Coastal seas, such as the Baltic Sea, link continents and oceans via freshwater and matter fluxes. However, before the supplied chemical substances reach the ocean, they undergo alternations that depend on physical, chemical, and biological conditions in the coastal seas. Changes due to eutrophication, deoxygenation, marine acidification, and climate change may also severely affect the carbon and nutrient cycles and therefore the marine ecosystems. CO2 and O2 dynamics (Fig. 1) are central to these changes in the marine biogeochemical cycles and for the ecosystem health.
Commonly, the term eutrophication is related to the excessive nutrient load of a sea area. A more appropriate definition considers eutrophication as “an increase in the rate of supply of organic matter to an ecosystem” (Nixon 1995). This definition is based on the fact that organic matter is a major control for the marine food web and oxygen depletion in deeper water layers. Hence, the modeling of eutrophication requires the explicit involvement of carbon as state variable that is no longer linked to the nutrient consumption by the traditional Redfield ratio (Redfield et al. 1963). Furthermore, the central role of carbon also implies that the input of organic carbon from the catchment must be included in the modeling. On the other hand, any production/mineralization of organic carbon affects not only the oxygen conditions but is also directly connected with the consumption/release of CO2. Including these processes in biogeochemical models and taking into account the input of inorganic carbon from land facilitate the simulation of the Baltic Sea acid–base system and thus address the marine acidification caused by increasing atmospheric CO2. Based on these considerations, a model framework was developed and complemented by measurements and data analysis that accounted for processes both in the sea and in the catchment which are relevant for the Baltic Sea O2–CO2 system (Fig. 1).
Eutrophication, acidification, and climate change are connected through the primary production and mineralization of organic matter from the sea and land. The coupling is complex, involving interconnection between organisms throughout the drainage basin and human activity. Human actions directly influence the carbon and nutrient cycles and may cause severe damage. Scientific knowledge, improved monitoring, and developing models addressing the carbon and nutrient cycles are therefore essential. These were addressed by BONUS+ in the Baltic-C program (http://www.baltex-research.eu/baltic-c/), where extensive fieldwork, database development, and modeling were the main activities during a 3-year research program starting in 2009. Baltic-C science was based on interdisciplinary cooperation among scientists from seven institutions and four countries. The present paper reviews some of the Baltic-C findings.
Biogeochemical models are based on mathematical process descriptions that include various empirical parameters. Due to the complexity of biogeochemical processes, these parameterizations only crudely approximate reality and are not universal laws applicable to all marine ecosystems. This particularly refers to the brackish Baltic Sea with its special hydrographic characteristics, permanent anoxic areas, and exposure to nutrients and carbon inputs from its catchment. Baltic-C biogeochemical modeling was therefore supported by a comprehensive measurement program and by monitoring data analysis to improve process parameterizations and provide model validation data (Leinweber et al. 2005; Kuznetsov et al. 2011). Activities focused on the marine CO2–O2 system, since almost all biogeochemical transformations entail CO2–O2 consumption or release. This also implies that the biogeochemical modeling was evaluated for its ability to simulate seasonal and spatial variations in the marine CO2–O2 system.
Relationship Between Surface Water pCO2 and Net Community Production
An automated measurement system for determining CO2 partial pressure (pCO2) was deployed on a cargo ship to investigate the seasonality and spatial distribution of surface water pCO2 (Schneider et al. 2006). The ship commutes 2–3 times per week between Luebeck in the southwest and Helsinki in the northeast Baltic Sea. This corresponds to a mean temporal resolution of the data acquisition of about 2 days. The spatial resolution given by ship speed and the measurement system response time was 1–2 nautical miles. Measurements were made with the Finnish Alga line Project, which records chlorophyll fluorescence and automatically samples surface water for nutrient analysis. pCO2 measurements started in summer 2003, stopped for 1.5 years when another ship took over the Luebeck–Helsinki route, and resumed in the long-term observation program of the Baltic Sea Research Institute (IOW, Warnemuende). For particular years and seasons, the data were used to estimate production and nitrogen fixation (Schneider et al. 2006, 2009). We present an overview of the data and draw conclusions regarding the seasonality of net community production and its relationship with nutrient availability following the analytical methods given by Koroleff (1983).
Figure 2a shows the seasonality of mean pCO2 in the northeastern Gotland Sea (57.5°–58.5°), 2004–2011. From April to about October, the pCO2 was clearly below atmospheric pCO2, which was 385–400 μatm in these years. This indicates that CO2 consumption by biological processes controlled pCO2 in this period, dominating the effect of rising temperatures in spring and summer that would increase pCO2. The seasonal pCO2 distribution is characterized by the two minima observed in spring and mid-summer, resulting from interplay between production peaks and increasing temperatures. The pCO2 increase after the main productive period coincides with the deepening of the mixed layer transporting CO2-enriched water masses to the surface. This process causes oversaturation of the surface water relative to atmospheric CO2, so CO2 is released into the atmosphere from November to March.
Based on pCO2 data, seasonal changes in total CO2 (CT) were calculated, which together with estimated CO2 gas exchange yield the net community production (NCP). Calculations were facilitated by the virtual absence of calcifying plankton from the central Baltic Sea (Tyrrell et al. 2008), so internal alkalinity changes were negligible. The mean total alkalinity in the northeastern Gotland Sea could be used to calculate seasonal CT changes from the pCO2, temperature and salinity data (Schneider et al. 2009).
The sharp CT drop occurring in all years in almost the same week by the end of March indicated the start of spring phytoplankton bloom (Fig. 2b). The CT decreased until mid-May, although nitrate was already entirely depleted in all years by mid-April (Fig. 2c). This indicates the continuation of net community production, since most excess phosphate left after nitrate depletion was concurrently consumed (Fig. 2d). This raises the question of the nitrogen source required for this production. Nitrate input from vertical mixing can be excluded, as nitrate is entirely exhausted to depths of 50–60 m after early spring bloom, while the mixed layer is only 20–30 m deep in the post-nitrate production period. Likewise, lateral transport cannot occur because nitrate concentrations in the surface water of the entire Baltic Proper, including coastal areas, are nearly zero. It has been speculated that nitrogen is preferentially mineralized and transferred from the existing biomass pool to new production using the phosphate excess after nitrate depletion (Thomas et al. 1999). This implies that the mean N/P ratio of the produced organic matter (POM) must approximately correspond to the low winter nitrate/phosphate ratio which on average is about 8 (Nausch et al. 2008). However, measurements yielded N/P ratios in POM close to or even above the Redfield ratio (16) during spring (Schneider et al. 2003). This indicates that preferential nitrogen mineralization plays only a minor role and cannot explain the continuation of net community production after nitrate depletion. Hence, another nitrogen source must exist; since atmospheric deposition is far too small to cause short-term effects, we speculated that either dissolved organic nitrogen was used for production or early nitrogen fixation took place despite low water temperatures in late April and early May. However, neither hypothesis could be substantiated by field measurements. But the analysis of monitoring data for total nitrogen and phosphorus concentrations in the eastern Gotland Sea (Swedish National Monitoring Program, SMHI) indicated that total nitrogen increased after the nitrate depletion, while total phosphorus decreased continuously from the start of spring bloom due to sedimentation (Fig. 3). These findings suggest an external nitrogen source, such as nitrogen fixation.
From mid-May to mid-June, CT did not display a clear trend. This indicates that net organic matter production was small and that the biological activity was based on regenerated production during which nutrients are recycled in the trophic layer. Since the post-nitrate bloom differed considerably between years, the ensuing regenerated production started at different CT levels causing the broad CT range in this phase (Fig. 2b).
A second distinct drop in CT was observed by mid-June when the well-documented mid-summer production based on nitrogen fixation started. The minimum generally occurred in July and indicated strong interannual variation in the minimum levels, which do not necessarily reflect variations in integrated production and nitrogen fixation in the trophic layer. The CT minima were confined to a shallow water layer about 2–3 m deep and occurred only during extremely calm weather conditions that produced high temperatures at the water surface. Finally, the phosphorus supply for production during nitrogen fixation must be considered. Excess phosphate was almost completely consumed by the mid-June start of nitrogen fixation. Furthermore, the continuous decrease in total phosphorus (Fig. 3) indicates that this phosphate was widely removed from the surface and was no longer a significant source of production during nitrogen fixation. Although it has been speculated that dissolved organic phosphorus and/or upwelling events (Nausch et al. 2009) may provide phosphorus for production, there is clearly a phosphorus shortage in the nitrogen fixation period. The lack of phosphorus obviously does not limit nitrogen fixation, and organic matter production results in C/P and N/P ratios that may exceed the corresponding Redfield ratios by a factor of up to four (Larsson et al. 2001; Schneider et al. 2003).
Deep Water Carbon Mineralization and Carbon Burial in the Sediment
To support Baltic-C modeling of organic matter mineralization, deep water total CO2 data were analyzed in Baltic-C. The measurements were made as part of the IOW’s long-term observation program. CT profiles were measured five times per year at the central station (BY15) in the eastern Gotland Sea. The vertical resolution was 25 m in the deeper part of the basin. From May 2004 to July 2006, temperature and salinity distributions indicated almost ideal stagnant conditions in the water masses below 150 m. The basin could thus be considered a biogeochemical reaction vessel, i.e., a closed system, unaffected by lateral water exchange, and could be used to trace the kinetics of biogeochemical transformation related to organic matter mineralization. CT accumulation below 150 m during the beginning, middle, and end of the stagnation period is shown in Fig. 4. Based on a mass balance accounting of the vertical exchange, the CT increase was used to calculate mineralization rates for different depth intervals (Schneider et al. 2010). We found that mineralization occurred mainly at the sediment surface and that the rates did not depend on redox conditions. The mean mineralization rate for the area below 150 m was 2.0 mol-C m−2 year−1, consistent with a previous finding of 1.8 mol-C m−2 year−1 (Schneider et al. 2002). However, our value is higher than that of Gustafsson and Stigebrandt (2007), who reported a rate of 1.3 mol-C m−2 year−1 based on oxygen and hydrogen sulfide data from 14 stagnation periods between 1965 and 2004. These estimates refer to different years and time spans which may differ with regard to the organic matter input. This may partly explain the differences in the calculated mineralization rates.
Not all organic material entering the deep basins is mineralized. A significant fraction of the organic matter either produced in the sea or transported to the sea from land is deposited to the sediments. Substantial portion of the deposited organic matter is mineralized, and defuses back to the overlaying water. The unmineralized fraction is permanently buried in sediments. Preliminary estimates indicate that the amount of organic carbon buried in sediment is about 2.7 Tg-C year−1 (0.58 mol-C m−2 year−1) (Kuliński and Pempkowiak 2011). However, these burial rates are associated with considerable uncertainty due to the limited data on both sediment accumulation rates and the range of organic carbon concentrations in bottom sediments.
Profiles of organic carbon concentrations in bottom sediments of the Baltic Sea indicate that organic matter concentrations decrease with sediment depth (Emeis et al. 2000; Szczepańska et al. 2012). This is attributed to the recent increased organic matter deposition caused by eutrophication (Emeis et al. 2000; Voss et al. 2000) and to ongoing mineralization of labile organic matter in deeper sediment layers (Kuliński and Pempkowiak 2011; Szczepańska et al. 2012). The mineralization occurs in two stages: the first lasting some 10 years, and the second lasting 50–60 years (Kuliński and Pempkowiak 2011).
Organic carbon burial in bottom sediments of the Baltic Sea was calculated as the difference between organic carbon accumulated in deep depositional areas of the Baltic Sea and organic matter losses due to long-term mineralization and diffusion into the water column. Carbon accumulation rates were determined from sediment accumulation rates based on the 210Pb method and validated against the 137Cs distribution (Pempkowiak 1991; Szczepańska et al. 2012) and from organic carbon concentrations in the sediments. Carbon losses caused by long-term mineralization were calculated from concentrations of inorganic carbon dissolved in pore water and from diffusion into the water column, where they contribute to accumulated total CO2. Likewise, the profiles of dissolved organic matter in pore water yielded the reflux into the water column. The details of organic carbon burial rate quantification in Baltic Sea sediments have been described by Kuliński and Pempkowiak (2011) and Szczepanska et al. (2012). Based on 23 sediment cores, the carbon accumulation, burial, and reflux rates were determined for the major depositional areas of the Baltic Sea (Table 1). The differences in burial rates between the basins are large. The highest rate observed in the Gotland Basin is 3.5 times larger than that in the Gulf of Bothnia, partly due to the lower productivity in the Gulf of Bothnia, and also due to the high lateral organic matter input into the Gotland Basin (Schneider et al. 2009).