John E. Reuter

Historical Food Web Structure and Restoration of Native Aquatic Communities in the Lake Tahoe (California-Nevada) Basin

Plans for the restoration of aquatic ecosystems are increasingly focusing on the restoration and rehabilitation of self-sustaining native fish communities. Such efforts have not traditionally adopted an ecosystem-based perspective, which considers species as embedded within a broader food web context. In this study, we quantify food web changes in Lake Tahoe (California-Nevada) over the last century based on stable isotope analysis of museum-archived, preserved fish specimens collected during 4 historical periods and under present conditions. We also examine the contemporary food web of nearby Cascade Lake, which is free from most exotic species and contains a species assemblage resembling that of Lake Tahoe prior to historical species introductions. During the last century, the freshwater shrimp Mysis relicta and lake trout (Salvelinus namaycush) have been introduced and established in Lake Tahoe, and the native top predator, Lahontan cutthroat trout (Oncorhynchus clarki henshawi; hereafter LCT), has been extirpated. Isotope analysis indicates that lake trout now occupy a trophic niche similar to that of historical LCT. Fish production has shifted from benthic to pelagic, corresponding with the eutrophication of Lake Tahoe during recent decades. The current Cascade Lake food web resembles that of the historical Lake Tahoe food web. Our isotope-based food web reconstructions reveal long-term food web changes in Lake Tahoe and can serve as the basis for setting historically relevant restoration targets. Unfortunately, the presence of nonnative species, particularly Mysis and lake trout, have dramatically altered the pelagic food web structure; as such, they are barriers to native fish community restoration. Fish community restoration efforts should focus on adjacent ecosystems, such as Cascade Lake, which have a high likelihood of success because they have not been heavily affected by nonnative introductions.

Atmospheric deposition of nitrogen and phosphorus in the annual nutrient load of Lake Tahoe (California-Nevada)

Atmospheric deposition provides most of the dissolved inorganic nitrogen (DIN) and total nitrogen in the annual nutrient load of Lake Tahoe. Deposition also contributes significant amounts of soluble reactive phosphorus (SRP) and total phosphorus loading but plays less of a role than in the case of nitrogen. Most of the DIN probably originates outside of the drainage basin in urban and agricultural areas to the south and west. Spatial patterns of SRP deposition differ from those of DIN and suggest a within-basin terrestrial source, such as leachate from windblown dust or other particles. Because of atmospheric N deposition, the N:P (molar) ratio in combined loading is well above the Redfield ratio of 16 and consistent with an observed shift from colimitation by N and P to persistent P limitation in the lake phytoplankton.

Water clarity modeling in Lake Tahoe: Linking suspended matter characteristics to Secchi depth

Abstract.An additive semi-analytic model of water clarity for the forward problem of calculating apparent optical properties (AOPs) of diffuse attenuation and Secchi depth from the inherent optical properties (IOPs) due to suspended matter in oligotrophic waters is presented. The model is general in form, taking into account algal concentration, suspended inorganic sediment concentration, particle size distribution, and dissolved organic matter to predict Secchi depth and diffuse attenuation. The model’s application to ultra-oligotrophic Lake Tahoe, California-Nevada, USA is described. The function of the clarity model is to quantify the relative effect of phytoplankton or phytoplankton-derived organic materials, other particles such as suspended mineral sediment, and dissolved organic matter on the lake’s clarity. It is concluded that suspended inorganic sediments and phytoplanktonic algae both contribute significantly to the reduction in clarity, and that suspended particulate matter, rather than dissolved organic matter, are the dominant causes of clarity loss.

Daily phosphorus variation in a mountain stream

Monthly diel monitoring studies for phosphorus content were conducted (1995–1996 period) for multiple stations on Incline Creek, a mountain stream in the Lake Tahoe basin (California-Nevada). Large discharge and particulate P (PP) concentration fluctuations occurred during June in the early evening as snowmelt from higher elevations arrived at the lower stream reaches. June diel dissolved organic P (DOP) concentrations steadily increased, while soluble reactive P (SRP) concentrations remained constant. June diel PP concentrations associated with sand-sized particles (PPsand: >63 μm) exhibited a clockwise hysteresis, indicating possible sediment source depletion on a daily timescale. June diel PP associated with silt- and clay-sized particles (PPSC: >0.45 μm and <63 μm) exhibited counterclockwise hysteresis behavior, suggesting a potential groundwater contribution to PPSC. PPSC comprised the majority of PP concentration, except during high-discharge events when PPsand concentration was dominant. Areal PP loading, specifically PPsand, appears to originate primarily from the lower eastern branch of Incline Creek during the spring snowmelt season. Possible sources include a ski resort/parking lot and a golf course. DOP and SRP areal loads were greatest from the undeveloped upper subwatershed, suggesting that natural factors such as slope are influencing loading of small-sized P fractions.

The use of wetlands for nutrient removal from surface runoff in a cold climate Region of California. Results from a newly constructed wetland at Lake Tahoe

Pollutant removal by wetlands represents a potential mitigation technique for treating urban runoff in the cold climates of the Sierra Nevada. This is especially encouraging in the Lake Tahoe Basin, where research has demonstrated the link between nitrate and phosphorus, and a precipitous increase of algal growth in the lake. In September 1987 a plastic-lined, gravel-filled wetland was constructed for experimental purposes and is the first demonstration of its kind in a subalpine region of the western United States. The primary objective of our research was to evaluate the effectiveness of this system in removing nitrogen, phosphorus, iron, suspended sediments and other constituents from runoff. Data collected between August 1987 and March 1989 show that the average per cent removal of total Kjeldahl-N as it passed through the wetland was low at −3%. In contrast, nitrate concentrations declined by greater than +85−90%. Changes in particulate phosphorus concentrations between inflow and outflow decreased by +47%, however, the soluble-P component showed a −28% reduction due to a contamination of the gravel during construction. The reduction of iron was very high at +84% for total reactive iron and +78% for soluble reactive iron. Finally, both turbidity and suspended solids were removed by the wetlands at an efficiency of approximately +85%.

Littoral zone production of oligotrophic lakes

Littoral zone primary production can be expressed as the summation of the integral production of its phytoplanton and benthic components:

Stream Phosphorus Transport in the Lake Tahoe Basin, 1989–1996

littoral prod.(mgC \cdot {m^{ - 1}}tim{e^{ - 1}}) = \mathop \smallint \limits_0^z \mathop \smallint \limits_0^x phyto prod, dzdx + \mathop \smallint \limits_0 benthic prod. dx.

Nitrogen fixation in periphyton of oligotrophic Lake Tahoe

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