The surface film of the hydrosphere covers more than 70% of the world�s surface. The surface microlayer (SML) or �skin� of oceans is a sink for natural and anthropogenic materials originating from both the atmosphere and the water column.
Organisms living in this SML are called �neuston� and our knowledge on the biology of the SML is still in its infancy. Research of the sea surface microlayer requires the use of appropriate sampling techniques and strategies and the question of what is the most suitable device has not yet been answered. In the present study, we have compared the efficiency of the Harvey glass plate and the Garrett metal screen to analyse of wide range of microbiological parameters from viruses to ciliates in SML samples collected at two coastal stations along the north Mediterranean coast.
Two types of membranes (Teflon and polycarbonate) were also used to collect the bacterioneuston. For most biological parameters, the metal screen was the most appropriate technique providing higher concentrations in the SML compared to the glass plate and therefore, the highest enrichments compared to underlying waters.
Phytoneustonic parameters were generally depleted compared to the underlying water when sampled by the glass plate. The metal screen is more efficient than the glass plate to collect larger neustonic organisms. We also provide clear evidence that membranes strongly overestimate the abundance of bacterioneuston due to selective adsorption. We conclude that the classic screen method is the most suitable technique allowing a short sampling time and the collection of large sample volumes when a large diversity of parameters are simultaneously analysed.
The sea surface microlayer (SML) is also a poorly characterized marine compartment regarding the organic matter accumulation and the atmosphere ocean exchanges of persistent organic pollutants (POPs). A major constraint for these studies has been its reliable sampling. In the present work, over 30 SML samples from two contrasting sites in the North-Western Mediterranean -Barcelona (Spain) and Banyuls-sur-Mer (France)- were collected using four different sampling devices, namely, glass plate, metal screen, Harvey�s roller and a surface slick sampler, and compared with the corresponding underlying water (15 samples). Enrichment factors (EF) regarding different parameters (individual and total PCBs, PAHs, DOC, POC and SPM) were compared using statistical tests. From the results obtained it was concluded that the MS is the more advantageous system for sampling organic pollutants in terms of sampling efficiency under a variety of meteorological conditions.
Obviously, the sampling for trace metals analysis is restricted to the glass plate device. Similar conclusions were drawn for particulate and dissolved fatty acids, as well as for the hydrophilic compounds (carbohydrates and amino-acids).
These results have been included in several publications that have been submitted to international journals. The thinner the layer collected, the closer the biological and chemical composition of the sample to the original distribution in the SML will be. Under this point of view, the GP is preferable.
However, the MS offers the smallest contact area between the water film to be sampled and the sampler, which probably results in the lowest contamination of the SML.
Furthermore, the MS offers the advantage to fit better the size range of neustonic organisms since large cells cannot attach to the surface of the glass plates. Another advantage is that MS collects larger volumes in shorter periods of time (around 10 litres per hour and per screen), something essential when a lot of parameters are to be analysed that require large water volumes.
In contrast, the GP has a much lower sampling capacity (1 liter per hour and per plate). The time of sampling is of a great importance since the longer the time of sampling, the highest is the effect of temporal and spatial variability. Thus, we recommend the use of MS when the aim is the comparison of different parameters. In all cases, control experiments should be performed to check for the potential bias that can be introduced by the sampler itself.
There is clear evidence from this study that membranes have important drawbacks and should not be used for quantification purposes. Both hydrophobic and hydrophilic membranes overestimated the concentration of living organisms by selective surface adsorption.
However, sampling with membranes remains of interest for rapid and easy collection of bacteria living in the surface microlayer. Finally, the depth at which UW samples are collected must be carefully determined depending on the question to be answered, as it was illustrated by the depth profiles of dissolved carbohydrates in the first meter below the surface. Furthermore, any comparison with other reports in the literature should be made not only in reference to the sampler but also to this depth.