Ceratophyllum demersum hampers phytoplankton development in some small Norwegian lakes over a wide range of phosphorus concentrations and geographical latitude

Data on submerged and floating‐leafed macrophytes, phytoplankton, nutrients (N, P) and calcium were collected from twenty‐four small lakes ( 1 km 2 ) over a wide range of latitudes in Norway. The majority of the investigated lakes were mesotrophic or eutrophic, and most of the lakes were markedly affected by diffuse and point‐source runoff from agriculture. According to their macrophyte species composition, the majority of the lakes can be classified as Potamogeton lakes or Chara lakes, or a combination of these. 2. This study is consistent with the ‘two alternative stable states’ hypothesis. We observed clearwater lakes with dense macrophyte cover over a wider range of total P concentration than has been reported previously: from 30 to more than 700 mg P m –3 . The clearwater state was only observed in lakes with mean depths of less than 1.9 m. 3. Most clear lakes with high cover of submerged vegetation showed indications of N limitation. 4. In this study nearly all the macrophyte‐dominated lakes with P concentrations above 30 mg m –3 had dense stands of Ceratophyllum demersum (L.). This indicates that Ceratophyllum may also play an important role in stabilizing and maintaining a clearwater state at high P concentrations.