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

Abstract

Seabirds concentrate 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 to those 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 as well as reference islands without nesting cormorants in the Stockholm archipelago in the northern Baltic Sea, Sweden. First, we showed, with δ 15 N 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 F. vesiculosus. Third, 3 of 5 invertebrate taxa (Jaera albifrons, Gammarus spp., and Chironomidae) showed increased biomasses near islands with high nest density; but, contrary to former 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 to the effects of eutrophication of marine systems.

The complexity of ecological impacts induced by great cormorants

“…In a study conducted on the Baltic Sea, the abundance and total biomass of such invertebrates as Jaera albifrons Leach, chironomids and Gammarus sp. increased significantly in areas with high cormorant nest densities (Kolb et al, 2010). As these species feed on microand macroalgae, it is likely that cormorants promote nutrient loading and subsequent phytoplankton biomass in inhabited areas.…”

“…Even though the littoral zone in the proximity of bird-inhabited areas may reveal increased concentrations of bioavailable nutrients during the breeding or roosting season (Kolb et al, 2010;Klimaszyk, 2012;Klimaszyk et al, 2014Klimaszyk et al, , 2015a, wave action and water currents dilute them throughout the ecosystem to an extent which does not exhibit any biological changes (Gwiazda et al, 2014). Therefore, biological outcomes of cormorant activities are more likely to be observed in smaller and shallow reservoirs.…”

“…Littoral zones near colonies have been found to contain increased concentrations of orthophosphates, ammonium and nitrates-the nutrient forms readily utilized by phytoplankton (Klimaszyk, 2012;Klimaszyk et al, 2014;Klimaszyk et al, 2015a). The availability of N from cormorant faeces for macro-and microalgae has also been demonstrated using 15 N signatures (Kolb et al, 2010).…”

“…As these birds are generally gregarious, appear collectively, gather in flocks, nest in colonies and can feed on a relatively large area on various aquatic ecosystems simultaneously, they may constitute a very important biological factor triggering environmental changes (Kolb et al, 2010;Klimaszyk et al, 2015b). Therefore, the impact of cormorants on various elements of both the terrestrial and aquatic environment has been largely addressed in different studies.…”

Contamination by trace metals and their trophic transfer to the biota in a Mediterranean coastal system affected by gull guano

“…With regard to the trophic transfer of ornithogenic elements, very few studies have been carried out, and these have dealt almost exclusively with the transfer of nitrogen and POPs, PCBs and DDT into terrestrial food chains (Harding et al 2004, Choy et al 2010b, Kolb et al 2010. As for ornithogenic trace metal biomagnification along food chains, the only inherent study did not detect biomagnification of ornithogenic THg in an Arctic terrestrial food web (Choy et al 2010a).…”