Effects of Nesting Cormorants (Phalacrocorax carbo) on Soil Chemistry, Microbial Communities and Soil Fauna

Seabirds act as vectors transporting marine nutrients to land by feeding on fish while nesting and roosting on islands. By depositing large amounts of nutrient-rich guano on their nesting islands they strongly affect island soils, vegetation and consumers. However, few studies have investigated how nesting seabirds affect soil communities. In this study, we investigated how cormorant nesting colonies affect soil chemistry, soil microbes and soil and litter fauna on their nesting islands in the Stockholm archipelago, Sweden. We found that cormorant colonies strongly increase organic soil N and P concentrations, and the effect is stronger close to cormorant nests. Microbial communities were studied by extracting phospholipid fatty acids (PLFA) from the soil. The total amounts of PLFA and the amount of PLFA indicating bacterial biomass were lower on active cormorant islands than on reference islands. Furthermore, PLFA structure and thus microbial community structure differed between cormorant and reference islands. Among ten investigated soil and litter arthropod groups three groups (Thysanoptera, Araneae and Oribatida) showed lower densities and one group (Astigmata) showed higher densities in soils on active cormorant than on reference islands. Some arthropod groups showed strong spatial variation on the cormorant islands. Astigmata, Mesostigmata and Diptera showed higher densities in soil samples close to cormorant nests, whereas Oribatida, Collembola and Hemiptera showed lower densities in litter samples close to cormorant nests than in samples taken 3-20 m away from nests. Overall, the cormorant colonies strongly affected soil ecosystems of their nesting islands, but causal correlations between arthropod densities and soil factors were difficult to reveal. One likely reason may be that nesting cormorant islands are very heterogeneous habitats showing large spatial variation in both soil properties as well as fauna densities.