Diel vertical migration rates of the dinoflagellate species Margalefidinium polykrikoides in a lower Chesapeake Bay tributary

<i>Margalefidinium polykrikoides</i> is a mixotrophic dinoflagellate harmful algal bloom (HAB) species that blooms annually in the lower Chesapeake Bay. <i>M. polykrikoides</i> undertakes a diel vertical migration (DVM) which may give it a competitive advantage over purely phototrophic organisms who cannot access deeper nutrient pools and allow it to form large toxic blooms. Laboratory-based estimates of <i>M. polykrikoides'</i> DVM rates suggest that it is one of the fastest known dinoflagellate swimmers and understanding this behavior is likely important for modeling and predicting <i>M. polykrikoides</i> blooms. However, to date, no field-derived estimates of <i>M. polykrikoides'</i> DVM rates have been made in the Chesapeake Bay. In this study, we conducted four targeted field experiments to investigate the DVM of <i>M. polykrikoides</i> in the Lafayette River, a sub-tributary of the Chesapeake Bay. Vertical profiles of chlorophyll <i>a</i> fluorescence collected at least every 2 h over diel periods were used to track the DVM of <i>M. polykrikoides</i> during blooms. The maximum observed DVM rate for <i>M. polykrikoides</i> was 2.5 m h^-1, with mean DVM rates around 1.3 m h^-1 for both ascents and descents. As in studies from other regions, our results show that <i>M. polykrikoides'</i> ascent to/descent from the surface initiates before sunrise/sunset, suggesting phototaxis is not the primary trigger of their DVM. However, unlike in other studies where <i>M. polykrikoides</i> was observed to modulate its DVM to avoid excessively warm temperatures (≥30°C), we do not observe active thermotaxic avoidance, despite ambient temperatures exceeding their optimal threshold.