Water was collected from a circulating seawater system distributed via plastic tubing for POC/PON/POP around 3m deep. An underway system was chosen to vastly increase sampling coverage, replicate number, and sample volume. The water intake is located near the ship sea chest, which may have missed particle production in the subsurface. The circulating seawater was never turned off during the entirety of the transect and kept at a constant flow. Water was passed through a 30 μm nylon mesh (Small Parts #7050-1220-000-12) to remove larger plankton and particles from the sample. Each replicate was collected into a separate 8.5L plastic carboys (Thermo Scientific, Waltham, Massachusetts). In between stations, carboys were rinsed with 30 μm filtered sample water just prior to collection. Six 8 L seawater samples were divided into POC/PON and POP triplicates. Carboys were placed at ~45° angle to avoid particle settling below the nozzle. Each replicate was passed through a 25 mm pre-combusted (500°C for 5 h) GF/F filter (Whatman, Florham Park, New Jersey) with a nominal pore size of 0.7 μm. The vacuum filtration was an in-line setup with 25 mm filter holders connected to an aspirator pump at -0.08 MPa. POP filters were rinsed with 5 ml of 0.17 M Na₂SO₄ to remove traces of dissolved phosphorus from the filter. All filters were stored in pre-combusted aluminum packets and immediately frozen at -80°C during the cruise and -20°C for shipment.
Particulate Organic Carbon/Nitrogen: Prior to analysis, the filters for POC and PON were dried according to the JGOFS protocol (Knap et al., 1996). The protocol has a detection range of 0.43-43.13 µM for POC and 0.037-7.39 µM for PON in sea water (Knap et al., 1996). First, the filters were dried in an incubator at 55°C for 24-48 h and then stored in a desiccator with concentrated HCl fumes for 24 h to remove inorganic carbonates. Secondly, the filters were dried again at 55°C for 48 h before being folded and packed into pre-combusted tin capsules (CE Elantech, Lakewood, New Jersey). The packaged filters are analyzed on a CN FlashEA 1112 Elemental Analyzer (Thermo Scientific, Waltham, Massachusetts) against an atropine standard curve (chemical formula C₁₇H₂₃NO₃).
Particulate Organic Phosphorus: Particulate organic phosphorus (POP) were analyzed according to a modified ash-hydrolysis protocol (Lomas et al., 2010). Thawed filters were placed in along with a corresponding standard curve of KH₂PO₄. 2 mL of 0.017M MgSO₄ was added to the acid-washed glass vials containing filters and covered with pre-combusted aluminum foil. The vials were placed in an incubator at 90°C for 24 h and then combusted (500°C, 2 h). Once cooled, 5 mL 0.2 M HCl was added and incubated at 90°C for at least 30 min. Next, the supernatant plus 5 mL milli-Q water was mixed with 2:5:1:2 parts ammonium molybdate tetrahydrate, 5N sulfuric acid, potassium antimonyl tartrate, and ascorbic acid for 30 min. Finally the standards and samples were analyzed on a spectrophotometer (Genesys 10vis, Thermo Scientific, Waltham, Massachusetts) at a wavelength of 885 nm to determine POP concentration with an assay detection limit ~0.1 nmol l⁻¹.
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