To Örebro University

The concentrations of chemical elements dissolved in seawater cover a range of far more than ten orders of magnitude due to natural abundance and solubility. Regarding to their mass concentrations, Cl, Na, Mg, S, Ca, and K (in this order) normally cover 99,95% of the dissolved matter. Trace elements are present in lower concentrations, but can play important roles in many biogeochemical processes, including nutrient cycling and the formation of marine sediments. Some of them can be highly toxic to marine organisms and may bioaccumulate in the food chain. Simultaneous multielement determination is challenging and requires analytical methods with high dynamic range. Inductively coupled plasma mass spectrometry (ICP-MS) gives the possibility to quantify all major elements and about 20 trace and ultra-trace elements in seawater at natural concentration levels without time-consuming sample pre-treatment. The samples were diluted 1:60 and the external calibration was based on matrix matched standard solutions. The method was optimised for the accurate determination of major and trace elements in seawater samples, such as off the coast of Tanzania and Mozambique in the West Indian Ocean (WIO). This water body is warmer than many other aquatic systems and characterized by high salinity. Surface water as well as deep-water samples were analysed to provide a snapshot as to the occurrence of 42 elements in a water body at low latitudes, with monsoonal winds and reversing currents.

Fourteen of the 42 elements determined were below the limits of quantification in most of the samples. The results of the remaining elements were assessed based on sampling site (location), depth, pressure, salinity, oxygen content, conductivity, and water temperature. Only for Si and P changes in concentrations with depth (or pressure) were observed, having correlation coefficients of R=0.94 and R=0.97. The increases with depth were more pronounced in the samples from Mozambique (greater depths) than Tanzania. Conductivity, temperature, and dissolved oxygen decreased with depth (vertical distance from the water surface) and pressure.Some alkali and alkaline earth metals, namely Li, Na, K, Rb, Mg, Ca, but also U and V were highly correlated with Cl and B but also with S.

A comparison of 18 elements determined in plankton from the same locations (van Aswegen, 2020) and in the water samples revealed very distinct patterns: whereas the plankton samples were dominated by Ca, followed by Mg, and Sr, the order in the water samples was Mg >> Ca > K.

Van Aswegen, J. D. (2020). Metal composition of zooplankton from the Western Indian Ocean (Doctoral dissertation, North-West University (South Africa))