Indirect effects of fish predation on calcite supersaturation, precipitation and turbidity in a shallow prairie lake

SUMMARY.

• 1 We tracked calcite saturation and seston composition during 1987 and 1988 in a shallow, hardwater prairie lake, 1 year before and 1 year after a lake-wide fish removal. We also measured the contribution of calcite to turbidity during 1988.
• 2 In both years calcite saturation increased rapidly after all ice had thawed and peaked during mid-late summer with the mineral saturation index of calcite (SI) sometimes exceeding 30.
• 3 Removal of calcite from lake seston by gentle acidification of unfiltered water samples showed suspended calcite to be an important source of lurbidity in this lake.
• 4 The lake-wide fish removal produced detectable changes in the annual cycle of calcite saturation and precipitation. Increased grazing by Daphnia galeata and Daphnia pulex apparently reduced calcite saturation during early summer by suppressing the phytoplankton, and lowering the demand for CO₂.
• 5 Lower calcite precipitation, as well as direct removal of calcite by Daphnia grazing, probably contributed to the improvement in water transparency observed after the fish kill.

     

Saprophytic Behaviour of Some Cereal Root-Rot Fungi

The ability of four cereal root-rot fungi, Helminthosporium sativum, Curvularia ramosa, Ophiobolus graminis and Fusarium culmorum , to colonize wheat straw as saprophytes has been investigated. Test pieces of straw were buried in a graded dilution series of maizemeal-sand culture of the test fungus with unsterilized soil. For assessing percentage straws colonized, three methods were compared: (1) isolation on agar plates, (2) Garrett's (1938 b ) wheat-seedling test and (3) incubation on moist sand, to promote sporulation (the 'sand-plate' test). Curvularia ramosa and Fusarium culmorum were found to behave as vigorous competitive saprophytes of the soil-inhabiting type, whereas Helminthosporium sativum and Ophiobolus graminis proved to be weak saprophytic colonizers of the specialized root-inhabiting type.