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

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 plus 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 8 observed Strombidium species, while 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 spp. 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 yr time series with lower taxonomic resolution revealed seasonality in some taxa and showed near-complete loss of Mesodinium spp. and Tontonia-like species during the longer 2014-2016 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 plus dinoflagellate biomass may increase trophic transfer efficiency and contribute to high fisheries yields.