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Breeding waterbirds of eutrophic lakes


Waterfowl populations in eutrophic inland waters have declined. Populations were stable on average in the early years of the monitoring period, but have declined since the mid-1990s. The deterioration of water quality, in particular eutrophication and darkening, is one of the main factors behind the decline of waterbirds. The population of the most abundant game duck, the mallard, has been increasing over the long term, but has not increased any further in the last 20 years. Of the indicator species, all species that nest exclusively in eutrophic lakes have declined.

Status: Bad
Direction: Deteriorating


The status of the indicator is bad. With a confidence level of 90 percent, the populations of naturally eutrophic inland breeding birds are currently at least 44% lower than at the beginning of the monitoring period (1986-1990).


The indicator has a deteriorating trend. In the 2000s, there is a 90 percent probability that breeding bird populations in eutrophic inland waters have declined by at least about 2.6% per year. In the early years of the period (1986-1999), populations were stable on average, but have declined since then.

Individual species have both clearly increasing and decreasing populations. Species in clear decline include the tufted duck, common pochard, northern showeler, northern pintail, Eurasian widgeon, Eurasian coot, red-breasted grebe, great crested grebe and horned grebe For example, the population of the whooper swan has more than quadrupled during the monitoring period. However, the increase in the abundance of the whooper swan is not linked to the decline of other species (Holopainen et al. 2022). The most abundant and important game bird species, the mallard, has experienced a long-term population increase, but the increase has not continued over the last 20 years.

Species-specific population trends can be found in the annual waterfowl surveys published by the Natural Resources Institute Finland, for example Piha et al. (2023).


This indicator describes the state of biodiversity in inland waters. The population trend of inland waterbirds indicates the viability of the species. Locally, this is reflected in the diversity and abundance of inland birds.

The bird surveys constitute one of the most reliable monitoring data on biodiversity in Finland (see “Data used” below).  In addition, the habitat needs of birds are well known compared to many other species groups. Thus, such bird species can be identified whose population trends are closely linked to the status of eutrophic inland waters.

The decline of waterfowl populations in eutrophic lakes is caused by habitat changes, especially over-eutrophication and darkening of waters (Lehikoinen et al. 2016). Over-eutrophicated water bodies are muddy and have fewer important underwater plants and bottom-dwelling animals. Overgrowth is also a problem in eutrophic waters, limiting the breeding of species that depend on open water. In addition to over-eutrophication, inland bird populations are being depleted by invasive species.

The population fluctuations inland waterbirds are not only explained by changes in habitat conditions, but are also influenced by factors such as conditions in wintering areas and along migratory routes, climate change, natural predation pressure and hunting pressure throughout the species’ range.

The indicator species that are currently hunted contain: Eurasian widgeon, Eurasian teal, mallard, common goldeneye, northern pintail, northern showeler and tufted duck.


Data used

The indicator is based on monitoring censuses coordinated by the Natural Resources Institute Finland and the Finnish Museum of Natural History and carried out by volunteer hunters and birdwatchers. The indicator for inland waterbirds is based on waterbird census data dating back to 1986.  

The indicator includes seven species that breed in eutrophic waters and five common species that breed in both eutrophic and oligotrophic water bodies. Species that breed almost exclusively in eutrophic inland waters are northern showeler, northern pintail, common pochard, red-breasted grebe, great crested grebe, horned grebe and Eurasian coot. Species living both in eutrophic and oligotrophic lakes include whooper swan, Eurasian teal, Eurasian widgeon, common goldeneye and tufted duck. Only the observations from eutrophic monitoring sites of these species were included in the indicator.

Species included in the indicator: 

Eurasian widgeon – Mareca penelope
Red-breasted grebe – Podiceps grisegena
Northern pintail – Anas acuta
Northern showeler – Spatula clypeata
Whooper swan – Cygnus cygnus
Horned grebe – Podiceps auritus
Eurasian coot – Fulica atra
Common pochard – Aythya ferina
Great crested grebe – Podiceps cristatus
Mallard – Anas platyrhynchos
Eurasian coot – Anas crecca
Common goldeneye – Bucephala clangula
Tufted duck – Aythya fuligula

More information: 

Indicator calculation

The Inland Breeding Bird Indicator is an indicator produced by the Natural Resources Institute Finland, which combines estimates of population trends of individual species into a single index number following the principle developed earlier by Lehikoinen et al. (2016). The index number describes the relationship to the reference year, which for the inland waterfowl indicators is 1995. A more detailed description of the calculation methods can be found in the Natural Resources Institute Finland’s report on waterfowl monitoring (Piha et al. 2023).

 The calculation of the multispecies indicator starts with an assessment of the population trends of individual species. Species population trends are estimated from the number of bird pairs observed in the censuses using Bayesian generalised mixed models (GLMM), which take into account differences between census sites and density differences between sites (see Piha et al. 2023).  

 Species-specific estimates based on sampling-dependent statistical models always contain uncertainty. This uncertainty is evaluated by Monte Carlo simulations and incorporated it into the multispecies indicator (Soldaat et al. 2017).

More information:

Ask for further information

Markus Piha

Senior Scientist (Luke), Wildlife ecology


+358 295 322726

Aleksi Lehikoinen

Senior Curator (Luomus), Head of the working Group on Birds


+358 294 128851

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