IOP PUBLISHING ENVIRONMENTAL RESEARCH LETTERS Environ. Res. Lett. 6 (2011) 014015 (9pp) doi:10.1088/1748-9326/6/1/014015 Opportunities and limitations to detect climate-related regime shifts in inland Arctic ecosystems through eco-hydrological monitoring Johanna Mard �� Karlsson1,2,4, Arvid Bring1,2, Garry D Peterson3, Line J Gordon3 and Georgia Destouni1,2 1 Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91 Stockholm, Sweden 2 Bert Bolin Center for Climate Research, Stockholm University, SE-106 91 Stockholm, Sweden 3 Stockholm Resilience Center, Stockholm University, SE-106 91 Stockholm, Sweden E-mail: johanna.maard@natgeo.su.se Received 31 December 2010 Accepted for publication 2 March 2011 Published 18 March 2011 Online at stacks.iop.org/ERL/6/014015 Abstract This study has identified and mapped the occurrences of three different types of climate-driven and hydrologically mediated regime shifts in inland Arctic ecosystems: (i) from tundra to shrubland or forest, (ii) from terrestrial ecosystems to thermokarst lakes and wetlands, and (iii) from thermokarst lakes and wetlands to terrestrial ecosystems. The area coverage of these shifts is compared to that of hydrological and hydrochemical monitoring relevant to their possible detection. Hotspot areas are identified within the Yukon, Mackenzie, Barents/Norwegian Sea and Ob river basins, where systematic water monitoring overlaps with ecological monitoring and observed ecosystem regime shift occurrences, providing opportunities for linked eco-hydrological investigations that can improve our regime shift understanding, and detection and prediction capabilities. Overall, most of the total areal extent of shifts from tundra to shrubland and from terrestrial to aquatic regimes is in hydrologically and hydrochemically unmonitored areas. For shifts from aquatic to terrestrial regimes, related water and waterborne nitrogen and phosphorus fluxes are relatively well monitored, while waterborne carbon fluxes are unmonitored. There is a further large spatial mismatch between the coverage of hydrological and that of ecological monitoring, implying a need for more coordinated monitoring efforts to detect the waterborne mediation and propagation of changes and impacts associated with Arctic ecological regime shifts. Keywords: Arctic, climate change, regime shifts, eco-hydrology, hydrology, biogeochemical cycling, permafrost, ecosystem dynamics, feedbacks, monitoring S Online supplementary data available from stacks.iop.org/ERL/6/014015/mmedia 1. Introduction Detecting, monitoring, and anticipating the ecological consequences of climate change is a particular challenge in the 4 Author to whom any correspondence should be addressed. Arctic. The consequences of Arctic change are complicated by a dynamic cryosphere, shifting hydrological connections, and ecological dynamics that can cause surprising reorganizations of ecological structure and function. Climate change is rapidly transforming the Arctic (White et al 2007, Francis et al 2009). In the past century, the 1748-9326/11/014015+09$33.00 �� 2011 IOP Publishing Ltd Printed in the UK 1

Environ. Res. Lett. 6 (2011) 014015 J M Karlsson et al Arctic has warmed much faster than the planet as a whole. Temperatures in the north (40���70 ���N) have increased up to 2���3 ���C over the past 50 years (ACIA 2005), much more than the 0.65 ���C increase in global mean surface air temperature during the same period (IPCC 2007). Current temperatures are the highest experienced in the Arctic in the past 400 years (Overpeck et al 1997), and these highs are forecast to be exceeded by a further 2.5 ���C by the mid-21st century, and up to 5���7 ���C by the end of the 21st century (ACIA 2005). Arctic warming has triggered substantial changes in the cryosphere. Annual snow cover has declined by 10% since 1972 (Serreze et al 2000, Hinzman et al 2005, White et al 2007, Francis et al 2009). The September sea ice extent has declined about 30% (���0.7 million km2/decade, linear trend) during the period 1979���2008 (Holland et al 2010). Retreat or melting of glaciers and ice caps has increased melt water flows to the Arctic Ocean, with the glacier contribution increasing by up to 42% between 1961���1992 and 1993���2006 (Dyurgerov et al 2010). Warming of the air has warmed the permafrost, ground that has been frozen for two or more consecutive years, which underlies and extends over 20���25% of the exposed land surface in the Arctic (Osterkamp and Romanovsky 1999, Serreze et al 2000). For instance, in northern Alaska, deep boreholes have measured a temperature increase of between 2 and 4 ���C during the last 50���100 years, with an additional warming of 3 ���C since the late 1980s (Lachenbruch and Marshall 1986, Nelson et al 2001, Yoshikawa and Hinzman 2003, Hinzman et al 2005). Similar patterns have been observed in the Nordic area with increasing active layer depths and rising permafrost temperatures during the past 10 years (Christiansen et al 2010), and in Russia, measurements in boreholes show substantial warming of permafrost during the past 20���30 years (Romanovsky et al 2010). Continued warming is expected to deepen the active layer (seasonally frozen ground) and cause a northward movement of the permafrost boundaries between areas of continuous, discontinuous, and sporadic permafrost (ACIA 2005). Ecosystem responses to Arctic warming are largely determined by changes in the cryosphere, which are in turn also reflected in observable hydrological changes (Rowland et al 2010). The latter have been demonstrated in terms of changes in the characteristic behavior of hydrological discharge dynamics (Lyon et al 2009, Lyon and Destouni 2010), as well as in soil moisture, drainage patterns and surface runoff (Prowse et al 2006) in permafrost regions. Frey and McClelland (2009) and Rowland et al (2010) have shown that permafrost degradation may have significant consequences on the Arctic freshwater system by causing a transition from a surface water-dominated system to a groundwater- dominated system. Furthermore, Bense et al (2009) have presented a process-based model clarification of the essential links between groundwater hydrology and permafrost change, while Lyon et al (2010) have done so for the links between groundwater hydrology and the waterborne carbon cycle in permafrost regions, and a series of other studies for the groundwater hydrology links to nitrogen and phosphorus flux dynamics (Baresel and Destouni 2005, 2006, Lindgren et al 2007, Darracq et al 2008, Destouni and Darracq 2009, Destouni et al 2010, Basu et al 2010). These studies all support earlier assessments of Hodkinson et al (1999) and Chapin et al (2006) that changes in hydrological conditions both reflect and contribute to essential transformations of the functioning of inland Arctic ecosystems, because water in its various forms couples the biotic and the abiotic components of the Arctic environment. The combination of gradual climate change and multiple ecological feedback processes, many of which include and are propagated by water, can cause Arctic ecosystems to shift, from one set of mutually reinforcing feedbacks to another (Holling 1973, Scheffer and Carpenter 2003) (see also supplementary information S1 and figure S1 available at stacks.iop.org/ERL/6/014015/mmedia). Such ecological regime shifts have been described for a diverse set of ecosystems worldwide, including tropical coral reefs, savannas, temperate lakes, and coastal zones (Folke et al 2004, Drever et al 2006, Gordon et al 2008). In the inland Arctic, a number of ecological regime shifts have been observed but not yet been systematically analyzed. Understanding if and which ecological changes represent rapid regime shifts is important because these shifts can in turn quite rapidly reduce important ecosystem services. Analyzing regime shifts that have already occurred and their spatio-temporal occurrence patterns is also important for identification of the key variables, characteristics and types of related changes that can trigger and be used to detect or monitor Arctic ecological change. Detecting early warning signs of ecological regime shifts requires substantial monitoring data from both biotic and abiotic ecosystem components (Scheffer et al 2009). Rowland et al (2010) have outlined how and why ecosystem responses to Arctic warming are largely determined by cryosphere changes, but these changes are difficult to observe directly. However, as discussed above, changes in the cryosphere can be observed in changes in hydrological fluxes. Hydrological flux changes thus reflect cryosphere changes that are in turn related to Arctic ecological change (Hodkinson et al 1999, Chapin et al 2006). Changes in both water fluxes and waterborne mass fluxes should therefore be expected to reflect climate, cryosphere and ecological condition changes in the catchment areas that feed into the flux observation points. Support for this expectation has, for instance, been demonstrated by quantification of the links between groundwater hydrology and waterborne carbon cycling (Lyon et al 2010), and changes in essential characteristics of hydrological discharge dynamics (Lyon et al 2009, Lyon and Destouni 2010) in permafrost regions. However, the use of hydrological monitoring to detect ecological changes may be limited by gaps and decline in available Arctic hydrological monitoring data, in particular with regard to the largely unmonitored waterborne nutrient transport (Bring and Destouni 2009) and the fact that, across the Pan-Arctic, river basins with the largest expected climatic changes have the least dense and most declining hydrological monitoring (Bring and Destouni 2010). This study aims to identify key opportunities and limitations in the Arctic eco-hydrological monitoring windows, i.e., in the systematic hydrological monitoring records 2