Crater Lake Limnological Studies Final Report
General Methods for Monitoring and Sampling Schedule
An array of variables should be monitored (Table 4) following methods developed during the initial 10-year study and described in various chapters of this report. Water-quality determinations (e.g., pH, total alkalinity, conductivity, and dissolved oxygen) are included in the program because they provide a basis for an understanding of processes associated with deep-lake circulation. Measurements of these water quality variables also are needed for monitoring the productive capacity of Crater Lake. The weight of abiotic particles should be estimated by filtering known volumes of lake water and then analyzing material retained by the filter to determine the total mass and the concentration of aluminum, a tracer for particulate material from the caldera walls. Although concentrations of total coliform, fecal coliform and fecal streptococcus bacteria have been very low in the springs, a continuation of bacterial sampling at selected springs is recommended, at least for several more years to continue to monitor for possible changes following the recent removal of the sewage gallery.
Table 4. Monitoring parameters at Crater Lake and at intra-caldera springs of the lake.
Precipitation, wind speed and direction, temperature, humidity and solar radiation
Lake temperature and conductivity
Conductivity, temperature, and depth probe (CTD) from surface to a depth of 550 m.
Optical properties of the lake
20-cm Secchi disk
Photometer (to 150 m)
Transmissometer (to 550 m)
Abiotic properties of the lake
Chemical properties of the lake
Total alkalinity, pH, specific conductance, and concentrations of dissolved oxygen, total phosphorus, orthophosphate, nitrate-nitrogen, total Kjeldahl-nitrogen, ammonia-nitrogen, silica, and trace elements at selected depths from the following depth sequence: 0, 5,10, 20, 60,100, 200, 500, and 550 m
Biological properties of the lake
Chlorophyll a concentration
In vitro chlorophyll concentration according to the following depth sequence: 5-m from 0 to 10 m, 20-m intervals from 20 to 200 m, and 25-m intervals from 200 to 300m, including contribution from picoplankton collected through differential filtering primary production (C-14 light/dark bottle)
C-14 primary production at the chlorophyll sampling depths to 180 m, including contribution picoplankton collected through differential filtering Phytoplankton
Species, density, and biovolumes at all chlorophyll sampling depths to a maximum 200 m
Species, density, and biomass in samples obtained by a vertical tow of a .5 m diameter, number 25 dosing net
Species, abundance, and biomass, spatial distribution, age, sex, growth and food habits; Samples collected with gill nets, hook and line, and down rigger. Pelagic distributions estimated using an echo-sounder
Bulk atmospheric deposition to the lake
Springs 20, 38, 39, 42, 48
Temperature, pH, conductivity, alkalinity, nutrients, trace elements, and coliform, fecal coliform, and fecal streptococcus)
Access to Crater Lake is a major consideration in the design of a monitoring program. The lake can be reached by foot via the Cleetwood trail only during summer and early fall (June through September). In winter and spring, the lake only can be reached by helicopter or by descending and ascending the caldera wall. Accessibility for sampling in winter, spring, and late fall (October – November) is restricted further by frequent periods of poor weather. Recognizing these limitations and the difficulty of working on Wizard Island in winter, the lake should be sampled at monthly intervals between June and September, when the lake is accessible by foot and when boats can be moored on the lake. Researchers also should strive to sample the lake during periods of good weather at other times of the year. Although weather conditions in the park are not predictable from year to year, sampling should be attempted each year in January and April to match the “off-season” sampling conducted during the 10-year program. Off-season data are considered essential to evaluate seasonal changes in the lake ecosystem. January and April samples provide estimates of the amount of deep-water circulation, concentrations of nutrients, primary productivity, and characteristics of the phytoplankton and zooplankton communities. Sampling from June through September provide information on the limnological conditions of the lake during the period of thermal stratification. With few exceptions, samples should be taken at Station 13 to be consistent with the major sampling site used over the last 10 years.