No abstract

The diatom composition in surface sediments from 119 northern Swedish lakes was studied to examine the relationship with lake-water pH, alkalinity, and colour. Diatom-based predictive models, using weighted-averaging (WA) regression and calibration, partial least squares (PLS) regression and calibration, and weighted-averaging partial least squares (WA-PLS) regression and calibration, were developed for inferences of water chemistry conditions. The non-linear response between the diatom assemblages and pH and alkalinity was best modelled by weighted-averaging methods. The lowest prediction error for pH was obtained using weighted averaging, with or without tolerance downweighting. For alkalinity there was still some information in the residual structure after extracting the first weighted-averaging component, which resulted in a slight improvement of predictions when using a two component WA-PLS model. The best colour predictions were obtained using a two component PLS model. Principal component analysis (PCA) of the prediction errors, with some characteristics of the training set included as passive variables, was performed to compare the results for the different alkalinity predictive models. The results indicate that calibration techniques utilizing more than one component (PLS and WA-PLS) can improve the predictions for lakes with diatom taxa that have broad tolerances. Furthermore, we show that WA-PLS performs best compared with the other techniques for those lakes that have a high relative abundance of the most dominant taxa and a corresponding low sample heterogeneity.

One of the most useful approaches to long-term monitoring of aquatic systems is the analysis of lake sediments. Biological indicators, such as diatoms, preserved in the sediments are widely used. We suggest that use of near-infrared reflectance (NIR) spectroscopy of lake sediments could become a rapid and cost-effective technique for environmental monitoring to follow long-term changes in water quality. NIR spectra of surface sediments from Swedish lakes were used to establish relationships between sediment properties and measured lake water chemistry. Predictive models for inferring total phosphorus (TP), pH, and total organic carbon (TOC) from sediment NIR data were developed using partial least squares regression. The model for inferring lake water TP (n ) 33 lakes) captured 83% of the variance, while the explained variance for pH (n ) 52 lakes) and TOC (n ) 25 lakes) was 85 and 68%, respectively. We also used the TP model to evaluate the effect of inaccuracy in measured lake water chemistry for the model performance, i.e., the amount of explained variance. The inaccuracy in measured lake water chemistry corresponds to 10.5% of the total variance in the model. The highest possible variance to model then being 89.5%. This evaluation indicated that the obtained modeled variance almost equaled the variance possible to model, which suggests that further improvement of the models should be focused on enlargement of the calibration data set to include more lake types.

The effect of recent land use and subsequent vegetation changes per se on surface-water acidity is difficult to ascertain because of the complicating factor of enhanced levels of anthropogenic acid deposition that occurs at the same time as land-use changes. Expansion of conifers is a major contemporary vegetation change in Sweden and other countries. The immigration of Norway spruce (Picea abies (L.) Karst.) into Sweden about 3000 years ago provides, however, a suitable means of assessing the acidification ability of spruce per se on surface waters. Pollen analysis was used to identify the arrival of spruce in the catchments of eight acid-sensitive Swedish boreal-forest lakes. pH and dissolved organic carbon (DOC) were inferred from diatom assemblages for time periods covering some hundred years before and after the establishment of spruce. Redundancy analysis (RDA) was used to assess whether catchment vegetation had significant effects on the diatom assemblages. At four sites there were significant changes in the diatom assemblages associated with the arrival of spruce, but none of the lakes acidified. At three sites, however, diatom- inferred DOC increased with the arrival of spruce, probably as a result of the accumulation of raw humus.

Abstract. Lysevatten, a lake in southwest Sweden, has experienced both acidification and recent changes in the amount of lake-water organic carbon (TOC), both causing concern across Europe and North America. A range of paleolimnological tools -diatom-inferred pH, inferred lake-water TOC from visible-near-infrared spectroscopy (VNIRS), multielement geochemistry and pollen analysis, combined with geochemical modeling were used to reconstruct the lake's chemistry and surroundings back to the most recent deglaciation 12 500 years ago. The results reveal that the recent anthropogenic impacts are similar in magnitude to the longterm variation driven by natural catchment changes and early agricultural land use occurring over centuries and millennia. The combined reconstruction of both lake-water TOC and lithogenic element delivery can explain the major changes in lake-water pH and modeled acid neutralizing capacity during the past 12 500 years. The results raise important questions regarding what precisely comprises "reference" conditions (i.e., free from human impacts) as defined in the European Water Framework Directive.