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					                                                                            tahoe: state of the L ake RepoRt 2008

      e x e C u t I v e s u m m a Ry
         The long-term data set collected       essential to refine the accuracy of        are expressed as daily variations in          7.5)
      on the Lake Tahoe ecosystem by the        those models. In these times of rapid      weather. In the long term, they are        •	 Every	month	in	2007,	except	Feb-
      University of California, Davis, is a     change, reliable predictive models are     expressed as normal cyclical varia-           ruary and September, was drier
      valuable tool for understanding eco-      indispensable tools for Lake Tahoe         tions such as wet and dry cycles, and         than the 97-year average. (Fig. 7.6)
      system function and change. It has        Basin resource managers.                   long-term trends related to global
                                                                                                                                      •	 Snow	represented	37.6	percent	of	
      become essential to public agencies         This report is available on the UC       climate change.
                                                                                                                                         total precipitation at lake level.
      tasked with restoring and managing        Davis Tahoe Environmental Research         Historical record:                            (Fig. 7.7)
      the Tahoe ecosystem, in part because      Center website (         •	 The	nightly	minimum	tempera-
      it allows us to monitor progress                                                                                                P h ys i c A l P r o P e r t i e s
                                                  Here are some of the highlights             tures recorded at Tahoe City have
      toward reaching Tahoe’s restoration
                                                presented in the following pages.             increased by more than 4 degrees          Lake Tahoe’s physical properties
                                                                                              F. since 1910. (Fig. 7.1)               are largely a response to external fac-
         This annual Tahoe: State of the        Angor A Fire                                                                          tors, especially meteorology. Physical
                                                                                           •	 Days	when	air	temperatures	
      Lake Report presents 2007 data in         •	 During	the	Angora	Fire,	atmo-                                                      properties, in turn, determine the
                                                                                              averaged below freezing have
      the context of the long-term record.         spheric deposition of nitrogen and                                                 environment for all the lake’s chemi-
                                                                                              decreased by 30 days per year
      It includes new data about impacts           phosphorus was 2½ to 7 times                                                       cal and biological processes (see next
                                                                                              since 1910. (Fig. 7.2)
      of the Angora Fire, and the effects of       normal summer rates, but still rep-                                                sections).
      climate change on snowmelt timing,                                                   •	 Since	1910,	the	percent	of	precipi-
                                                   resented only 1 to 2 percent of the                                                Historical record:
      lake water temperature and density                                                      tation that fell as snow decreased
                                                   annual loads from all sources. (Fig.
      stratification. It also shows how                                                       from 52 percent to 34 percent.          •	 Water	temperature	(volume	aver-
      much of the pollutants that reduce                                                      (Fig. 7.7)                                 aged) rose by more than 1 degree
      lake clarity (fine sediment particles,    •	 Atmospheric	deposition	from	                                                          F. in the past 37 years. (Fig 8.3)
                                                                                           •	 Peak	snow	melt	averages	2	½	
      nitrogen and phosphorus) are con-            the Angora Fire had a negligible
                                                                                              weeks earlier than in the early         •	 Average	surface	water	temperature	
      tributed by different sources.               impact on lake clarity and algal
                                                                                              1960s. (Fig. 7.8)                          rose by more than 1.5 degrees F. in
                                                   biomass. (Figs. 6.2 and 6.3)
         The UC Davis Tahoe Environmen-                                                    Previous year1:                               the past 37 years, to 51.9 degrees F.
      tal Research Center has developed         M e t e o r o l o gy                                                                     (Fig. 8.4)
                                                                                           •	 2007	was	the	14th	driest	year	on	
      sophisticated computer models that                                                      record. Precipitation at Tahoe City     •	 Winter	surface	water	temperatures	
                                                   The Lake Tahoe ecosystem is
      help scientists more accurately pre-                                                    was 19.7 inches, two-thirds of the         were the coldest measured in the
                                                largely driven by meteorology. In the
      dict how Lake Tahoe’s ecosystem                                                         annual average of 31.6 inches. (Fig.
                                                short term, meteorological conditions                                                      (c o n t i n u e d o n n e x t PA g e )
      behaves. Long-term data sets are

      “Previous year” for some parameters means data collated in terms of the water year, which runs from October 1 through September 30; for other parameters, it means

      data for the calendar year, January 1 through December 31. Therefore, for this 2008 report, water year data are from Oct. 1, 2006 through Sept. 30, 2007. Calendar year
      data are from Jan. 1, 2007 through Dec. 31, 2007.                                                                                                                                                                     2.1
                                                                             tahoe: state of the L ake RepoRt 2008

      e x e C u t I v e s u m m a Ry
      (c o n t i n u e d F r o M PA g e 5)       sured at various depths at TERC’s          B i o l o gy                               in 2007. (Figs. 10.11 to 10.12)
        last 10 years, with the lowest maxi-     mid-lake and western lake stations.          The longest data sets for lake biol-
        mum surface water temperature of           One form of nitrogen—nitrate—            ogy are on the base of the food web –
        41.11 degrees F. (Fig. 8.5)              enters the lake through stream and         the algae (or phytoplankton) and the        Clarity remains the indicator of
                                                 urban runoff, groundwater and              zooplankton (microscopic aquatic         greatest interest about Lake Tahoe
      •	 Density	stratification	of	Lake	
                                                 atmospheric deposition. Phosphorus         animals that graze on algae). Algae      because it tracks degradation and the
         Tahoe has increased over the last
                                                 occurs naturally in Tahoe Basin soils      and zooplankton influence the lake’s     community’s efforts to restore clarity
         37 years as surface water warmed
                                                 and enters the lake from soil distur-      food web, clarity and aesthetics.        to historic levels. Secchi depth (the
         due to climate change. (Fig. 8.8)
                                                 bance and erosion, as well as atmo-                                                 point below the lake surface at which
      Previous year:                                                                        Historical record:
                                                 spheric deposition.                                                                 a 10-inch white disk disappears from
      •	 In	2007,	lake	level	fell	to	a	low	of	                                              •	 Primary	productivity,	the	rate	       view) has been measured continu-
                                                 Historical record:                            at which algae produce biomass
         6224.7 feet in December. (Fig. 8.2)                                                                                         ously since 1968, and is the longest
                                                 •	 Stream	inputs	of	particles,	nitrogen	      through photosynthesis, has been      continuous measure of Lake Tahoe’s
      •	 Lake	Tahoe	mixed	all	the	way	to	
                                                    and phosphorus are directly linked         increasing since 1959. (Fig. 10.1)    water clarity.
         the bottom in March 2007, the first
                                                    to the annual amount of precipita-      •	 Since	1984,	the	annual	average	
         deep mixing since 2001. (Fig. 8.9)                                                                                             In 2007, the Secchi depth was 70.1
                                                    tion. (Figs. 9.3 to 9.5)                   depth of the deep chlorophyll         feet, an increase of 2.4 feet over 2006.
      nutrients And                              •	 Atmospheric	deposition	of	nutri-           maximum has declined. (Fig. 10.4)     In the last seven years, Secchi depth
      PA r t i c l e s                              ents, both in concentration and         •	 Diatoms	remain	the	dominant	          measurements have been better than
        Lake Tahoe’s clarity is determined          total loads, are also linked to pre-       algal species and provide high        predicted by the long-term linear
      especially by fine sediment particles,        cipitation. (Figs 9.6 and 9.7)             quality food for aquatic species.     trend. There is statistical support
      and also by nutrients. Tahoe’s urban       •	 Nitrogen	concentrations	in	the	            (Fig 10.6)                            that Lake Tahoe’s clarity decline has
      areas contribute 72% of fine particles,       lake have remained generally con-       Previous year:                           slowed significantly, and is now best
      despite representing only 10% of the          stant for many years. (Fig. 9.8)                                                 represented by a curve. (Fig. 11.1)
                                                                                            •	 Primary	productivity	in	2007	was	
      land base.                                 •	 Phosphorus	reached	a	minimum	              the highest on record, five times     e d u c At i o n A n d
         Nutrients affect lake clarity by pro-      in 1999 and has increased slightly         the 1959 level. (Fig. 10.1)           o u t r e Ac h
      moting algae growth. Offshore, algae          since. (Fig. 9.9)
                                                                                            •	 The	maximum	deep	chlorophyll	            The public can learn about the sci-
      make the water greenish and less           Previous year:                                depth increased in 2007 to a mean     ence behind Lake Tahoe restoration
      clear. Along the shoreline, algae are a
                                                 •	 The	watersheds	that	contributed	           of 180 feet. (Fig. 10.4)              at TERC’s Incline Village education
      problem because it coats rocks with
                                                    the most particles and nutrients to     •	 Periphyton	(attached	algae)	con-      center (the Thomas J. Long Founda-
      green slime.
                                                    Lake Tahoe were the Upper Truc-            centrations were above average in     tion Education Center). In 2007, over
         The two nutrients that most affect         kee River, Blackwood Creek, Trout          2007. (Fig. 10.9)                     6,900 people participated in our edu-
      algal growth are nitrogen and phos-           Creek, Ward Creek and Incline
                                                                                            •	 Zooplankton,	an	important	part	of	    cation and outreach activities. (Fig.
      phorus. These nutrients are mea-              Creek. (Fig. 9.2)
                                                                                               the food web, were at a 10-year low   12.1)                                                                                                                                                                2.2

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