April is here and that means that Earth Day 2013 will soon inspire us to reflect on the health of our planet. Although Earth Day has grown into a global event that energizes millions of participants in 192 countries, for those of us who visit or live at Lake Tahoe this annual renewal of a grassroots environmental movement should have special meaning. All will agree that we are blessed to enjoy this inspirational environment, but, over the long term, Tahoe is facing an uncertain future.
It’s no secret that Lake Tahoe’s ecology and general environmental health have been stressed since the basin was first commercially logged a century and a half ago. Rapid residential development after World War II continued the negative impact on Tahoe’s legendary water quality. In recent decades, mandated better management practices in the Tahoe Basin have reduced erosional sedimentation and organic compounds flowing into the lake, which should decrease the amount of nutrients currently feeding algae that cloud the water.
Within the past decade, however, annual observations of Big Blue’s water clarity have been a mixed bag. Data indicate definite improvement during the winter months, a trend that may reflect the substantial investment in capital improvements built to reduce the load of fine particles from urban storm water. (To quantify the results, scientists need a comprehensive, regional monitoring plan to track changes in how urban storm water is impacting Lake Tahoe.) This storm water discharge is the basin’s largest source of fine particles, and generally enters the lake during the winter/storm months. Indicative of how complex and challenging it is to analyze progress in the battle to improve water clarity, winter measurements may inspire optimism, but summer clarity readings continue to decline due to algal growth and other factors.
Tahoe is one of the worlds’ most studied lakes and holds a unique position among large lakes. In fact, when NASA began gathering temperature information on 167 large lakes worldwide, Tahoe’s in-lake measurements were used to calibrate the satellite data for this global assessment. For 25 years, researchers have watched Tahoe’s water temperature warm more rapidly than surrounding air temperatures due to climate change. NASA has confirmed the same trend in play on virtually all observed lakes around the world. Warming water temperatures impact lakes in a variety of ways, including biological productivity and creating a more hospitable environment for invasive species.
Like most lakes located in temperate latitudes, water in Tahoe occasionally mixes, bringing oxygen-starved water up from depth while infusing the deeper zones with oxygenated surface water. The extent of the mixing varies between relatively shallow zones or a complete turnover of water — top to bottom. In 2011, Tahoe’s liquid volume mixed to the bottom, which pushed cold water up from the depths to make for invigorating water sports that summer. There was only a shallow mixing in 2006. According to the U.C. Davis-based Tahoe Environmental Research Center, past studies suggest that climate change will reduce the extent of lake mixing, and in particular the mixing of oxygen, down to deep water. But, whether this possibility verifies remains an open question for Tahoe and other lakes worldwide.
Lake Tahoe mixes when its vertical water column reaches the same temperature and density from top to bottom. Surface winds trigger the turn-over. The worry for the future is what happens if a buoyant surface cap of warm water prevents this hydrological flip and what would the impact be on Lake Tahoe? One concern is that oxygen depletion will do more than affect biology at depth. Scientists are warning that the anaerobic conditions might release phosphorus and nitrogen from bottom sediments, nutrients that are known drivers for algae production. In 2013, researchers hope to install equipment to measure these mixing events in real-time so that they will be able to monitor the evolution of the process as it happens and learn more about it.
Have you ever noticed that some summers Big Blue is warmer than others? The most obvious answer kicked around by locals is that after a big winter the snowpack melts later and dumps cold water into the lake well into spring. The main culprit, however, is the mixing of Tahoe’s waters at great depth. Consider that during the summer of 2011, Tahoe’s surface water was the coldest in a decade. Makes sense right? Winter 2011 was epic with skiing into July. Actually, it was deep mixing in 2011 that made the lake so cold. More snow fell during the winter of 2006 than 2011, but because there was only a shallow mixing in 2006, Tahoe’s surface temperature was one of the balmiest since 1968.
Other climate trends reveal that over the past century rain has become a significantly greater ratio of precipitation than snow at Tahoe City, and that since 1961 the lake level snowpack is melting about two weeks earlier in spring. However, at the Central Sierra Snow Laboratory at 6,900’, there has been no meaningful change in spring melt timing since measurements began in 1946. It’s important to remember that although scientists are quantifying regional climate change, the impacts will vary depending on location, particularly elevation.
Research informs us that we can create solutions and tools to help protect Big Blue, but there is no guarantee of success despite our best efforts. The water in Lake Tahoe is still clean and relatively pristine, and it is up to us and future generations to keep it that way.
Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. His award-winning books are available at local stores or at thestormking.com. You can reach him atmark@thestormking.com. Check out Mark’s blog at tahoenuggets.com.
