Comparison of bioassay procedures for growth-limiting nutrients in the Laurentian Great Lakes

Summary

A series of experiments conducted on the effects on phosphorus, nitrogen and silica on the growth of algae showed that phytoplankton were phosphorus limited in Lake Huron and Lake Michigan but not in Lake Erie. Three types of experiments were used: a spiked test with Selenastrum capricornutum Printz., a spiked test with natural phytoplankton assemblages and a fixed-level test with natural phytoplankton assemblages in which depleted nutrients were replenished periodically to maintain specified levels. A replicated factorial design was used in all experiments. Levels of nutrients were ambient (existing levels), plus three additions of phosphorus ranging from 5 to 15 μg/1, two additions of silica, 0.8 and 1.6 mg SiO²/l, and one addition of nitrate, 0.30 mg N/l. Although other variables were used, changes in chlorophyll standing crops were utilized most frequently to determine whether treatments affected algal growth. Results indicate that experiments with natural populations can be conducted either in the field or laboratory and that the naturally occurring phytoplankton assemblages respond to treatment containing low levels of nutrients and are not excluded by experimental conditions. Such experiments with small nutrient additions therefore can be used to obtain valuable information on the ecology of specific populations in natural phytoplankton assemblages.