Seasonal trophic resource partitioning by Pacific oyster Crassostrea gigas and Pacific blue mussel Mytilus trossulus in an Alaskan estuary

Shellfish mariculture has expanded in Alaska, yet potential ecological interactions between cultured and native taxa have rarely been considered. Shellfish such as the Pacific oyster Crassostrea gigas and the Pacific blue mussel Mytilus trossulus are already farmed in Alaska at a small scale, with production expected to increase in coming years. C. gigas and M. trossulus selectively feed on naturally occurring particulate organic matter (POM) sources; thus, understanding the partitioning of trophic resources by these bivalves has implications for mariculture site selection and marine management. We identified temporal patterns in food source consumption by C. gigas and M. trossulus in Jakolof Bay, Alaska. We estimated endmember contributions (phytoplankton, macroalgae, terrestrial organic matter, and zooplankton) to the suspended POM pool and to the 2 bivalves using bulk stable isotopes (δ 13 C and δ 15 N) in April, June, August, and December 2023. While δ 13 C values remained comparable for both bivalves across sampling periods, C. gigas had higher δ 15 N values than M. trossulus , indicating that although both bivalves ultimately derive carbon from similar sources of primary production, C. gigas feeds at a higher trophic level. Mixing model results indicate that macroalgae contributed 4-58% of the assimilated carbon in C. gigas and 25-75% in M. trossulus . These results suggest that farming shellfish in areas with macroalgae populations (i.e. farmed in co-culture or wild) could offer C. gigas and M. trossulus an additional source of organic matter.