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Determining the Impact of Shoreline Retrogressive Thermokarst Slumping to Tundra Lakes
Biological Oxygen Demand Measured with the Fibox 3 LCD trace
Paul Moquin & Fred Wrona
University of Victoria, CA
Permafrost temperatures in the high Arctic have been rising causing a deepening of the active layer and an increase in thermokarst activity. Permafrost degradation, often occurring directly adjacent to lakes, has been associated with terrestrial inputs to the lacustrine environment. This study used an in situ mesocosm approach to determine the potential impacts of permafrost degradation on the basal components of the food web. Varying amounts of permafrost related sediments were added to mesocosms which were installed in an undisturbed Arctic lake thus mimicking permafrost degradation and terrestrial inputs. Biological oxygen demand (BOD) in the pelagic and benthic environments was measured weekly throughout one open water season using the Fibox 3 LCD trace oxygen meter together with chemical optical sensors. Gross primary productivity (GPP) was found to be the greatest source of carbon to the food webs of these systems regardless of treatment. GPP in the pelagic zone decreased by up to 70 % in the high sediment treatment relative to the control enclosures which received no treatment. Benthic GPP did not change significantly.
Permafrost temperatures in the high Arctic have been rising causing a deepening of the active layer and an increase in thermokarst activity. The degradation of permafrost occurring directly adjacent to lakes has been associated with terrestrial inputs to the lacustrine environment. Formally called shoreline retrogressive thermokarst slumping (SRTS), these inputs have been associated with changes in chemical and biological parameters in permafrost embedded lakes throughout the northern regions of the globe. A number of synoptic studies on lakes situated in the uplands of the Mackenzie Delta have revealed that slumping in the lakes of this region is associated with overall decreases in turbidity and water color, water-column nutrient availability as well as a number of biological differences including greater macrophyte biomass in slumped lakes (Fig. 2). In an effort to further our understanding of the underlying mechanisms of the transformations observed in disturbed lakes, we carried out an in situ mesocosm experiment in which treatments consisted of sediment additions varying in volume. The experiment took place in an undisturbed Mackenzie Delta upland lake and consisted of twelve 1 meter square mesocosms installed at 1 meter depth: three replicates for each of three levels of sediment addition plus three replicates of a control in which no additions were made (Fig. 1). Sediments for the additions were sourced from an SRTS-affected lake about a kilometer away. Mesocosms were dosed in the spring and monitored over the course of the open-water growing season (early June to mid-September). We sampled weekly for water chemistry, primary and bacterial production in both the benthic and pelagic environments and zooplankton community structure and biomass. Primary productivity was determined by measuring the BOD in light and dark bottle experiments. Therefore, an innovative light and dark bottle measurement system was developed applying optical sensor technology. With an optical sensor spot glued to the inside of the bottles oxygen concentrations at very low level could be measured in situ. The fiber optic measurement system was very suitable for performing oxygen monitoring out in the field. Our interests focused on bacterial production and primary production in both the pelagic and benthic environments as these are the basal components of the food web and the carbon source for higher trophic levels. Changes in these components with the addition of permafrost related sediments could indicate how SRTS are impacting Arcitc lakes. Furthermore, the study may help identify if SRTS affect the carbon budget of permafrost embedded lakes which are known to be a large potential source of carbon on a global scale.
Primary productivity was assessed using BOD incubations which involves measuring the oxygen concentration of water samples in light and dark bottles both before and after a given incubation period. Whereas BOD experiments are typically performed in the laboratory and involve potentially tedious techniques such as titrations, our experiments needed to be performed in situ as the experimental treatment would affect the optical properties of the water and therefore affect the results. As such, an optrode-based system (Fibox 3 LCD trace, by PreSense) was used to measure oxygen concentration. Optical oxygen sensors work according to the principle of fluorescence quenching, where the degree of quenching correlates with the partial pressure of oxygen. The system offers a number of advantages over traditional oxygen methods and in particular, the ability to perform precise low level measurements without chemicals in real time. This also allowed for short incubation periods, reducing the concern of community shifts in the container during the incubation period. BOD experiments in the pelagic zone were conducted in light and dark bottles while light and dark tubes of approximately 20 cm in diameter were used in the benthic environment (Fig. 3). Sensor spots for trace oxygen measurement were glued to the inside of the bottles and their signals read out with the Fibox 3 LCD trace. Bacterial production was assessed with radio-labeled leucine.
Results of in situ Experiments
Results showed that primary productivity in the pelagic zone was the main source of carbon to these systems regardless of sediment treatment outpacing bacterial production by two orders of magnitude regardless of treatment. Sediment treatment was associated with reductions in gross primary production in the pelagic zone whereas no significant differences were found in the benthos. Bacterial production decreased in the pelagic zone but increased by up to 500 % in the benthos. Analysis of zooplankton data revealed that the decrease in pelagic primary production was due to increased grazing pressure from Cladocera which also increased with sediment treatment.
Determining primary productivity by measuring BOD with the Fibox 3 LCD trace allowed in situ analysis and therefore guaranteed realistic conditions. The optical sensor technology turned out to be an ideal tool for these experiments giving correct measurements at low oxygen concentrations. Results identify thermokarst slumping as a source of enrichment, at least in the initial stages of a slump. While much of the studies to date have focused on the physio-chemical transformations in slumped and unslumped lakes, this study suggests that trophic interactions likely play an important role in the contrasts observed between lake types. Our results do not suggest however that the status of these lakes as sources of CO2 will change as a result of increased thermokarst slumping. However, it is important to note that the experiment represents only the initial stages of the transformative process, long term experiments are planned to better address these questions.