Pc filters

Dissolved primary production (¹⁴C-DOCp) rates were determined after a 2–3 h incubation period, from the 72-mL samples sets, using four different types of filters, and after a 9 h incubation, using 0.2-μm PC filters (Whatman, 25 mm ∅). The different types of filters used during the short incubations were borosilicate (25 mm Whatman GF/F, 0.7 μm), silver membrane (SM) (25 mm Steriltech, 0.2 μm), polycarbonate (25 mm Whatman nucleopore, 0.2 μm) and mixed acetate and nitrate cellulose (25 mm Millipore GSWP, 0.22 μm). For each incubation, two 5-mL replicates were taken from each 72-mL bottle and were filtered under low-vacuum pressure. Filtrates were subsequently acidified to a pH of ~2 with 100 μL of 50% HCl and kept overnight in open scintillation vials (20 mL) placed on an orbital shaker. After the removal of inorganic ¹⁴C, 15 mL of scintillation cocktail was added to each filtrate. The radioactivity on each filtrate and filter was determined following the procedure previously detailed in ¹⁴C-POCp measurements section.

Samples (15 mL) for the analysis of dissolved inorganic nutrients [ammonium, nitrates and nitrites (NO_X) and phosphates] were pre-filtered through Whatman PC filters (0.2 μm pore diameter) and measured spectrophotometrically according to Hansen and Koroleff (1999) using an automated constant flow analyzer Seal Analytical QuAAtro (for nitrates, nitrites and phosphates) or spectrophotometer Shimadzu, UV-1202 UV-Vis (for ammonium) with the detection limits of 0.2 μM L^–1 for nitrate, 0.05 μM L^–1 for nitrite, 0.1 μM L^–1 for phosphate and 0.05 μM L^–1 for ammonium.