Nutrient and Phytoplankton Dynamics on the Inner Shelf of the Eastern Bering Sea

In the Bering Sea, the nitrogen cycle near Nunivak Island is complicated due to limited nutrient replenishment across this broad shelf, and substantial nitrogen loss through sedimentary processes. While diffusion at the inner front may periodically support new production, the inner shelf in this region is generally described as a regenerative system. This study combines hydrographic surveys with measurements of nitrogen assimilation and benthic fluxes to examine nitrogen cycling on the inner shelf, and connectivity between the middle and inner shelves of the southern and central Bering Sea. Results establish the inner shelf as primarily a regenerative system even in spring, although new production can occur at the inner front. Results also identify key processes that influence nutrient supply to the inner shelf and reveal coupling between the middle shelf nutrient pool and production on the inner shelf

Results of the first Arctic Heat Open Science Experiment

Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 99 (2018): 513-520, doi:10.1175/BAMS-D-16-0323.1.Seasonally ice-covered marginal seas are among the most difficult regions in the Arctic to study. Physical constraints imposed by the variable presence of sea ice in all stages of growth and melt make the upper water column and air–sea ice interface especially challenging to observe. At the same time, the flow of solar energy through Alaska’s marginal seas is one of the most important regulators of their weather and climate, sea ice cover, and ecosystems. The deficiency of observing systems in these areas hampers forecast services in the region and is a major contributor to large uncertainties in modeling and related climate projections. The Arctic Heat Open Science Experiment strives to fill this observation gap with an array of innovative autonomous floats and other near-real-time weather and ocean sensing systems. These capabilities allow continuous monitoring of the seasonally evolving state of the Chukchi Sea, including its heat content. Data collected by this project are distributed in near–real time on project websites and on the Global Telecommunications System (GTS), with the objectives of (i) providing timely delivery of observations for use in weather and sea ice forecasts, for model, and for reanalysis applications and (ii) supporting ongoing research activities across disciplines. This research supports improved forecast services that protect and enhance the safety and economic viability of maritime and coastal community activities in Alaska. Data are free and open to all (see www.pmel.noaa.gov/arctic-heat/).This work was supported by NOAA Ocean and Atmospheric Research and the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA15OAR4320063 and by the Innovative Technology for Arctic Exploration (ITAE) program at JISAO/PMEL. Jayne, Robbins, and Ekholm were supported by ONR (N00014-12-10110)

Sea ice‐derived dissolved iron and its potential influence on the spring algal bloom in the Bering Sea

Observational and modeling studies in the Bering Sea indicate that changes in the seasonal ice cover and time of ice retreat influence open‐water productivity. In particular, the timing of the spring bloom and its phytoplankton community composition are affected. Dissolved iron (DFe) data in the water column and ice cores collected during the 2007‐ Bering Ecosystem Study (BEST) cruise indicate that the melting ice provided substantial DFe to the water column. The additional DFe input from melting sea ice could be biologically important along the outer shelf and shelf break where in ice‐free areas insufficient DFe (20 μM). Variability in sea ice dynamics are likely to result in a varying supply of DFe to the outer shelf and shelf break in early spring, and to contribute to the observed changes in the timing and community composition of the spring phytoplankton bloom

Inorganic carbon dynamics during northern California coastal upwelling

Coastal upwelling events in the California Current System can transport subsurface waters with high levels of carbon dioxide (CO 2) to the sea surface near shore. As these waters age and are advected offshore, CO 2 levels decrease dramatically, falling well below the atmospheric concentration beyond the continental shelf break. In May 2007 we observed an upwelling event off the coast of northern California. During the upwelling event subsurface respiration along the upwelling path added ∼35 μmol kg −1 of dissolved inorganic carbon (DIC) to the water as it transited toward shore causing the waters to become undersaturated with respect to Aragonite. Within the mixed layer, pCO 2 levels were reduced by the biological uptake of DIC (up to 70%), gas exchange (up to 44%), and the addition of total alkalinity through CaCO 3 dissolution in the undersaturated waters (up to 23%). The percentage contribution of each of these processes was dependent on distance from shore. At the time of measurement, a phytoplankton bloom was just beginning to develop over the continental shelf. A box model was used to project the evolution of the water chemistry as the bloom developed. The biological utilization of available nitrate resulted in a DIC decrease of ∼200 μmol kg −1, sea surface pCO 2 near ∼200 ppm, and an aragonite saturation state of ∼3. These results suggest that respiration processes along the upwelling path generally increase the acidification of the waters that are being upwelled, but once the waters reach the surface biological productivity and gas exchange reduce that acidification over time. ► Carbon transformations off US West Coast were evaluated during 3 phases of upwelling. ► Subsurface respiration on upwelling path added ∼35 μmol kg −1 of inorganic carbon. ► Respiration along upwelling path can significantly increase coastal acidification. ► Mixed layer productivity can reduce pCO 2 of the upwelled water to 200 ppm. ► Once at the surface, biology and gas exchange reduce upwelled water acidification

Water Mass Evolution and Circulation of the Northeastern Chukchi Sea in Summer: Implications for Nutrient Distributions

Synoptic and historical shipboard data, spanning the period 1981–2017, are used to investigate the seasonal evolution of water masses on the northeastern Chukchi shelf and quantify the circulation patterns and their impact on nutrient distributions. We find that Alaskan coastal water extends to Barrow Canyon along the coastal pathway, with peak presence in September, while the Pacific Winter Water (WW) continually drains off the shelf through the summer. The depth-averaged circulation under light winds is characterized by a strong Alaskan Coastal Current (ACC) and northward flow through Central Channel. A portion of the Central Channel flow recirculates anticyclonically to join the ACC, while the remainder progresses northeastward to Hanna Shoal where it bifurcates around both sides of the shoal. All of the branches converge southeast of the shoal and eventually join the ACC. The wind-forced response has two regimes: In the coastal region the circulation depends on wind direction, while on the interior shelf the circulation is sensitive to wind stress curl. In the most common wind-forced state—northeasterly winds and anticyclonic wind stress curl—the ACC reverses, the Central Channel flow penetrates farther north, and there is mass exchange between the interior and coastal regions. In September and October, the region southeast of Hanna Shoal is characterized by elevated amounts of WW, a shallower pycnocline, and higher concentrations of nitrate. Sustained late-season phytoplankton growth spurred by this pooling of nutrients could result in enhanced vertical export of carbon to the seafloor, contributing to the maintenance of benthic hotspots in this region

Return of warm conditions in the southeastern Bering Sea: Physics to fluorescence

<div><p>From 2007 to 2013, the southeastern Bering Sea was dominated by extensive sea ice and below-average ocean temperatures. In 2014 there was a shift to reduced sea ice on the southern shelf and above-average ocean temperatures. These conditions continued in 2015 and 2016. During these three years, the spring bloom at mooring site M4 (57.9°N, 168.9°W) occurred primarily in May, which is typical of years without sea ice. At mooring site M2 (56.9°N, 164.1°W) the spring bloom occurred earlier especially in 2016. Higher chlorophyll fluorescence was observed at M4 than at M2. In addition, these three warm years continued the pattern near St. Matthew Island of high concentrations (>1 μM) of nitrite occurring during summer in warm years. Historically, the dominant parameters controlling sea-ice extent are winds and air temperature, with the persistence of frigid, northerly winds in winter and spring resulting in extensive ice. After mid-March 2014 and 2016 there were no cold northerly or northeasterly winds. Cold northerly winds persisted into mid-April in 2015, but did not result in extensive sea ice south of 58°N. The apparent mechanism that helped limit ice on the southeastern shelf was the strong advection of warm water from the Gulf of Alaska through Unimak Pass. This pattern has been uncommon, occurring in only one other year (2003) in a 37-year record of estimated transport through Unimak Pass. During years with no sea ice on the southern shelf (e.g. 2001–2005, 2014–2016), the depth-averaged temperature there was correlated to the previous summers ocean temperature.</p></div

Ice and temperature at M2.

<p>(A) The average percent of areal ice cover in a 2° × 1° box (163–165°W, 56.5–57.5°N) around M2 during March–April. (B) The depth-averaged hourly temperature at M2. The circles are a replotting of the percent ice cover data found in (A). (C) The depth-averaged temperature anomaly (relative to 1995–2009) at M2.</p

Winds, currents and sea ice.

<p>Panels (A)–(C) are March–May time series from 2014 (left), 2015 (middle), and 2016 (right). (A) 5-day average winds at 57.32°N, 166.32°W. Vectors are color-coded according to air temperature. (B) Time series of contours of daily areal ice concentration for 0.25° latitude bands in the light-shaded area in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0185464#pone.0185464.g001" target="_blank">Fig 1</a>. The latitudes of the two long-term moorings on the southern Bering Sea shelf are indicated with dashed lines, as is M5 in the transition zone between the southern and northern shelf. Daily maximum ice extent is indicated for 2010 (blue) and 2001, 2002, and 2003 (red). (C) Daily mean derived currents through Unimak Pass. Lines are color-coded by SST anomaly upstream near the Shumagin Islands (star in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0185464#pone.0185464.g001" target="_blank">Fig 1</a>, 54.625°N, 161.125°W). Positive indicates inflow into the Bering Sea.</p

Deployment period of moorings deployed in Unimak Pass.

<p>Deployment period of moorings deployed in Unimak Pass.</p