Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 89

Report on sediment accumulation rate changes in European lakes

Sediment accumulation rate (SAR) is one of the most important physical parameters affecting the functioning of lake systems. In many lakes across the world an increase in SAR has been observed over the last c.100 years. Reasons for this include changes in land−use and land−management, causing accelerated catchment soil erosion, and eutrophication. Climate change can influence these processes and is likely to play a progressively more important role in future.

The EU Water Framework Directive (WFD) requires that biological, hydromorphological and chemical elements of water quality should be based on the degree to which present day conditions deviate from those expected in the absence of significant anthropogenic influence, termed reference conditions. Currently however, the reference condition for a ?basal sediment accumulation rate? for lakes of different types is undefined, and the main driving factors remain unidentified. It is therefore necessary to improve our understanding of the controls on SARs, including changing climate, to determine which are the most susceptible lake types and what the biological consequences might be. This is one of the aims of Euro−limpacs WP2 Task 1.2.

SAR data were compiled for 337 sediment cores from 278 lakes. However, data on latitude, longitude, alkalinity, altitude, maximum depth and lake area were only available for 207. The 207 lake dataset includes sites in 19 countries. 60% are from the UK and smaller clusters exist in Italy, Finland, Estonia, Svalbard, Spain and Denmark. Major gaps occur in mainland central Europe, from the Low Countries eastwards, in south−east Europe and in mid−northern latitudes including central Norway and Sweden and southern Finland. European Russia is particularly poorly represented. In terms of climatic zones, most sites fall within a humid−oceanic zone, while humidcontinental and sub−arctic regions are under−represented.

Consideration of the sites with >3 dated cores shows that where multiple cores are taken from representative deep−water areas then there is good agreement in terms of general SAR trends and in ?basal? SARs. We conclude that SAR data from sediment cores taken from deep water areas are likely to be representative of that lake sediment basin. 66% of the sediment cores showed surface SARs higher than basal rates. 19% showed no change while 15% showed a decline. For classes showing SAR increases, little change occurred prior to 1900. Most increases seemed to occur in more recent periods in particular 1950 ? 1975 and post−1975. It may be that this observation identifies a general acceleration in SAR in European lakes.

Lakes were classified into lake−types using four parameters: alkalinity (3 classes), altitude (3 classes), maximum depth (2 classes) and lake area (2 classes). This generated a possible 36 lake classes of which 25 were represented in the dataset. Nine classes contained >10 lakes. The largest class (1221) includes many upland sites in the UK used in the Surface Water Acidification Project and those now forming part of the UK Acid Waters Monitoring Network.

Reference SARs were estimated for 8 lake classes. Mountain lakes had the lowest reference SAR (0.005 ± 0.003 g cm−2 yr−1) while large, lowland, high alkalinity sites had the highest (0.03 ? 0.04 g cm−2 yr−1). Others range between 0.012 and 0.024 g cm−2 yr−All would benefit from further data.

Next steps include: (i) addition of further SAR and typology data (ii) further statistical analysis (iii) more detailed sedimentological work to identify those components of the sediment responsible for SAR increases in key lake classes and determining the causes of these increases.

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