Factors regulating the coastal nutrient filter in the Baltic Sea

Ambio https://doi.org/10.1007/s13280-019-01282-y REVIEW Factors regulating the coastal nutrient filter in the Baltic Sea Jacob Carstensen , Daniel J. Conley, Elin Almroth-Rosell, Eero Asmala, Erik Bonsdorff, Vivi Fleming-Lehtinen, Bo G. Gustafsson, Camilla Gustafsson, Anna-Stiina Heiskanen, Urzsula Janas, Alf Norkko, Caroline Slomp, Anna Villnäs, Maren Voss, Mindaugas Zilius Received: 15 March 2019 / Revised: 17 September 2019 / Accepted: 23 September 2019 Abstract The coastal zone of the Baltic Sea is diverse with strong regional differences in the physico-chemical setting. This diversity is also reflected in the importance of different biogeochemical processes altering nutrient and organic matter fluxes on the passage from land to sea. This review investigates the most important processes for removal of nutrients and organic matter, and the factors that regulate the efficiency of the coastal filter. Nitrogen removal through denitrification is high in lagoons receiving large inputs of nitrate and organic matter. Phosphorus burial is high in archipelagos with substantial sedimentation, but the stability of different burial forms varies across the Baltic Sea. Organic matter processes are tightly linked to the nitrogen and phosphorus cycles. Moreover, these processes are strongly modulated depending on composition of vegetation and fauna. Managing coastal ecosystems to improve the effectiveness of the coastal filter can reduce eutrophication in the open Baltic Sea. Keywords Biogeochemistry
Climate change
Coastal filter
Eutrophication
Hypoxia
Nutrient management INTRODUCTION The open Baltic Sea is one of the most studied marine systems in the world, with a profound scientific basis for integrated management of the sea and its watershed (Reusch et al. 2018). This includes managing nutrient inputs from land and the atmosphere to mitigate the most prominent regional problem, eutrophication. Although the adverse effects of eutrophication in the open Baltic Sea have received most attention (Carstensen et al. 2014a; Kahru and Elmgren 2016), coastal eutrophication prevails around large parts of the Baltic Sea, manifested by massive algal blooms, loss of benthic vegetation and fauna as well as spreading hypoxia (Bonsdorff et al. 1997; Conley et al. 2011). Nutrient and organic matter inputs from land enter the Baltic Sea through a broad variety of coastal ecosystems, including lagoons, archipelagos, river-dominated estuaries, embayments and open coastal stretches, and these coastal systems transform, retain and remove these substances through different biogeochemical processes with rates spanning several orders of magnitude across the Baltic Sea coastal zone (Asmala et al. 2017). Importantly, sediments play a key role in the retention and permanent removal of nutrients and organic matter. However, most studies have focused on processes in deep dark and muddy sediments, whereas less is known about the role of shallower sandy sediments, with or without sufficient light to support benthic primary producers, despite the fact that such habitats are prevalent in many coastal ecosystems (McGlathery et al. 2007). The heterogeneity of sediment types and the inhabiting biota are dictated mainly by the complex coastal bathymetry, which implies large spatial variability within coastal ecosystems. In addition, temporal variability of biogeochemical processes should in general be substantially larger in the coastal zone compared to the open Baltic Sea, due to the larger sensitivity to variable inputs from land and changing physico-chemical conditions. Consequently, more measurements are needed to constrain transformation, retention and removal processes in the coastal zone due to the enormous variability in space and time. The attenuation of material fluxes from land to sea is often termed the ‘‘coastal filter’’ (Billen et al. 1991; Bouwman et al. 2013), but the effectiveness of the filter Ó The Author(s) 2019 www.kva.se/en 123 Ambio varies broadly with the physico-chemical attributes as well as the biological configuration of the ecosystem. Particularly, erosion of the filter function with hypoxia and loss of deep-burrowing benthic macrofauna has been shown (Conley et al. 2009; Norkko et al. 2012; Carstensen et al. 2014b). Whereas the individual biogeochemical processes underlying the coastal filter are generally well understood, the rates of these processes and the influence of various environmental factors on these rates are not well constrained (e.g. Ruttenberg 2003; Giblin et al. 2013). Due to the variable nature of the coastal environment, the dominant pathways can also change considerably and abruptly over time and space. Consequently, integrative biogeochemical models, based on established relationships of different pathways as a function of environmental factors, are needed to assess the coastal filter function. The main objective of this review is to identify the major pathways for nutrients (N, P and Si) and organic matter, as well as their drivers, within the Baltic Sea coastal zone. The biogeochemical processes transforming, retaining and removing nutrients and organic matter have only been measured directly for a few selected coastal ecosystems, providing fragmented and limited knowledge to assess the coastal filter across the broad range of coastal ecosystems in the Baltic Sea. Here, we will synthesize the recent developments in our understanding of coastal biogeochemistry, and we will discuss consequences for coastal management. COASTAL ECOSYSTEMS OF THE BALTIC SEA The Baltic Sea is a large tideless inland sea covering almost 400 000 km2 with a coastal periphery of about 8 000 km. However, due to the complex morphometry and fractal-like properties, it is difficult to quantify. The geological processes during the last glacial period and the Holocene have created diverse land- and seascapes around the Baltic Sea, changing from boreal archipelagos in the north, over long open coasts interrupted by lagoons in the southeast, to a drowned moraine landscape with many estuaries and embayments to the southwest. Consequently, coastal ecosystems exhibit large environmental and ecological gradients across the Baltic Sea from the entrance in the southwest to the three gulf extensions of the Baltic Proper to the north and east. Here, coastal systems in six regions are considered (Fig. 1). The majority of the coastal systems are shallow (\ 20 m; Fig. 2a) with surface areas typically ranging from 2 to 500 km2 (Fig. 2b). The deepest systems are located in the Baltic Proper and Bothnian Sea, whereas the shallowest are found in the Danish Straits. All regions have a broad size span of coastal systems. There are marked latitudinal 123 differences in both temperature and salinity. The annual mean temperature increases gradually from typically 5–6 °C in the Bothnian Bay to 9–10 °C in the Danish Straits (Fig. 2c). Similarly, mean salinity ranges from 1.5 to 4.9 in the Bothnia (...truncated)