Remote sensing of lake dynamics in Alaska

Thesis (Ph.D.) University of Alaska Fairbanks, 2016Lakes are abundant in high northern latitude permafrost regions. They are important ecosystem components forming a complex and dynamic landscape with repeated cycles of lake formation and drainage affecting regional hydrological and terrestrial characteristics, biogeochemical processes and carbon cycling, wildlife habitats, and human communities living in the permafrost region. Remote sensing is a useful tool to map the spatial distribution of lakes and assess its change, understand lake dynamics, and to extract useful information to study their associated feedbacks in a changing climate. In this dissertation, I focused on remote sensing studies associated with (1) methane ebullition from a thermokarst lake, (2) post-drainage succession patterns in drained thermokarst lake basins, and (3) lake change dynamics. I developed a semi-automatic classification method based on an Object-based Image Analysis (OBIA) framework to detect methane ebullition bubbles trapped in a snow-free ice-covered lake using high-resolution airborne images of Goldstream lake, Fairbanks, Alaska acquired following freeze up in October of 2011 and 2012. This study showed that remote sensing is a valuable tool to map ebullition bubbles (bubble patches) on the entire lake surface with an accuracy of > 95%, a task that is difficult to achieve through field-based survey alone. The image analysis performed by combining the mapping results from the OBIA and field-based observations showed a relationship between bubble patch brightness and ground-measured methane flux, which was then used to estimate the whole-lake methane flux. A strong inverse exponential relationship (R2 >= 0.79) was found between the percent of the surface area of lake ice covered with bubble patches and distance from the active thermokarst lake margin, indicating high methane production as a response to thermokarst activity that released labile organic-rich carbon along the eroding lake margin. Despite the influence of atmospheric pressure conditions on distribution of ebullition bubble patches following the lake freeze-up events, the spatiotemporal regularity of bubble patches revealed that a larger number of seeps are stable over at least annual timescales. This remote sensing technique is applicable to other regions for mapping ebullition bubbles trapped in snow-free ice-covered lakes, identifying their relative flux, and assessing their spatiotemporal dynamics. By using TerraSAR-X (TSX) Synthetic Aperture Radar (SAR) backscatter data and the Normalized Difference Vegetation Index (NDVI) derived from a Landsat-5 image from the year 2009, I characterized drained thermokarst lake basins (DTLBs) of various age ranging between 0 to 10,000 years since drainage in the northern Seward Peninsula, Alaska. In the study I found logarithmic relationships of basin age from 0 to 10,000 years with mean basin TSX backscatter (R2 = 0.36) and with mean basin NDVI (R2 = 0.53). However, TSX data performed much better to discriminate older basins in the age class 50–10,000 years with R2 = 0.58, while no significant relationship was found between NDVI and basin age. Results of this study demonstrated the potential application of X-band SAR data in combination with NDVI data to enhance differentiation of soil moisture and vegetation status on drained basins for mapping long-term succession dynamics of DTLBs. Finally, I demonstrated the utility of Landsat imagery to identify lake distribution patterns and changes between 1972 and 2014 in six major lake-rich study regions across various permafrost zones covering an area of 68,830 km2 in western Alaska. Even though lake area change was found to be positive (increase by less than 4%) in some study areas while negative (decrease by 4-15%) in others, there was a widespread drainage of mainly large lakes in all regions creating remnant ponds that increased the abundance of lakes <10 ha in all study regions by 2-27%. The average lake area decline observed in various permafrost zones did not represent the trend of individual sites due to spatial heterogeneity in lake characteristics. While lake drainage dominated the non-continuous permafrost zones, areas of continuous permafrost showed both trends of negative and positive lake area change accompanied by major lake drainage events that led to a regional lake area loss in the continuous permafrost zone. This remote sensing technique proved to be useful in identifying ongoing lake drainage and expansion events within study regions and a regional shift in lake distribution (i.e. lake area loss) that is happening in western Alaska. Based upon my research, there is an immense opportunity to use and combine various remote sensing tools to study lake dynamics and to evaluate associated environmental changes. Future work should be directed towards collaborative research for combining field-based observations and remote sensing tools to improve the understanding of how lakes and drained lake basins change in a changing climate as well as extend the scale of observations of methane ebullition features by covering many lakes in an environmentally diverse set of regions. This will guide us to understanding the feedback of lake dynamics to the surrounding ecosystem, global carbon budget, and to upscale the response of lakes to changing climate and permafrost environments to larger regions.Chapter 1. Introduction -- Chapter 2. An object-based classification method to detect methane ebullition bubbles in early winter lake ice -- Chapter 3. Detection and spatiotemporal analysis of methane ebullition on thermokarst lake ice using high-resolution optical aerial imagery -- Chapter 4. Characterizing post-drainage succession in thermokarst lake basins on the Seward Peninsula, Alaska with TerraSAR-X backscatter and Landsat-based NDVI data -- Chapter 5. Landsat-based lake distribution and changes in western Alaska between 1972 and 2014 -- Chapter 6. Summary

Landsat-based lake distribution and changes in western Alaska permafrost regions between the 1970s and 2010s

Lakes are an important ecosystem component and geomorphological agent in northern high latitudes and it is important to understand how lake initiation, expansion and drainage may change as high latitudes continue to warm. In this study, we utilized Landsat Multispectral Scanner System (MSS) images from the 1970s (1972, 1974, and 1975) and Operational Land Imager (OLI) images from the 2010s (2013, 2014, and 2015) to assess broad-scale distribution and changes of lakes larger than 1 ha across the four permafrost zones (continuous, discontinuous, sporadic, and isolated extent) in western Alaska. Across our ca 70,000 km2study area, we saw a decline in overall lake coverage across all permafrost zones with the exception of the sporadic permafrost zone. In the continuous permafrost zone lake area declined by -6.7 % (-65.3 km2), in the discontinuous permafrost zone by -1.6 % (-55.0 km2), in the isolated permafrost zone by -6.9 % (-31.5 km2) while lake cover increased by 2.7 % (117.2 km2) in the sporadic permafrost zone. Overall, we observed a net drainage of lakes larger than 10 ha in the study region. Partial drainage of these medium to large lakes created an increase in the area covered by small water bodies <10 ha, in the form of remnant lakes and ponds by 7.1 % (12.6 km2) in continuous permafrost, 2.5 % (15.5 km2) in discontinuous permafrost, 14.4 % (74.6 km2) in sporadic permafrost, and 10.4 % (17.2 km2) in isolated permafrost. In general, our observations indicate that lake expansion and drainage in western Alaska are occurring in parallel. As the climate continues to warm and permafrost continues to thaw, we expect an increase in the number of drainage events in this region leading to the formation of higher numbers of small remnant lakes

Decadal-scale hotspot methane ebullition within lakes following abrupt permafrost thaw

Thermokarst lakes accelerate deep permafrost thaw and the mobilization of previously frozen soil organic carbon. This leads to microbial decomposition and large releases of carbon dioxide (CO2) and methane (CH4) that enhance climate warming. However, the time scale of permafrost-carbon emissions following thaw is not well known but is important for understanding how abrupt permafrost thaw impacts climate feedback. We combined field measurements and radiocarbon dating of CH4 ebullition with (a) an assessment of lake area changes delineated from high-resolution (1–2.5 m) optical imagery and (b) geophysical measurements of thaw bulbs (taliks) to determine the spatiotemporal dynamics of hotspot-seep CH4 ebullition in interior Alaska thermokarst lakes. Hotspot seeps are characterized as point-sources of high ebullition that release 14C-depleted CH4 from deep (up to tens of meters) within lake thaw bulbs year-round. Thermokarst lakes, initiated by a variety of factors, doubled in number and increased 37.5% in area from 1949 to 2009 as climate warmed. Approximately 80% of contemporary CH4 hotspot seeps were associated with this recent thermokarst activity, occurring where 60 years of abrupt thaw took place as a result of new and expanded lake areas. Hotspot occurrence diminished with distance from thermokarst lake margins. We attribute older 14C ages of CH4 released from hotspot seeps in older, expanding thermokarst lakes (14CCH4 20 079 ± 1227 years BP, mean ± standard error (s.e.m.) years) to deeper taliks (thaw bulbs) compared to younger 14CCH4 in new lakes (14CCH4 8526 ± 741 years BP) with shallower taliks. We find that smaller, non-hotspot ebullition seeps have younger 14C ages (expanding lakes 7473 ± 1762 years; new lakes 4742 ± 803 years) and that their emissions span a larger historic range. These observations provide a first-order constraint on the magnitude and decadal-scale duration of CH4-hotspot seep emissions following formation of thermokarst lakes as climate warms

PeRL:A circum-Arctic Permafrost Region Pond and Lake database

Ponds and lakes are abundant in Arctic permafrost lowlands. They play an important role in Arctic wetland ecosystems by regulating carbon, water, and energy fluxes and providing freshwater habitats. However, ponds, i.e., waterbodies with surface areas smaller than 1. 0 × 104ĝ€m2, have not been inventoried on global and regional scales. The Permafrost Region Pond and Lake (PeRL) database presents the results of a circum-Arctic effort to map ponds and lakes from modern (2002-2013) high-resolution aerial and satellite imagery with a resolution of 5ĝ€m or better. The database also includes historical imagery from 1948 to 1965 with a resolution of 6ĝ€m or better. PeRL includes 69 maps covering a wide range of environmental conditions from tundra to boreal regions and from continuous to discontinuous permafrost zones. Waterbody maps are linked to regional permafrost landscape maps which provide information on permafrost extent, ground ice volume, geology, and lithology. This paper describes waterbody classification and accuracy, and presents statistics of waterbody distribution for each site. Maps of permafrost landscapes in Alaska, Canada, and Russia are used to extrapolate waterbody statistics from the site level to regional landscape units. PeRL presents pond and lake estimates for a total area of 1. 4 × 106ĝ€km2 across the Arctic, about 17ĝ€% of the Arctic lowland ( &lt; ĝ€300ĝ€mĝ€a.s.l.) land surface area. PeRL waterbodies with sizes of 1. 0 × 106ĝ€m2 down to 1. 0 × 102ĝ€m2 contributed up to 21ĝ€% to the total water fraction. Waterbody density ranged from 1. 0 × 10 to 9. 4 × 101ĝ€kmĝ'2. Ponds are the dominant waterbody type by number in all landscapes representing 45-99ĝ€% of the total waterbody number. The implementation of PeRL size distributions in land surface models will greatly improve the investigation and projection of surface inundation and carbon fluxes in permafrost lowlands. Waterbody maps, study area boundaries, and maps of regional permafrost landscapes including detailed metadata are available at https://doi.pangaea.de/10.1594/PANGAEA.868349

Detection and spatiotemporal analysis of methane ebullition on thermokarst lake ice using high-resolution optical aerial imagery

Thermokarst lakes are important emitters of methane, a potent greenhouse gas. However, accurate estimation of methane flux from thermokarst lakes is difficult due to their remoteness and observational challenges associated with the heterogeneous nature of ebullition. We used high-resolution (9–11 cm) snow-free aerial images of an interior Alaskan thermokarst lake acquired 2 and 4 days following freeze-up in 2011 and 2012, respectively, to detect and characterize methane ebullition seeps and to estimate whole-lake ebullition. Bubbles impeded by the lake ice sheet form distinct white patches as a function of bubbling when lake ice grows downward and around them, trapping the gas in the ice. Our aerial imagery thus captured a snapshot of bubbles trapped in lake ice during the ebullition events that occurred before the image acquisition. Image analysis showed that low-flux A- and B-type seeps are associated with low brightness patches and are statistically distinct from high-flux C-type and hotspot seeps associated with high brightness patches. Mean whole-lake ebullition based on optical image analysis in combination with bubble-trap flux measurements was estimated to be 174 ± 28 and 216 ± 33 mL gas m−2 d−1 for the years 2011 and 2012, respectively. A large number of seeps demonstrated spatiotemporal stability over our 2-year study period. A strong inverse exponential relationship (R2 >  =  0.79) was found between the percent of the surface area of lake ice covered with bubble patches and distance from the active thermokarst lake margin. Even though the narrow timing of optical image acquisition is a critical factor, with respect to both atmospheric pressure changes and snow/no-snow conditions during early lake freeze-up, our study shows that optical remote sensing is a powerful tool to map ebullition seeps on lake ice, to identify their relative strength of ebullition, and to assess their spatiotemporal variability

Western Alaska Lake Database

This vector data layer covers 6 major lake districts (Baldwin Peninsula, Kobuk Delta, Selawik Lowland, Northern Seward Peninsula, Central Seward Peninsula, and Yukon-Kuskokwim Delta) in the northern and central sub-regions of the Western Alaska Landscape Conservation Cooperative (WALCC) region and consists of polygons of lakes with areas equal or greater than 1.0 ha. More than 58000 Lakes were mapped from Landsat TM and ETM+ imagery acquired between 1972 and 1975, 2002 and 2009, and 2013 and 2014 using Object-Based Image Analysis (OBIA) techniques with an classification accuracy of 96%. The spatial image resolution of Landsat TM and ETM+ is 30 m. Permafrost characteristics and surficial geology associated with lake polygons were determined from the Alaska permafrost map (Jorgenson et al. 2008)