148-17 Benthic Foraminiferal Assemblage in a Gravel Beach in Puget Sound, Washington State, USA

Session: Insights from Microfossils and Their Modern Analogs: From Traditional to Emerging Approaches (Posters)

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The distribution of benthic foraminifera in environments 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 distribution of benthic foraminifera in a coarse-grained (sand and gravel) beach in Southern Puget Sound, Washington State, USA. Triplicate samples were collected from 8 stations at Jacoby Park beach, Shelton, Washington. The samples were stained with Rose Bengal at the time of collection. During laboratory processing, each sample was divided into three size fractions:> 1mm, 1mm –125 μm, and < 125 μm. All foraminifera in the 1mm –125 μm fraction were picked out and identified to the species level. Counts of each species were separated into living (stained) and dead (unstained) tests. The Assemblage in this location is comprised of 16 species 9 calcareous and 7 agglutinated. Overall, the samples exhibited a low diversity (5-15 species per sample) with highly variable abundance (16- 7330 total specimens per sample). The assemblage is dominated by 3 species: Ammonia beccarii, Elphidella hannai, and Trochammina pacifica. These species comprise approximately 77% of the total number of specimens. Three additional species, Buccella frigida, Cribroelphidium excavatum, and Deuterammina rotaliformis, comprise another 21% of the assemblage. All other species comprise less than 1% of the total.

 A previous study has documented a relationship between two of the species in the assemblage, E. hannai and D. rotaliformis, and coarse-grained sediments (Shaw and Frederick, 2023). Ammonia beccarii has been identified as an indicator of anthropogenic contamination, including increased organic and nutrient levels. The high percentage of T. pacifica in this site has not been previously noted in Puget Sound intertidal areas and may represent the occurrence of the invasive Trochammina hadai. The presence of abundant Trochammina in this study site may reflect the spread of T. hadai to Puget Sound.

The foraminiferal assemblage of Jacoby Park locality likely reflects a mix of natural and anthropogenic factors and shows that caution must be used in applying foraminiferal distribution in biomonitoring.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11218

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