2.1. Sampling Sites and Strategy

The ATOS1 cruise took place from 27 June to 28 July 2007. Nineteen stations were sampled in the west and northern Greenland Sea and Arctic Ocean (68.5°–80.9° N, 2.6° W–19.5° E; Figure 1A) close to Svalbard Island. In the ATOS2 cruise, (28 January to 25 February 2009), we visited 17 stations around the Antarctic Peninsula located in the Bransfield Strait and the Weddell and Bellingshausen Seas (61.0°–69.5° N, 51.5°−76.1° W; Figure 1B). Both cruises were conducted on board the R/V BIO-Hespérides (for more details see [19,20,21]).

Map of sampled stations in the Arctic (Greenland Sea and around Svalbard) (A) and Antarctic waters (Bransfield Strait, Weddell Sea, and Bellingshausen Sea) (B) during ATOS1 and ATOS2 cruises, respectively.

Environmental parameters, microbiological abundances, and prokaryotic production were measured at all visited stations during both cruises in the SML and SSW layers, except temperature, salinity, and DOC, which were only measured in the SSW for Antarctic waters (ATOS2) (Figure 1). Viral production, virus-mediated prokaryotic mortality, and lysogeny were determined at 9 stations in the Arctic Ocean (6, 9, 12, 15, 23, 33, 39, and 43; Figure 1A) and at 6 stations in Antarctic seawaters (2, 5, 7, 10, 13, and 17; Figure 1B) in both layers.

The surface microlayer (SML) water was sampled under calm sea conditions from a rubber boat deployed 2 km away from the research vessel in order to avoid contamination of the samples from the vessel’s influence. The SML samples were collected using a glass plate sampler [22], which had been previously cleaned with acid overnight and rinsed thoroughly with ultrapure water (MQ-water). To quantify any procedural contamination, we collected field SML blanks by rinsing and collecting 0.5 L of ultrapure water. We used a glass plate of a 975 cm² surface area, and about 100 dips were required to collect 500 mL of the SML water. SSW samples were collected by hand at 0.1 m depth in an acid-cleaned plastic carboy from the same site as the SML samples. For chemical and microbiological parameters, we collected respectively 1 L and 600 mL from the SML and 2 L from the SSW layers.

The enrichment factor (EF) for chemical and biological variables was assessed as the ratio of the concentration or rate of the respective parameter in the SML to that in the SSW. An EF of >1.0 indicated an enrichment in the SML relative to the SSW; an EF of <1.0 indicated a depletion in the SML relative to the SSW.