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Widespread ciliate and dinoflagellate mixotrophy may contribute to ecosystem resilience in a subarctic sea: the northern Gulf of Alaska

Suzanne L. Strom*, Kelley J. Bright, Kerri A. Fredrickson

*Corresponding author:

ABSTRACT: Mixotrophy among ciliates and dinoflagellates in the northern Gulf of Alaska (NGA) was widespread during spring and summer, with mixotrophs contributing a median of 38 to 61% of total ciliate+dinoflagellate biomass depending on season and year. The proportional contribution of mixotrophs was higher during a heatwave year (2019) than during a year of average temperatures (2018). The most common mixotrophic ciliates included Mesodinium spp. and several of eight observed Strombidium species, whilst for dinoflagellates, the most common mixotrophs were Gymnodinium-like cells and Tripos (formerly Ceratium) spp. Onshore-offshore distribution gradients were seen mainly in summer when elevated freshwater inputs create a horizontal salinity gradient. A nearshore mixotroph assemblage consisted of nutritionally related Mesodinium and dinoflagellate Dinophysis, as well as Tripos spp., while an offshore assemblage included Tontonia-like ciliates and dinoflagellates including Gymnodinium-like cells and Torodinium robustum. An 11-year time series with lower taxonomic resolution revealed seasonality in some taxa and showed near-complete loss of Mesodinium and Tontonia-like species during the longer 2014-16 North Pacific marine heatwave. The constellation of nutritional strategies represented by NGA mixotrophs may be an important component of lower trophic level resilience to marine heatwaves, while high mixotroph contribution to ciliate+dinoflagellate biomass may increase trophic transfer efficiency and contribute to high fisheries yields.