N-Isotopes in Feathers and Abundance of Eiders Respond to Nutrients in Seawater

Abstract

Nitrification of the environment has resulted from tremendous increases in the use of fertilizers for crop plants. This has increased runoff to coastal marine areas with consequences for primary production, benthos and upper trophic-level consumers, including sea ducks such as eiders Somateria mollissima. This species primarily relies on filter-feeding bivalves, especially blue mussels Mytilus edulis. Stable isotopes of nitrogen (measured as δ15N) are incorporated in the feathers of eiders during molt at a constant rate reflecting the amount in food eaten. We examined if δ15N values in feathers can link eiders to the nitrogen loads in the marine waters surrounding Denmark, which is the main wintering grounds for the Baltic/Wadden Sea eider population. We also assessed how the abundance of eiders is related to nutrients and dead zones with oxygen deficit. During 2014–2018 we collected samples from 489 eiders, and of these 100 were analyzed for stable isotopes δ15N. Eider feather δ15N was positively related to nitrogen abundance in marine waters. Local aggregations of eiders in late summer (the molting period) and in winter increased with the amount of nitrogen in the marine environment. Large eiders took longer to molt, they molted later and the daily growth increments of feathers were larger. Finally, larger eiders and eiders in higher numbers occurred in areas with hypoxia. These findings show that anthropogenic inputs to the coastal marine environment positively influence the abundance and aggregations of eiders.