Contribution of Small Phytoplankton to Primary Production in the Northern Bering and Chukchi Seas

The northern Bering and Chukchi seas are biologically productive regions but, recently, unprecedented environmental changes have been reported. For investigating the dominant phytoplankton communities and relative contribution of small phytoplankton (<2 µm) to the total primary production in the regions, field measurements mainly for high-performance liquid chromatography (HPLC) and size-specific primary productivity were conducted in the northern Bering and Chukchi seas during summer 2016 (ARA07B) and 2017 (OS040). Diatoms and phaeocystis were dominant phytoplankton communities in 2016 whereas diatoms and Prasinophytes (Type 2) were dominant in 2017 and diatoms were found as major contributors for the small phytoplankton groups. For size-specific primary production, small phytoplankton contributed 38.0% (SD = ±19.9%) in 2016 whereas 25.0% (SD = ±12.8%) in 2017 to the total primary productivity. The small phytoplankton contribution observed in 2016 is comparable to those reported previously in the Chukchi Sea whereas the contribution in 2017 mainly in the northern Bering Sea is considerably lower than those in other arctic regions. Different biochemical compositions were distinct between small and large phytoplankton in this study, which is consistent with previous results. Significantly higher carbon (C) and nitrogen (N) contents per unit of chlorophyll-a, whereas lower C:N ratios were characteristics in small phytoplankton in comparison to large phytoplankton. Given these results, we could conclude that small phytoplankton synthesize nitrogen-rich particulate organic carbon which could be easily regenerated.

On the circulation, water mass distribution, and nutrient concentrations of the western Chukchi Sea

Substantial amounts of nutrients and carbon enter the Arctic Ocean from the Pacific Ocean through the Bering Strait, distributed over three main pathways. Water with low salinities and nutrient concentrations takes an eastern route along the Alaskan coast, as Alaskan Coastal Water. A central pathway exhibits intermediate salinity and nutrient concentrations, while the most nutrient-rich water enters the Bering Strait on its western side. Towards the Arctic Ocean, the flow of these water masses is subject to strong topographic steering within the Chukchi Sea with volume transport modulated by the wind field. In this contribution, we use data from several sections crossing Herald Canyon collected in 2008 and 2014 together with numerical modelling to investigate the circulation and transport in the western part of the Chukchi Sea. We find that a substantial fraction of water from the Chukchi Sea enters the East Siberian Sea south of Wrangel Island and circulates in an anticyclonic direction around the island. This water then contributes to the high-nutrient waters of Herald Canyon. The bottom of the canyon has the highest nutrient concentrations, likely as a result of addition from the degradation of organic matter at the sediment surface in the East Siberian Sea. The flux of nutrients (nitrate, phosphate, and silicate) and dissolved inorganic carbon in Bering Summer Water and Winter Water is computed by combining hydrographic and nutrient observations with geostrophic transport referenced to lowered acoustic Doppler current profiler (LADCP) and surface drift data. Even if there are some general similarities between the years, there are differences in both the temperature–salinity and nutrient characteristics. To assess these differences, and also to get a wider temporal and spatial view, numerical modelling results are applied. According to model results, high-frequency variability dominates the flow in Herald Canyon. This leads us to conclude that this region needs to be monitored over a longer time frame to deduce the temporal variability and potential trends.

Variability in spring phytoplankton blooms associated with ice retreat timing in the Pacific Arctic from 2003–2019

The Arctic is experiencing rapid changes in sea-ice seasonality and extent, with significant consequences for primary production. With the importance of accurate monitoring of spring phytoplankton dynamics in a changing Arctic, this study further examines the previously established critical relationship between spring phytoplankton bloom types and timing of the sea-ice retreat for broader temporal and spatial coverages, with a particular focus on the Pacific Arctic for 2003–2019. To this end, time-series of satellite-retrieved phytoplankton biomass were modeled using a parametric Gaussian function, as an effective approach to capture the development and decay of phytoplankton blooms. Our sensitivity analysis demonstrated accurate estimates of timing and presence/absence of peaks in phytoplankton biomass even with some missing values, suggesting the parametric Gaussian function is a powerful tool for capturing the development and decay of phytoplankton blooms. Based on the timing and presence/absence of a peak in phytoplankton biomass and following the classification developed by the previous exploratory work, spring bloom types are classified into three groups (under-ice blooms, probable under-ice blooms, and marginal ice zone blooms). Our results showed that the proportion of under-ice blooms was higher in the Chukchi Sea than in the Bering Sea. The probable under-ice blooms registered as the dominant bloom types in a wide area of the Pacific Arctic, whereas the marginal ice zone bloom was a relatively minor bloom type across the Pacific Arctic. Associated with a shift of sea-ice retreat timing toward earlier dates, we confirmed previous findings from the Chukchi Sea of recent shifts in phytoplankton bloom types from under-ice blooms to marginal ice zone blooms and demonstrated that this pattern holds for the broader Pacific Arctic sector for the time period 2003–2019. Overall, the present study provided additional evidence of the changing sea-ice retreat timing that can drive variations in phytoplankton bloom dynamics, which contributes to addressing the detection and consistent monitoring of the biophysical responses to the changing environments in the Pacific Arctic.

The climatic trends of temperature and salinity in the Bering straight in the last decades

Представлены результаты расчетов линейных климатических трендов температуры и солености поверхностных вод Берингова пролива и прилегающих акваторий за временной период 1950 – 2020 гг., отдельно за август и февраль месяцы. Отмечается нагрев поступающей тихоокеанской воды в Чукотское море в летний период на +0,120С/10лет, в зимний период +0,140С/10лет. Одновременно происходит временной тренд в сторону уменьшения солености поступающих вод, это ‒0,060/00/10лет в летний сезон и ‒0,03 S0/00/10лет в зимний сезон. Таким образом в случае инерционного изменения климата через 100 лет вода в Беринговом проливе будет на 1,30С теплее и на 0,450/00 менее соленой. Сделаны оценки роста потока тепла через Берингов пролив в Чукотское море вследствие климатического тренда, который составляет +2,4*1019Дж/10лет. Отмечено, что направления трендов температуры и солености в Беринговом море и Беринговом проливе в сторону нагрева и уменьшения солености совпадают, а в Чукотском море климатические тенденции противоположные. The calculation of the climatic linear trends of the surface waters temperature and salinity in the Bering Strait and nearby water areas are presented. The time period is 1950 – 2020 years. The Time data for the warm season - August and the cold season - February series are shown separately. The heating of the incoming Pacific water into the Chukchi Sea is note. There is the summer period +0.120C/decade, in the winter period +0.140C/decade. At the same time, there is some trend towards decrease in salinity the straight water, this is ‒0.06psu/decade in the summer season and this is ‒0.03psu/decade in the winter season. Following of an inertial climate change in 100 years there is the Bering Strait water will be +1.30Cwarmer and ‒0.45psu/decade salty less. The estimate of the heat flux increase through the Bering Strait to the Chukchi Sea due to the climatic trend is +2.4*1019J/decade. There is a peculiarity that the time trends of temperature and salinity in the Bering Sea and the Bering Strait have the same direction to the heating and the salinity decrease, but at the same time the Chukchi Sea has the opposite tendency. An explanation of this discrepancy is given.

Genetic history and stock identity of beluga whales in Kotzebue Sound

We investigate the recent history and stock identity of beluga whales (Delphinapterus leucas) in Kotzebue Sound in the Chukchi Sea, a region long frequented by large numbers of belugas in summer until their near disappearance in the 1980s. Wide variation in numbers since then suggests a complex recent history that hinders recovery efforts. Analysis of teeth sampled during the historical (pre-decline) era using ancient DNA (aDNA) methods found that the original Kotzebue Sound whales were differentiated for mitochondrial DNA (mtDNA) from other summering concentrations across the Pacific Arctic revealing a demographically distinct subpopulation where long-established migratory culture likely facilitated population divergence. Analysis of microsatellite (nDNA) and mtDNA markers in belugas from the contemporary (post-decline) era revealed that whales from other stocks likely visited Kotzebue Sound, including during two low ice years when relatively large numbers of whales were present. Some mtDNA lineages were found only in Kotzebue Sound, with one recorded in both the historical and contemporary eras. Exclusion tests found a number of whales in Kotzebue Sound during the contemporary era that had nDNA genotypes unlikely to arise in other contemporary stocks in the Pacific Arctic. Our findings indicate that the Kotzebue Sound belugas comprised a unique stock of which a few remnants likely still co-occur with belugas from other larger stocks. We recommend that the US government work through the co-management process to greatly reduce or eliminate the taking of belugas, especially adult females, likely to belong to the Kotzebue Sound stock, until they recover.

Ringed seal (Pusa hispida) breeding habitat on the landfast ice in northwest Alaska during spring 1983 and 1984

There has been significant sea ice loss associated with climate change in the Pacific Arctic, with unquantified impacts to the habitat of ice-obligate marine mammals such as ringed seals (Pusa hispida). Ringed seals maintain breathing holes and excavate subnivean lairs on sea ice to provide protection from weather and predators during birthing, nursing, and resting. However, there is limited baseline information on the snow and ice habitat, distribution, density, and configuration of ringed seal structures (breathing holes, simple haul-out lairs, and pup lairs) in Alaska. Here, we describe historic field records from two regions of the eastern Chukchi Sea (Kotzebue Sound and Ledyard Bay) collected during spring 1983 and 1984 to quantify baseline ringed seal breeding habitat and map the distribution of ringed seal structures using modern geospatial tools. Of 490 structures located on pre-established study grids by trained dogs, 29% were pup lairs (25% in Kotzebue Sound and 33% in Ledyard Bay). Grids in Ledyard Bay had greater overall density of seal structures than those in Kotzebue Sound (8.6 structures/km2 and 7.1 structures/km2), but structures were larger in Kotzebue Sound. Pup lairs were located in closer proximity to other structures and characterized by deeper snow and greater ice deformation than haul-out lairs or simple breathing holes. At pup lairs, snow depths averaged 74.9 cm (range 37–132 cm), with ice relief nearby averaging 76 cm (range 31–183 cm), and ice deformation 29.9% (range 5–80%). We compare our results to similar studies conducted in other geographic regions and discuss our findings in the context of recent declines in extent and duration of seasonal cover of landfast sea ice and snow deposition on sea ice. Ultimately, additional research is needed to understand the effects of recent environmental changes on ringed seals, but our study establishes a baseline upon which future research can measure pup habitat in northwest Alaska.

Environmental Filtering Influences Functional Community Assembly of Epibenthic Communities

Community assembly theory states that species assemble non-randomly as a result of dispersal limitation, biotic interactions, and environmental filtering. Strong environmental filtering likely leads to local assemblages that are similar in their functional trait composition (high trait convergence) while functional trait composition will be less similar (high trait divergence) under weaker environmental filters. We used two Arctic shelves as case studies to examine the relationship between functional community assembly and environmental filtering using the geographically close but functionally and environmentally dissimilar epibenthic communities on the Chukchi and Beaufort Sea shelves. Environmental drivers were compared to functional trait composition and to trait convergence within each shelf. Functional composition in the Chukchi Sea was more strongly correlated with environmental gradients compared to the Beaufort Sea, as shown by a combination of RLQ and fourth corner analyses and community-weighted mean redundancy analyses. In the Chukchi Sea, epibenthic functional composition, particularly body size, reproductive strategy, and several behavioral traits (i.e., feeding habit, living habit, movement), was most strongly related to gradients in percent mud and temperature while body size and larval development were most strongly related to a depth gradient in the Beaufort Sea. The stronger environmental filter in the Chukchi Sea also supported the hypothesized relationship with higher trait convergence, although this relationship was only evident at one end of the observed environmental gradient. Strong environmental filtering generally provides a challenge for biota and can be a barrier for invading species, a growing concern for the Chukchi Sea shelf communities under warming conditions. Weaker environmental filtering, such as on the Beaufort Sea shelf, generally leads to communities that are more structured by biotic interactions, and possibly representing partitioning of resources among species from intermediate disturbance levels. We provide evidence that environmental filtering can structure functional community composition, providing a baseline of how community function could be affected by stressors such as changes in environmental conditions or increased anthropogenic disturbance.

Thermal Conductivity of Sea Ice and Antarctic Permafrost

<p>We present results from measurements of the thermal conductivity of sea ice, ksi, using two different techniques. In the first, ice temperatures were measured at 10 cm and 30 minute intervals by automated thermistor arrays deployed in land-fast first-year (FY) and multi-year (MY) ice in McMurdo Sound, Antarctica, and in FY ice in the Chukchi Sea and shallow Elson Lagoon, near Point Barrow, Alaska. Conductivity profiles through the ice were calculated from the coupled time- and depth- dependence of the temperature variations using a conservation of energy analysis, and a graphical finite difference method. These profiles show a reduction in the conductivity of up to 25% over the top ~ 50 cm, consistent with similar previous measurements. From simulations and a detailed analysis of this method, we have clearly identified this reduction (for which physical explanations had previously been invoked) as an analytical artifact, due to the presence of temperature variations with time scales much less than the 30 min sampling interval. These variations have a penetration depth that is small compared with the thermistor spacing, so the effect is shallow. Between 50 cm and the depth at which the method becomes noise-limited, we calculate average conductivities of 2.29 +/- 0.07 W/m degrees C and 2.26 +/- 0.11 W/m degrees C at the FY McMurdo Sound and Chukchi Sea sites, and 2.03 +/- 0.04 W/m degrees C at the MY site in McMurdo Sound. Using a parallel conductance method, we measured the conductivity of small (11 x 2.4 cm diameter) ice cores by heating one end of a sample holder, and with the other end held at a fixed temperature, measuring the temperature gradient with and without a sample loaded. From several different cores in each class, we resolved no significant difference, and certainly no large reduction, in the conductivity of FY surface (0-10 cm) and sub-surface (45-55 cm) ice, being 2.14 +/- 0.11 W/m degrees C and 2.09 +/- 0.12 W/m degrees C respectively. The conductivity of less dense, bubbly MY ice was measured to be 1.88 +/- 0.13 W/m degrees C. Within measurement uncertainties of about +/-6%, the values from our two methods are consistent with each other and with predictions from our modification of an existing theoretical model for ksi(p, S, T). Both our results and previous measurements give conductivity values about 10% higher than those commonly used in Arctic and Antarctic sea ice models. For FY ice, we tentatively propose a new empirical parameterisation, ksi = 2.09 - 0.011T + 0.117S/T [W/m degrees C], where T is temperature [degrees C] and S salinity [0/00]. We expect this parameterisation to be revised as thermal array data from other researchers are made available. We also report thermal array measurements in ice-cemented permafrost at Table Mountain in the Antarctic Dry Valleys, between November 2001 - December 2003. From 13 months of temperature data with a sampling interval reduced from 4 hours to 1 hour (November 2002 - December 2003), we have modified some aspects of an already published initial analysis [Pringle et al., 2003]. Using thermal diffusivity profiles calculated from measured temperatures, and a heat capacity estimated from recovered cores, we have determined thermal conductivity profiles at two sites that show depth- and seasonal- variations that correlate well with core compositions, and the expected underlying temperature dependence. The conductivity generally lies in the range 2.5 +/- 0.5 W/m degrees C, but is as high as 5.5 +/- 0.4 W/m degrees C in a quartz-rich unit at one site. The wintertime diffusivity is 4 +/- 7% higher than the summertime value, which we understand to reflect the underlying temperature dependence. In this analysis we find our graphical finite difference method more versatile and more accurate than common 'Fourier' time-series methods.</p>