• Diversity and community structure of eukaryotic phototrophs in the Bering and Chukchi seas

      Lekanoff, Rachel M.; Collins, R. Eric; McDonnell, Andrew M.P.; Danielson, Seth L. (2020-05)
      The phytoplankton of the Bering and Chukchi seas support highly productive ecosystems characterized by tight benthic-pelagic coupling. In this study, we focus on the northern Bering and Chukchi seas, considering them as one ecosystem. This community has historically been dominated by diatoms; however, climate change and accompanying warming ocean temperatures may alter primary producer communities. Using metabarcoding, we present the first synoptic, high-throughput molecular phylogenetic investigation of phytoplankton diversity in the Bering and Chukchi seas based on hundreds of samples collected from June to September in 2017. We identify the major and minor taxonomic groups of diatoms and picophytoplankton, relative abundances of genera, exact sequence variants (201 for diatoms and 227 for picophytoplankton), and describe their biogeography. These phylogenetic insights and environmental data are used to characterize preferred temperature ranges, offering insight into which specific phytoplankton (Chaetoceros, Pseudo−nitzschia, Micromonas, Phaeocystis) may be most affected as the region warms. Finally, we investigated the likelihood of using shipboard CTD data alone as predictive variables for which members of phytoplankton communities may be present. We found that the suite of environmental data collected from a shipboard CTD is a poor predictor of community composition, explaining only 12.6% of variability within diatom genera and 14.2% variability within picophytoplankton genera. Clustering these communities by similarity of samples did improve predictability (43.6% for diatoms and 32.5% for picophytoplankton). However, our analyses succeeded in identifying temperature as a key driver for certain taxa found commonly throughout the region, offering a key insight into which common phytoplankton community members may be affected first as the Alaskan Arctic continues to warm.
    • Particles in the Pacific: how productivity and zooplankton relate to particles in the deep sea

      Pretty, Jessica L.; McDonnell, Andrew; Johnson, Mark; Hopcroft, Russ (2019-05)
      The magnitude and spatio-temporal patterns of particulate material flux from the surface ocean through mesopelagic and bathypelagic depths determines sequestration of atmospheric carbon and the food supplied to deep-dwelling ocean life. The factors that influence how and where this organic material is exported from euphotic depths are poorly understood. Zooplankton are thought to play a key role in modulating the transport of surface-produced particles to depths through consumption, fragmentation, active diel vertical migration, and fecal pellet production, thus it is important to study both particulate matter and zooplankton in tandem. In this study, I use an in-situ optical instrument, the Underwater Video Profiler 5 (UVP5), to describe broad scale patterns of large (> 100 μm) particles and zooplankton across a longitudinal transect of the Pacific Ocean during April to June 2015. Satellite-derived surface chlorophyll-a was employed to describe the timescales over which particles arrive in meso- and bathypelagic depths after a productivity peak. High abundances and volumes of particles are noticeable beyond the euphotic zone across the Equator, transition zone, and the sub-arctic Pacific, indicating increased export in these high-nutrient low-chlorophyll (HNLC) areas. In two of these areas, the Equator and transition zone, large abundances and volumes of particles extend into bathypelagic depths. High abundances of zooplankton were seen in all areas where high abundances of particles are seen in bathypelagic waters. Rhizaria were revealed to be pervasive across all biogeographic regions, and appear to play a role in particle attenuation in the sub-arctic Pacific. The insight into patterns between particles, zooplankton, and productivity identify HNLC regions as deserving more detailed examination in future studies of biological pump efficiency.
    • Tracing sea ice algae into various benthic feeding types on the Chukchi Sea shelf

      Schollmeier, Tanja; Iken, Katrin; Wooller, Matthew; Hardy, Sarah (2018-12)
      Climate change in the Arctic is expected to have drastic effects on marine primary production sources by shifting ice-associated primary production to an overall greater contribution from pelagic primary production. This shift could influence the timing, amount, and quality of algal material reaching the benthos. We determined the contribution of sea ice particulate organic matter (iPOM) to benthic-feeding invertebrates by examining concentrations and stable carbon isotope values (expressed as δ¹³C values) of three FAs prominent in diatoms: 16:4(n-1), 16:1(n-7) and 20:5(n-3). Our underlying assumption was that diatoms make up the majority in sea ice algal communities compared with phytoplankton communities. According to the FA concentrations, subsurface deposit feeders consumed the most iPOM and suspension feeders the least. Conversely, there were little differences in δ¹³C values of FAs between deposit and suspension feeders, but the higher δ¹³C values of 16:1(n-7) in omnivores indicated greater consumption of iPOM. We suggest that omnivores accumulate the ice algal FA biomarker from their benthic prey, which in turn may feed on ice algae from a deposited sediment pool. The dissimilar results between FA concentrations and isotope values suggest that the FAs used here may not be sufficiently source-specific and that other unaccounted for production sources, such as bacteria, may also contribute to the FA pool. We propose that FA isotope values are a more indicative biomarker than FA concentrations, but there is a further need for more specific ice algal biomarkers to resolve the question of ice algal contributions to the Arctic benthic food web.