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Geology topics

R.L. Hoffman

Publications and source records attributed to R.L. Hoffman.

6 recordsLinked to original sources
USGS2007

Thermal, chemical, and optical properties of Crater Lake, Oregon

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km 2 and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm) clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study, August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m) between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of 1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements provided additional evidence about the exceptional clarity of Crater Lake.

Oregon↗
USGS2007

Predicting Secchi disk depth from average beam attenuation in a deep, ultra-clear lake

We addressed potential sources of error in estimating the water clarity of mountain lakes by investigating the use of beam transmissometer measurements to estimate Secchi disk depth. The optical properties Secchi disk depth (SD) and beam transmissometer attenuation (BA) were measured in Crater Lake (Crater Lake National Park, Oregon, USA) at a designated sampling station near the maximum depth of the lake. A standard 20 cm black and white disk was used to measure SD. The transmissometer light source had a nearly monochromatic wavelength of 660 nm and a path length of 25 cm. We created a SD prediction model by regression of the inverse SD of 13 measurements recorded on days when environmental conditions were acceptable for disk deployment with BA averaged over the same depth range as the measured SD. The relationship between inverse SD and averaged BA was significant and the average 95% confidence interval for predicted SD relative to the measured SD was ??1.6 m (range = -4.6 to 5.5 m) or ??5.0%. Eleven additional sample dates tested the accuracy of the predictive model. The average 95% confidence interval for these sample dates was ??0.7 m (range = -3.5 to 3.8 m) or ??2.2%. The 1996-2000 time-series means for measured and predicted SD varied by 0.1 m, and the medians varied by 0.5 m. The time-series mean annual measured and predicted SD's also varied little, with intra-annual differences between measured and predicted mean annual SD ranging from -2.1 to 0.1 m. The results demonstrated that this prediction model reliably estimated Secchi disk depths and can be used to significantly expand optical observations in an environment where the conditions for standardized SD deployments are limited. ?? 2007 Springer Science+Business Media B.V.

Hydrobiologia↗
USGS2007

Impacts of a gape limited Brook Trout, Salvelinus fontinalis, on larval Northwestern salamander, Ambystoma gracile, growth: A field enclosure experiment

The formation of amphibian population structure is directly affected by predation. Although aquatic predators have been shown to have direct negative effects on larval salamanders in laboratory and field experiments, the potential impacts of gape-limited fish on larval salamander growth has been largely underexplored. We designed an enclosure experiment conducted in situ to quantify the effects of gape-limited Brook Trout (Salvelinus fontinalis) on larval Northwestern Salamander (Ambystoma gracile) growth. We specifically tested whether the presence of fish too small to consume larvae had a negative effect on larval growth. The results of this study indicate that the presence of a gape-limited S. fontinalis can have a negative effect on growth of larval A. gracile salamanders. Copyright 2007 Society for the Study of Amphibians and Reptiles.

Journal of Herpetology↗
USGS2002

Persistence of an unusual pelagic zooplankton assemblage in a clear, mountain lake

The planktonic zooplankton assemblage in Mowich Lake, Mount Rainier National Park (MORA), was composed almost entirely of rotifers in 1966 and 1967. Adult pelagic crustacean taxa were rare. Their paucity was attributed to predation by kokanee salmon (Oncorhynchus nerka), which had been stocked in 1961. During a park-wide survey of 24 lakes in 1988, Mowich Lake was the only one that did not contain at least one planktonic crustacean species. Given the apparent persistence of the unusual pelagic zooplankton assemblage in Mowich Lake, the first objective of this study was to document the interannual variation in the taxonomic structure of the zooplankton assemblages in the lake from 1988 through 1999. A second objective was to determine if it was possible to predict the taxonomic composition of the pelagic crustacean zooplankton assemblage in Mowich Lake prior to the stocking of kokanee salmon. The Mowich Lake zooplankton assemblages in 1988-1999 were consistent with those in 1966 and 1967. Crustacean taxa were extremely rare, but they included most of the primary taxa collected from 23 MORA lakes surveyed in 1988. Nonetheless, the 1988 collections showed that the September rotifer assemblage in Mowich Lake was similar to 10 of the 24 lakes sampled. Seven of the 10 lakes were dominated by cladocerans, primarily Daphnia rosea and Holopedium gibberum. Therefore, it appeared that either one or both of these species may have numerically dominated the crustacean zooplankton assemblage in the lake prior to 1961.

Hydrobiologia↗