Spatial links of phytoplankton abundance and community structure between shelf and slope waters off Oregon, USA

Abstract

Abstract Coastal upwelling is known for inducing nearshore primary production, which can be advected to the offshore environment through offshore Ekman transport. However, this nearshore−offshore phytoplankton link has been less characterized at the community level. This study focuses on two shelf and slope sites off Oregon during years of 2013−2017 to examine the shelf−slope covariations in diatom and dinoflagellate abundance and community structure at seasonal and annual timescales. In general, the temporal changes of diatom and dinoflagellate abundance were coherent between shelf and slope. However, some differences were also noticeable. For example, diatom and dinoflagellate bloom magnitudes were higher in 2017 and 2014 on the shelf, but were higher in 2014−2016 at the slope. Diatom abundance had a stronger shelf−slope correlation than dinoflagellate correlation particularly in spring. The community structure was significantly correlated between shelf and slope during the annual upwelling period and in spring season. Ekman transport and nutrients played a significant role in shaping the diatom abundance and community structure temporal patterns at the slope. This shelf−slope correlation study infers that the benefits of nearshore upwelling blooms to the offshore food chains are season dependent and under high interannual variability.