Pond or lake: does it make any difference?

To investigate the importance of lake size, we analysed the chemical and biological characteristics of nearly 800 Danish lakes ranging from 0.01 to 4200 ha. Most of the lakes were shallow (median depth = 1.5 m) and eutrophic (lake water mean total phosphorus = 0.26 mg P1(-1) and mean chlorophyll-a = 60 mu g 1(-1)). Phosphorus and nitrogen concentrations were unaffected by lake size, but positively related to agricultural exploitation. Lakes < 1 ha showed a higher variability in phosphorus concentrations, but had a lower chlorophyll yield per unit of both nitrogen and phosphorus, which is indicative of less importance of nutrients in small lakes. Fish were absent in most lakes smaller than 0.1 ha and mean fish biomass was markedly lower in lakes < 1 ha than in lakes > 1 ha. The absence of fish did, however, not result in higher abundance of Daphnia, suggesting a higher impact by invertebrate predators in small lakes. Taxon richness of both zoo- and phytoplankton was weakly related to lake size, whereas the number of submerged macrophyte and fish species increased steadily with lake size. Also species richness of macrophytes increased with increasing alkalinity. The low impact of lake size on the species richness of several taxonomic groups suggests that ponds and small lakes are important biodiversity components in the agricultural landscape.