Controls on zooplankton assemblages in the northeastern Chukchi Sea

Thesis (Ph.D.) University of Alaska Fairbanks, 2016The Chukchi Sea is a broad and shallow marginal sea of the western Arctic Ocean that lies between the Bering Sea and the deeper Amerasian basin. It plays a pivotal role as the only gateway for transporting heat, carbon, nutrients, and plankton from the North Pacific into the Arctic Ocean. I examined the seasonal and inter-annual variability of the zooplankton communities in the northeastern region of the Chukchi Sea as part of a high-resolution multidisciplinary ecosystem study. Specifically, I examined how the physical onset of each open water season influenced the composition, abundance, and biomass of zooplankton assemblages from the 2008 to 2010 field seasons. Copepods in the genus Pseudocalanus are key members of the Chukchi community, and may be undergoing species-level biogeographic shift in response to climate change. I determined the degree of gene flow and population connectivity in the Chukchi Sea through comparative phylogeographic analysis of the Pseudocalanus species complex to the northern Gulf of Alaska and Beaufort Sea. I then investigated the extent to which biogeochemical factors influence these zooplankton assemblages by relating a portion of the seasonal production to concurrent changes in herbivorous mesozooplankton biomass during 2010 and 2011. This work demonstrates just how complex and variable marine ecosystems of the western Arctic are, where multidisciplinary and analytical approaches will become essential in detecting change, especially with the rate of present-day climate perturbations.Chapter 1: Seasonal and interannual variation in the planktonic communities of the northeastern Chukchi Sea during the summer and early fall -- Chapter 2: Phylogeography and connectivity of the Pseudocalanus (Copepoda: Calanoida) species complex in the eastern North Pacific Ocean and the Pacific Arctic Region -- Chapter 3: Community production in the northeastern Chukchi Sea and its relationship to phytoplankton and mesozooplankton biomass, 2010-2011 -- General Conclusions -- References

Spatial and Temporal Variations in Active Layer Thawing and Their Implication on Runoff Generation in Peat-Covered Permafrost Terrain

The distribution of frost table depths on a peat-covered permafrost slope was examined in a discontinuous permafrost region in northern Canada over 4 consecutive years at a variety of spatial scales, to elucidate the role of active layer development on runoff generation. Frost table depths were highly variable over relatively short distances (0.25–1 m), and the spatial variability was strongly correlated to soil moisture distribution, which was partly influenced by lateral flow converging to frost table depressions. On an interannual basis, thaw rates were temporally correlated to air temperature and the amount of precipitation input. Simple simulations show that lateral subsurface flow is governed by the frost table topography having spatially variable storage that has to be filled before water can spill over to generate flow downslope, in a similar manner that bedrock topography controls subsurface flow. However, unlike the bedrock surface, the frost table is variable with time and strongly influenced by the heat transfer involving water. Therefore, it is important to understand the feedback between thawing and subsurface water flow and to properly represent the feedback in hydrological models of permafrost regions

Comparing temporal patterns in body condition of ringed seals living within their core geographic range with those living at the edge

Ecological theory suggests that demographic responses by populations to environmental change vary depending on whether individuals inhabit central or peripheral regions within the species’ geographic range. Here, we tested this prediction by comparing a population of ringed seals Pusa hispida located at high latitudes as part of their core range (core) with a population located at the southern extremity of their range (peripheral). First, we compared the two regions’ environmental trends in timing of sea-ice breakup and freeze-up, open-water duration and the North Atlantic Oscillation (NAO). We found that the core region shifted to progressively warmer conditions in the early 1990s; whereas, in the peripheral region, the warming trend shifted in 1999 to one with no warming trend but high inter-annual variability. Next, we examined how body condition, inferred from blubber depth, responded to temporal changes in sea-ice and climatic variables – variables that have been shown to influence ringed seal demography. Core seals displayed minimal seasonal changes in body condition; whereas peripheral seals displayed a 20–60% amplitude seasonal change in body condition with a phase shift to earlier initiation of fat accumulation and loss. Finally, we tested for interannual differences and found that both core and peripheral seals responded similarly with decreased body condition following more positive NAO. Environmental variables influenced body condition in opposite directions between the two regions with core seals declining in body condition with later spring breakup and shorter open-water duration, whereas peripheral seals showed opposite relationships. Seals living at the core likely benefit from an evolved match between adaptation and environmental variation resulting in dampened seasonal and interannual fluctuations in body condition. Knowledge of how different populations respond to environmental change depending on geographic location within a species range can assist in anticipating population specific responses to climate warming

Scaling and balancing carbon dioxide fluxes in a heterogeneous tundra ecosystem of the Lena River Delta

The current assessments of the carbon turnover in the Arctic tundra are subject to large uncertainties. This problem can (inter alia) be ascribed to both the general shortage of flux data from the vast and sparsely inhabited Arctic region, as well as the typically high spatiotemporal variability of carbon fluxes in tundra ecosystems. Addressing these challenges, carbon dioxide fluxes on an active flood plain situated in the Siberian Lena River Delta were studied during two growing seasons with the eddy covariance method. The footprint exhibited a heterogeneous surface, which generated mixed flux signals that could be partitioned in such a way that both respiratory loss and photosynthetic gain were obtained for each of two vegetation classes. This downscaling of the observed fluxes revealed a differing seasonality in the net uptake of bushes (−0.89 µmol m−2 s−1) and sedges (−0.38 µmol mm−2 s−1) in 2014. That discrepancy, which was concealed in the net signal, resulted from a comparatively warm spring in conjunction with an early snowmelt and a varying canopy structure. Thus, the representativeness of footprints may adversely be affected in response to prolonged unusual weather conditions. In 2015, when air temperatures on average corresponded to climatological means, both vegetation-class-specific flux rates were of similar magnitude (−0.69 µmol m−2 s−1). A comprehensive set of measures (e.g. phenocam) corroborated the reliability of the partitioned fluxes and hence confirmed the utility of flux decomposition for enhanced flux data analysis. This scrutiny encompassed insights into both the phenological dynamic of individual vegetation classes and their respective functional flux to flux driver relationships with the aid of ecophysiologically interpretable parameters. For comparison with other sites, the decomposed fluxes were employed in a vegetation class area-weighted upscaling that was based on a classified high-resolution orthomosaic of the flood plain. In this way, robust budgets that take the heterogeneous surface characteristics into account were estimated. In relation to the average sink strength of various Arctic flux sites, the flood plain constitutes a distinctly stronger carbon dioxide sink. Roughly 42 % of this net uptake, however, was on average offset by methane emissions lowering the sink strength for greenhouse gases. With growing concern about rising greenhouse gas emissions in high-latitude regions, providing robust carbon budgets from tundra ecosystems is critical in view of accelerating permafrost thaw, which can impact the global climate for centuries

Insight into the diet history of ice seals using isotopic signatures of muscle tissue and claws

Thesis (M.S.) University of Alaska Fairbanks, 2012Climate change and sea ice reduction in the Arctic may impact foraging of ice-associated predators. The goal of my thesis work was to examine interannual differences in the diet of ringed, bearded, spotted, and ribbon seals as described by stable nitrogen and carbon isotope ratios of muscle tissue and claws to assess foraging plasticity. Isotopic mixing models from muscle data were used to describe the proportional contribution of prey groups during 2003, 2008-2010. Results showed a higher proportional contribution of smelt (Osmeridae) and benthic prey to ringed and bearded seal diets in 2003 compared to 2008-2010. Seasonal keratin layers deposited in claws can document trophic history up to about 10 years. During 2007 (record ice minimum), proportionally more ringed seals fed at a lower trophic level, while spotted seal adults and young-of-the-year fed at a lower trophic level during 2006. Bearded seals may have been foraging more pelagically from 2008 to 2010. Ice seals may be taking advantage of more abundant pelagic crustaceans as the Arctic ecosystem changes to a pelagic-dominated food web. Interannual variations and high variability among species and individual diets illustrate the opportunistic nature and flexibility of ice seals to changes in prey composition.1. General introduction -- 2. Interannual variations in the diet of ice seals assessed by isotopic mixing models -- 3. Diet history of ice seals using stable isotope ratios in claw growth bands -- 4. General discussion