Responses of a Marl Lake to Fertilization

Abstract

A shallow marl lake was fertilized during three consecutive years with inorganic fertilizer. Application was made in midsummer and evaluation of its effect was made by comparing data gathered in a pretreatment and a posttreatment period each year. Fertilization brought about an immediate increase in suspended solids and a decrease in transparency. These changes appeared to be caused by a coalescence of suspended marl rather than by chemical precipitation or by an increase in phytoplankton. A large fraction of the added phosphorus disappeared from the water along with the suspended marl. When solids were separated from the water with a centrifuge, we were unable to detect an increase in the organic fraction of the solids after fertilization. However, during the third year of the study a small increase was noted when a Millipore filter was employed. Inorganic nitrogen and phosphorus remained in the water for a very short period; however, an organic phosphorus fraction persisted 25 days or more. In 1954 and 1955 we were unable to detect living phytoplankton cells in solids separated from the water either before or after fertilization when a Foerst centrifuge was used to separate suspended matter. In 1956 a sparse flora of nannoplankton was concentrated from the water using a Millipore filter, and the number of nannoplankton genera increased following fertilization. This response was weak and did not affect the transparency of the lake water. Fertilization brought about a well-marked increase in the production of periphyton algae in all three years. The data suggest that benthic algae are more effective than planktonic algae in utilizing nutrients added to marl lakes.