In this type of incubation, samples are taken from different depths, placed in transparent tubes and exposed to solar radiation in their natural habitat and at the depth from where they were collected. If considering also the effects of UVR (280-400 nm) phytoplankton will be exposed to the in situ radiation field by using UV-transparent tubes placed in trays which are incubated at different depths in the water column. Different treatments can be implemented (i.e. by the use of selective filters) to assess the effects of UV-B (280-315 nm) and UV-A (315-400 nm) in addition to that of PAR. The principal disadvantage of in situ incubations is that cells are kept at a fixed depth throughout incubation period (e.g. a few hours) thus receiving a constant proportion of the surface incident radiation. In the water column however, cells move within the upper mixed layer (UML) and thus they are exposed to a variable irradiance field. In fact, relatively few studies have addressed the importance of mixing on phytoplankton photosynthesis, not only due to fluctuations in PAR but also in UVR. These studies have determined a wide range of responses, with vertical mixing enhancing, reducing or having no effects on primary production. Although in situ incubations will result in the most realistic responses of phytoplankton exposed to various depths, they are strongly conditioned by weather conditions; as a result, certain areas of the World Ocean (i.e. polar) are relatively under-sampled.
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