Environmental change in Cook Inlet: A stable isotope analysis of Pacific razor clams
2025 Research Seed Awards
Professor of Geology and Earth Sciences, Director of MSES
ҹAV Pacific University
Project Overview and Research Question
Pacific razor clams are an important component of the Cook Inlet ecosystem, providing a major food source for marine animals such as sea otters and sea birds. However, razor clam abundance in the eastern Cook Inlet has declined significantly over the past several years. In 2015, all east-side Cook Inlet beaches were permanently closed to sport harvesting. The cause of this decline remains unknown, although habitat modification and shifting environmental conditions have been hypothesized as underlying factors. In collaboration with the ҹAV Department of Fish and Game (ADF&G) in Homer, AK, we propose to utilize the long-term clam shell size-age archive to construct a decadal-scale stable isotope record. Construction of this record will place important constraints on the environmental and hydrologic conditions impacting razor clam habitat in the northern Gulf of ҹAV.
Research Question
How does the stable isotope composition (δ13C and δ18O) of razor clam shells relate to clam age, growth rate, geographic location and year of shell synthesis? Year of shell synthesis (as determined by combining visual age bands with year of collection) can be tied to historical environmental data such as sea surface temperatures (SST).
Our hypothesis is that above-average SSTs in the Gulf of ҹAV/Cook Inlet are contributing to a spatial and temporal match-mismatch between clam spawning, winter storms, and the availability of phytoplankton. These interactions are manifesting as poor recruitment rates, slower clam growth, and smaller clams overall. We also hypothesize that faster clam growth rates will correlate with increased freshwater input, as indicated by a lower δ18O of clam shell carbonate.
To test this hypothesis, we propose to produce a long-term (decadal) δ13C and δ18O dataset from Cook Inlet razor clam shells. Such a record could quantify relationships between clam growth rate, SSTs in the Gulf of ҹAV, and primary productivity. For over 15 years, ҹAV Pacific University (APU) has maintained a collaborative relationship with ҹAV Department of Fish and Game (ADF&G) in Homer, AK. They have provided APU with a large collection of Pacific razor clam shells (hundreds) to analyze, collected from Kenai Peninsula beaches, with an agreement to provide more as needed. Razor clam surveys have been conducted by ADF&G since 1989. With each clam providing a 10-15-year record of growth, this means there is potential for assembling a record of environmental change spanning up to the past 40 years. Utilizing clams collected from across Cook Inlet (both east and west side beaches) will allow us to analyze both spatial and temporal trends in the resulting dataset.
Background
Bivalves living at high latitudes, where water temperatures vary seasonally, undergo seasonal variation in growth. Faster growth occurs with increased food availability and warmer water during summer months, and slower growth occurs with limited food availability and colder waters during the winter months. As nutrients from the water are mineralized into calcium carbonate for shell growth, bivalves retain an isotopic signature within the biogeochemistry of the shell (Yan et al., 2014). Moreover, bivalves typically precipitate their shell in isotope equilibrium with their host waters (Elliot et al., 2003). Therefore the δ18O signature of the shell reflects the composition of the water in which the clams live and grow, which will indicate the degree of freshwater input. Higher values of δ18O would reflect a greater marine component, while lower δ18O will indicate relatively more freshwater input (Epstein and Mayeda, 1953; Fairbanks, 1982). Such measurements may help to explain the difference in growth rates between clams throughout Cook Inlet. Previous work by ADF&G indicates significantly different clam growth rates among eastern Cook Inlet beaches, with the fastest growth rates at Ninilchik and the slowest at Clam Gulch (Szarzi et al., 2010).
Shell δ13C in bivalves is dominantly controlled by environmental conditions, and in aquatic mollusks such as razor clams, the source of the shell carbon is generally attributed to ambient dissolved inorganic carbon (DIC; McConnaughey and Gillikin, 2008). As such, δ13C values in biogenic carbonate, while sometimes complicated by vital effects, can be interpreted to reflect nutrient upwelling and productivity (Krantz et al, 1987). Ocean conditions may have a strong effect on clam growth and survival, as temperature is known to influencing the timing of spawning and maturation rates (Weymouth et al., 1925). Sea surface temperatures (SST) also impact primary productivity, creating a potential temporal and spatial predator-prey mismatch. The Gulf of ҹAV (GOA) ecosystem recently experienced anomalously high SSTs, instigating widespread effects that are still cascading through the ecosystem. The largest decline ever surveyed of Pacific cod (Gadus inacrocephalus), toxic phytoplankton blooms, and marine mammal and seabird die-offs have been observed in the GOA in recent years. These have been attributed to a period of above-average SSTs, nicknamed “the blob” (Cavole et al., 2016; Walsh et al., 2018).
Methods
We currently have 109 clam shells in our APU collection, acquired from ADF&G and ranging in age from 2 to 16 years old. These clams were collected from beaches located on the eastern side of the Kenai Peninsula, bounded by the Kasilof River to the north and the Anchor River to the south. In addition to the Kasilof and Anchor Rivers, the Ninilchik River and Deep Creek flow from large drainages off the Kenai Peninsula and into Cook Inlet. Shell length and length-to-each annuli were measured by ADF&G using calipers at the longest axis. Additional shells will be requested from ADF&G as needed.
Carbonate will be extracted from each shell for carbon and oxygen stable isotopic analysis at the ҹAV Stable Isotope Facility (ASIF) at the University of ҹAV Fairbanks (UAF). To expose the carbonate layer, clam shells will be first bleached and rinsed to aid in the removal of the periostracum (outermost organic layer; Szarzi et al., 2010). Carbonate powder will be extracted from each shell using a 4200 Dremel Drill attached to a Dremel drill press, equipped with a 300 μm carbide tungsten steel micro drill bit. Shells will be sampled along a transect from the outer aspect towards the umbonal area. The resulting carbonate powder will be collected into small sample vials for isotopic analysis. Subsamples of carbonate (0.5 - 1.0 mg) will be analyzed for δ18O and δ13C at ASIF. We aim to analyze 160 subsamples from at least 15 different shells from varying locations (~10 samples/shell depending on clam age, allowing for some duplicates).
Resulting δ13C and δ18O transects from all clam shells will be analyzed for spatial and temporal trends across Cook Inlet sample sites. These isotope profiles will be plotted by year and compared to historical data (e.g., clam abundance, ocean temperature) collected by other agencies (e.g., ADF&G, NOAA). Maximum δ13C, minimum δ13C and average δ13C from each site will also be compared to calculated growth rates to determine if there is a relationship. We will also compare δ18Oshell, as a proxy for freshwater contribution, with clam growth rate. If there is a correlation, as hypothesized, this could help explain spatial trends in recruitment and abundance, indicating habitat modification could be a major factor influencing clam mortality.
Alignment with Interface of Change goals
This project aligns with the goals of Interface of Change by generating data that will help understand the environmental and hydrologic conditions impacting razor clam habitat in the northern Gulf of ҹAV/Cook Inlet. This understanding is critical to the long-term sustainability of the Kenai Peninsula razor clam fishery, and can advise future management decisions regarding ҹAV’s largest sport and personal use razor clam fishery. Specifically, this project focuses on quantifying nutrient delivery to clam habitat via riverine inputs, which are a key land-to-ocean linkage potentially influencing clam survival. Stable oxygen isotope composition (δ18O) of clam shells will serve as a proxy for freshwater input, while carbon isotope composition (δ13C) will provide insights into nutrient sources and marine productivity.
This project also fits within the vision of Interface of Change by using scientific research to aid Gulf of ҹAV coastal communities, Homer and Ninilchik, in their understanding of the decline of the Pacific razor clam. It is likely that climatic and environmental changes over the past several decades are a major factor in the decline. In order for these communities to build resilience and adapt to future climate change, we must first characterize the present challenges to razor clam critical habitat.
Sharing Project Outcomes
The PI and/or students will attend the ҹAV Marine Science Symposium (AMSS) in January 2026 to present research results and participate in a science communications workshop. This symposium is held locally in downtown Anchorage and will require no travel resources. The audience for AMSS includes scientists, policy managers, coastal community leaders, educators and industry professionals. Additionally, we will take advantage of the marketing resources here at APU to produce blog posts and newsletter articles, keeping the community updated on all scientific progress, student presentations and newsworthy findings.
Continued Support for Research
Using results obtained from this seed research grant, the PI will apply for additional awards from a major funding agency such as NSF or NPRB. The proposed research would be appropriate for NPRB’s Core Program, which is focused on fishery management issues and marine ecosystem science. Another potential program is NSF’s CREST-RISE (Centers of Research Excellence in Science and Technology – Research Infrastructure for Science and Engineering), which is specifically for Minority-Serving Institutions (MSIs), of which APU is designated. This program provides support to enhance the research capabilities of MSIs, promoting the development of new knowledge and enhancements of the research productivity of individual faculty. The PI would begin to apply for these awards near the end of this project period, i.e., spring 2026.
References
Cavole, L.M., Demko, A.M., Diner, R.E., Giddings, A., et al., 2016, Biological Impacts of the 2013—2015 Warm-Water Anomaly in the Northeast Pacific: Winners, Losers, and the Future. Oceanography, 29(2). https://doi.org/10.5670/oceanog.2016.32 Elliot, M., deMonocal, P.B., Linsley, B.K, and Howe, S.S., 2003, Environmental controls on the stable isotopic composition of Mercenaria mercenaria: Potential application to paleoenvironmental studies. Geochemistry, Geophysics, Geosystems. 4(7), 1056, doi:10.1029/2002GC000425.
Epstein, S., and T. Mayeda, 1953, Variation of 18O content of waters from natural sources. Geochimica Cosmochimica Acta. 4: 213- 224.
Fairbanks, R.G., 1982, The origin of continental shelf and slope water in the New York Bight: Evidence from 180/160 ratio measurements. Journal of Geophysical Research. 87: 5796-5808.
Krantz, D. E., D. F. Williams, and D. S. Jones, 1987, Ecological and paleoenvironmental information using stable isotope profiles from living and fossil molluscs, Palaeogeography Palaeoclimatology Palaeoecology, 58, 249 – 266.
McConnaughey, T.A., and Gillikin, D.P., 2008, Carbon isotopes in mollusk shell carbonates, Geo-Marine Letters, 28: 287. https://doi.org/10.1007/s00367-008-0116-4
Szarzi, N.J., Hansen, P.A., and Hasbrouck, J.J., 2010, Harvest, Abundance, Age and Length Characteristics of Razor Clams from Eastern Cook Inlet Beaches, 1993 – 2003. ҹAV Department of Fish and Game, Fishery Data Series No. 10-49, 81 pp. Weymouth, F.W., H.C. McMillan, and H.B. Holmes, 1925, Growth and age at maturity of the Pacific razor clam Siliqua patula (Dixon). U. S. Department of Commerce, Bureau of Fisheries Bulletin 41:201-236.
Walsh, J.E., Thoman, R.L., Bhatt, U.S., Bieniek, P. A., Brettschneider, B., et al., 2018, The High Latitude Marine Heat Wave of 2016 and Its Impacts on ҹAV. Bulletin of the American Meteorological Society, 99 (1), S39-S43. https://doi.org/10.1175/BAMS D-17-0105.1
Yan, H., Chen, J., and Jun, X., 2014, A review on bivalve shell, a tool for reconstruction of paleo-climate and paleo-environment. Chinese Journal of Geochemistry, 33. doi:10.1007/s11631-014-0692-0.