IW6 Amount of algae
The abundance of planctonic algae in lakes is estimated by measuring the concentration of chlorophyll-a in standard monitoring sites. Freely flowing planktonic algae or phytoplankton react rapidly to changes in lake conditions, and also cause most of the harmful effects of algae. Epiphytic and benthic algae growing on vegetation and rocks for instance, are left outside this examination.
Average chlorophyll-a concentration has remained quite steady in Finnish lakes during the last 25 years. In some lakes the trend has clearly been towards smaller concentrations, since nutrient load from for example industry and municipalities has significantly decreased. On the other hand, several small lakes have experienced an increase in the amount of algae, escpecially in areas where loading from diffuse sources (mainly agriculture and forestry) has been continuous.
Chlorophyll-a concentrations differ notably between clear-water and humic lakes. These lake types have naturally quite different nutrient levels, and are therefore examined separately.
Abundance of algae is affected by weather conditions as well as the availability of nutrients. Sunshine and high water temperature enhance their growth. Because of varying weather conditions, the abundance of algae in one year may differ significantly from that of preceding or following years. However, increase in the long-term average concentration of chlorophyll-a is usually a result of increased nutrient load.
Eutrophication and algal blooms
Eutrophication refers to increased primary production (or algal growth). Phytoplankton biomass reflects the lake's nutrient concentration, mainly the concentration of phosphorus and nitrogen. As the phytoplankton biomass increases, water becomes more turbid, which is the first visible sign of eutrophication.
Eutrophication becomes even obvious in algal blooms. These mass occurences of planktonic algae are natural to eutrophic lakes as such, but as they become more common they cause problems. Algal blooms disturb the natural balance of lake ecosystem, since turbid water and algal blooms on the water surface allow less light to pass through to subsurface water column. Submerged and benthic vegetation suffer then from lack of light and become sparser, which in turn leads to decreases in other benthic organims.
Algal blooms are possible only if they grow faster than other organisms can consume them. Phytoplankton is mainly grazed by zooplankton, water fleas and copepods among others. Phytoplankton communities of eutrophic lakes are dominated by large-sized and colony forming species, since they are best able to resist grazing. In addition, some phytoplankton species are toxic, which restricts grazers. For example some species of cyanobacteria produce toxic substances, which are harmful or toxic for humans too. If weather conditions are favourable, these algae may become highly abundant.
Eventually alga sinks to the bottom of the water body. Near bottom there usually is not enough light for photosynthesis, and the alga dies. If algae are abundant in water a lot of dead organic matter accumulates to the bottom. There it is decomposed by microbes which consume oxygen. In eutrophic and small volume lakes this, in part, promotes oxygen depletion in the near botttom water layer.
| This indicator will be updated annually.|
- Updated (14.05.2013)