Vertical distribution of primary production and phytoplankton in two small lakes with different humus concentration in southern Finland

Abstract

Vertical distribution of primary production and phytoplankton was studied in a polyhumic brownwater lake and in an oligo‐mesohumic lake. During summer both lakes were thermally, chemically and biologically stratified. In the brownwater lake primary production was restricted to the uppermost layer of 1–1.5 m of epilimnion. In the oligo‐mesohumic lake noticeable primary production was detected down to depths of 2–3 m. The ice‐free period primary production was about 20% higher in the oligo‐mesohumic lake, though occasionally the surface production was 2–3 times higher in the brownwater lake. Epilimnetic total phosphorus and total nitrogen concentrations were higher in the brownwater lake, while nitrate‐nitrite, ammonium and phosphate concentrations were very low in both lakes.Phytoplankton was confined to the uppermost productive layer in the brownwater lake. In the oligo‐mesohumic lake phytoplankton was distributed more evenly, though the mean maximum biomass was at the depth of 3–4 m. Below the oxic water layer biomass decreased abruptly in both lakes. In the oligo‐mesohumic lake chlorophyll concentration was extremely high (max. 320 mg chl a m−3) in the anoxic hypolimnion, due to green sulphus bacteria.Flagellated chlorophytes and thrysophytes dominated in the brownwater lake; in spring Chlamydomonas species, followed by Mallomonas caudata. In the oligo‐mesohumic lake small coccal green algae, such as Oocystis, Scenedesmus and Westella‐like species, dominated in mid‐summer, and chrysophytes and cryptomonads in autumn.