For many aquatic organisms, temperature is the key factor determining distribution range and annual life cycle from reproduction to resting stages. Rapidly warming shallow archipelago areas have the best energetic options for high ecological production. For this reason, they are also the best reproduction sites for spring spawning fish species like pike, perch and pikeperch.

In a recent study by researchers from Natural Resources Institute Finland and University of Helsinki, spring and summer temperature development was analyzed and visualized for the entire Finnish coastal area. Small, automatic temperature data loggers were used to record water temperature in various coastal sites around the year. This data was used to model coastal temperature development in springtime across the whole coastal region. The coastal temperatures vary highly annually, the maximum recorded difference between summer and winter temperatures was over 28 °C. The high range and variability of water temperatures makes Finnish coastal areas rather challenging for many aquatic species.

Water temperature is determined by solar radiation, depth and wind exposition

Depth and exposition are the most important variables explaining coastal temperature gradients in spring and early summer when water is heated by increasing solar radiation. Deep areas warm slowly and in summertime occasional upwelling caused by wind forcing can decrease temperature rapidly. Cumulative water temperature during spring is the highest in innermost areas, lowest in open water areas and variation in daily averages was highest in the middle region of the archipelago.

Differences in water temperature level off at the end of summer

Difference in surface water temperatures between the innermost and outermost coastal areas diminish at the time when cooling begins in August -September. Differences in cumulative temperature from inner to outer coastal areas are largest in early summer and the cumulative water temperature sum differs more within coastal area than between the basins like Archipelago Sea and Bothnian Bay. For example, in innermost coastal areas the cumulative temperature sum from ice-break up to July is over twice that in the open water area. The cumulative temperature sum can be used to classify coastal areas into biologically relevant coastal regions.

Temperature sum map layer extends to species distribution modelling and estimation of potential causes of climate change for fisheries

Spatial knowledge of cumulative water temperature can be used to model and predict spatial distribution of aquatic biota, like fish reproduction areas studied in VELMU -project.  The data will also help evaluation of possible causes of larger scale climatological changes in a biological context, including productivity and extent of fish reproduction areas.  For example, pikeperch favors warm areas for reproduction and extent of suitable spawning habitats for it are likely to increase in the future. At the same time, reproduction of whitefish in the southern Gulf of Bothnia is predicted to suffer from high winter temperatures. In the SmartSea project we use this kind of basic data to estimate influences of environmental changes for purposes of marine spatial planning.

The temperature sum model indicates the rapidly warming areas (red colour) and colder areas (blue colour) in the entire Finnish coastal area.
The temperature sum model indicates the rapidly warming areas (red colour) and colder areas (blue colour) in the entire Finnish coastal area.


Veneranta, L., Vanhatalo, J., & Urho, L. (2016). Detailed temperature mapping – Warming characterizes archipelago zones. Estuarine, Coastal and Shelf Science182, 123-135.

Lari Veneranta, Jarno Vanhatalo