Grazing rates of mixotrophic nano- and dinoflagellates in the Northern Gulf of Alaska in response to gradients in light, inorganic nutrients, and prey availabilities

Experimental studies of mixotrophic dinoflagellates and nanoflagellates have shown complex relationships between environmental factors and grazing rates. Hypotheses concerning mixotrophy have often focused on feeding as a mechanism for additional energy acquisition in times of low irradiance or for supplemental nutrient intake when dissolved inorganic nutrients are scarce. This has been confirmed by some studies while others have shown the opposite or no significant effect, indicating that mixotrophic behavior may have multiple evolutionary drivers and is likely species- and/or ecosystem-specific.

In the Northern Gulf of Alaska, understanding how environmental conditions influence food web dynamics is particularly important due to the highly variable nature of the region. A high abundance of mixotrophs have been observed across the shelf but have not previously been quantified or studied under various environmental conditions. Experiments exposing the natural plankton community to gradients in the availability of light, inorganic nutrients, or prey were conducted at various sites across the Northern Gulf of Alaska in the context of the Long-Term Ecological Research program in that region; grazing rates on added Synechococcus sp. were obtained. Rates and results from individual experiments were interpreted with respect to large-scale environmental gradients and species presence and abundance varied across sites. Preliminary results indicate that, across mixotrophic flagellate taxa, light was the strongest dictator of mixotrophic behavior, with decreased feeding occurring at lower irradiance levels. Inorganic nutrient concentrations did not have an effect on grazing for the dinoflagellate or nanoflagellates groups observed. The grazing rates and functional responses obtained in this study will be used to inform modeling efforts of the region, increasing our understanding of how mixotrophy may influence key ecosystem properties and ecosystem resilience.