Black spots for aquatic and terrestrial ecosystems: impact of a perennial cormorant colony on the environment

• The complexity of ecological effects of cormorants colonies were investigated.
• Soil chemistry and plant vegetation were altered via extreme nutrients deposition.
• Promotion of nitrophilous and invasive alien species was observed.
• Large loads of nutrients were transferred to nearby lake through surface runoff.
• The colony increased coliform numbers in the nearby littoral zone.

The global growth of populations of different cormorant species has raised concern on the consequences of their presence in the environment. This study examined the impact of a perennial colony (160 breeding pairs) of great cormorants on terrestrial and aquatic ecosystems. The deposition of bird-originating nutrients within the area of colony, their accumulation in soils and the fluxed of chemical substances to a nearby lake were investigated. The impact of cormorants on terrestrial vegetation and microbial pollution of the lake were also studied. The soils beneath the colony were found to contain extremely high concentrations of nitrogen and phosphorus. The overgrowing vegetation was largely limited with nitrophilous and invasive species being more abundant. Increased loads of organic matter, nitrogen and phosphorus were also found in groundwater and particularly, surface runoff. The colony area delivered significant amounts of nutrients to the lake also when the birds were absent. The lake water near colony was also characterized by increased nutrient content and additionally higher number of faecal bacteria. The present results demonstrate the complexity through which the effect of cormorant colonies can be manifested simultaneously in terrestrial and aquatic ecosystem.

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