Epsilon Archive for Student Projects

Abstract

In this paper phytoplankton is studied in a gradient from a macrophyte covered inner part of the bay of Dalbyviken of Lake Mälaren to the mouth area into the Ekoln Basin. The object was to obtain clarity upon the function of the macrophyte community as a recruit habitat for phytoplankton. Another aim was to compare the macrophyte community with the community investigated twentynine years earlier in the same bay. The results showed that the bay has been silted up during the passed time, the reed vegetation has spread and increased in biomass. The water of the littoral zone had low concentrations of nitrogen but very high of phosphorus . Concentrations of these nutrients in the mouth area was the inverse - high considering nitrogen and lower considering phosphorus. Life-form of phytoplankton differed significantly in the shallowest parts from that of the open water areas. Among the tested environmental variables inorganic nitrogen and light climate were especially related to differences in taxa composition of the phytoplankton community. When separating the phytoplankton in eight different groups of life-forms a co-variation with phosphorus, light climate and temperature was significant. Plankton splitted up in groups related to competitors, ruderals and stress tolerants were most related to nutrients as phosphorus and nitrogen. The phytoplankton and environmental relationsships were elucidated by the use of canonical correspondence analysis. Phytoplankton´s biovolume reached its maximum earliest during the summer in the open water zone and later in the inner parts of the bay. Therefore this study could not confirm that the littoral zone acts as a recruit habitat for phytoplankton, causing higher biomass in the littoral parts before high biomasses were reached in the open water areas. The wind direction during the whole studied period was mainly southern or south-western causing a main transport of phytoplankton from the mouth of the bay to its inner region. In the shallow inner areas the light reached to the bottom layer which could have had a photoinhibiting effect and be a possible explanation for the small quantities of non-motile plankton there.