FROM SINK TO RESURGENCE: THE BUFFERING CAPACITY OF A CAVE SYSTEM IN THE TONGASS NATIONAL FOREST, USA OD PONORA DO IZVIRA: PUFRSKA KAPACITETA JAMSKIH SISTEMOV V NARODNEM GOZDU TONGASS, ZDA Melissa R. HENDRICKSON1 & Chris GROVES2 Abstract UDC 551.444(739.8) Melissa R. Hendrickson & Chris Groves: From sink to resurgence: the buffering capacity of a cave system in the Tongass national forest, USA The Tongass National Forest of Southeast Alaska, USA, provides a unique environment for monitoring the impact of the cave system on water quality and biological productivity. The accretionary terrane setting of the area has developed into a complex and heterogeneous geologic landscape which includes numerous blocks of limestone with intense karstification. During the Wisconsian glaciation, there were areas of compacted glacial sediments and silts deposited over the bedrock. Muskeg peatlands developed over these poorly drained areas. The dominant plants of the muskeg ecosystem are Sphagnum mosses, whose decomposition leads to highly acidic waters with pH as low as 2.4. These waters drain off the muskegs into the cave systems, eventually running to the ocean. In accordance with the Tongass Land Management Plan, one of the research priorities of the National Forest is to determine the contributions of karst groundwater systems to productivity of aquatic communities. On Northern Prince of Wales Island, the Conk Canyon Cave insurgence and the Mop Spring resurgence were continuously monitored to understand the buffering capacity of the cave system. Over the length of the system, the pH increases from an average 3.89 to 7.22. The insurgence water temperature, during the summer months, ranged from between 10oC to 17oC. After residence in the cave system, the resurgence water had been buffered to 6oC to 9oC. Over the continuum from insurgence to resurgence, the specific conductance had increased by an order of magnitude with the resurgence waters having a higher ionic strength. The cave environment acts as a buffer on the incoming acidic muskeg water to yield resurgence water chemistry of a buffered karst system. These buffered waters contribute to the Izvleček UDK 551.444(739.8) Melissa R. Hendrickson & Chris Groves: Od ponora do izvira: Pufrska kapaciteta jamskih sistemov v Narodnem gozdu Tongass, ZDA Narodni gozd Tongass, na jugovzhodni Aljaski, ZDA, je izjemo okolje za spremljanje vpliva jamskega sistema na kvaliteto vode in z njo povezano biološko produkcijo. Geološko pestro območje vključuje številna območja apnenca z intenzivnim zakrasevanjem. Med zadnjo ledeno dobo so se na karbonatno podlago ponekod odložili ledeniški sedimenti in melji. Tu so se zaradi slabega odvodnjavanja razvila barja (muskegi), na katerih prevladuje šotni mah. Zaradi razpada šotnih mahov iz barij v kras odtekajo zelo kisle vode, z minimalnim pH 2,4. Prostorsko ureditveni načrt območja Tongass med prioritetne naloge uvršča tudi določitev pomena kraških vodonosnikov v produktivnosti vodnih združb. Z namenom določitve pufr-ske kapacitete kraških sistemov smo zvezno na severnem delu otoka Walškega princa opazovali parametre na ponoru v jami Cong Canyon in izviru Mop. Med opazovanima točkama pH v povprečju naraste iz 3,89 na 7,22. Temperatura ponorne vode je v poletnih mesecih med 10°C in 17°C, voda na izviru pa med 6°C in 9°C. Med ponorom in izvirom specifična električna prevodnost naraste skoraj za red velikosti. Jamsko okolje deluje za kot pufer za kisle vode, ki dotekajo z barij. Vode, ki se pre-cejajo skozi kras, zato pomembno vplivajo na dolvodno produkcijo vodnih združb. Vode iz opazovanega sistema se stekajo v Whale Pass, ki predstavlja pomembno mesto v industriji lososa. Naša opazovanja so potrdila, da pufrski učinek kraškega sistema zagotavlja kemično in temperaturno ugodno vodo in s tem pomembno vpliva na produktivnost mladih lososov vrste Oncorhynchus kisutch (angl. coho salmon). Ključne besede: kras, pufrska kapaciteta, barje (muskeg), Narodni gozd Tongass, Aljaska. 1 Ashley National Forest, Vernal, Utah, e-mail: melissarhendrickson@gmail.com 2 Western Kentucky University, Bowling Green, Kentucky, e-mail: chris.groves@wku.edu Received/Prejeto: 4.3.2010 productivity in aquatic environments downstream. The waters from this system drain into Whale Pass, an important location for the salmon industry. The cool, even temperatures, as well as buffered flow rates delivered by the karst systems are associated with higher productivity of juvenile coho salmon. Keywords: karst, buffering, muskeg, Tongass National Forest, Alaska. INTRODUCTION: THE KARST ENVIRONMENT OF SOUTHEAST ALASKA The Tongass National Forest is comprised of 6.9 million hectares of coastal temperate rainforest. The geologic history of the area led to the development of the highly fractured and fragmented Alexander Terrane that makes up the pan handle of Alaska. Blocks of limestone are numerous and have undergone intense karstification. Tills deposited during the Wisconsin (Marine Isotope Stage 2) glacial episode created areas of impermeable clays and silts perched over carbonate bedrock. Muskeg peat bogs, composed of upper layers of living sphagnum moss and lower layers of partially decomposed sphagnum moss, developed on these poorly drained glacial deposits. The combination of these layers makes up the fibrous brown peat which characterizes a muskeg. Decomposition of dead plants is inhibited by the absence of oxygen; therefore the muskegs are rich in humic substances. The presence of these humic substances result in peatlands, typically having highly acidic water with pH ranging from 2.4 to 5.8 (Elliot 1994). In some locations, the highly acidic waters from these muskegs run onto the carbonate strata and enter the groundwater system through developed cave systems. These waters later resurge downstream as buffered karst streams which play an important role in the fishing industry. It has been suggested that karst dominated aquatic systems support higher biodiversity than nonkarst areas because of the high calcium concentrations and may even have higher growth rates for some species (Swanston 1993). One research priority of the National Forest, as stated in the Tongass Land Management Plan, is to determine the contributions of the karst groundwater systems to the productivity of the aquatic communities. The first step in investigating this relationship is to understand the changes in water chemistry that occur from acidic insurgence to buffered resurgence. STUDY AREA The study area is located in the Tongass National Forest, Northern Prince of Wales Island, Alaska in the Conk Canyon - Mop Spring drainage system (Fig. 1). Dye tracing has shown that water draining off the peatlands flows into the karst of Conk Canyon, which resurges at Mop Spring and eventually flows out Whale Pass (Prussian & Baichtal 2003). While the Conk Canyon muskeg is not the only input to the system, it is assumed for this study that it is representative of the multiple muskeg drain- ages that also resurge at Mop Spring. Conk Canyon is a single muskeg catchment area of around 2 hectares (GIS approximation); whereas Mop Spring has inputs from multiple sources over a 200 hectare watershed. While this may be a small fraction of the spring flow, other allogenic inputs in the area coming off the muskeg have similar chemistry signatures. Due to the geology of the area with the glacial hardpan overlying the karst, most of the water in the system occurs as allogenic recharge. Fig. 1: Study location with acidic insurgence of Conk Canyon and the karst Mop Spring. Twin Island Lake then flows to whale Pass. STUDY METHODS Continuous data measurements of pH, temperature, stage, and specific conductance were taken in the waters of the Conk Canyon muskeg and the Mop Spring resurgence. A Campbell Scientific CR10x datalogger was used with a two minute resolution to capture the changing natural conditions. The pH was measured with Cole-Parmer double-junction industrial in-line ATC pH sensors connected to a 3 meter shielded coaxial cable which terminated in the instrument box at a Cole-Parmer preamplifier to increase signal stability. Measurements of pH were recorded in millivolts and had an accuracy of +/- 0.01 SU. The values the probes recorded were assumed to drift over the course of the study. To correct for this drift, they were calibrated with three pH standards, 4.01, 7.00, and 10.00 to create a regression and line equation to obtain the pH values. CSI CS547A-L specific conductance/temperature sensors were used with accuracy of +/-0.1oC and +/- 0.001 mS. Stage was measured by Campbell Scientific Druck PDCR 1830-8388 submersible pressure transducers with an accuracy of +/- 0.1% FSO. The dataloggers were installed on Julian Day 166 of 2006 (June 15) and taken offline on Julian Day 236 (August 24) giving a data collection period of 70 days. The rainfall data was collected once daily at a gauge located in the area. The continuous temperature data was obtained from the weather monitoring station in Thorne Bay, Alaska. RESULTS AND DISCUSSION The continuous data was collected and analyzed to determine the evolution of the water from insurgence to resurgence. One buffering effect of the cave system is on the temperature of the water. The muskeg waters of the Conk Canyon insurgence are open to the atmosphere, Fig. 2: Water temperature at the Conk Canyon muskeg and air temperature. Fig. 3: water temperature and stage height for the Mop Spring datalogger site. and thus the water temperature reflects the average daily air temperature for the study period (Fig. 2). The larger fluctuations in air temperature, as seen around Julian Day 180 are during good weather. Because of the clear skies, the temperatures tend to be higher during the day and lower at night because there is no cloud cover to keep the heat overnight. When there was cloudy weather, the night and day temperature differences were less. The temperature range for the insurgence waters was between 10°C to 17°C. The water temperature exiting the cave system at Mop Spring showed a greater correlation on the stage height of the water coming out of the system (Fig. 3). After residence in the cave system, the resurgence water had been buffered to 6 °C to 9 °C. During the beginning of the study, the water coming out of the karst system was a little above 6 °C. From June 5'h, 2006 to June 22nd, 2006 there were no large precipitation events. From the 22nd onwards, when there was an increase in stage from precipitation, there was an increase in the water temperature resurg-ing at Mop Spring. This increase reflects the input of water into the system that hasn't had time to reach equilibrium with the groundwater temperature. After the first rain event, the relaxation time was never long enough before the next storm for the water temperature to return to equilibrium with the groundwater temperature. Over the course of the study period, there is a increasing trend in the resurgence water temperature. This increase in temperature is both from the increase in water coming through the system that hasn't had time to equilibrate as well as the increase in the water temperature from the input system (seen in Fig. 2) over the study period. The most apparent change in water chemistry from the insurgence to the resurgence is the increase in pH (Fig. 4). The pH averaged 3.89 in the acidic waters of the muskeg at Conk Canyon. It ranged from a low of 3.60 to a high of 4.27 over the study period. The lowest pH values were recorded at the beginning of storm events when the stage increased. During the increased rainfall, there is more water traveling through the muskeg and draining into the karst. The 7 - 6 - I Q. 5 - 4 - Mop Spring V-—^—Nv------- Conk Canyon Musl