Factors regulating the coastal nutrient filter in the Baltic Sea

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.

Fig. 1

The Baltic Sea and its catchment with coastal ecosystems and their regions considered in the present study. Coastal ecosystems are delineated according to national definitions of water bodies according to the European Water Framework Directive

Fig. 2

Distribution of mean depth (a), area (b), temperature (c), salinity (d), TN (e) and TP (f) across coastal systems in different regions of the Baltic Sea. Coastal systems were defined based on the national implementation of the European Water Framework Directive. The number of coastal systems used to characterize the distribution is inserted in each histogram. Mean temperature, salinity, TN and TP for the subsets of coastal systems were calculated from monitoring data (http://nest.su.se/bed/)

Fig. 3

Major pathways for nitrogen (a), phosphorus (b), silica (c) and organic carbon (d) in the coastal zone. The coastal ecosystem box includes both the water column and the “biologically active” top sediment layer. Coastal stocks of different forms are shown as grey circles. Aquatic transports are shown in orange, removal processes in green, production processes in red and internal processes in dark blue. For the in–out net transport fluxes, the arrow sizes indicate relative magnitudes of the different components in a typical coastal ecosystem with significant inputs from land, whereas this is not the case for the arrows representing biological processes

Fig. 4

Conceptual diagram of seasonal differences in the denitrification pathway for the oligotrophic Öre River estuary and the eutrophic Curonian Lagoon. Denitrification in the Curonian Lagoon fuelled by the spring bloom is relatively more important than denitrification fuelled by the summer cyanobacteria bloom as indicated by the sizes of the boxes. Note that the sediment composition differs between the two sites

Fig. 5

Results of a sensitivity analyses of a reactive transport model for a site in the coastal zone of the Bothnian Sea illustrating the role of salinity in controlling P burial in the region (Lenstra et al. 2018)

Fig. 6

Changing organic matter characteristics along the salinity gradient: a DOC-specific UV absorbance (SUVA₂₅₄), b C:N ratio and c stable isotope δ¹³C values in dissolved and particulate organic matter pools. Redrawn from Massicotte et al. (2017) (a), and Jilbert et al. (b, c)

Fig. 7

The effects of plants and animals on turnover time in the coastal filter, which are largely dependent on what biological traits the organisms express. Large and long-lived species are likely to promote the removal and/or retention of carbon and nutrients (left). In contrast, species characterized as small and short lived will enhance the turnover rates of carbon and nutrients within the coastal zone (right). Increasing eutrophication can result in such a shift with major implications for the coastal nutrient filter. Images by Tracey Saxby, Dieter Tracey, Jane Thomas, Integration and Application Network, University of Maryland Center for Environmental Science (https://ian.umces.edu/imagelibrary/)