1.2.1 Hydrological and environmental characteristics of the Baltic Sea
The Baltic Sea is situated in northern Europe, spanning latitudes from 53°N to 66°N. It is a semi-enclosed marginal sea with the only connection to the North Sea through a few narrow, shallow channels in the Danish Straits. In general the water body is very shallow with just 12% of the total area under 100 m deep, with Landsort Deep the deepest part of the Baltic Sea at 459 m deep (Leppäranta and Myrberg 2009). This shallow nature confers many important characteristics to water mass exchange and oxygenation of deep basins in the Baltic Sea.
At only 18 m deep, the shallow and narrow Darss Sill in the Danish Straits presents the greatest barrier to saline water inflow from the North Sea via the Kattegat to the adjacent basins in the southern end of the Baltic Sea. This dense seawater of salinity 15 - 25 collects in the deep basins (Voipio, 1981). In combination with freshwater input from rivers in northern and coastal areas which remains in the surface layer, this leads to steep salinity gradients from north to south as well as maintaining the strong, permanent halocline (Leppäranta and Myrberg, 2009) present at around 60 m deep (Schiewer, 2008). The Baltic Sea is classified as a brackish water body with a surface water salinity of between 6.5 - 8, much lower than the open ocean (East Gotland Basin, Leppäranta and Myrberg, 2009). Although the permanent halocline restricts physical water mass exchange, sinking organic matter can sink below the halocline. As it is remineralised through aerobic bacterial processes, this consumes oxygen, thereby depleting the deeper waters in oxygen. Periods of persistent westerly winds can lead to sporadic, short and intensive pulses of saline oxygenated water over the Darss Sill and into the Baltic Sea (Leppäranta and Myrberg, 2009). While there is always a small degree of subsurface inflow and exchange, this is the only process which substantially replenishes oxygen below the halocline.
Anthropogenic pressures in the Baltic Sea ecosystem
Around 85 million people in 14 countries live in the drainage basin which is almost four times larger than the sea itself (Hannerz and Destouni, 2006) meaning that anthropogenic activity from agriculture, urban centres, recreational activities, fishing activities and ship traffic have a large influence on the water quality in the Baltic Sea. The anthropogenic influence on the Baltic Sea has intensified over the past two centuries (Elmgren, 2001; HELCOM, 2013; Viitasalo et al., 2015). Substantial nutrient run-off and ensuing changes in phytoplankton productivity have
been of particular concern in the region due to the importance of the biological diversity, stable ecological state, and recreational area in this primarily coastal ecosystem (HELCOM, 2009).
1.2.2 N supply and seasonal plankton succession
The Baltic Sea is a region where fixed N concentrations are low in summer and limit net autotrophic production in the surface layers (Granéli et al., 1990), similar to the open ocean (Section 1.1.2). Nutrient supply in the Baltic Sea is not strictly in Redfield proportion as hypoxia in the bottom water drives preferential phosphate release under hypoxia from the sediments and N loss processes of annamox and denitification below the halocline and at the oxic/anoxic interface (Risgaard-Petersen et al., 2005; Lam and Kuypers, 2011). Hence mixing events, such as coastal upwelling (Kahru et al., 1995), bring up P-rich and N-deplete deep water. Hence the generally diatom-dominated spring bloom (Wasmund et al., 1998) draws down inorganic N leaving an excess of inorganic P (Granéli et al. 1990, Fig. 1.1).
Temperature
Chl a Nitrate
Spring Summer Autumn
New production (NO3-)
Regenerative production (NH4+) N2-fixation
Diatoms
Filamentous diazotrophic cyanobacteria
Diatoms Dinoflagellates
Picoplankton (<2 µm)
Dinoflagellates
Dominant N sourcePhytoplankton successionEnvironmental variables Phosphate
Figure 1.1: Schematic summarising common variations in environmental variables, dominant N source and succession of select phytoplankton groups in different regions in the Baltic Sea between spring and autumn. Based on data obtained from Andersson et al. (1996); Wasmund and Siegel (2008). In some regions, the order of succession of diatoms and dinoflagellates may be reversed e.g. Lignell et al. (1993).
8 CHAPTER 1. INTRODUCTION The residual phosphate, increasingly stratified water column with warm surface waters and high light availability during the summer, leads to a seasonal diazotrophic niche (Section 1.1.3), which supports the development of annual blooms of diazotrophic filamentous cyanobacteria.
Due to their buoyancy regulation, these organisms tend to form large aggregates, which accu-mulate in the surface. These scums occur regularly over large areas of the Baltic Sea and are highly visible as indicated in this satellite photo as the light green swirls against the dark water background (Fig. 1.2).
These common annual blooms are generally dominated by filamentous diazotrophic cyanobacteria with two main genera: Nodularia spumigenain the open Baltic Sea and Aphani-zomenon flos-aquaein more coastal areas (Fig. 1.3, Olli et al., 2015). This geographical distinc-tion between these two genera may be attributed to dissolved phosphate (Degerholm et al., 2006;
Olli et al., 2015) or salinity and solar irradiance (Lehtimaki et al., 1997). Both of these genera have heterocysts, specialised compartments to fix N. This spatially separates the nitrogenase en-zyme from carbon fixation and associated O2production from C-fixation in carboxysomes in the neighbouring vegetative cells as the nitrogenase enzyme is irreversibly inhibited by O2(Postgate, 1998). In contrast, other autotrophic diazotrophs use temporal rather than physical separation of C-fixation and the O2-sensitive N2-fixation (Berman-Frank et al., 2003). N. spumigena is a toxic species, known outside of scientific circles due to the hepatotoxins it produces which can lead to beach closures during major blooms.
Baltic Sea Sweden
Poland Germany
Figure 1.2: Satellite image of a surface bloom of filamentous cyanobacteria in the southern Baltic Sea taken on 27/7/2012 by the MODIS satellite. Source: M. Kahru.
N. spumigena A. flos-aquae
Figure 1.3: Microscopy pho-tographs of Aphanizomenon flos-aquae and Nodularia spumigena, two common, bloom-forming filamentous N2-fixing cyanobacteria in the Baltic Sea. Source: A. Stuhr.
Estimations vary, however, new N input through N2-fixation in the Baltic Sea is reportedly on the same order of magnitude as riverine inputs and atmospheric N deposition (Fig. 1.4, Voss et al. 2011) indicating the importance of diazotrophic organisms in supporting N turnover in the region. In addition, N2-fixation counteracts the nitrogen lost via anammox and denitrifica-tion in the anoxic layers below the halocline and in the sediments (Fig. 1.4). This may also act as a natural feedback system promoting organic matter production, oxygen consumption, phos-phate release from the sediments under anoxia which in turn increases the niche for N2-fixing filamentous cyanobacteria (Vahtera et al., 2007).
N2-fixation Atmospheric N
deposition
Riverine N input
Denitrification/
anammox
N burial/
sedimentation halocline
Denitrification/
anammox
370 201
686
47
426 - 652 North Sea
water
exchange 43 13
113
Baltic Sea N fluxes
Microbially-driven N assimilation and remineralisation
sediment upper
water column
Units = kt y-1 deep oxic layer deep anoxic layer
Figure 1.4: Key N fluxes in Baltic Sea as summarised by Voss et al. (2011). N sources to the Baltic Sea are indicated as black and N loss processes in red text and arrows. The budget is not balanced, possibly due to underestimated N loss processes.
10 CHAPTER 1. INTRODUCTION