The distribution ofPeridinium willeiandP. volziiwas studied in Danish lakes. Both species were confined to lakes with concentrations of Total P<0.15 mg 1‐1, with the majority of occurrences at Total P concentration between 0.020–0.040 mg 1‐1and concentrations of PO4P between detection limit and 0.040 mg 1‐1. The occurrence of the species in relation to inorganic N compounds (NH4N and NO2+ NO3N) was significantly broader forP. willeithan forP. volzii: P. willeihad an almost even distribution within a wide range of NH4N, whereasP. volziimainly occurred between 0.001 and 0.10 NH4N 1‐1.P. willeihad an almost even distribution at values beween 0.005 and 0.42 mg NO2+ NO3N 1‐1, whereasP. volziimainly occurred below 0.050 mg NO2+ NO3N 11.P. willeiwas found at pH values between 4.2 and 8.5, whereasP. volziiwas confined to lakes with a slightly basic pH. The study confirmed the broad limits ofP. willeiand the much more narrow limits ofP. volziiin relation to seasonal occurrence and pH, as well as an affinity of the former to ponds and lakes with a rich bottom vegetation. The study also showed, however, that the species were not as widespread and common in recent Danish lake phytoplankton as generally stated by previous authors. The use of different ecological factors to give weight to species separation is discussed. The inclusion ofP. volziiinP. willeiproposed by Popovsky&Phiester is not supported by the present study, as the two taxa appear to have different ecological
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