Description: The low surface nitrate concentration and high atmospheric iron input in the tropical eastern North Atlantic provide beneficial conditions for N-2 fixation. Varying abundances of diazotrophs have been observed and an Fe- and P-colimitation of N-2 fixation was reported in this ocean region. It is however unclear, how different limiting factors control the temporal variability of N-2 fixation and what the role of Fe-limitation is in a region with high fluxes of dust deposition. To study the environmental controls on N-2 fixation, an one-dimensional ecosystem model is coupled with a physical model for the Tropical Eastern North Atlantic Times-series Station (TENATSO), north of the Cape Verde Islands. The model describes diazotrophy according to the physiology of Trichodesmium, taking into account a growth dependence on light, temperature, iron, dissolved inorganic (DIP) and organic phosphorus (DOP). The modelled total Chl a is compared with satellitederived total Chl a and modelled Trichodesmium (Tri) compared with satellite-derived cyanobacterial Chl a as well as with High Performance Liquid Chromatography data. Model results show a complex pattern of competitive as well as mutually beneficial interactions between diazotrophs and non-diazotrophic phytoplankton. High DOP availability after spring blooms of non-diazotrophic phytoplankton and the ability of Trichodesmium to take up DOP are crucial for allowing a maximal abundance of Tri in autumn. Part of the reactive nitrogen newly fixed by diazotrophs is directly excreted or released through mortality, significantly fuelling the growth of non-diazotrophic phytoplankton in autumn and winter. Fe consumption by non-diazotrophic phytoplankton earlier in the year makes Fe limitation of Tri in late summer more acute, whereas Tri growth in surface waters reduces phytoplankton abundance deeper in the water column by light limitation. Overall, the atmospheric iron input at the TENATSO site is required to enable diazotrophic growth and to support the observed abundance of non-diazotrophic phytoplankton

Description: The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.