Fertilizer Leakage to the Marine Environment, Ecosystem Effects and Population Trends of Waterbirds in Denmark

Abstract

Leakage of nutrients from farmland to freshwater and marine environments results in fertilization that increases primary production with cascading effects in the ecosystem. Leakage of fertilizers may initially increase availability of food for primary and secondary consumers, although part of this effect may subsequently be followed by a reduction in food abundance caused by hypoxia. We hypothesized that leakage of nutrients from farmland increased primary and secondary production and subsequently the population size of waterbirds. The amount of phosphorus in the marine environment in Denmark during 1975–2010 increased with fertilizer use on farmland and this effect was stronger when winter precipitation was high. Leakage of fertilizer had ecosystem effects on phytoplankton, zooplankton, and ultimately biodiversity in the marine environment. We found linear relationships between fertilizer use and population size of breeding and wintering waterbirds, with additional quadratic relationships with fertilizer use and linear effects of temperature. The linear effect of fertilizer use depended on the diet of waterbirds with stronger effects in herbivorous and piscivorous species than in species eating benthos. These findings have implications for management of waterbird populations because a reduction in fertilizer use should result in increasing populations of herbivores and piscivores.