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

1. Data on submerged and floating-leafed macrophytes, phytoplankton, nutrients (N, P) and calcium were collected from twenty-four small lakes (less than or equal to 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.