Epsilon Open Archive

Abstract

In order to protect or improve surface waters ecosystem response to pressures needs to be quantified. Diatoms are frequently used for assessing ecological status in streams and for reconstructing water quality of lakes. However, ecological status assessment of European lakes based on extant diatom assemblages is rare.

The overall aim of this thesis is to facilitate the application of benthic diatoms in water quality assessment of boreal lakes, using methods developed for stream assessment. Therefore, I investigated (i) the main environmental drivers of lentic diatom assemblages, (ii) the structural differences between lentic and lotic diatom assemblages (iii) the implications of differences between lake and stream diatom assemblages for assessment, (iv) the power of different aquatic organisms as indicators of nutrient and acidity status in lakes, and (v) ecological thresholds of diatom and phytoplankton assemblage structure along a nutrient gradient in boreal lakes.

The major environmental drivers of benthic diatom assemblage composition in boreal lakes were gradients in acidity and nutrient status. In contrast, the distribution of ecological diatom guilds seems to be decoupled from nutrient availability in acidic compared to high pH aquatic systems. Diatom assemblage composition can thus be recommended for ecological status assessment of boreal lakes, whereas species-specific growth morphology and the drivers affecting growth morphology need further study. Most diatom taxa occurred in Swedish lakes and streams, but differences in frequency and abundance among lakes and streams were noted. However, the diatom indices IPS and ACID, developed for streams, responded similarly to nutrient and acidity gradients, respectively, in Swedish lakes and streams and can thus also be recommended for ecological status assessment of lakes. In nutrient poor boreal lakes, both primary producers and primary consumers were correlated with nutrient concentrations. Primary producers, especially diatoms, displayed a stronger response to pH than macro-invertebrates, possibly due to shorter generation times implying faster response. Nutrient thresholds in taxonomic and guild structure of benthic diatom assemblages in boreal lakes were found at low TP concentrations (18-26 μg L⁻¹). However, caution is advised in interpreting the response of some species to nutrients, as pH dependencies might obscure the response of some nutrient sensitive diatom species to TP.