The carbon budget of the Baltic Sea
Biogeosciences, 8, 3219–3230, 2011
www.biogeosciences.net/8/3219/2011/
doi:10.5194/bg-8-3219-2011
© Author(s) 2011. CC Attribution 3.0 License.
Biogeosciences
The carbon budget of the Baltic Sea
K. Kuliński and J. Pempkowiak
Institute of Oceanology, Polish Academy of Sciences, ul. Powstańców Warszawy 55, 81-712 Sopot, Poland
Received: 26 April 2011 – Published in Biogeosciences Discuss.: 16 May 2011
Revised: 24 October 2011 – Accepted: 24 October 2011 – Published: 9 November 2011
Abstract. This paper presents the results of a comprehensive study of the Baltic Sea carbon budget. The Baltic Sea
is very much influenced by terrestrial carbon input. Rivers
are the largest carbon source, and their input amounts to
10.90 Tg C yr−1 (Tg = 1012 g) with a 37.5 % contribution of
organic carbon. On the other hand, carbon is effectively
exported from the Baltic to the North Sea (7.67 Tg C yr−1 )
and is also buried in bottom sediments (2.73 Tg C yr−1 ). The
other sources and sinks of carbon are of minor importance.
The net CO2 emission (1.05 Tg C yr−1 ) from the Baltic to the
atmosphere was calculated as the closing term of the carbon
budget presented here. There is a net loss of organic carbon,
which indicates that the Baltic Sea is heterotrophic.
1
Introduction
Shelf seas play a key role in the global fluxes of matter and
energy between the land, ocean and atmosphere (Thomas et
al., 2009). Although they make up a little over 7 % of the
global sea surface and less than 0.5 % of the ocean volume,
shelf seas are responsible for 15–30 % of marine primary
production and as much as 80 % of organic matter burial
(Walsh, 1991; Borges, 2005; Bozec et al., 2005; Chen and
Borges, 2009). These features of shelf seas are due to the
high biological activity they support, which is driven by nutrient inputs from all of the adjacent environments (Gattuso
et al., 1998; Pätsch and Kühn, 2008; Thomas, 2009).
As a consequence of this high biological productivity,
most global shelf seas are believed to act as net sinks for anthropogenic CO2 (e.g. Chen et al., 2003; Borges et al., 2005;
Chen and Borges, 2009; Laruelle et al., 2010). Moreover, the
CO2 loads absorbed by shelf seas exceed those reported from
Correspondence to: K. Kuliński
()
the open ocean (Chen and Borges, 2009; Takahashi et al.,
2009). It has recently been suggested that in contrast to open
shelf seas, some near-shore zones are identified as sources of
CO2 to the atmosphere (Chen and Borges, 2009; Liu et al.,
2010b; Laruelle et al., 2010). Consequently, detailed studies
of the carbon cycle in shelf seas are still required in order to
clarify its role in the global carbon cycle. Although several
attempts have been made to quantify the role of shelf seas
in global CO2 fluxes (Tsunogai et al., 1999; Andersson and
Mackenzie, 2004; Thomas et al., 2004), validation of the outcome of these studies must be based on compilations of the
results of local studies. These enable the multifarious locally
specific processes, which influence CO2 exchange between
seawater and the atmosphere and these processes must be
taken into consideration (Borges, 2005; Borges et al., 2005;
Chen and Borges, 2009).
The Baltic Sea is a spatially and temporally highly diverse ecosystem (Dippner et al., 2008; HELCOM, 2009).
The ecosystem diversity is extended in the direction from
South-West, influenced by the high saline North Sea water
inflows, to North-East – being under high influence of the
freshwater inflow. This salinity gradient induces the biodiversity gradient with a minimum in the Gulf of Bothnia
(Fig. 1). Such a diversity pattern is strengthened with the
temperature and irradiation gradients that influence duration
of the vegetation period. Biological activity is much higher
in the southern, warmer, part of the Baltic Sea. Additional
force here include significant amounts of nutrients entering
the Baltic with large continental rivers draining agriculturally
transformed catchment areas (Wasmund and Uhlig, 2003;
Wasmund and Siegel, 2008; HELCOM, 2009). The biological activity, including the ratio between photosynthesis and
respiration in particular, determines the level and dynamics of CO2 partial pressure (pCO2 ) in seawater and hence
a strength and direction of CO2 exchange through the seawater/atmosphere interface. These are, most likely, the reasons behind the significant discrepancies in the CO2 air-sea
Published by Copernicus Publications on behalf of the European Geosciences Union.
3220
K. Kuliński and J. Pempkowiak: The carbon budget of the Baltic Sea
Fig. 1. Map of the Baltic Sea showing its division into natural basins
and sub-basins (modified after Omstedt et al., 2009).
exchange results reported in the literature (Ohlson, 1990;
Thomas and Schneider, 1999; Thomas et al., 2003; Algesten
et al., 2004 and 2006; Kuss et al., 2006; Wesslander et al.,
2010; Beldowski et al., 2010; Schneider et al., 2003). On the
one hand, low productive and highly influenced by terrestrial
organic carbon water of the Gulf of Bothnia is believed to be
a CO2 source to the atmosphere (Algesten et al., 2004 and
2006). On the other hand, highly productive, open waters
of the southern Baltic act as an effective sink of atmospheric
CO2 (Ohlson, 1990; Thomas and Schneider, 1999; Thomas
et al., 2003; Kuss et al., 2006; Chen and Borges, 2009). However, recent data (Wesslander et al., 2010) have identified the
southern and central Baltic as a significant source of CO2 to
the atmosphere as well.
The results reported above are based on the pCO2 measurements made at stations located in the open waters of the
Baltic Sea. The near-shore zones and areas adjacent to river
mouths are often not included in the pCO2 measurements.
However, these regions of the Baltic Sea could be of special
importance for the CO2 cycling, since it has been demonstrated worldwide that the near-shore zones and river mouths
are important sources of CO2 to the atmosphere (Frankignoulle et al., 1998; Borges, 2005; Chen and Borges, 2009;
Liu et al., 2010a). This is due to the significant input of terrestrial carbon. The rivers flowing into the Baltic Sea drain
an area that is more than four times larger than that of the
sea itself. Moreover, the water volume the rivers supply anBiogeosciences, 8, 3219–3230, 2011
nually to the Baltic Sea amounts to almost 2 % of the total
water volume of the sea (Lass and Matthäus, 2008).
Although numerous studies on CO2 exchange through the
seawater/atmosphere interface have been performed in the
Baltic Sea in comparison with other shelf seas, there is no
straightforward understanding of the part played by the entire Baltic Sea in the CO2 air-sea exchange. There are discrepancies between reported results, even though they relate
to the same area (Thomas and Schneider, 1999; Wesslander
et al., 2010). Similarly, the other carbon inputs and outputs to
and from the Baltic Sea, reported in the literature, are incomplete or require revision (Thomas et al., 2003 and 201 (...truncated)
