Water is abundant in Iceland, both surface and groundwater. Iceland experiences high precipitation, but comparatively little evaporation. The geology of Iceland and the nature of the bedrock play a major role in shaping hydrological conditions here. One factor is the permeability of geological strata, i.e., their ability to transmit water. Precipitation flows mainly along the surface where the geological strata are densest and least permeable in the oldest bedrock. In postglacial lava fields, precipitation percolates down and there is virtually no surface runoff, but voluminous springs emerge from the lava margins.
Most Icelandic lakes are oligotrophic. The concentration of nutrients, together with light levels and temperature, regulates primary production in lakes and largely determines their ecosystems. In Iceland, the pH of lakes is almost with exception around and above 7, which is somewhat higher than is common in most other parts of the world. This is primarily a result of the alkalinity of the bedrock in Iceland.
Classification of lake habitat types is based mainly on the species composition and distribution of aquatic vegetation, together with the nutrient status of the lake. Lakes are classified on the basis of their phosphorous (P), nitrogen (N), and chlorophyll concentrations. Other factors also have an impact, such as water depth, height above sea level, vegetation cover within the catchment area, lakebed type, connections to glaciers, and salinity. The majority of Icelandic lakes are classified as oligotrophic and may be grouped into a total of nine different habitat types.
Running water has been classified on the basis of a number of factors, including river regime and flow. These two factors can affect the vegetation found in the water. The traditional Icelandic classification of flowing water is into spring-fed, run-off, and glacial rivers. Surface running water habitat types are here chiefly classified on the basis of their flow, as per the EUNIS system, which distinguishes watercourses with turbulent flow from those with slower flow, approaching laminar. Geysers and cold springs are grouped into an additional category. Most Icelandic surface running waters are characterised by a fast, turbulent flow rate. There are eight different habitat types for Icelandic surface running waters.
Classification is chiefly based on the EUNIS system. The EUNIS definitions for subcategories of standing and running water are not always descriptive of natural conditions in Iceland. While an attempt was made to use existing categories in the EUNIS system, this was not always possible. Several proposals have been made for new habitat types within EUNIS.
Fact sheets provide concise but detailed descriptions of each habitat type. Accompanying these descriptions is a list of typical aquatic vegetation species (where known), based on field surveys and information from published sources. The distribution of freshwater habitat types is shown on a 10x10 km grid square map. A more detailed habitat map can be accessed in the map viewer.