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1. Asymmetry in the rate of warming and the phenology of seasonal blooms in the Northeast US Shelf Ecosystem

   https://doi.org/10.1093/icesjms/fsad007  

   Friedland, Kevin D; Record, Nicholas R; Pendleton, Daniel E; Balch, William M; Stamieszkin, Karen; Moisan, John R; Brady, Damian C (February 2023, ICES Journal of Marine Science)

   Woodson, C Brock (Ed.)

   Abstract Predicting the impact of marine ecosystem warming on the timing and magnitude of phytoplankton production is challenging. For example, warming can advance the progression of stratification thereby changing the availability of nutrients to surface phytoplankton, or influence the surface mixed layer depth, thus affecting light availability. Here, we use a time series of sea surface temperature (SST) and chlorophyll remote sensing products to characterize the response of the phytoplankton community to increased temperature in the Northeast US Shelf Ecosystem. The rate of change in SST was higher in the summer than in winter in all ecoregions resulting in little change in the timing and magnitude of the spring thermal transition compared to a significant change in the autumn transition. Along with little phenological shift in spring thermal conditions, there was also no evidence of a change in spring bloom timing and duration. However, we observed a change in autumn bloom timing in the Georges Bank ecoregion, where bloom initiation has shifted from late September to late October between 1998 and 2020—on average 33 d later. Bloom duration in this ecoregion also shortened from ∼7.5 to 5 weeks. The shortened autumn bloom may be caused by later overturn in stratification known to initiate autumn blooms in the region, whereas the timing of light limitation at the end of the bloom remains unchanged.  These changes in bloom timing and duration appear to be related to the change in autumn thermal conditions and the significant shift in autumn thermal transition. These results suggest that the spring bloom phenology in this temperate continental shelf ecosystem may be more resilient to thermal climate change effects than blooms occurring in other times of the year. 

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2. Distinct responses to warming within picoplankton communities across an environmental gradient

   https://doi.org/10.1111/gcb.17316  

   Stevens, Bethany_L F; Peacock, Emily E; Crockford, E Taylor; Shalapyonok, Alexi; Neubert, Michael G; Sosik, Heidi M (May 2024, Global Change Biology)

   Abstract Picophytoplankton are a ubiquitous component of marine plankton communities and are expected to be favored by global increases in seawater temperature and stratification associated with climate change. Eukaryotic and prokaryotic picophytoplankton have distinct ecology, and global models predict that the two groups will respond differently to future climate scenarios. At a nearshore observatory on the Northeast US Shelf, however, decades of year‐round monitoring have shown these two groups to be highly synchronized in their responses to environmental variability. To reconcile the differences between regional and global predictions for picophytoplankton dynamics, we here investigate the picophytoplankton community across the continental shelf gradient from the nearshore observatory to the continental slope. We analyze flow cytometry data from 22 research cruises, comparing the response of picoeukaryote andSynechococcuscommunities to environmental variability across time and space. We find that the mechanisms controlling picophytoplankton abundance differ across taxa, season, and distance from shore. Like the prokaryote,Synechococcus, picoeukaryote division rates are limited nearshore by low temperatures in winter and spring, and higher temperatures offshore lead to an earlier spring bloom. UnlikeSynechococcus, picoeukaryote concentration in summer decreases dramatically in offshore surface waters and exhibits deeper subsurface maxima. The offshore picoeukaryote community appears to be nutrient limited in the summer and subject to much greater loss rates thanSynechococcus. This work both produces and demonstrates the necessity of taxon‐ and site‐specific knowledge for accurately predicting the responses of picophytoplankton to ongoing environmental change. 

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3. Changing phytoplankton phenology in the marginal ice zone west of the Antarctic Peninsula

   https://doi.org/10.3354/meps14567  

   Turner, JS; Dierssen, H; Schofield, O; Kim, HH; Stammerjohn, S; Munro, DR; Kavanaugh, M (April 2024, Marine Ecology Progress Series)

   Climate change is altering global ocean phenology, the timing of annually occurring biological events. We examined the changing phenology of the phytoplankton accumulation season west of the Antarctic Peninsula to show that blooms are shifting later in the season over time in ice-associated waters. The timing of the start date and peak date of the phytoplankton accumulation season occurred later over time from 1997 to 2022 in the marginal ice zone and over the continental shelf. A divergence was seen between offshore waters and ice-associated waters, with offshore bloom timing becoming earlier, yet marginal ice zone and continental shelf bloom timing shifting later. Higher chlorophylla(chla) concentration in the fall season was seen in recent years, especially over the northern continental shelf. Minimal long-term trends in annual chlaoccurred, likely due to the combination of later start dates in spring and higher chlain fall. Increasing spring wind speed is the most likely mechanism for later spring start dates, leading to deeper wind mixing in a region experiencing sea ice loss. Later phytoplankton bloom timing over the marginal ice zone and continental shelf will have consequences for surface ocean carbon uptake, food web dynamics, and trophic cascades. 

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4. Spatial and temporal variations in Synechococcus microdiversity in the Southern California coastal ecosystem

   https://doi.org/10.1111/1462-2920.15307  

   Nagarkar, Maitreyi; Wang, Maggie; Valencia, Bellineth; Palenik, Brian (November 2020, Environmental Microbiology)

   Summary TheSynechococcuscyanobacterial population at the Scripps Institution of Oceanography pier in La Jolla, CA, shows large increases in abundance, typically in the spring and summer followed, by rapid declines within weeks. Here we used amplicon sequencing of the ribosomal RNA internal transcribed spacer region to examine the microdiversity within this cyanobacterial genus during these blooms as well as further offshore in the Southern California coastal ecosystem (CCE). These analyses revealed numerousSynechococcusamplicon sequence variants (ASVs) and that clade and ASV composition can change over the course of blooms. We also found that a large bloom in August 2016 was highly anomalous both in its overallSynechococcusabundance and in terms of the presence of normally oligotrophicSynechococcusclade II. The dominant ASVs at the pier were found further offshore and in the California Current, but we did observe more oligotrophic ASVs and clades along with depth variation inSynechococcusdiversity. We also observed that the dominant sequence variant switched during the peak of multipleSynechococcusblooms, with this switch occurring in multiple clades, but we present initial evidence that this apparent ASV switch is a physiological response rather than a change in the dominant population. 

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5. Seasonal modulation of phytoplankton biomass in the Southern Ocean

   https://doi.org/10.1038/s41467-020-19157-2  

   Arteaga, Lionel A.; Boss, Emmanuel; Behrenfeld, Michael J.; Westberry, Toby K.; Sarmiento, Jorge L. (December 2020, Nature Communications)

   null (Ed.)

   Abstract Over the last ten years, satellite and geographically constrained in situ observations largely focused on the northern hemisphere have suggested that annual phytoplankton biomass cycles cannot be fully understood from environmental properties controlling phytoplankton division rates (e.g., nutrients and light), as they omit the role of ecological and environmental loss processes (e.g., grazing, viruses, sinking). Here, we use multi-year observations from a very large array of robotic drifting floats in the Southern Ocean to determine key factors governing phytoplankton biomass dynamics over the annual cycle. Our analysis reveals seasonal phytoplankton accumulation (‘blooming’) events occurring during periods of declining modeled division rates, an observation that highlights the importance of loss processes in dictating the evolution of the seasonal cycle in biomass. In the open Southern Ocean, the spring bloom magnitude is found to be greatest in areas with high dissolved iron concentrations, consistent with iron being a well-established primary limiting nutrient in this region. Under ice observations show that biomass starts increasing in early winter, well before sea ice begins to retreat. The average theoretical sensitivity of the Southern Ocean to potential changes in seasonal nutrient and light availability suggests that a 10% change in phytoplankton division rate may be associated with a 50% reduction in mean bloom magnitude and annual primary productivity, assuming simple changes in the seasonal magnitude of phytoplankton division rates. Overall, our results highlight the importance of quantifying and accounting for both division and loss processes when modeling future changes in phytoplankton biomass cycles. 

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