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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).

¹Total alkalinity, pH, conductivity, and dissolved oxygen.

A key aspect of the proposed sampling program (Table 3) is that it is designed to focus on features of the lake system that affect water column clarity and are also sensitive to changing environmental conditions. Thus, the proposed long-term monitoring program is structured to optimize park management’s ability to detect and track changes in the lake. The hypotheses of the program are:

1. Lake clarity is controlled more by abiotic light scattering particles from the caldera walls and suspended lake sediments than by phytoplankton.

2. Secchi disk clarity and the clarity of the water column below a depth of 40 m exhibit a significant, positive correlation.

3. Thermal structure of the water column has no direct influence on lake clarity.

4. Changes in climate, local weather conditions, and lake levels alter clarity by affecting the amount of turbidity entering the lake from the caldera walls and by affecting erosion of the lake shore.

5. Changes in primary productivity alter lake clarity by affecting phytoplankton cell densities and the concentration of chlorophyll.

6. Climate change and air pollution significantly affect primary productivity by increasing nutrient input to the lake and by affecting the amount of upwelling of nutrients from the deep lake.

7. Changes in the species composition, cell densities, and vertical distribution of the phytoplankton are significantly correlated with changes in primary productivity.

8. Picoplankton, the portion of the phytoplankton community that is too small to be seen under a light microscope, contribute significantly to changes in lake clarity and primary productivity.

9. Grazing by zooplankton affects lake clarity by affecting densities and cell sizes of phytoplankton assemblages.

10. Nutrient regeneration by zooplankton contributes significantly to changes in primary productivity and lake clarity.

11. Changes in primary productivity have a significant impact on the abundance and species composition of the zooplankton community.

12. Predation by kokanee salmon affects lake clarity indirectly by altering the taxanomic structure of the zooplankton community.

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