Effects of seabird nesting colonies on algae and aquatic invertebrates in coastal waters

• Seabirds concentrate marine nutrients, from large marine areas, on their nesting islands. The high nutrient load may cause runoff into surrounding waters and affect marine communities in similar ways as reported from marine fertilization experiments. In order to test if cormorant colonies affect algae and invertebrates in surrounding coastal waters, we collected Fucus vesiculosus fronds, its epiphytic algae and associated invertebrate fauna near abandoned and active cormorant nesting islands and reference islands without nesting cormorants in the Stockholm archipelago in the northern Baltic Proper, Sweden. First, we showed, with δ15N analyses, that ornithogenic nitrogen provided a significant nitrogen source for algae and invertebrate consumers near islands with high nest density. Second, the nitrogen and phosphorus content of algae near active cormorant islands with high nest density was elevated and epiphytic algae increased relative to Fucus. Third, 3 of 5 invertebrate taxa (Jaera albifrons, Gammarus spp. and Chironomidae) showed increased biomasses near islands with high nest density, but contrary to earlier fertilization studies only J. albifrons increased in abundance compared to reference islands. We conclude that runoff from seabird colonies has a profound effect on primary producers and some consumers in the surrounding water, but only if the colonies exceed a certain nest density. Thus, seabirds not only affect marine communities via top-town forces as commonly assumed, but also via bottom-up forces by concentrating nutrients around their nesting islands. Consequently, seabird islands can be seen as natural fertilization experiments and give important insights in the effects of eutrophication of marine systems.

Ämnesord

NATURVETENSKAP  -- Biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences (hsv//eng)

Nyckelord

nitrogen

stable isotopes

bottom-up

eutrophication

Baltic Sea

cormorants

Biology

Biologi

Plant Ecology

växtekologi

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