Long-term changes in the ecosystem of a lake (Lake Strzyżminskie) and an island induced by a colony of Great Cormorants (Phalacrocorax carbo sinensis L.)

Cormorants feed in aquatic ecosystems and transport a large amount of biomass and chemical substances to colonies or roosts situated on land adjacent to the water. This leads to significant enrichment of soils in nutrients. Some loads of nutrients may be transferred to a nearby lake. A long-term impact of cormorants may be followed by the destruction of vegetation beneath the colony. Within a few decades, cormorants lose places suitable for nesting or roosting and abandon the colonies. Then the content of nutrients in the soil decreases and natural regeneration of vegetation occurs. We found that despite the passage of several years since the abandonment of a cormorant colony, the concentration of nutrients in the soil remained high. Groundwater beneath the colony was also strongly enriched with nutrients. Thus, despite the absence of a bird being a source of nutrients, the area of the former colony constantly supplies nitrogen and phosphorus into the nearby lake. The occurrence of vegetation in the area of the former cormorant colony is limited to a few species. Nitrophilous black elder Sambucus nigra, usually creates a dense canopy preventing the light penetration, thereby reducing the growth of other plants.