Vertical profiles

of photosynthetically active radiation (PAR) were measured at 10 cm intervals in the vertical profile of the water column using a submergible radiometer Li-Cor LI-192SB (Lincoln, Nebraska, USA). Thereafter, light extinction coefficient (k, m−1) was estimated considering that light is exponentially attenuated with depth. In addition, the mean light intensity in the mixed layer, Im, was calculated with the equation ( Riley, 1957): Im = I0 (1 − e(−kZm)) (kZm)−1, where I0 (in μE m2 s−1) is the light intensity received at the water surface and Zm is the depth of the mixed zone (in m), which corresponds Erlotinib in vivo to the water column depth with no vertical stratification (i.e. absence of thermocline and halocline). The limit see more of the euphotic zone (Zeu, m) was estimated as the depth at which irradiance is 1% of the surface value (i.e. Zeu = 4.6 k−1). During the dates of installation and recovery of the sediment collectors (during and after the winter bloom: July–November), vertical profiles of pH, temperature, dissolved oxygen, salinity and turbidity (1 m intervals) were measured in situ with the portable Horiba U-10 multi-probe. In addition, surface water samples were collected with a van Dorn bottle (2.5 l) to assess phytoplankton abundance, chlorophyll, phaeopigments,

dissolved nutrients, PSM and POM concentrations. In addition, the size of the suspended particles was analyzed from May to November in surface water. Sediment collectors were used to assess the sinking rates of PSM and the fate of phytoplankton cells. Nor fixatives were added (Varela et al., 2004) in order to evaluate the natural physical and chemical processes that affect the accumulation of organic matter in the collectors. The cylindrical container (PVC material) had a height to diameter ratio of 8:1 and a collecting area of 0.1 m2; the design was based on Lange and Boltovskoy (1995). The mooring system consisted of a 200 kg platform which was connected to a buoy by a line and a ballast positioned at a fixed distance from the collectors. This only system led to keep clear the water column above the collectors’ mouth without any lines.

Sampling devices were built at CCT-BB facilities, CONICET-Bahía Blanca, Argentina. The sediment collectors were moored at 300 m offshore in Puerto Cuatreros station, within a relatively undisturbed area from boats and fishing. The mouths of the collectors were positioned 2 m above the bottom, where the depth fluctuated between 9.5 m in high tide and 5.5 m in low tide. Sampling was carried out conducting a total of four deployments (D1–D4): D1 from 24 July to 7 August, 14 days; D2 from 15 to 22 August, 7 days; D3 from 22 August to 6 September, 15 days and D4 from 27 November to 30 November, 3 days. The accumulated material inside the collectors was homogenized in order to analyze PSM, POM, dissolved inorganic nutrients, chl and pha concentrations and C:N ratios.