Phytoplankton - inland waters

Changes in water nutrient status are rapidly reflected in phytoplankton biomass and species composition. According to current Swedish assessment criteria, phytoplankton are used as a quality factor based on a multimetric index combining total biomass (or biovolume) of phytoplankton, percentage of cyanobacteria, a trophic plankton index (TPI), number of phytoplankton taxa, and chlorophyll.

The three first parameters are combined and used to classify the ecological status of a lake along a trophic gradient. Chlorophyll is used primarily as a screening method in the absence of phytoplankton analysis. The number of phytoplankton taxa is used to assess acidity.

Phytoplankton in WATERS
The phytoplankton metrics currently used to assess ecological status along a nutrient gradient are intercalibrated within Europe. A new plankton index, PTI, is being developed that includes a large number of phytoplankton taxa. WATERS will test PTI on Swedish lakes, especially species-rich lakes in southern Sweden with few indicator species according to the currently applied index TPI.

According to the Swedish assessment method, lakes are divided into five types based on ecoregion and water colour: mountain region lakes above the tree line, clear and humic lakes of Norrland, and clear and humic lakes of southern Sweden. The boundary between northern and southern lakes is drawn at Limes Norrlandicus. This way of typing lakes is not in line with that used in intercalibration work or international reporting, which makes it difficult to compare boundaries between status classes. The current assessment method for phytoplankton will be adjusted to enable comparisons with the current EC standard that selects lakes based mainly on size, depth, altitude, water colour, and alkalinity.

Satellite images are used to track algal blooms in the Baltic Sea. Corresponding measurement systems with high-frequency sampling are not available for lakes. In lakes, algal pigments detected by sensors mounted on boats or buoys would be an alternative with high sampling frequency. No resources for developing such tools are available in WATERS, however, we will monitor the available tools for measuring algal pigments developed for marine waters in Sweden and in other countries.

Since the development of the currently used assessment tools, many lakes have been sampled for phytoplankton at both the regional and local levels. This new lake dataset captures a larger proportion of lakes affected by human pressures, which will help in developing and refining future assessment tools based on phytoplankton. Boundary setting will be more reliable with this new information added to the models, reducing the risk of misclassifying the ecological status of a lake.