Jens Daniel Müller

Highly Sensitive Poisoning-Resistant Optical Carbon Dioxide Sensors for Environmental Monitoring

A new optical carbon dioxide sensor for environmental monitoring is presented. It combines a robust and long-term stable sensing material with a compact read-out device. The sensing material relies on a NIR pH indicator immobilized into ethyl cellulose along with a quaternary ammonium base. The perfluorinated polymer Hyflon AD 60 used as a protection layer significantly enhances the long-term and mechanical stability of the sensor foils, as well as the robustness against poisoning gases, e.g. hydrogen sulfide. The sensor can be stored under ambient conditions for more than six weeks, whereas sensors covered with silicone rubber deteriorate within one week under the same conditions. The complete sensor device is applicable after a three-point (re)calibration without a preconditioning step. The carbon dioxide production and consumption of the water plant Egeria densa was measured in the laboratory. Furthermore, results of profiling carbon dioxide measurements during a research cruise on the Baltic Sea at water depths up to 225 m are presented.

Metrology of pH Measurements in Brackish Waters—Part 2: Experimental Characterization of Purified meta-Cresol Purple for Spectrophotometric pHT Measurements

Spectrophotometric pH measurements allow for an accurate quantification of acid-base equilibria in natural waters, provided that the physico-chemical properties of the indicator dye are well known. Here we present the first characterization of purified m-Cresol Purple (mCP) directly linked to a primary pH standard in the salinity range 5-20. Results were obtained from mCP absorption measurements in TRIS buffer solutions. The pHT of identical buffer solutions was previously determined by Harned cell measurements in a coordinated series of experiments. The contribution of the TRIS/HCl component to the ionic strength of the buffer solutions increases towards lower salinity: This was taken into account by extrapolating the determined pK2e2 to zero buffer concentration, thereby establishing access to a true hydrogen ion concentration scale for the first time. The results of this study were extended with previous determinations of pK2e2 at higher and lower salinity and a pK2e2 model was fitted to the combined data set. For future investigations that include measurements in the salinity range 5-20, pHT should be calculated according to this pK2e2 model, which can also be used without shortcomings for salinities 0-40 and temperatures from 278.15 – 308.15 K. It should be noted that conceptual limitations and methodical uncertainties are not yet adequately addressed for pHT determinations at very low ionic strength.

Metrology for pH Measurements in Brackish Waters—Part 1: Extending Electrochemical pHT Measurements of TRIS Buffers to Salinities 5–20

Harned cell pHT measurements were performed on 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) buffered artificial seawater solutions in the salinity range 5 20, at three equimolal buffer concentrations (0.01, 0.025, 0.04 mol·kg H2O 1), and in the temperature range 278.15 – 318.15 K. Measurement uncertainties were assigned to the pHT values of the buffer solutions and ranged from 0.002 to 0.004 over the investigated salinity and temperature ranges. The pHT values were combined with previous results from literature covering salinities from 20 to 40. A model function expressing pHT as a function of salinity, temperature and TRIS/TRIS·H+ molality was fitted to the combined data set. The results can be used to reliably calibrate pH instruments traceable to primary standards and over the salinity range 5 to 40, in particular, covering the low salinity range of brackish water for the first time. At salinities 5-20 and 35, the measured dependence of pHT on the TRIS/TRIS·H+ molality enables to extrapolate quantities calibrated against the pHT values, e.g. the dissociation constants of pH indicator dyes, to be extrapolated to zero TRIS molality. Extrapolated quantities then refer to pure synthetic seawater conditions and define a true hydrogen ion concentration scale in seawater media.

Surface Water Biogeochemistry as Derived from pCO 2 Observations

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.

The Main Hydrographic Characteristics of the Baltic Sea

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.

Organic Matter Mineralization as Reflected in Deep-Water C T Accumulation

Measurements of CT 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 CT 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 CT 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 CT were released at a C/P ratio close to the Redfield ratio for the composition of OM.

Progress Made by Investigations of the CO 2 System and Open Questions

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.

The Marine CO 2 System and Its Peculiarities 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.