Long-term changes in nutrients and mussel stocks are related to numbers of breeding eiders Somateria mollissima at a large Baltic colony.

Karsten Laursen,Anders Pape Møller,Gabriele Sorci

doi:10.1371/journal.pone.0095851

Abstract

BackgroundThe Baltic/Wadden Sea eider Somateria mollissima flyway population is decreasing, and this trend is also reflected in the large eider colony at Christiansø situated in the Baltic Sea. This colony showed a 15-fold increase from 1925 until the mid-1990's, followed by a rapid decline in recent years, although the causes of this trend remain unknown. Most birds from the colony winter in the Wadden Sea, from which environmental data and information on the size of the main diet, the mussel Mytilus edulis stock exists. We hypothesised that changes in nutrients and water temperature in the Wadden Sea had an effect on the ecosystem affecting the size of mussel stocks, the principal food item for eiders, thereby influencing the number of breeding eider in the Christiansø colony.Methodology/Principal FindingA positive relationship between the amount of fertilizer used by farmers and the concentration of phosphorus in the Wadden Sea (with a time lag of one year) allowed analysis of the predictions concerning effects of nutrients for the period 1925–2010. There was (1) increasing amounts of fertilizer used in agriculture and this increased the amount of nutrients in the marine environment thereby increasing the mussel stocks in the Wadden Sea. (2) The number of eiders at Christiansø increased when the amount of fertilizer increased. Finally (3) the number of eiders in the colony at Christiansø increased with the amount of mussel stocks in the Wadden Sea.Conclusions/SignificanceThe trend in the number of eiders at Christiansø is representative for the entire flyway population, and since nutrient reduction in the marine environment occurs in most parts of Northwest Europe, we hypothesize that this environmental candidate parameter is involved in the overall regulation of the Baltic/Wadden Sea eider population during recent decades.

Highlights

Leakage of nutrients from farmland to the marine environment has changed the benthic community by increasing primary and secondary production
Reduced nutrient loads affect the marine ecosystem by changing community structure of phytoplankton and benthos, it has been difficult to predict the consequences for the ecosystem due to high annual variation [8]
Mussel stocks in the Wadden Sea increased from 40,000 metric tonnes in 1986 to 117,000 metric tonnes in 1994, while varying considerably during the following years, and it decreased to reach 12,000 metric tonnes in 2007

Summary

Introduction

Leakage of nutrients from farmland to the marine environment has changed the benthic community by increasing primary and secondary production. Countries across Western Europe subsequently initiated programs in the 1970’s and 1980’s to reduce nutrient leakage from farmland, which led to reduced concentrations of nitrogen and phosphorus in the environment. In major rivers such as Elbe and the Rhine that receive water from large parts of European farmland and cities reduced concentrations of nutrients have been recorded since the late 1980’s [3,4,5]. We hypothesised that changes in nutrients and water temperature in the Wadden Sea had an effect on the ecosystem affecting the size of mussel stocks, the principal food item for eiders, thereby influencing the number of breeding eider in the Christiansø colony

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