Accurate, on-site determinations of macronutrients (phosphate (PO₄³⁻), nitrate (NO₃⁻), and silicic acid (H₄SiO₄)) in seawater in real time are essential to obtain information on their distribution, flux, and role in marine biogeochemical cycles. The development of robust sensors for long-term on-site analysis of macronutrients in seawater is a great challenge. Here, we present improvements of a commercial automated sensor for nutrients (including PO₄³⁻, H₄SiO₄, and NO₂⁻ plus NO₃⁻), suitable for a variety of aquatic environments. The sensor uses the phosphomolybdate blue method for PO₄³⁻, the silicomolybdate blue method for H₄SiO₄ and the Griess reagent method for NO₂⁻, modified with vanadium chloride as reducing agent for the determination of NO₃⁻. Here, we report the optimization of analytical conditions, including reaction time for PO₄³⁻ analysis, complexation time for H₄SiO₄ analysis, and analyte to reagent ratio for NO₃⁻ analysis. The instrument showed wide linear ranges, from 0.2 to 100 μM PO₄³⁻, between 0.2 and 100 μM H₄SiO₄, from 0.5 to 100 μM NO₃⁻, and between 0.4 and 100 μM NO₂⁻, with detection limits of 0.18 μM, 0.15 μM, 0.45 μM, and 0.35 μM for PO₄³⁻, H₄SiO₄, NO₃⁻, and NO₂⁻, respectively. The analyzer showed good precision with a relative standard deviation of 8.9% for PO₄³⁻, 4.8% for H₄SiO₄, and 7.4% for NO₂⁻ plus NO₃⁻ during routine analysis of certified reference materials (KANSO, Japan). The analyzer performed well in the field during a 46-day deployment on a pontoon in the Kiel Fjord (located in the southwestern Baltic Sea), with a water supply from a depth of 1 m. The system successfully collected 443, 440, and 409 on-site data points for PO₄³⁻, Σ(NO₃⁻ + NO₂⁻), and H₄SiO₄, respectively. Time series data agreed well with data obtained from the analysis of discretely collected samples using standard reference laboratory procedures and showed clear correlations with key hydrographic parameters throughout the deployment period.

Stable isotope ratio analysis offers a unique opportunity to obtain information on ecosystem processes. The increase in atmospheric CO₂ as a consequence of fossil fuel combustion and land-use change is altering the stable carbon isotope composition (δ¹³C) of the atmosphere and ocean. This work investigates the application of using δ¹³C measurements of seawater samples to explore the biogeochemical responses of marine ecosystems to anthropogenic CO₂ perturbations. The combination of isotopic and non-isotopic measurements from a subtropical North-Atlantic mesocosm experiment provided a holistic view of the biogeochemical mechanisms that affect carbon dynamics under a gradient of pCO₂ ranging from ~350 up to ~1,000 μatm during a phytoplankton succession. A clear CO₂ response was detected in the isotopic datasets with ¹³C shifts of up to ~5%0, but increased CO₂ levels only had a subtle effect on the concentrations of the dissolved and particulate organic carbon pools. Distinctive δ¹³C signatures of the particulate organic carbon pools in the water column and sediment traps were detectable for the different CO₂ treatments after a nutrient stimulated phytoplankton bloom. These signatures were strongly correlated (p < 0.05) with the δ¹³C signatures of the inorganic carbon but not with the δ¹³C of the dissolved organic carbon pools (p > 0.05). Fractionation of carbon isotopes in phytoplankton was positively affected (9.6 < ε < 16.5%0) by high CO₂ levels either because of the higher CO₂ availability or because of a shift in phytoplankton community composition. Nevertheless, phytoplankton bloom intensity and development was independent of CO₂ concentrations, and higher CO₂ levels had no significant effect on inorganic nutrient uptake. Results from this mesocosm experiment showed that variations in the carbon isotopic signature of the carbon pools depend on both physical (air-sea exchange) and biological (community composition) drivers opening the door to new approaches for investigations of carbon cycling in marine ecosystems.