Coastal erosion: long-term patterns and short-term processes
The TLSA is known to have some of the highest rates of coastline erosion in the Arctic (Jorgenson and Brown 2005; Reimnitz et al. 1988) with the most recent measurements for the entire north-facing coastline of 13.6 m/year between 2002 and 2007 compared to 8.7 m/year between 1979 and 2002 and 6.8 m/year between 1955 and 1979 (Jones et al. 2009b). We have updated the recent erosional patterns of this coastal segment by analyzing high-resolution satellite imagery acquired 14–20 July 2009. This comparison of coastline positions with the 2007 imagery indicates a mean annual erosion rate of 17.1 ± 3.1 m/year, therefore the increase in erosion rates documented between 2002 and 2007 has been sustained. We were also able to assess seasonal erosional patterns for a 750-m stretch of coast near Drew Point (Figs. 1, 3) in the context of driving processes. At this site, the bluff-face is relatively high, typically >3 m, thus representing a segment of coastline where we expected few storm-surge floods and only unidirectional loss of coastline habitat.
This more recent and higher temporal resolution analysis of land loss captures some interesting patterns of seasonal and inter-annual variability which allow us to compare environmental conditions during periods with differences in erosional magnitude (Table 1). During 5 nearly equal time periods when the coastline was exposed to the open ocean, erosional loss ranged from 2.1 m in late summer and fall 2008 to 12.2 m in the previous summer and fall 2007 (Table 1). The early summer period in 2009 also had high erosion losses. We found little correlation with higher temporal resolution measurements to storm events, SST, and coastline permafrost temperatures, factors that are known to regulate thermo-mechanical niche erosion along the Beaufort Sea coast (Jones et al. 2009b, Reimnitz et al. 1988). Particularly interesting is the lack of coherence between storm events and erosion rates with the single westerly storm corresponding to only moderate amounts of erosion and high erosion rates occurring in 2009 in the absence of any effective storm event. It is understood, however, that the westerly storm in 2008 produced a regional cooling affect on air and surface-waters and that 2007 had abnormally high air and surface water temperatures due to a persistent high pressure system (Arp et al. in press, Jones et al. 2009c). This sharp interannual climate variation was not captured in the mean SST and soil temperature for these periods; still we suspect that these conditions played a role in controlling the amount of thermal erosion of ice-rich bluffs. Currently, efforts are underway to improve data collection in the N-TLSA to better understand the processes driving this high degree of interannual variation in erosion rates observed in this study (Jones et al. 2009d).
Storms: long-term patterns and short-term responses
Coastal storms are known to have a profound effect on coastal erosion and can generate storm surge flooding during certain conditions (Manson et al. 2005). Longshore currents along the Beaufort Sea coast are consistently from the east (Walker 1985) such that westerly storms raise sea levels while easterly storms lower sea levels (Reimnitz and Maurer 1979; Reimnitz et al. 1988). Sea ice typically is absent from the coastline from July through September, but this may vary greatly from year to year and in 2007 sea ice reached a record minimum extent (Stroeve et al. 2007). Thus, the effectiveness of storms is not only determined by duration and magnitude, but also direction and timing. These processes coupled with sea ice extent interact to govern how effective storms are upon on coastline habitats through both flooding and erosion (Lynch et al. 2004).
Analysis of records from Barrow, AK in our study, along with more detailed analysis by Lynch et al. (2003), indicates both an increase in the frequency and magnitude of effective storms (Fig. 5). It appears, however, that storm severity may vary over relatively short distances along the ACP coastline. A comparison between the Barrow meteorological data and a short-term dataset from Drew Point (2005–2009) indicates that storms impacting the TLSA may not reach the same magnitude of storms recorded in Barrow (Fig. 5); however, this may also be due to differences in the height of the anemometer above the ground surface. Whether or not this relationship holds over longer time periods, it is notable that from 1971 to 1981 there were a low number and magnitude of storms, particularly westerly storms with only a low intensity storm in late October 1976. Prior to this low storm period, there was a major westerly storm around 13 September 1970 according to analysis by Reimnitz and Maurer (1979), which suggested that this was a 100-year storm event and produced a substantial storm surge exceeding 3 masl throughout much of the western and central Beaufort Sea coastline. Our analysis from Barrow, however, places this westerly event as relatively minor, further underscoring the variability in storm system behavior along the ACP coastline. Major storms impacted Barrow in 1963 and 2000 (Lynch et al. 2003) and it is uncertain to what degree these impacted the TLSA, although we note a number of lake flooding events in 2000 (Fig. 4). Another lull in westerly storms happened recently, starting in 2005 with only one storm occurring around 31 July 2008. The same period had seven easterly storms recorded at Barrow, yet only two were recorded in the TLSA at Drew Point (Fig. 5). These comparisons underscore the need for local monitoring networks to be established and maintained in areas of long-term research interest, such as the TLSA.
The role of easterly versus westerly winds on storm surge magnitude and impact to lakes was best described by comparing two storms in 2008—the westerly storm on 31 July and an easterly storm on 10 October (Fig. 6). These events were of similar duration, 12 and 18 h, respectively, and mean wind velocity, 10.1 and 11.4 m/s, respectively, yet had very different impacts on responses of four coastal lakes (Fig. 6). After the westerly storm, the low-lying lakes 53 and 132 with direct sea connectivity through outlet streams, peaked at 0.7 and 0.5 m, respectively, above mean lake levels of 0.2 masl. After the easterly storm, however, Lake 53 levels increased by only 0.2 m and there was no discernable response at Lake 132. The lack of response, at these very low elevation lakes during the easterly storm event, emphasizes the importance of storm direction in controlling regional sea levels in the Beaufort Sea to a much greater extent than angle of wave attack (Reimnitz and Maurer 1979). Tidal flux in this portion of the Beaufort Sea is low, 40 cm (NOAA Prudhoe Bay Tide Gage) and this signal can be observed in our gaging data at Lake 132 during non-storm periods. Lakes 190 and 195 are at slightly higher elevation (>1 masl) without direct sea connectivity, but occur <0.5 km from coastlines facing north to east and east to south, respectively (Fig. 1). Little to no response was recorded at either lake beside increased seiche; indicating that the storm surge for both events was <1 masl (Fig. 6). The lower-lying lakes that were impacted by the westerly storm actually are located much farther inland, but had well-defined surface connectivity to the coast. Such responses emphasize the need for high-resolution topographic data for these coastal environments, as well as local meteorological data and other environmental monitoring. Analysis of these events helps us place the longer-term record from Barrow into an appropriate context in terms of effects on the TLSA coastline and storm surges inland. These events likely play a consequential role in controlling both terrestrial and aquatic habitat and food webs, along with potentially creating direct disturbances to wildlife populations during short and dynamic Arctic summers.
Impacts to habitat from coastal erosion and storm surge flooding
Not only is the conversion of land to sea a dramatic change, but consideration of the proportion of various tundra types lost to coastal erosion is necessary from a habitat perspective (Table 2). Moss-peat shoreline and halophytic vegetation, types strongly preferred by black brant (Bollinger and Derksen 1996; Flint et al. 2008), comprised 0.1 and 1.6% of the landscape, respectively. Sedge meadow, heavily utilized forage by caribou during certain times of the year, comprised 24% of the landscape in the N-TLSA. Comparison of land loss due to erosion showed a proportionally higher loss of moss-peat shorelines, 6.1%, and moist sedge meadow, 32%, than the overall abundance of these tundra classes throughout the N-TLSA (Table 2). This mainly reflects differences in composition of coastal vegetation versus inland vegetation in the N-TLSA, but may also relate to how certain wildlife populations select forage and the resources adjacent to shoreline ecotones with very different microclimates and proximate habitats, such as the gradient from the Beaufort Sea coastline inland. Tundra vegetation types including moist grass/sedge meadow and dwarf shrub—graminoid classes were proportionally unchanged by erosional losses in the N-TLSA (Table 2).
Recent erosion along Drew Point also resulted in the tapping and drainage of a pond located on a relatively high landscape position (Fig. 3). Such small waterbodies provide important habitat for certain shore-bird species, particularly during early summer when larger waterbodies retain ice cover. While it is difficult to document tapping of such small waterbodies with moderate resolution remotely sensed imagery, we were able to identify in delimited periods of time the tapping and drainage of many other larger lakes, which also provide essential habitat. For example, black brant typically use larger oriented lakes during their molt, likely because they provide a refuge from predators coupled with ready access to shoreline grazing lawns (Bollinger and Derksen 1996, Flint et al. 2008). Several other lakes along this segment of coastline have been tapped and drained by coastal erosion (Fig. 4)—a common feature of the lake-rich coastal plains that provide a diverse habitat mosaic (Ruz et al. 1992). The role of such small or shallow lakes and wetlands in providing waterbird habitat may not be fully recognized in the ACP, but is likely shifting along these rapidly eroding coastlines with corresponding responses in avifauna and other wildlife populations. Remote sensing analysis indicated the timing of at least four lakes captured by coastal erosion since 1955, all of which were in the eastern portion of the TLSA (Fig. 4). Other accounts describe tapping of a very large lake in the TLSA, now Pogik Bay, somewhere between 1854 and 1919 (Jones et al. 2008), which emphasizes that this type of dramatic habitat conversion is not necessarily a new phenomena along this coastline, and has likely occurred throughout the Holocene. Still, with the documentation of much higher erosion rates that appear to correspond to low autumn sea ice extent (Jones et al. 2009b), it is reasonable to suppose that this type of habitat conversion is happening at a much faster rate with uncertain consequences to Arctic coastline biological communities (Flint et al. 2008).
Storm events, and their direction and timing, play a consequential role in shaping ACP coastlines (Ruz et al. 1992) and influence coastal vegetation communities and successional patterns (Taylor 1981). We defined the zone of potential saltwater flooding as areas ≤ 2.5 masl with direct connections to the coastline. This maximum flood elevation was based on an estimate from Reimnitz and Maurer (1979) of the 100-year storm surge. This flood-prone zone covered 477 km2 or 41% of the N-TLSA where vegetation has been classified (Markon and Derksen, 1994), including much of the eastern low-lying tundra and lakes (Table 2; Fig. 7). Areas of distinct salt-burned tundra, all were well within the maximum flooding zone and were 71 km2 in extent or 6% of the N-TLSA (Table 2; Fig. 7). Salt-burned tundra, likely due to salt-burn as indicated by necrosis (Taylor 1981), appeared most common in and around DTLBs being eroded into the sea and along the low gradient outlets of the Smith and Kogru Rivers and connected lakes. Most of the salt-burned tundra areas were formerly sedge meadow with varying moisture conditions (Table 2). Interestingly, our analysis identified little salt-affect on moss-peat shoreline types (brown mosses), though studies of storm surge flooding and salt-affects from the Yukon-Kuskokwim Delta suggest that bryophyte communities are most sensitive to saltwater, however, these are predominantly Sphagna at a differing landscape position (Jorgenson 2000, Jorgenson and Ely 2001). The rate at which vegetation recovers from salt-burn and how this may vary by plant community, soil type, and landscape position is uncertain, but warrants further study.
Lake water salinity, as measured by SC, within the flooding zone ranged from values near seawater of 49,000 μS/cm in Lake 188 and 41,000 μS/cm in Lake 181 to as low as 230 μS/cm in Lake 106 (Fig. 7). Much of the variation in lake salinity can be explained by elevation, which influences susceptibility to flooding, and the degree to which freshwater inflow, flushes and dilutes intruded saltwater (Arp et al. unpublished data), the latter related to watershed area. Salinity of individual lakes may shift dramatically over time; for example, SC in Lake 188 increased by >600% between 1977 and 2006. Hourly SC monitoring in lakes 181 and 132 in 2007 and 2008, however, shows strong day-to-day variation in lake salinization and freshening, related to tidal fluctuations, wind events (even weak ones), and rainfall-runoff patterns. Therefore, the exact timing of sample collection relative to storm surge inundation is critical, and long-term trends may not be safely interpreted unless short-term variability is accounted for.
Our remote sensing analysis of lake flooding from storm surges helped bracket chronology of lake change events since 1955 and show at least seven lakes that fully or partly flooded with seawater (Fig. 4). The precision of this analysis increased during recent periods where we have more imagery available, including SAR. Still, this chronology suggests that recent flooding of Lake 125, a large oriented thaw lake known to support molting brant, occurred between 2000 and 2002, along with three other small lakes and ponds. The Barrow storm record shows very strong westerly storms in August 2000 (Lynch et al. 2003) and October 2002 (Fig. 5), which may have been responsible for flooding of these lakes. A large shallow lake, 181, within an oriented DTLB, also became open to frequent seawater flooding sometime between 1985 and 2000, during which time at least 10 westerly storms of magnitude and duration >100 storm power value were recorded (Figs. 4, 5).
Erosion likely has opened up pathways inland for storm surge flooding exacerbating its effect, particularly along portions of the ACP where microtopograhic relief often far exceeds regional elevation gradients. With such slight elevational gradients in a landscape with an increasingly dynamic surface, due to thermokarst processes (Jorgenson et al. 2006, Lawrence et al. 2008), land-deflation (Couture and Pollard 2007), and dynamic coastline (Jones et al. 2009b) as in the N-TLSA, even moderate estimates of sea level rise may greatly enhance these two mechanisms of Arctic coastal habitat change. Many other circumpolar coastal zones have similar attributes, also with very high lake densities, and may be similarly susceptible to the interacting mechanisms of erosion and flooding with uncertain consequences to aquatic and terrestrial habitat.