Nutrient Facility

Marine biogeochemical cycles can be understood as the balance between sources and sinks of elements that are essential for life in the ocean, in particular carbon, nitrogen, phosphorus and silicon, as driven by physical, biological and chemical processes. Marine biogeochemical cycles play a relevant role in ocean productivity and link the latter to the planet’s climate system. The elements of the biogeochemical cycles are considered essential ocean variables (EOV) as defined by the Global Ocean Observing System. In order to study biogeochemical cycles, we need to measure compounds that contain such essential elements at appropriate temporal and spatial scales. We do so via the analyses of samples often collected during oceanographic expeditions and field campaigns. On occasion, samples also derive from laboratory assays. At the AWI Nutrient Facility, we therefore focus on the measurement of carbon, nitrogen, phosphorus and silicon containing compounds. Additionally, we also measure total alkalinity, which provides information about the buffer capacity of seawater, relevant to study ocean acidification. 

The facility is divided in three modules based on scientific topics, as follows.

Nutrients: Dissolved compounds in seawater that contain elements essential for life in the ocean; e.g., nitrogen, phosphorus, silicon, iron. Their availability sustains ocean productivity and carbon export to depth. Some of these nutrients occur at trace concentrations (e.g., iron, nickel, copper, zinc) and are termed micro-nutrients. Others occur at higher concentrations and are termed macro-nutrients (e.g., nitrate, phosphate, silicate), which are the compounds we are interested about.

Dissolved organic matter (DOM): Most DOM is produced by primary producers during photosynthesis. Microbial decay modifies the chemical structure and persistence of DOM leading to long residence times in the ocean. The amount of carbon that is stored in DOM in the ocean is one of the largest active organic carbon reservoirs on earth. The global amount of marine dissolved organic carbon (DOC) is comparable to the amount of carbon stored in atmospheric CO₂ and it exceeds the amount of carbon contained in marine animals, plants, bacteria and organic particles by a factor of 30. In addition to carbon, DOM also contains other chemical elements such as nitrogen, oxygen, hydrogen, sulfur or phosphorus. The analysis of the DOC concentration is typically used as a representative element for the total concentration of DOM. 

Dissolved inorganic carbon (DIC): Total dissolved inorganic carbon (DIC = [CO₂] + [HCO₃^-] + [CO₃^2-]) is by far the largest active carbon reservoir in the global carbon cycle, containing almost 50 times more carbon than the atmosphere. The high CO₂ storage capacity of the ocean is due to the fact that it reacts with water to form carbonic acid (H₂CO₃), which almost fully dissociates to bicarbonate (HCO₃^-) and hydrogen ions (H⁺). Rising atmospheric CO₂ has caused DIC concentrations to increase and pH and dissolved carbonate ion levels to drop, a phenomenon called ‘ocean acidification’. The CO₂ storage capacity is strongly affected by another property of the carbonate system, the total alkalinity (TA). With CO₂ uptake, the concomitant changes in [H⁺] influence the weak (not fully dissociated) acid-base systems in seawater, and thus the change in pH is smaller than expected from the simple stoichiometry of ‘one H⁺ per CO₂ molecule added’. This pH-buffering is due to high values of total alkalinity in seawater (TA = [HCO3^-] + 2 [CO₃^2-] + [B(OH)₄^-] + minor components). To determine the full carbonate system, two variables such as DIC and TA have to be measured.

To carry out the measurements of these EOV using state-of-the-art techniques, the facility counts with the following instruments; 1) a VINDTA 3C (Versatile Instrument for the Determination of Total inorganic carbon and titration Alkalinity) and a SI Analytics Titroline Alpha plus with adjusted TitriSoft for determination of Total Alkalinity.; 2) Seal Analytical continuous flow segmented nutrient analysers: a 4-channel QuAAtro analyser (dissolved inorganic carbon, nitrate+nitrite, phosphate and silicate), two 5-channel analysers, a QuAAtro and an AA-500 (nitrate+nitrite, nitrite, ammonium, phosphate and silicate), a 7-Channel  AA-500 (total nitrogen, nitrate+nitrite, nitrite, ammonium, total phosphorus phosphate and silicate) and a 2-Channel AA3 (total nitrogen and total phosphorus); 3) an AQ-300 Seal Analytical discrete analyser; 4) and last but not least, two high temperature catalytic oxidation TOC-L Shimadzu for total organic carbon and total organic nitrogen. 

The facility is supported by highly qualified technical experts: Laura Heitmann, Frederike Schmidt and Kai-Uwe Ludwichowski. Scientific support is provided by Prof. Dr. Boris Koch (DOC,DON), Prof. Dr. Björn Rost and Prof. Dr. Mario Hoppema (both carbon system), and facility head Dr. Sinhué Torres-Valdés (nutrients).