positive and negative correlations that were found within and
between the biological, pore water, and sediment groups of factors
indicated the multiplicity of direct and indirect interactions and supported
the contention that the tidal flat is a complex interrelated
Change in one or more of the major factors, such as precipitation,
river runoff, sedimentation, or tidal prism, can be expected
to have diverse effects on the littoral sediment environment.