SEDIMENT GRAIN SIZE AND DISTRIBUTION OF THE BENTHIC FORAMINIFERA DEUTERAMMINA ROTALIFORMIS AND ELPHIDIELLA HANNAI, PUGET SOUND WASHINGTON

The distribution of benthic foraminifera in environments with is controlled by both abiotic and biotic factors. Abiotic factors include grain size, sediment oxygen, nutrient concentration, salinity, and temperature (Murray, 2009). These factors are often covariant, sediment oxygen tends to increase with grain size. Conversely organic content tends to decrease with increased grain size. Biotic control on benthic foraminiferal distribution includes food availability, predation, competition and reproduction (Murray, 2009). This study examines the relationship of two species of benthic foraminifera to sediment grain size in Puget Sound, Washington, USA

Intertidal samples collected in Southern Puget Sound, Washington State, USA were examined for benthic foraminifera. Sample processing divided the sediment into three size fractions > 1mm, 1mm – 125 um, and < 125 um. Two species demonstrated a positive correlation with increasing grain size. Deuterammina rotaliformis and Elphidiella hannai both generally increase in abundance with an increase in grain size in the study area. Deuterammina rotaliformis is an agglutinated species found in a wide range of localities and water depths. Elphidiella hannai is a calcareous species that is common in intertidal to shelf depths. Both species are common in subtidal locations in Puget Sound.

In the samples studied D. rotaliformis occurs sporadically in samples of a range of grain sizes. However, it is uncommon in samples containing more than 40% of grains < 125 um (5 of 20, avg number specimens 1). Additionally, D. rotaliformis tends to be more abundant in samples having high percentages of grain > 1mm. E. hannai does not occur in any sample containing more than 40% <125 um fraction and is rare in samples with more than 30%. E. hannai is most abundant in samples containing R. rotaliformis is an attached species, thus requiring grains of sufficient size for attachment. As a result, the distribution of R. rotaliformis appears to be controlled by the presence of suitable attachment grains. Controls on the distribution of E. hannai are less clear and may reflect other covariant factors such as Oxygen content or food availability.

References

Murray, J.W., 2006. Ecology and Applications of Benthic Foraminifera. Cambridge University

Press, New York, p. 426.