Co-variance of dissolved Fe-binding ligands with phytoplankton characteristics in the Canary Basin

Dissolved Fe and ligand concentrations and the Fe-binding strength of the organic ligands were measured in samples from the upper water column (150 m) of the oligotrophic waters of the Canary Basin (eastern North Atlantic Ocean). Concentrations of major nutrients, phytoplankton abundance and photosynthetic characteristics were also measured in the same samples.The concentrations of dissolved Fe and dissolved organic ligands were low with mean values of 0.31 ± 0.18 nM Fe and 1.79 ± 0.73 nEq of M Fe(n = 47), respectively. The conditional binding constant varied between 1019.8–1022.7 (n = 47). The largest variation with depth in the ligand concentrations (between 4.78 and 1.1 nEq of M Fe) was observed in the upper layer, above the Deep Chlorophyll Maximum (DCM located between 80 and 100 m), with high surface values in stations at 18 and 34.At the DCM where Fe was depleted, the ligand concentrations were still relatively high showing the same trend with depth as the amount of phytoplankton cells. Here 62% of the vertical variation in ligand concentrations can be explained by parameters describing phytoplankton cell abundance or biomass and orthosilicic acid concentration, which could reflect diatom growth. Ligand concentrations below the maximum of the DCM (n = 4) showed good linear positive relationships with the total phytoplankton biomass as well as with 2 out of 4 distinguished groups of phytoplankton (Synechococcus and pico-eukaryote I).In the maximum of the DCM and below this maximum the phytoplankton origin of the dissolved organic ligands of Fe is very probable. Data suggest a release of ligands by cell lysis and not by an active production. However, the origin in the surface layer is more difficult to explain. Although the amount of phytoplankton cells in the surface layer is reduced, it is still ∼ 25% of the cell concentration observed in the DCM. High concentrations of organic ligands could then be a remnant of past blooms or present production under nutrient depleted conditions. Input of Sahara dust can be another source of ligands.