Occurrence of Cryptophyceae and katablepharids in boreal lakes

Abstract

One of the most important algal groups in Finnish lakes are the Cryptophyceae. Changes in the community structure of Cryptophyceae in a total of 22 lakes belonging to the Vuoksi river basin in eastern Finland were studied. The existence of lakes with water qualities varying from oligotrophic to eutrophic, often loaded by human activities, provides a good opportunity to study the effects of environmental variables on the occurrence and size variation of Cryptophyceae. In the Vuoksi river basin, the main soil type is moraine. Twelve of the lakes were large or moderately large and with clear, i.e. oligo-humic water, and one lake could be described as a small clear water lake. Eight large or moderately large lakes were humic, with a water colour number of 40–70 mg l⁻¹ Pt, including three lakes impacted by nutrient loads. One lake was naturally eutrophic, with a high water colour number of 100 mg l⁻¹ Pt, and was also impacted by municipal and pulping effluents. CCA-ordination analysis grouped the studied lakes into: (1) clear water lakes, (2) humic lakes and (3) the naturally eutrophic brown water lake. In the CCA-ordination analysis based on cell numbers small Cryptophyceae (Cryptomonadales), Rhodomonas lacustris and the katablepharid Katablepharis ovalis were grouped into the first axis, which was positively correlated with Secchi depth (r = 0.58) and NO₃N — nitrogen (r = 0.24) and negatively with Ptot (r = −0.69), PO₄P (r = −0.69) and water colour number (r = −0.66). In humic lakes, medium-sized Cryptophyceae were abundant. The naturally eutrophic lake was grouped into first axis, which is positively correlated with Ptot (r = 0.69), PO₄P (r = 0.69) and water colour number (r = 0.66). The lake formed a distinct group with large Cryptophyceae. Only in this lake was the heterotrophic Katablepharis ovalis rather abundant. However, large-sized taxa dominated the biomass of the Cryptophyceae assemblage in all lake types excluding large clear water lakes, where Rhodomonas lacustris dominated and large Cryptophyceae co-dominated.