The cruise D361 forms part of a study entitled 'Physical and chemical forcing of diazotrophy in the (sub)-tropical Atlantic Ocean'. The study is investigating the potential influence of iron and phosphorus availability on nitrogen fixation in regions of the tropical Atlantic Ocean. The cruise also undertakes deep trace metal clean CTD casts as part of the International GEOTRACES programme. The purpose of the cruise therefore is to undertake measurements of dissolved and particulate iron and phosporpus availability, their spatial and temporal variations, and their impact on diazotrophy in the surface ocean. Aim: To quantify the supply and determine the biogeochemical cycling of Fe and other nutrients, and relate this to N2 fixation, diazotroph species distribution and N* fields. 1. Quantify the distribution of nutrients and trace metals: Quantify surface water and water column distributions of dissolved inorganic/organic N, P, Fe, and DAl, DMn and particulate P, N, Fe, Al, Mn. 2. Quantify the rate of Fe, Al, Mn, P and N supply to surface waters: Assess the source fluxes of the key elements for diazotrophs and source tracers to the surface ocean from atmospheric deposition and internal transport via diapycnal mixing and lateral advection. 3. Identify the source of subsurface Fe enrichment: Identify whether Fe-rich subsurface waters of the tropical North Atlantic thermocline originate from the atmosphere or the shelf using Fe distributions and Al, Mn, and O2 source tracers. 4. Quantify the diazotrophic response to Fe, phosphate, DOP supply: Relate the spatial distributions of inorganic Fe and organically complexed Fe, and phosphate and DOP to diazotrophy. The specific uptake of Fe, phosphate and DOP by the whole microbial community and Trichodesmium will be assessed by shipboard incubations, radiotracer techniques and enzyme bioassays. In addition, we will identify the connection between N2 fixation rates and diazotroph community structure, by comparing size fractionated 15N2-derived rates of N2 fixation (Fig. 1) with abundance and diversity of diazotrophs using nifH phylogeny. Objective 5: Investigate how the large scale transport pathways of Fe and P influence the N* distribution: Use fine-scale isopycnic model to reveal the large-scale transport pathways of Fe and P in the (S)-T Atlantic, and their effect on the N* distribution.

The main testable hypothesis of the proposed work is: Atmospheric inputs control rates of primary production and microbial diversity in oceanic waters where nutrients are limiting. The objectives of the project are to: 1. Obtain an improved temporal and spatial estimate of atmospheric dust inputs to the tropical N Atlantic through collections on a dedicated SOLAS process cruise. 2. Obtain an improved estimate of the seawater dissolution of N, P, Fe and Zn species from aerosol dust. 3. Determine the impact of atmospheric dust derived micronutrients on microbial community production and species diversity in the surface microlayer and underlying waters. In addition, researchers from the Archer/Geider group were on-board. The overall aim of their research was to determine the extent to which the photoprotective roles of DMSP and QAs influence their production rates in marine surface waters and hence, the production of their volatile breakdown products. The cruise objectives for this work were to: 1. relate DMSP and QAC concentrations to plankton community structure, light regime, photoinhibition, xanthophyll cycle and MAA accumulation in varying oceanic provinces and over diel cycles. 2. determine the potential for photoinhibition and DMSP/GBT turnover in natural phytoplankton in contrasting oceanic provinces. The cruise departed Tenerife on February 5, 2008, and we have conducted regular stations (typically 2 per day) along the cruise track. The track took us into the oligotrophic Atlantic waters, productive Cape Verde waters, and tropical waters with very high nitrogen fixation (judged initially from the Trichodesmium concentrations). The various researchers and groups have also started a range of biological experiments at different sites along the cruise track. We have encountered a major dust event in week 2, and then from week 3 of the cruise we encountered large amounts of dust. We have visited the TENATSO time series site near the Cape Verdes and have undertaken an extensive set of measurements there. We have collected the atmospheric dust for elemental analyses (at UEA and NOCS), and also to produce leachates which are used on board for biological experiments. We have undertaken sampling of the water column to analyse for dissolved and particulate metals, nutrients, dissolved organic matter, amino acids, hemes, thiols and phytochelatin synthase expression. In addition, nitrogen fixation measurements were undertaken, in tandem with nifh gene sampling. Nitrate uptake experiments have been conducted. Furthermore, bacterial phosphate uptake experiments have been undertaken using addition of collected dust. Halocarbon and DMS gas measurements have been conducted during the cruise. In addition, experiments have been undertaken by the Archer/Geider group on effects of high sun light exposure on DMS and GBT production. The cruise has been very successful with a minimum of lost time.