Limnological Report – 04 Priorities, Questions, and Hypotheses

Crater Lake Limnological Studies Final Report

 Priorities, Questions, and Hypotheses


A long-term monitoring program at Crater Lake should focus on the collection of meaningful information about the status and trends of the lake. It should provide a minimum set of standards by which the health of the system can be evaluated as well as the baseline data needed to support specific investigations into lake processes that we do not understand. Understanding those processes that affect lake clarity is the highest priority of the recommended monitoring program. These processes involve complex interactions with other components and processes in the lake, and the following questions need to be addressed:

  1. How much of the variation in lake clarity can be explained by changes in the densities of abiotic and biotic light scattering particles, and how much does each type of particle contribute to changes in lake clarity?

  2. Is there a significant relationship between Secchi disk clarity and the clarity of the water column below a depth of 40 m where much of the biological production occurs?

  3. What are direct and indirect effects of thermal stratification on lake clarity?

  4. Do changes in weather, climatic conditions, and lake level alter clarity by affecting the amount of particles entering the lake from surface runoff, avalanches, mud slides and erosion of the shoreline?

  5. How do changes in primary productivity, concentration of chlorophyll, and phytoplankton cell densities relate to variation in lake clarity?

  6. Do changes in the abundance and distribution of zooplankton affect lake clarity by affecting cell size and densities of phytoplankton?

  7. Does predation by kokanee salmon on zooplankton affect lake clarity through indirect food-chain relationships with phytoplankton populations?

These questions provide a focus for the long-term monitoring program which is described below. Specific hypotheses, which will guide routine monitoring and other scientific studies, emerge from these questions. These hypotheses direct the sampling effort of the long-term monitoring program toward specific components and processes of the lake system. These components and processes are ranked on a relative scale from 1 to 2, with 1 being of highest importance for evaluating changes in lake clarity, and 2 being of least importance (Table 3).