Biostatistical evaluation of long-term lake clarity record

Lake Tahoe is a large (501 km') and deep (505 m) subalpine lake in the Sierra Nevada mounrain range of the western U.S. It is renowned for the clarity of its water and the majesty of its surrounding mountains an d forests. Because o f the lake' s unique beauty, protection of its water clarity has become an issue of pressing concern for watershed residents and the millions of annual recreational visitors. The Secchi depth is one common measure of optical clarity in lakes and the oceans. It is defined as the depth at which a 25-cm white disc disappears from view when lowered into the water. Secchi depth is historically important because of the large body of data that has been collected from many locations around the world. Our data demonstrate that Secchi depth, despite its simpliciry, is also a precise measurement of clarity when carefully executed ÜASSBY et al. 1999). Because of the importance of lake optical clarity and the ability of Secchi depth to quantify clarity as perceived by the human visual system, Secchi depth is a highly suitable managemenr endpoint for lakes. Certain lakes have a high Secchi depth because of various factors that result in less dissolved and suspended materials. The same characteristics that lend uniqueness to these lakes in the form of transparency and color, however, also render them unusually susceptible to small increases in dissolved and suspended matter. Large clarity declines have in fact occurred over the last few decades in some of our most highly transparent water bodies, including Lake Tahoe. Secchi depths of over 40 m occurred in the early years of the measurement program at Lake Tahoe and still occasionally exceed 30 m. The longterm decline, though, is a matter of great concern. Using a long time series of primary productiviry, previous studies have shown that algal growth rates have increased over the past few decades (GOLDMAN 1988). In the littoral zone, this increase has manifested in the proliferation of epilithic periphyton around the shores of the lake. In the pelagic wne, however, the relative roles of phytoplankton and other materials in the transparency decline have not been delineated. Certainly, in other lakes, terrigenous particle input as well as phytoplankton conrributes to variability in optical water qualiry. The resolution of this issue has important implications for management of the Tahoe watershed: it determines whether the focus should be on conrrolling phytoplankton growth, allochthonous materials such as mineral suspensoids or both. The long time series of Secchi depth for Lake Tahoe not only records variability in optical clarity but also offers the potenrial to distinguish the underlying causes. Secchi depth has been measured in Lake Tahoe an average of every 12 days since July 1967. Because of the unusual clarity of the lake, Secchi depth is responsive to small changes in light-attenuating particles and so the record exhibits strong variabiliry at the seasonal, interannual and decadal scales. Using recently developed methods of applied time series analysis, the mechanisms of change were delineated at each scale ÜASSBY 1999, }ASSBY et al. 1999): l. The seasonal pattern is a bimodal one with two minima, June and December, approximately. The June minimum is due to cumulative discharge of suspended sediments following melting of the snow pack; the spring bloom of phytoplankton may also play a role. The December minimum is probably a result of mixed-layer deepening as the thermocline passes through layers of phytoplankton and other light-attenuating particles that reach a maximum below the summer mixed layer. 2. The interannual scale exhibits two modes of variabiliry, one during the weakly stratified autumn-winrer period and the other during the more stratified spring-summer period. The first mode is a result of variable depth of mixing in this unusually deep lake, while the second results from year-to-year changes in spring runoff. 3. A decadal trend also exists (-0.25 m year-'). This longer-term trend results from the accumulation of particles in the water column rather than the inrensification of annual processes, but it is not yet understood how much is due to phytoplankton or recent phytoplankton-derived materials and how much to other materials. Based on the 0368-0770/00/0027-02634 $ 0.50 ©2000 E. Schweizerbarr'sche Verlagsbuchhandlung, D-70176 Stuttgart A. D. Jassby et al., Secchi depth recording as an evaluation too! 2635 available measurements and physical considerations, both categories may play a significant role. Further studies are required to delineate the relative roles of different light-attenuating categories. Specifically, the relative contribution to attenuation of chlorophyll-associated and other particles needs to be described more precisely. Related issues are the relative importance of autochthonous contributions (such as biogenic silica) to mineral suspensoids, as well as the role played by dissolved organic matter. In addition, the size distribution of particles entering and within the lake needs to be determined. Longterm clarity losses due to mineral particles are dependent on a certain size fraction, namely the fraction that will be retained in the lake and contribute to a build-up of light-attenuating particles. Furthermore, the residence time for mineral particles in the lake needs to be determined. Insofar as mineral particles contribute to long-term loss of clarity, the recovery time for optical water quality is dependent on their residence time. References GOLDMAN, C. R. , 1988: Primary productivity, nutrients, and transparency during the early onset of eutrophication in ultra-oligotrophic Lake Tahoe, California-Nevada. Limnol. Oceanogr. 33: 1321-1333. }ASSBY, A. D. , 1999: Uncovering mechanisms of interannual variability from short ecological time series. In: Scow K. M. , FoGG G. E. , HINTON D. E. , & }OHNSON M. L. {eds): lntegrated AJsessment of Ecosystem Health: 285-306. CRC Press, Boca Raton, Florida. }ASSBY, A. D. , GOLDMAN e. R. , REUTER J. E. , & RICHARDS R. C. , 1999: Origins and scale-dependence of temporal variability in transparency of Lake Tahoe, California-Nevada (USA). Limnol. Oceanogr. 44: 282-294.