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Abstract Seasonal sea ice impacts Arctic delta morphology by limiting wave and river influences and altering river‐to‐ocean sediment pathways. However, the long‐term effects of sea ice on delta morphology remain poorly known. To address this gap, 1D morphologic and hydrodynamic simulations were set up in Delft3D to study the 1500‐year development of Arctic deltas during the most energetic Arctic seasons: spring break‐up/freshet, summer open‐water, and autumn freeze‐up. The model focused on the deltaic clinoform (i.e., the vertical cross‐sectional view of a delta) and used a floating barge structure to mimic the effects of sea ice on nearshore waters. From the simulations we find that ice‐affected deltas form a compound clinoform morphology, that is, a coupled subaerial and subaqueous delta separated by a subaqueous platform that resembles the shallow platform observed offshore of Arctic deltas. Nearshore sea ice affects river dynamics and promotes sediment bypassing during sea ice break‐up, forming an offshore depocenter and building a subaqueous platform. A second depocenter forms closer to shore during the open‐water season at the subaerial foreset that aids in outbuilding the subaerial delta and assists in developing the compound clinoform morphology. Simulations of increased wave activity and reduced sea‐ice, likely futures under a warming Arctic climate, show that deltas may lose their shallow platform on centennial timescales by (a) sediment infill and/or (b) wave erosion. This study highlights the importance of sea ice on Arctic delta morphology and the potential morphologic transitions these high‐latitude deltas may experience as the Arctic continues to warm. 

ABSTRACT Eastern boundary upwelling currents are some of the most biologically productive and diverse regions in the world's oceans. Driven by equatorward winds and Ekman transport, surface waters are transported offshore and replaced by cold, nutrient‐rich deep waters that seed extensive phytoplankton blooms. Studying phytoplankton community succession and physiological acclimation during the initial stages of upwelling is critical to building a comprehensive understanding of phytoplankton responses to upwelling in these important regions. Additionally, factors like lateral transport, seed population dynamics and physiological and molecular shifts are conducive to shaping the community assemblage and primary productivity. This study examines how phytoplankton gene expression and resulting physiology change between early and later phases of upwelling. By incorporating metatranscriptomic analyses and stable isotope incubations to measure nutrient uptake kinetics into our assessment of early and later upwelling stages, we observed variability in phytoplankton assemblages and differential gene expression of phytoplankton that were de‐coupled from their physiology. We show that the gene expression response to a fresh upwelling event precedes their physiological response. Ultimately, understanding how phytoplankton change through the course of an upwelling event is critical to assessing their importance to regional biological rate processes, trophic systems and resulting biogeochemistry. 

The Galápagos Archipelago is a globally significant biodiversity hotspot. However, compared to the relatively well-known megafauna, the distribution and ecological significance of marine protists in this system are poorly understood. To gain an understanding of the protistan assemblages across trophic modes, an intensive oceanographic survey was conducted in the Galápagos Marine Reserve (GMR) in October of 2018. The Equatorial Undercurrent (EUC)-influenced region had higher chlorophyll- a (Chl- a ) concentrations than those of the eastern regions of the archipelago, along with higher abundances of protistan grazers. Specifically, proportions of autotrophic and potentially mixotrophic dinoflagellates were higher in the EUC, whereas in the eastern regions, heterotrophic dinoflagellates and chlorophytes dominated. Taxonomic composition and biochemical indicators suggested proportions of micrograzers and their associated heterotrophic biomass was higher in the oligotrophic, low Chl- a regions in the east. We also report observations from a dinoflagellate bloom in the western archipelago, which was heavily influenced by upwelling of the EUC. The red tide-forming dinoflagellate Scrippsiella lachrymosa was highly detected through light microscopy and DNA amplicon sequencing. In addition, the heterotrophic dinoflagellate Polykrikos kofoidii was detected and, based on cell densities observed in this study and grazing rates obtained from the literature, estimated to potentially graze up to 62% of S. lachrymosa bloom population. Our findings thus provide new insights into the composition of micrograzers and their potential roles in structuring protistan communities in the Galápagos Archipelago. 

ABSTRACT The Galápagos Islands are a biodiversity hotspot, largely due to the Equatorial Undercurrent (EUC) which supplies nutrient‐rich waters to the euphotic zone and supports enhanced levels of primary productivity performed by phytoplankton. Understanding phytoplankton responses to changing environmental conditions is crucial for regional conservation and management efforts. Research cruises conducted between 2014 and 2022, spanning a major El Niño event in 2015 and a La Niña event in 2022, observed varying oceanic conditions and diverse phytoplankton community composition. At most EUC‐influenced stations, larger‐sized phytoplankton groups (≥ 5 μm) were dominant while warmer, oligotrophic sites favoured smaller‐sized phytoplankton groups (< 5 μm). Predictably, nutrient supply was suppressed during the El Niño event associated with the weakening of the EUC and deepening of the thermocline. Counterintuitively, nutrient levels were not significantly enhanced during the La Niña event likely because increased stratification between the mixed and deep water layers reduced entrainment, particularly at Eastern stations. Protist community composition was evaluated using 18S rRNA gene metabarcoding; the majority of detected OTUs were associated with upwelling conditions prevalent around the archipelago. Taxonomic variability reflected heterogeneous environmental conditions generated by the convergence of multiple ocean currents. These results highlight the dynamic interplay of physical and biological factors shaping primary productivity in the Galápagos marine ecosystem.