EXCLUSION OF PHYTOPLANKTON IN THE PROXIMITY OF DOMINANT WATER-SOLDIER (STRATIOTES-ALOIDES)

SUMMARY. Profuse growth of the submerged macrophyte, Stratiotes aloides, in two similar locations, one in central Sweden and one in the Masurian lake district of north‐eastern Poland, exerted a negative influence upon contiguous populations of phytoplankton. The Polish location offered an additional possibility to investigate chemical and biological gradients between the littoral community of S. aloides and the prevailing limnetic population of cyanophytes (mainly Anabaena planctonica and Oscilatoria spp.). A considerable decline in planktonic chlorophyll was accompanied by decreases in electrolytic conductivity, carbonate alkalinity and contents of calcium, potassium and sodium. Low concentrations of molybdate‐reactive phosphorus in the water of the Stratiotes community was credited to co‐precipitation with calcium carbonate on the leaf surfaces of the submerged plants. Thus, rather than allelopathy, competition for esssential nutrients together with changes in the ionic composition of the water seemed to be a more likely explanation for the decline in phytoplankton. Copyright © 1979, Wiley Blackwell. All rights reserved