Water quality in streams typically changes fast, and sensitive biological indicators are crucial for monitoring these changes in water quality. Diatoms are widely used in biological stream quality assessment. However, interannual and intra-annual variation of diatom population densities is large, often hampering a reliable quality assessment. We studied the importance of different species traits on temporal occurrence of diatom species. We also examined whether temporal occurrence of diatoms is related to species' local abundance or regional distribution using a data set collected in Finnish streams. According to the general linear model (GLM), temporal occurrence of diatoms increased with increasing local abundance and regional distribution. Species that occurred more frequently also had larger niche breadths and nonmarginal niche positions. In addition, cell size and attachment ability were positively related to species temporal occurrence. Our results imply that abundant and widely distributed species with large niches and ability to attach sustain persistent populations in varying environmental conditions typical for streams. We suggest that future studies could concentrate on monitoring these common (abundant) species when detecting the possible changes in the biological state of streams. We advise, however, to consider a relatively large number of species as many of the most common species may have low indicator values.

The effects of hypolimnetic aeration on oxygen penetration depth and oxygen flux into the sediment (DOU) were studied during two successive years. The study was conducted by comparing data from the aerated Enonselk€ a basin with the non-aerated Kajaanselk€ a basin of Lake Vesij€ arvi (southern Finland). Aeration increased the average O 2 concentration in nearbottom layers. This was mainly due to shortened stratification period in the aerated basin, which was again attributed to the positive effect of aeration on hypolimnetic water temperature. Aeration did not prevent hypoxia during summer stratification and oxygen penetration depth in the sediment and DOU did not increase. During summer stratification DOU was even lower at the aerated than at the non-aerated basin. The weak effect of aeration on DOU and oxygen penetration was mostly due to the minor effect on hypolimnetic oxygen concentrations. The positive effect of aeration on hypolimnetic O 2 concentration was negated by the positive effects of aeration on hypolimnetic temperature and turbulence that increase oxygen consumption in the water column. Due to the generally low effect on DOU, the present aeration effort in Enonselk€ a does not have positive effects on sediment quality.

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A 3D-model was developed to study the effects of hypolimnetic aeration on the temperature profile of a thermally stratified Lake Vesijärvi (southern Finland). Aeration was conducted by pumping epilimnetic water through the thermocline to the hypolimnion without breaking the thermal stratification. The model used time transient equation based on Navier-Stokes equation. The model was fitted to the vertical temperature distribution and environmental parameters (wind, air temperature, and solar radiation) before the onset of aeration, and the model was used to predict the vertical temperature distribution 3 and 15 days after the onset of aeration (1 August and 22 August). The difference between the modelled and observed temperature was on average 0.6°C. The average percentage model error was 4.0% on 1 August and 3.7% on 22 August. In the epilimnion, model accuracy depended on the difference between the observed temperature and boundary conditions. In the hypolimnion, the model residual decreased with increasing depth. On 1 August, the model predicted a homogenous temperature profile in the hypolimnion, while the observed temperature decreased moderately from the thermocline to the bottom. This was because the effect of sediment was not included in the model. On 22 August, the modelled and observed temperatures near the bottom were identical demonstrating that the heat transfer by the aerator masked the effect of sediment and that exclusion of sediment heat from the model does not cause considerable error unless very short-term effects of aeration are studied. In all, the model successfully described the effects of the aerator on the lake's temperature profile. The results confirmed the validity of the applied computational fluid dynamic in artificial aeration; based on the simulated results, the effect of aeration can be predicted.