Alanna L. Lecher

Methane transport from the active layer to lakes in the Arctic using Toolik Lake, Alaska, as a case study

Significance Methane, a greenhouse gas, contributes to global warming. We show that methane-rich water from the seasonally thawed active layer in the Arctic flows into Toolik Lake, Alaska. This may be an important previously unrecognized conduit for methane transport and emissions in Arctic lakes. The controls on methane input from the active layer are fundamentally different than those affecting methane production within lakes, and the response of these processes to climate and environmental change is also distinct. The accuracy of predictions of methane emissions and ultimately the extent of climate change that can be expected in the Arctic depend on a better understanding of methane dynamics in the region, including the controls over methane production and transport processes within the active layer. Methane emissions in the Arctic are important, and may be contributing to global warming. While methane emission rates from Arctic lakes are well documented, methods are needed to quantify the relative contribution of active layer groundwater to the overall lake methane budget. Here we report measurements of natural tracers of soil/groundwater, radon, and radium, along with methane concentration in Toolik Lake, Alaska, to evaluate the role active layer water plays as an exogenous source for lake methane. Average concentrations of methane, radium, and radon were all elevated in the active layer compared with lake water (1.6 × 104 nM, 61.6 dpm⋅m−3, and 4.5 × 105 dpm⋅m−3 compared with 1.3 × 102 nM, 5.7 dpm⋅m−3, and 4.4 × 103 dpm⋅m−3, respectively). Methane transport from the active layer to Toolik Lake based on the geochemical tracer radon (up to 2.9 g⋅m−2⋅y−1) can account for a large fraction of methane emissions from this lake. Strong but spatially and temporally variable correlations between radon activity and methane concentrations (r2 > 0.69) in lake water suggest that the parameters that control methane discharge from the active layer also vary. Warming in the Arctic may expand the active layer and increase the discharge, thereby increasing the methane flux to lakes and from lakes to the atmosphere, exacerbating global warming. More work is needed to quantify and elucidate the processes that control methane fluxes from the active layer to predict how this flux might change in the future and to evaluate the regional and global contribution of active layer water associated methane inputs.

Nutrient Loading through Submarine Groundwater Discharge and Phytoplankton Growth in Monterey Bay, CA.

We quantified groundwater discharge and associated nutrient fluxes to Monterey Bay, California, during the wet and dry seasons using excess (224)Ra as a tracer. Bioassay incubation experiments were conducted to document the response of bloom-forming phytoplankton to submarine groundwater discharge (SGD) input. Our data indicate that the high nutrient content (nitrate and silica) in groundwater can stimulate the growth of bloom-forming phytoplankton. The elevated concentrations of nitrate in groundwater around Monterey Bay are consistent with agriculture, landfill, and rural housing, which are the primary land-uses in the area surrounding the study site. These findings indicate that SGD acts as a continual source of nutrients that can feed bloom-forming phytoplankton at our study site, constituting a nonpoint source of anthropogenic nutrients to Monterey Bay.

Current Magnitude and Mechanisms of Groundwater Discharge in the Arctic: Case Study from Alaska

To better understand groundwater-surface water dynamics in high latitude areas, we conducted a field study at three sites in Alaska with varying permafrost coverage. The natural groundwater tracer ((222)Rn, radon) was used to evaluate groundwater discharge, and electrical resistivity tomography (ERT) was used to examine subsurface mixing dynamics. Different controls govern groundwater discharge at these sites. In areas with sporadic permafrost (Kasitsna Bay), the major driver of submarine groundwater discharge is tidal pumping, due to the large tidal oscillations, whereas at Point Barrow, a site with continuous permafrost and small tidal amplitudes, fluxes are mostly affected by seasonal permafrost thawing. Extended areas of low resistivity in the subsurface alongshore combined with high radon in surface water suggests that groundwater-surface water interactions might enhance heat transport into deeper permafrost layers promoting permafrost thawing, thereby enhancing groundwater discharge.

Sources of methane to an Arctic lake in Alaska: An isotopic investigation

Sources of dissolved methane (CH4) at Toolik Lake, Alaska, include both diffusion from lake sediments and groundwater entering the lake from its perimeter. Here we use hydrogen and oxygen isotopes in water (H2O), carbon and hydrogen isotopes in CH4, and carbon isotopes in dissolved inorganic carbon (DIC) to calculate the relative importance of lake sediment and groundwater discharge as sources of dissolved CH4 to Toolik Lake. We also resolve the relative importance of the source contribution spatially within the lake and determine the processes controlling CH4 concentrations in groundwater surrounding the lake. Our findings, from a mixing model based on isotopes in CH4, suggest that groundwater is a more important source of CH4 at the perimeter of the lake where the water-to-air flux is high. Additionally, we find on the local scale that high groundwater methane concentrations may be better linked to areas around the lake where rain is the dominant source of water to the active layer, indicating that changes in precipitation and active layer thaw depth will impact methane concentrations in the active layer and, ultimately, the groundwater associated flux to Toolik Lake.

Piecing together the plastic cycle

piecing together the plastic cycle Dissecting the word biogeochemistry harks at the root of the field. Biogeochemists study how chemical elements and compounds cycle through biological and geological processes in order to build a global conceptual model. Some of the most well-known biogeochemical cycles first introduced to students include the water, carbon and nitrogen cycles. These cycles have existed on Earth of millennia, and I spent my PhD researching how humans modified these cycles. However, after graduate school, a new emerging cycle caught my eye: the plastic cycle. I first encountered the idea of the plastic cycle when Law and colleagues reported their efforts to understand plastic content and transport in the Northwest Atlantic Ocean (Science 329, 1185–1188; 2010). Although the cycle was not explicitly named, throughout the text they described how plastic entered, was transported within, and exported from the ocean — hallmark terms and processes of a biogeochemical cycle. The oceanic plastic cycle begins with plastic entering the ocean, which can occur through different means. Plastic litter being blown into the ocean from land or dumped off sea-going vessels are two canonical pathways. However, as Law et al. pointed out, another substantial source of plastics comes from shipping containers filled with preproduction plastic pellets, often called virgin plastic. These containers fall off cargo ships while en route to a factory; however, the flux of plastic via this source is decreasing due to programs instituted to prevent pellet pollution. Once in the ocean, plastic is transported by currents. Due to the buoyant nature and long deterioration time of plastic, slow currents can move plastic long distances. How plastic was transported and where it ended up in the Atlantic was at the heart of Law and colleagues’ experiments. They towed nets behind boats for 20 years to determine the oceanic abundance of plastic pieces, and tracked drifters meant to mimic plastic transport. Law et al. found that currents induced an accumulation of floating plastic in the Northwestern Atlantic Gyre convergence zone off the coast of the Mid-Atlantic Bight. Once in the convergence zone, plastic is removed from the surface mixed layer through sinks and export processes. A substantial sink within the mixed layer is ingestion by animals, such as sea turtles that live in the algal mats of the Gulf Stream (Mar. Poll. Bull. 28, 154–158; 1994). Plastic is also exported to the deep ocean when its density increases due to fouling organisms that grow on the plastic. Once in the deep ocean, plastic can be either ingested by animals or buried (Annu. Rev. Mar. Sci. 9, 205–229; 2017). I am currently focusing on these export processes, namely the flux of plastic into coastal sediments, and how this flux is affected by oceanic plastic abundance. My lab is comparing the concentration of small plastic pieces in coastal sediment adjacent to and removed from areas of plastic convergence. Our preliminary results indicate significantly higher concentrations of plastics in sediment along the strandline of beaches adjacent to areas of high oceanic plastic concentrations. It will take many more individual experiments to understand each part of the plastic cycle as well as we do other biogeochemical cycles. Law et al. provided the foundation to build these experiments.

Assessing Cumulative Effects of Climate Change Manipulations on Phosphorus Limitation in a Californian Grassland

Grasslands throughout the world are responding in diverse ways to changing climate and environmental conditions. In this study we analyze indicators of phosphorus limitation including phosphorus concentrations, phosphorus to nitrogen, and carbon ratios, oxygen isotope ratios of phosphate in vegetation, and phosphatase enzyme activity in soil to shed light on potential effects of climate change on phosphorus availability to grassland vegetation. The study was conducted at the Jasper Ridge Global Change Experiment (JRGCE), California where manipulations mimicking increases in temperature, water, nitrogen and carbon-dioxide have been maintained for over 15 years. We compare our results to an earlier study conducted 3 years after the start of the experiment, in order to assess any change in the response of phosphorus over time. Our results suggest that a decade later the measured indicators show similar or only slightly stronger responses. Specifically, addition of nitrogen, the principle parameter controlling biomass growth, increased phosphorus demand but thresholds that suggest P limitation were not reached. A study documenting changes in net primary productivity (NPP) over time at the JRGCE also could not identify a progressive effect of the manipulations on NPP. Combined these results indicate that the vegetation in these grassland systems is not very sensitive to the range of climate parameters tested.