Water was collected via Niskin bottles mounted on a rosette, equipped with a CTD.

From the Niskin bottle, water was dispensed into smaller glass containers that were cleaned and pre-rinsed three times with water from the Niskin bottle prior to dispensing. This water was used to measure the activities of polysaccharide hydrolases. A separate glass Duran bottle was filled with seawater from the Niskin bottle and sterilized in an autoclave for 20-30 minutes to serve as a killed control for microbial activity measurements.

Polysaccharide hydrolase activity was measured by filling three 50 mL falcon tubes with seawater and one 50 mL falcon tube was filled with autoclaved seawater to serve as a killed control, for each substrate. Polysaccharide substrate was added at 3.5 μM monomer-equivalent concentrations, except for fucoidan, which was added at 5 μM concentrations (a higher concentration was necessary for sufficient fluorescence signal). Two 50 mL falcon tubes – one with seawater and one with autoclaved seawater – with no added substrate served as blank controls. Incubations were stored in the dark at as close to in situ temperature as possible.

Subsamples of the incubations were collected at time zero, and at a sequence of subsequent time points. At each time point, 2 mL of seawater was collected from the 50 mL falcon tube using a sterile syringe, filtered through a 0.2 μm pore size syringe filter, and stored frozen until processing.

Molecular weight distributions were determined by sequential size-exclusion chromatography using a Bio-Rad Econo-Column packed with ~20 cm of Sephadex G-50 resin followed by ~18 cm of Sephadex G-75 resin, and quantified on Shimadzu 10ADvp HPLC systems equipped with Hitachi fluorescence detectors (set to excitation and emission wavelengths of 490 and 530 nm, respectively) controlled by EZStart software. Hydrolysis rates were calculated based on the change in molecular weight distribution from higher molecular weight initially to lower molecular weight of the substrate over the course of the incubation time, as described in detail in Arnosti (2003).