Biogeochemical Transformations in the Baltic Sea

Research on the marine CO₂ system has its roots in the Baltic Sea, where it commenced at the beginning of the last century. Physico-chemical studies were performed that resulted in the determination of equilibrium constants describing the dissociation of carbonic acid, and they were still being used 50 years later. Comprehensive field studies in the northern Baltic Sea facilitated the first characterization of the seasonality and regional peculiarities of the CO₂ system. It was not until the 1980s that research on the CO₂ system received greater interest, a product of the growing awareness of the oceans role for absorbing anthropogenic CO₂. Studies of the marine CO₂ system were also revived in the Baltic Sea. However, rather than examining the fate of anthropogenic CO₂, investigations of the CO₂ system were considered as an ideal tool to study fundamental biogeochemical processes, such as the production and mineralization of organic matter, which are connected to the consumption and release of CO₂, respectively. It is the aim of this book to demonstrate this and to present related progress in biogeochemical research in the Baltic Sea.

Carbonic acid formed by the dissolution of CO₂ in seawater dissociates to yield hydrogen carbonate, carbonate, and hydrogen ions, which are linked to each other by dissociation constants and constitute the marine CO₂ system. To determine the composition of the CO₂ system, requires that the values of two of four measurable variables are known: total CO₂ (sum of the CO₂ species), alkalinity (excess of base equivalents over hydrogen ions), pH (log of the hydrogen ion concentration), and the CO₂ partial pressure (pCO₂ in air at equilibrium with the respective water). Alkalinity plays a central role because it controls the status of the CO₂ system in case that the sea is at equilibrium with CO₂ in the atmosphere. In the Baltic Sea, alkalinity inputs via river water are subject to strong regional differences. Together with the alkalinity input by inflowing North Sea water different alkalinity regimes are formed which lead to characteristic regional distributions of the total CO₂ and pH. Alkalinity also affects the relationships between the variables of the CO₂ system. The magnitude of the change in pCO₂ in response to a change in total CO₂ increases with decreasing alkalinity. This effect is important when pCO₂ measurements are used to estimate biological production, but it also influences CO₂ gas exchange with the atmosphere, by increasing the equilibration time at high alkalinities.

Strong inputs of river water, the topographic succession of sills and basins, and the narrow entrance to the North Sea are the main features that determine the hydrography of the Baltic Sea. An estuarine circulation is established which transports low-salinity water towards the North Sea. This surface current consists of river water that is modified by mixing with the high-salinity deeper water layers originating from the North Sea. Consequently, a permanent halocline is formed in the Baltic Proper at a depth of ~60 m. In addition to preventing full mixing of the water column, the presence of the halocline widely inhibits the transport of oxygen into deeper water layers. A thermal stratification of the water above the halocline develops during spring and summer such that a warm surface layer forms at depths above 20–30 m. In autumn and winter, the cooling of the surface water and the increasing winds cause erosion of the thermocline such that full mixing of the water column down to the permanent halocline is finally restored. The below-halocline water of the deep basins in the central Baltic Sea may be subjected to stagnation for many years. Water renewal by lateral inputs of high-salinity, oxygenated water occurs irregularly, at intervals of up to 10 years, and only under specific meteorological conditions.

Early measurements of pCO₂ in the surface waters of the Baltic Sea clearly pointed to biological production as the major controlling factor underlying the seasonality of the pCO₂. However the temporal resolution of the measurements was too low to allow reasonable quantitative conclusions regarding biological production. This problem was overcome in 2003, when a fully automated measurement system that continuously recorded the surface-water pCO₂ was deployed on a cargo ship. The ship commuted at regular intervals of 2–3 days between the southwest Baltic Sea (Lübeck) and the western Gulf of Finland (Helsinki) and thus traversed the entire Baltic Proper. Between 2003 and 2015, pCO₂ data for 1600 transects were obtained. Investigations of CO₂ accumulation in deep water also started in 2003, in conjunction with IOW’s long-term observation program. The measurements were confined to a central station in the eastern Gotland Basin and were made four to five times per year. The vertical resolution of the data with respect to total CO₂ and the accompanying variables, such as nutrient and oxygen/H₂S concentrations, was 25 m.

The surface-water pCO₂ in the central Baltic Sea shows a distinct seasonality, with minima in May and July. This pattern can be unambiguously attributed to the net community production (NCP) during the spring bloom and to the mid-summer NCP fueled by nitrogen fixation. Converting the pCO₂ data to concentrations units for the total CO₂ facilitated a detailed and quantitative analysis of the chronology of the NCP. The start of the spring bloom was triggered by the year’s first increase in the surface-water temperature and during the study period regularly occurred in the central Baltic Sea by the end of March. The first phase of NCP was based on the availability of nitrate and lasted, on average, until mid-April. However, NCP continued until the end of May despite the absence of dissolved inorganic nitrogen (nitrate + ammonia). This observation has led to questions regarding the occurrence of nitrogen fixation already during spring. A period of regenerated production that did not contribute to NCP followed the termination of the spring bloom. Mid-summer NCP fueled by nitrogen-fixing cyanobacteria was detected as discrete pulses and coincided with sudden increases in temperature. Distinct linear correlations between temperature and the accumulated NCP for the individual production events suggested that solar radiation controls and limits the efficiency of nitrogen fixation. The role of phosphate as limiting factor could not be confirmed.

Measurements of C_T in the deep water (below 150 m) of the Gotland Basin spanned the stagnation period between two major inflow events that occurred in 2003 and 2014. Whereas the salinity in the basin decreased widely in a steady way during stagnation, the increase of C_T and other products of OM mineralization was not continuous. This could be explained by vertical mixing, which must be taken into account when mineralization rates are determined on the basis of a C_T mass balance. Since the stagnation period was interrupted by a short-lasting lateral inflow of water in 2006, mineralization rates where calculated separately for the time before and after this event. For both stagnation phases almost identical mineralization rates were obtained (2.0 and 1.8 mol m⁻²) which did not change following a switch from oxic to anoxic conditions. Soon after the start of the stagnation period that followed the major inflow of oxygen-rich water in 2003, the release of phosphate was disproportionately high in relation to the OM mineralization. This was attributed to the anoxic dissolution of the iron-III-oxy-hydroxy-phosphates that had formed during deep-water oxygenation. After this initial phase, phosphate and C_T were released at a C/P ratio close to the Redfield ratio for the composition of OM.

In the introduction we stated that investigations of the marine CO₂ system are an ideal tool for biogeochemical studies because almost all biogeochemical processes are ultimately driven by the interplay between organic matter (OM) production and mineralization in an everlasting cycle driven by solar radiation. Since these processes are connected with the consumption or production of CO₂, CO₂ mass balance calculations based on measured changes in total CO₂, were successfully used to estimate net production and mineralization rates in the Baltic Sea.