Sampling and analytical procedures:
Bioassay experiments were conducted at station 1 and stations 3. At each station, inorganic and organic phosphate amendments were performed on seawater with and without nitrogen enrichment (NH4Cl, NaNO3). Bioassay Experiments consisted in incubating, over an incubation period of 48h, surface seawater (5m) with inorganic or organic phosphate compounds (20 µM; final concentration of P) including, polyphosphate (polyp), inorganic phosphate (Pi), nucleotides (ATP or AMP) and methylphosphonate (Mepn). In each incubation experiment, a control treatment (surface seawater without amendment) was included.
To investigate a potential abiotic hydrolysis of inorganic and organic phosphate compounds, duplicates of surface seawater sampled inshore (5 m) and previously autoclaved (120°C; 30 min), were incubated in parallel. The autoclaved seawater (1L) was amended with either Mepn, AMP, ATP or PolyP (final concentration 20 µM P).
In bioassay experiments, subsamples (50 mL) were taken for SRP determination from each experimental incubation bottle and then filtered through pre-combusted (450°C, 4.5 h) and acid washed (1N HCl) GF/F filter using a syringe (25 mm). The filtrates were then transferred into acid cleaned (10% HCl) HDPE bottles (60 mL) and kept frozen until analysis. SRP concentrations were analyzed following a spectrophotometric method based on the molybdenum reagent (Murphy and Riley, 1962). SRP concentrations (all samples except Pi amended treatments and T=48h of AMP and ATP amended treatments) were analyzed using the Liquid Waveguide Capillary Cell® (LWCC, optical length path=2.5 m). Pi amended treatments as well as T=48h of AMP and ATP amended treatments were analyzed using a conventional spectrophotometer (Genesys®) with a 10 cm cuvette.
Instruments: Measurements of SRP were performed using the Liquid wave guide capillary cell (2.5 m length path, model 3250, World Precision Instrument) and using a conventional spectrophotometer (Genesys®,10 cm cuvette).
Location: Northwestern Atlantic surface waters. Depth: surface-50 m.
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