Nutrient Retention in the Swedish Coastal Zone

Moa Edman,Iréne Wåhlström,H. E. Markus Meier,Lars Arneborg,Kari Eilola,Elin Almroth-Rosell

doi:10.3389/fmars.2018.00415

Abstract

In this study, the average nutrient filter efficiency of the entire Swedish coastline is estimated to be about 54% and 70% for nitrogen and phosphorus, respectively. Hence, significantly less than half of the nutrient input from land (defined as river discharge and point sources) can be assumed to be exported from coastal waters to the open sea. However, some coastal areas retained more than 100% of the land load and thus, also filter the open Baltic Sea water. These areas with effective filtering of nutrients have low land load per unit coastal area. The filter efficiency was calculated from a 30-years model simulation (1985-2014) of water exchanges and nutrient cycling within the Swedish coastal zone. The average model skill was evaluated to be good or acceptable compared to observations. In addition to the entire Swedish coast, the retention of total nutrient loads in seven larger coastal sub-regions and selected key sites representing different coastal types was also estimated. The modelled long term nutrient retention was found to be associated with the physical characteristics of a water body, such as the surface area, but also the mean depth and residence time of water. In addition, high retention efficiency is associated with high ratio of sediment nutrient content to pelagic nutrient concentrations. On interannual timescales temporal changes in the coastal nutrient pool can have a large influence on perceived nutrient retention. At one site, the phosphorus filter efficiency was actually negative, i.e. the coastal zone transported more phosphorus to the open Baltic Sea than it received from land. The nutrient removal is most efficient close to land, where the area specific retention efficiency is the highest. The variability of both the filter and retention efficiency between coastal regions was found to be large, with a range of approx. 4-1,200% and 0.1-8.5%, respectively.

Highlights

Most of the Swedish coast boarders the brackish, semi-enclosed Baltic Sea while the Swedish West Coast is connected to the Kattegat and Skagerrak system that acts as a transitional zone between the North Sea and the Baltic Sea
In order to understand what governs the retention efficiency in a specific water body we explore the concept of steady state retention as discussed in Eilola et al (2017)
The model skill averaged over salinity, DIN, DIP, and oxygen, expressed as C and r, is acceptable in all water districts (Figure 2, numbers colored after WD)

Summary

Introduction

Most of the Swedish coast boarders the brackish, semi-enclosed Baltic Sea while the Swedish West Coast is connected to the Kattegat and Skagerrak system that acts as a transitional zone between the North Sea and the Baltic Sea. The Baltic Sea is shallow with a mean water depth of 54 m [Gotland Deep and Landsort Deep are about 250 and 459 m, respectively; see Seifert and Kayser (1995)]. The Baltic Sea is shallow with a mean water depth of 54 m [Gotland Deep and Landsort Deep are about 250 and 459 m, respectively; see Seifert and Kayser (1995)] It is characterized by large vertical and horizontal salinity gradients (e.g., Leppäranta and Myrberg, 2009) from the entrance up to the Bothnian Bay. The Baltic Sea is connected to the Kattegat through the Danish straits of Öresund, the Great Belt, and the Little Belt where the entrance area is very shallow. Further north of the Kattegat, in the Skagerrak, both water depth and salinity increase considerably

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Conclusion