Dataset: Arabian Sea phytoplankton

During the cruise, seawater samples were collected with a 5-l Niskin® sampler mounted on a Sea Bird Electronics® CTD rosette using JGOFS prescribed clean techniques (Knap et al., 1996). Calibrated sensors mounted on the CTD provided vertical profiles of salinity, temperature and dissolved O2. The salinity sensor was calibrated against Autosal® Salinometer measurements, while the dissolved O2 sensor was calibrated against measurements using the Winkler method with the high-precision amperometric end point detection (Langdon, 1984). Quality control was guided by the National Oceanographic Data Center Report and included duplicate profile, depth duplication and inversion checks, as well as data range and large temperature and inversion checks (Boyer and Levitus, 1994).

Chlorophyll-a concentrations corrected for phaeophytin were measured by the fluorometric method, whereas phytoplankton production rates were measured using 14C labeled NaHCO3 incubations under simulated in-situ conditions in plexiglass tanks placed on the deck in full sunlight and in running seawater (Knap et al., 1996).

Microscopic identification and enumeration of phytoplankton was undertaken on duplicate samples that were fixed with 1% Lugol’s iodine in 3% buffered formaldehyde solution and stored in cool and dark conditions to allow the cells to settle. Prior to microscopic analysis, samples were concentrated to 5-10 ml by carefully siphoning out the top layer through a 10um Nitex® filter wrapped around the tip of a thin tube. Although examination of the siphoned layer under the microscope indicated that no visible cells were lost by this method of concentration, cell abundance estimated using this method only applies to >10um cells. Cell density was counted by transferring 1ml replicates of the concentrated sample on to a Sedgwick-Rafter® slide and counted with an Olympus Inverted microscope (Model IX 50) at 200x magnification. Identification of phytoplankton was based on standard taxonomic keys, and was in most cases undertaken up to the species level (Tomas, 1997).