Relationships between phytoplankton light absorption, pigment composition and size structure in offshore areas of the Mediterranean Sea

Abstract

Phytoplankton absorption is mainly determined by pigment composition and packaging into the cell, in turn affected by pigment concentration and cell size and shape. The variations of the chlorophyll-specific absorption coefficients at two selected wavelengths (443 nm and 675 nm) were analysed for offshore areas of the Mediterranean Sea (Alboran, Algerian, Tyrrhenian, Ionian Basins and Sardinia Channel) investigated in autumn 2006, 2007 and 2008. The spatial distribution of Tchl a, accessory pigments, size structure and taxonomic composition of phytoplankton assemblages was diversified among the areas, characterized by higher contribution of photoprotective pigments in oligotrophic surface waters, while accessory chlorophylls increased in deeper layers and where microphytoplankton dominated. a à ph (443) values decreased from the oligotrophic waters to the more mesotrophic ones where microphytoplankton prevailed, while for a à ph (675) a small range of variation was observed. The differentiation of a à ph (443) was explained by the variations in the phytoplankton assemblage mean cell size, that consequently can be assumed as the main factor determining package effect. Comparison with previous parameterizations supported the existence of a Mediterranean differentiation for which the use of global models might determine inaccuracy for predicting phytoplankton absorption coefficients and thus refining ecological models.

Light Absorption by Suspended Particles in the Red Sea: Effect of Phytoplankton Community Size Structure and Pigment Composition

“…A broad range of variation in [TChl a ] (0.008–1 mg m −3 ) is associated with a narrower variability in a ph *(676) values (0.011–0.036 m −1 ), whereas a ph *(440) values vary widely (0.029–0.152 m −1 ). This observation is consistent with anterior studies in other oligotrophic environments (Bouman et al, ; Organelli et al, ; Perez et al, ; Vijayan & Somayajula, ). The large variability observed in a ph *(440) and a ph *(676) for a given [TChl a ] can be attributed to changes in phytoplankton community size structure and pigment composition.…”

“…On average, the phytoplankton community size structure seems to be slightly smaller than those in the Mediterranean Sea and slightly larger than those in the Atlantic Ocean (Figure b). As reported in previous studies, the [PPC]/[TChl a ] ratio decrease according to depth (higher values in surface water) in inverse relation to [TChl a ] (Bricaud et al, ; Organelli et al, ). A group of low [PPC]/[TChl a ] values (>0.2) is also identified in the deeper layer and appears to not be related to [TChl a ] (Figure c) as reported in the Mediterranean Sea by Organelli et al ().…”

“…As reported in previous studies, the [PPC]/[TChl a ] ratio decrease according to depth (higher values in surface water) in inverse relation to [TChl a ] (Bricaud et al, ; Organelli et al, ). A group of low [PPC]/[TChl a ] values (>0.2) is also identified in the deeper layer and appears to not be related to [TChl a ] (Figure c) as reported in the Mediterranean Sea by Organelli et al (). As for SI values, the [PPC]/[TChl a ] values are within the range of those observed in the other areas of the global ocean (Bricaud et al, ).…”

Multivariate approach for the retrieval of phytoplankton size structure from measured light absorption spectra in the Mediterranean Sea (BOUSSOLE site)

“…The minimum [Tchl a] value during the nine-year BOUSSOLE time series was 0.047 mg m −3 (Fig. 4), which is slightly larger than [Tchl a] measured in extremely clear, picophytoplankton-dominated, Mediterranean waters observed during the PROSOPE cruise [30], or during other surveys in the Ligurian Sea [64] and other Mediterranean areas [50,54]. Nanophytoplankton was the dominant size class even in the clearest waters of the BOUSSOLE site (SI values close to 10 μm).…”

“…As already noted [11,51], such a distribution of DPs may yield some errors and uncertainty in the evaluation of the algal size classes because some pigments can be shared by various phytoplankton groups and some groups can be found in more than one size class. In spite of these possible sources of error and ambiguity, this method has been shown to provide reasonable information on the size structure and taxonomic composition of algal communities at global scale [30,41,48,[52][53][54]. Note, however, that slight modifications in the repartition of pigments within size classes have been recently proposed by Brewin et al [14] and Hirata et al [21].…”