BS6 Algae

>> Background information

Development

The abundance of phytoplankton, also known as microalgae, is estimated by measuring the chlorophyll concentration of the water. The concentration of chlorophyll-a has increased markedly in the Finnish coastal areas during the past three decades. While the overall trend is clearly increasing, there are great differences between different sea areas. Concentrations have increased the most in the Gulf of Finland where they have more than doubled from their level in the 1980s.

Algal blooms have been observed weekly during the summer months (June?August), usually on approximately 50 monitoring locations. Most of the observers are trained volunteers, who estimate the amount of algae on a scale from 1?3 (from "little algae" to "high abundance of algae"). The algal bloom index is then calculated based on the number of observations in each class of the scale.

The number of algal bloom observations varies a lot annually, because the blooms are strongly related to weather conditions. During the relatively short observation period of ten years, a slight increase is however detectable. Algal blooms are most usual in the middle or late summer (july and august) when the water is warmest.

Impact on biodiversity

Phytoplaktons are important primary producers and make the greatest contribution to the production of organic matter in the Baltic Sea. Abundance of phytoplankton is highly correlated with the amount of nutrients (mainly phosphorus and nitrogen) in the water. Therefore they may be considered as an indicator of eutrophication.

Increasing phytoplakton biomass decreases water transparency (visibility depth) and thus the availability of light deeper in the water columns. This leads to decrease in submerged vegetation and changes in seabed community compositions. As a result, several microhabitats may be lost and species diversity reduced.

Phytoplankton blooms affect the ecosystem also by increasing sedimentation of organic matter to the seabed. Decomposing the increased organic matter enhances oxygen consumption which in turn increases the risk of anoxia in the seabed. Cyanobacterial blooms may in addition directly harm organisms since some cyanobacteria produce compounds which are toxic to several species, including humans.