An experiment in >1000 river and riparian sites found spatial patterns and controls of carbon processing at the global scale.

status of rivers using benthic diatoms were compared. Ecological status is estimated as the ratio between the observed value of a biological element and the value expected in the absence of significant human impact. Approaches to defining the 'reference sites', from which these 'expected' values were derived, varied from country to country. Minimum criteria were established as part of the exercise but there was still considerable variation between national reference values, reflecting typological differences that could not be resolved during the exercise. A simple multimetric index was developed to compare boundary values using two widely used diatom metrics. Boundary values for high/good status and good/moderate status set by each participant were converted to their equivalent values of this intercalibration metric using linear regression. Variation of ±0.05 EQR units around the median value was considered to be acceptable and the exercise provided a means for those Member States who fell significantly above or below this line to review their approaches and, if necessary, adjust their boundaries.

The goal of this study was a harmonization of diatom identification and counting among diatomists from the Scandinavian and Baltic countries to improve the comparison of diatom studies in this geographical area. An analysis of the results of 25 diatomists following the European standard EN 14407 during an intercalibration exercise showed that a high similarity was achieved by harmonization and not because of a long experience with diatoms. Sources of error were wrong calibration scales, overlooking of small taxa, especially small Navicula s.l., misidentifications (Eunotia rhomboidea was mistaken for Eunotia incisa) and unclear separation between certain taxa in the identification literature. The latter was discussed during a workshop with focus on the Achnanthes minutis-M. Kahlert (*) : sima group, the separation of Fragilaria capucina var. gracilis from F. capucina var. rumpens, and Nitzschia palea var. palea from N. palea var. debilis. The exercise showed also that the Swedish standard diatom method tested here worked fine with acceptable error for the indices IPS (Indice de Polluo-sensibilité Spécifique) and ACID (ACidity Index for Diatoms) when diatomists with a low similarity (Bray-Curtis <60%) with the auditor in at least one of the samples are excluded.

We traced the origin of dissolved organic matter (DOM) in the large, shallow, eutrophic Lake Võ rtsjärv in Estonia. Allochthonous DOM (Al-DOM) had higher d 13 C values than autochthonous DOM (Au-DOM). The d 13 C of inflow DOM varied from 228.2% to 225.4% (mean 226.7%) and in-lake DOM varied from 228.4% to 226.1% (mean 227.2%). Low stable isotope (SI) signatures of Au-DOM were caused by relatively 13 C-depleted values of its precursors (mainly phytoplankton) with mean d 13 C of 228.9%. SI signatures of dissolved inorganic carbon (DIC) in the inflows and in the lake were also relatively low (from 215.1% to 23.28%). SI values of DOM were lower during the active growing season from May to September and higher from October to April, with the corresponding estimated average proportions of Al-DOM 68% and 81%. The proportion of Al-DOM decreased with increasing water temperature, chlorophyll a, and pH and increased with increasing water level and concentration of yellow substances and DIC. The high proportion of Al-DOM in Võ rtsjä rv shows that, even in this highly productive ecosystem, the labile Au-DOM produced is rapidly utilized and degraded by microorganisms and thus makes a relatively small contribution to the instantaneous in-lake DOM pool.

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