Problems in estimating phytoplankton nitrogen limitation in Shallow Eutrophic Lakes

In three shallow eutrophic lakes, phytoplankton nutrient limitation was inv estigated. Special emphasis was laid on nitrogen limitation. Two different methods were applied: growth response after nutrient supply (enrichment res ponse) and ammonium enhancement response. In two of the three lakes, nitrog en limitation occurred, but the two methods led to different results. The e nrichment response was related to the ambient nitrogen concentrations where as the ammonium enhancement response corresponded more closely with the phy toplankton C:N ratios. Thus, it is difficult to estimate nitrogen limitatio n in systems with rapid changing nutrient conditions.