NORPEC- Diatoms

General
Diatoms make up a large group of single-celled freshwater and marine plant algae (division Bacillariophyta) usually varying in size from 0.0025 to 0.5 mm. They can also be found in wet terrestrial habitats. Their cell wall is split into two valves. These two valves consist of silica, and they are therefore usually well preserved in sediments. The taxonomy of the diatoms is based on shape and sculpturing of the valves. The distribution of the different diatom taxa is closely related to water chemistry and the diatoms have therefore been used as ecological sensitive indicators to a range of water variables such as pH, nutrients, salinity, dissolved organic carbon (DOC), and conductivity.

Quantitative environmental reconstruction
With the development of statistical analyses and the so-called transfer functions in the 1980 and 1990 it has been possible to reconstruct the ecological parameters quantitatively. The reconstruction can both be applied in space (e.g. using surface sediment samples or epilithic samples) and time (e.g. using sediment sequences). The quantitative reconstruction of the ecological parameters demands a data set of diatom assemblages covering the gradient you want to reconstruct with related chemical parameters, a so-called calibration set.

Strengths and weaknesses by quantitative reconstruction
During the last decades an important aim in diatom research has been to demonstrate the diatoms response to climate change. There has been produced several calibration sets between diatom assemblages and mean July air or water temperature. These works have demonstrated a unique response to temperature change but have also demonstrated that the diatoms respond to a range of other environmental variables covarying with the temperature gradient.
For quantitative reconstruction purposes, there are two very important assumptions:
1. The environmental variable to be reconstructed is, or is linearly related to an ecological important variable in the system of interest. This assumption is usually followed in the construction of a calibration set were lakes are selected along altitudinal or latitudinal gradients.
2. The relationship between the important variables that influence the diatom assemblages in the core is the same as the relationship between the variables in the calibration set used. This assumption is very difficult to follow. The calibration set studies have shown the temperature signal from diatom assemblages to be unique but small and has a high probability to be overridden by other environmental variables such as e.g. pH and DOC that usually are important for the diatom content and changes through time. pH can under certain conditions covary with temperature but can also be dependent on other environmental changes such as soil stability, soil development, human disturbance, etc.

These two assumptions are of course also important when reconstruction other variables than temperature. The problem of separating the effect of two correlated variables will always make the interpretation difficult to what variable is an artefact and what variable has a significant change.

Project aims and sites
In the NORPEC project the main aims of the diatom study have been to reconstruct lake ecosystems and study their responses and sensitivity to both climatic and non-climatic derived changes. Both long Holocene sequences and shorter time periods covering the Little Ice Age have been collected along a climatic gradient from the oceanic west to the continental east in the alpine region in southern Norway (Figure 1 and 2). Potential indirect climate changes such as pH, DOC, and phosphorous are reconstructed using existing calibration sets such as the Surface Water Acidification Project (SWAP), the Acidification of Mountain Lakes: Palaeolimnology and Ecology project (AL:PE), and a Swiss data set available on web.

Diatoms
General Diatoms make up a large group of single-celled freshwater and marine plant algae (division Bacillariophyta) usually varying in size from 0.0025 to 0.5 mm. They can also be found in wet terrestrial habitats. Their cell wall is split into two valves. These two valves consist of silica, and they are therefore usually well preserved in sediments. The taxonomy of the diatoms is based on shape and sculpturing of the valves. The distribution of the different diatom taxa is closely related to water chemistry and the diatoms have therefore been used as ecological sensitive indicators to a range of water variables such as pH, nutrients, salinity, dissolved organic carbon (DOC), and conductivity. Quantitative environmental reconstruction With the development of statistical analyses and the so-called transfer functions in the 1980 and 1990 it has been possible to reconstruct the ecological parameters quantitatively. The reconstruction can both be applied in space (e.g. using surface sediment samples or epilithic samples) and time (e.g. using sediment sequences). The quantitative reconstruction of the ecological parameters demands a data set of diatom assemblages covering the gradient you want to reconstruct with related chemical parameters, a so-called calibration set. Strengths and weaknesses by quantitative reconstruction During the last decades an important aim in diatom research has been to demonstrate the diatoms response to climate change. There has been produced several calibration sets between diatom assemblages and mean July air or water temperature. These works have demonstrated a unique response to temperature change but have also demonstrated that the diatoms respond to a range of other environmental variables covarying with the temperature gradient. For quantitative reconstruction purposes, there are two very important assumptions: 1. The environmental variable to be reconstructed is, or is linearly related to an ecological important variable in the system of interest. This assumption is usually followed in the construction of a calibration set were lakes are selected along altitudinal or latitudinal gradients. 2. The relationship between the important variables that influence the diatom assemblages in the core is the same as the relationship between the variables in the calibration set used. This assumption is very difficult to follow. The calibration set studies have shown the temperature signal from diatom assemblages to be unique but small and has a high probability to be overridden by other environmental variables such as e.g. pH and DOC that usually are important for the diatom content and changes through time. pH can under certain conditions covary with temperature but can also be dependent on other environmental changes such as soil stability, soil development, human disturbance, etc. These two assumptions are of course also important when reconstruction other variables than temperature. The problem of separating the effect of two correlated variables will always make the interpretation difficult to what variable is an artefact and what variable has a significant change. Project aims and sites In the NORPEC project the main aims of the diatom study have been to reconstruct lake ecosystems and study their responses and sensitivity to both climatic and non-climatic derived changes. Both long Holocene sequences and shorter time periods covering the Little Ice Age have been collected along a climatic gradient from the oceanic west to the continental east in the alpine region in southern Norway (Figure 1 and 2). Potential indirect climate changes such as pH, DOC, and phosphorous are reconstructed using existing calibration sets such as the Surface Water Acidification Project (SWAP), the Acidification of Mountain Lakes: Palaeolimnology and Ecology project (AL:PE), and a Swiss data set available on web.
		Figure 1. Vestre Øykjamyrtjønn is the lake situated furthest to the west in the transect. It has the lowest pH of the five lakes, varying around pH 5.5. Dissolved organic carbon has probably been an important variable determining the diatom assemblages during part of the Holocene sequence as pine was growing around the lake during warmer periods in the Holocene.

		Figure 2. Råtåsjøen is the lake situated furthest to the east in the transect. It is oligotrophic with the highest pH of the five lakes, varying around pH 7.0.