SEITZ Carina
informe técnico
Impacts of Smoke-Ash from the 2021 Wildfires to the Ecology of Lake Tahoe
Fecha inicio/fin:
Naturaleza de la

Producción Tecnológica:
Conservación de ecosistemas acuáticos
Campo de Aplicación:
Rec.Hidr.-Cuencas superficiales
Quantifying the disturbances from wildfire smoke generated from regional to continental wildfires is particularly important as mega-fires are predicted to increase with changes in climate. Understanding the direct influences of wildfire smoke is of particular importance to policymakers who are already confronted with managing ecosystems important to the regional economy, like Lake Tahoe, that are undergoing accelerated environmental change from multiple stresses. Questions remain about how wildfires augment or exacerbate the current changes and whether these disturbances from wildfires have lasting impacts.In 2021, while still under COVID-19 pandemic conditions and limited capacity at their institutions, a team of 34 scientists and early career trainee students (undergraduate and graduate) from 5 institutions, lobbied for a ?rapid response? study which focused on quantifying the influence of wildfire smoke to Lake Tahoe and a smaller lake representative of other lakes in the area. With support from non-profit organizations, agencies, private donors, and their individual institutions, the scientists were able to initiate immediate studies to quantify the impacts of wildfire smoke to the water quality of these ecosystems. Without the support of many organizations who contributed to these efforts, and the staff available at the time from the institutions to conduct such a study, this rapid action to understand the impacts of wildfire smoke could not have occurred. The team used existing information from monitoring programs and publicly available data, opportunistic sampling events using novel technologies, laboratory experiments, and basic concepts in science to determine the impacts of wildfire smoke from the 2021 season to lake dynamics. At the heart of the investigations was the augmentation and interpretation from the long-term monitoring information (physical and biological) collected at Lake Tahoe combined with analysis of freely available weather and air quality monitoring information.The publicly available ?purple air? sensors are useful to understand the air quality conditions around the basin from wildfire smoke. These sensors and resulting calculations for particulate matter of smaller size (2.5 micron in size) can be used to understand the variability in smoke conditions around the lake, which was highly variable over time. In general, smoke conditions interpreted as changes in potential particulate matter deposition and light were strongest in the southern part of the lake, but sites across the lake would exhibit synchronous and asynchronous behavior indicating the potential for changing light conditions across the lake surface. As air quality changes within the basin, we observe differential changes to surface or incident light conditions measured at a continuous sensor in Tahoe City. Changes in surface light influence biological, chemical, and physical conditions in freshwaters. Considering the variability of smoke across the basin exhibited by the publicly available air sensors, having calibrated surface light radiometers across the spectral band paired with weather stations around the basin could assist in determining nearshore and offshore changes around the lake from changes in atmospheric conditions from wildfires.5Climate and snowpack are known to influence the relative loads of particulate matter and nutrients to the lake. Years with higher snow condition led to increased loads of particulate matter to the lake. Thus, we adopted an approach to compare the 2021 wildfire smoke conditions to other climatic years (dry to wet). Since 2021 was a relatively drier year on record, we expected transparency to start relatively high compared to years with larger snowpack. As expected, before smoke conditions were observed in the basin in 2021, we observed high water transparency measured by secchi disk and increased penetration of ultraviolet and visible light, known as photosynthetically active radiation. Ultraviolet light has been shown to control many biological and chemical processes in lakes and photosynthetically active light conditions determine the depth of algal production and organismal foraging behavior. However, after the smoke period, the transparency and ultraviolet and visible light was immediately reduced by 4.5m, 17.5m, and 7.6m, respectively. The reduction in light conditions was similar to years when there was higher snowpack indicating that wildfire smoke and resulting depositional impacts may act similar to years with more snow.From a single fixed monitoring station at a buoy in the lake, the atmospheric deposition of particles was a relatively small contributor to overall lake particles where the mean addition was approximately 0.2% of the population of particles in the upper 20 meters of the lake each day. It is not known what fraction of the in-lake particle number was organic or inorganic. Evaluating the loading from another view with broader spatial layout using collectors distributed around the lake indicated the highest ash deposition rates occurred during the first sampling period (Aug. 26th ? Sept. 10th, 2021) and around the southern end of the lake. Average ash deposition around the lake during this period was 240.5 mg m-2 d-1. The heterogeneous deposition in space and time around the lake, differential quality of material, disappearance of this material, and likely contribution of this material to open water productivity and water quality conditions (see below) indicates the need for a more robust atmospheric deposition and air quality measurements around the basin. Using an autonomous vehicle, we were able to determine the concentration of fine particles in space and time after the wildfires. There is a rapid decrease in fine suspended particulate matter concentrations indicating the particles entering the lake are rapidly cleared from the water. As deposition decreased by mid-September, concentrations of small particles in Lake Tahoe followed a similar trend with little lag indicating that these small particles disappear either through physical settling, photodegradation, or consumption.The deposition and resulting degradation of wildfire smoke-ash material and changes in atmospheric light condition influence the production and composition of open water algae producing organic material in the water column that contributed to the observed decline in clarity and light conditions. The monitoring data suggests changes to where productivity in the water column occurs, the composition of plankton, and the overall amount of phytoplankton production, but little change in algal biomass as measured via chlorophyll. Clearwater lakes with depth can have two layers of algal production, one in the shallower water and one in the deeper waters where light levels are lower, near or below 1 to 0.1% of the surface, and nutrients can be higher in availability. The deep-water layers of algae can be important for zooplankton energy and feeding supporting the flow of carbon to fish consumers. Prior to the6arrival of smoke conditions in the basin, the maximum amount of chlorophyll occurred in the deeper water around the typical summer depth of 60-80 meters. After the arrival of smoke and changes in the surface levels of light and decrease in visible light depth, chlorophyll levels rise in depth. A previous published study at Castle Lake, a moderately productive lake similar to Emerald Bay or other small water bodies in the region, showed smoke in the atmosphere and changes in light conditions in the water column that led to the disappearance of the chlorophyll a maximum. Thus, having a high frequency measurements of water quality, including chlorophyll a in time and space, during these large-scale disturbances can inform us of the changes in ecological conditions in these large lakes which can influence zooplankton.Together with the change in deep chlorophyll maximum there was a change in the phytoplankton community. Compared to historical information which shows that Lake Tahoe?s algal community is predominantly dominated by diatoms, the 2021 information indicates a change to the larger bodied Leptolyngbya, which is a cyanobacteria. Some cyanobacteria can fix nitrogen supplying this nutrient to the lake after they die and many also produce toxins that are generally inedible for filter feeding animals like zooplankton due to their size and toxicity. Some have part of their life history in the bottom habitats of the lakes and Leptolyngbya is found in the nearshore of Lake Tahoe. Along with changes in composition, likely due to changing light conditions and the addition of nutrients into the lake from ash (see below), the amount of open water algal production was the highest annual value recorded at 334 g carbon meter-2 year-1. The value is 20% higher than the record set in 2019 although the data is still provisional by UC Davis. This finding is consistent with the very high algal abundance in 2021, and with the occurrence of very high cell counts of Leptolyngbya. Previous studies by Charles R. Goldman and colleagues that evaluated the impacts of wildfire smoke from the Wheeler fire in the 1980s also suggested the highest production on record at the time of the fire. Together this indicates the importance of wildfire smoke in influencing open water algal production.Information from experiments which manipulated both the light and ash sources, and visual observations by our scientists on the ground nearly each day during the wildfires, complement the lake monitoring data and elucidates the mechanisms affecting production. While there is more ash deposited in the south/east and less in the north/west (see above), the quality of ash deposits varies in space. Generally, less ash is deposited in the north/west part of the lake, but the ash deposited is much higher quality contributing to algal and bacterial production. This occurs when there is simulated reduced light conditions in the atmosphere from wildfire smoke, or normal light conditions for smaller lakes, and we see that ash deposited around the lake along with ash collected from the burn sites, which can run off into the lake at a later time, stimulate net algal production. This suggests that lakes can have complex responses to smoke ash deposits and under different atmospheric light conditions and it may depend on the lake?s starting conditions of algal and bacterial community structure and nutrient state at the time of deposition. Future work should couple fields of knowledge and expertise that are exploring the development of wildfires, creation of wildfire smoke and weather, distribution in the atmosphere, deposition of ash deposits, and effects to lake dynamics (light, nutrients, biological production, and behaviour).7Incubating lake water with ash and manipulating light conditions also suggests that the lakes start off as net heterotrophic (dominated by bacteria that consume organic carbon rather than algal dominance). The responses of the lake water quality dynamics during the experiments suggest that the lakes take time to process the organic matter, which is dominated by bacterial and respiration processes. Once this initial flux of organic matter is processed then the phytoplankton and light driven utilization of carbon and nutrients occurs. While the method for using oxygen may be useful for understanding initial changes in water quality, it will take time to detect changes in experiments or the water column unless there is a substantial change to the community. The experimental studies combined with the fact that particles deposited to the lake ?disappeared quickly? suggests that the microbial dynamics in the lake are likely very important for processing organic matters. Currently the monitoring and experimental investigations funded at the lake do not include the evaluation of microbial activity, biological players or micro zooplankton grazers which feed on them.Immediately during the wildfire smoke conditions in the basin and after the wildfire, continuing into the summer of 2022, our teams and many members of the public observed changes to the nearshore environment, specifically reporting increases in nearshore and bottom algal growth in some parts of the lake but not others. During our collections of ash deposited during the wildfire of 2021, we visually observed ash washing up on beaches and trapped between sediment particles at the bottom of the lake. Using high frequency sensing for oxygen and temperature, coupled with a published model, we determined that nearshore productivity (algal and bacteria) increased immediately after the wildfire smoke entered the basin, but the influence was relatively short-lived. We do not have other years to compare nearshore production, nor are there measurements at depth through the lighted zone of the lake (40-50 meters). Given the variability in nearshore algal and bacterial production due to variable habitats around the lake (cobble, sand, rocks) and nearshore and offshore mixing dynamics, we recommend that additional high frequency sensors and models incorporating metabolic and mixing processes are developed to understand how light, ash deposition, and nutrient run off influence the nearshore environments in the lake. We predict that not every part of the nearshore will respond the same way due to the variability within the environment.While not explicitly required as a part of this study, we did measure metal toxicants in the ash deposited in the lake. We observed higher levels of metal deposits in the north/west sites that exhibited lower loads to the lake compared to the south/east shores. In the future, analyzing information collected by the US Environmental Protection Agency?s IMPROVE program, the loading of toxic metals, and conditions pre- and post-wildfire may be warranted. Burning structures associated with wildfire may result in the deposits of metals and other contaminants which influence public health and the ecology of the lake.In conclusion, this rapid response effort by scientists shows the importance of wildfire connections to fundamental lake processes. The efforts would not have been possible without the generous support of private donors, our local non-governmental organizations and resources provided to the Tahoe Science Advisory Council).