Food for thought: Phytoplankton abundances in the Arctic Ocean have been increasing over recent decades as the region has warmed and sea ice has disappeared. The presumptive causes of this increase were expanding open water area and a longer growing season—at least until now. Lewis et al. show that although these factors may have driven the productivity trends before, over the past decade, phytoplankton primary production rose by more than half because of increased phytoplankton concentrations (see the Perspective by Babin). This finding means that there has been an influx of new nutrients into the region, suggesting that the Arctic Ocean could become more productive and export additional carbon in the future. Science , this issue p. 198 ; see also p. 137

As the Arctic Ocean (AO) is increasingly altered by anthropogenic climate change, it is critical that we accurately assess ongoing changes in its capacity to support marine life. Ocean color remote sensing provides an effective tool to estimate phytoplankton biomass and net primary production in the remote and undersampled AO. However, standard algorithms have been parameterized using global data sets that severely underrepresent the AO. Because the optical properties of AO waters differ markedly from those of lower‐latitude waters, standard algorithms perform poorly in the AO. Here, we use the largest bio‐optical database ever assembled for AO waters to examine seasonal and regional patterns of AO optical properties and to quantify their impact on ocean color algorithms. We find that the standard algorithms fail to accurately retrieve the primary photosynthetic pigment chlorophyll a (Chl a) in the AO; it is overestimated at low phytoplankton biomass due to exceptionally high absorption of colored dissolved organic matter (CDOM), particularly on the interior shelves, and underestimated at high biomass due to the photoacclimation of phytoplankton growing in low light leading to high pigment packaging effect. Using this large bio‐optical database that includes in situ measurements from across the AO and through the entire growing season, we parameterize a new empirical (AOReg.emp) and semianalytical (AO.GSM) algorithm which represent the unique bio‐optical properties of the AO to successfully retrieve Chl a, CDOM absorption, and particle backscattering with better accuracy than any algorithm yet applied to the AO.

As the physical environment of the Arctic Ocean shifts seasonally from ice‐covered to open water, the limiting resource for phytoplankton growth shifts from light to nutrients. To understand the phytoplankton photophysiological responses to these environmental changes, we evaluated photoacclimation strategies of phytoplankton during the low‐light, high‐nutrient, ice‐covered spring and the high‐light, low‐nutrient, ice‐free summer. Field results show that phytoplankton effectively acclimated to reduced irradiance beneath the sea ice by maximizing light absorption and photosynthetic capacity. In fact, exceptionally high maximum photosynthetic rates and efficiency observed during the spring demonstrate that abundant nutrients enable prebloom phytoplankton to become “primed” for increases in irradiance. This ability to quickly exploit increasing irradiance can help explain the ability of phytoplankton to generate massive blooms beneath sea ice. In comparison, phytoplankton growth and photosynthetic rates are reduced postbloom due to severe nutrient limitation. These results advance our knowledge of photoacclimation by polar phytoplankton in extreme environmental conditions and indicate how phytoplankton may acclimate to future changes in light and nutrient resources under continued climate change.

The Palmer Long‐Term Ecological Research program has sampled waters of the western Antarctic Peninsula (wAP) annually each summer since 1990. However, information about the wAP prior to the peak of the phytoplankton bloom in January is sparse. Here we present results from a spring process cruise that sampled the wAP in the early stages of phytoplankton bloom development in 2014. Sea ice concentrations were high on the shelf relative to nonshelf waters, especially toward the south. Macronutrients were high and nonlimiting to phytoplankton growth in both shelf and nonshelf waters, while dissolved iron concentrations were high only on the shelf. Phytoplankton were in good physiological condition throughout the wAP, although biomass on the shelf was uniformly low, presumably because of heavy sea ice cover. In contrast, an early stage phytoplankton bloom was observed beneath variable sea ice cover just seaward of the shelf break. Chlorophyll a concentrations in the bloom reached 2 mg m⁻³ within a 100–150 km band between the SBACC and SACCF. The location of the bloom appeared to be controlled by a balance between enhanced vertical mixing at the position of the two fronts and increased stratification due to melting sea ice between them. Unlike summer, when diatoms overwhelmingly dominate the phytoplankton population of the wAP, the haptophyte Phaeocystis antarctica dominated in spring, although diatoms were common. These results suggest that factors controlling phytoplankton abundance and composition change seasonally and may differentially affect phytoplankton populations as environmental conditions within the wAP region continue to change.

Spring phytoplankton growth in polar marine ecosystems is limited by light availability beneath ice‐covered waters, particularly early in the season prior to snowmelt and melt pond formation. Leads of open water increase light transmission to the ice‐covered ocean and are sites of air‐sea exchange. We explore the role of leads in controlling phytoplankton bloom dynamics within the sea ice zone of the Arctic Ocean. Data are presented from spring measurements in the Chukchi Sea during the Study of Under‐ice Blooms In the Chukchi Ecosystem (SUBICE) program in May and June 2014. We observed that fully consolidated sea ice supported modest under‐ice blooms, while waters beneath sea ice with leads had significantly lower phytoplankton biomass, despite high nutrient availability. Through an analysis of hydrographic and biological properties, we attribute this counterintuitive finding to springtime convective mixing in refreezing leads of open water. Our results demonstrate that waters beneath loosely consolidated sea ice (84–95% ice concentration) had weak stratification and were frequently mixed below the critical depth (the depth at which depth‐integrated production balances depth‐integrated respiration). These findings are supported by theoretical model calculations of under‐ice light, primary production, and critical depth at varied lead fractions. The model demonstrates that under‐ice blooms can form even beneath snow‐covered sea ice in the absence of mixing but not in more deeply mixed waters beneath sea ice with refreezing leads. Future estimates of primary production should account for these phytoplankton dynamics in ice‐covered waters.