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Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N 2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N 2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region. Key Points Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edge Enhanced N2 fixation within the eddy core is proposed to have driven increased phosphate consumption No substantial total phytoplankton biomass increase was found within the eddy core

A key challenge in oceanography is to capture and quantify processes that happen on short time scales, seasonal changes and inter-annual variations. To address this problem the P&O European Ferries Ltd. Ship MV Pride of Bilbao was fitted with a FerryBox from 2002 to 2010 and data returned to NOC in real time providing near continuous measurements between UK (Portsmouth) and Spain (Bilbao) of temperature, salinity, chlorophyll-fluorescence and oxygen. Additional monthly samples were collected on manned crossings. Over 6000 samples were analysed for nitrate (nitrate and nitrite) concentrations. The timing of nitrate concentration increases (with winter mixing) and decreases (with the spring bloom) are different on and off shelf and in autumn nitrate concentrations remain high on the shelf. Off shelf in the Bay of Biscay, the mixed layer depth assessed using Argo floats, was found to vary from 212 m in relatively mild winters (such as 2007/2008) to 476 m in cold winters (2009/2010). Years with deeper mixing were associated with an increase in nitrate concentrations in the surface waters (~3 μmol l−1) and the increased vertical nutrient supply resulted in higher productivity the following spring. Bloom progression could be seen through the increase in oxygen anomaly and decrease in nitrate concentrations off shelf prior to changes further north on the shelf and phytoplankton growth was initiated as shoaling begins. The full dataset demonstrates that ships of opportunity, particularly ferries with consistently repeated routes, can deliver high quality in situ measurements over large time and space scales that currently cannot be delivered in any other way.

The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific, and Indian Oceans, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non-SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity-normalized alkalinity with that of the open ocean end-member. Non-SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin, and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end-member can only be reliably derived from the y intercept of TA-S regression (TA(S0)) in river-dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification, etc.) are more influential, TAS0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TAS0 can result from non-SDC TA removal at the low salinity end (relative to the salinity of the oceanic end-member) and/or non-SDC TA addition at high salinities (as occurs in the Mediterranean Sea).

The surface ocean plays an important role in the marine carbon cycle linking the atmosphere and the deep ocean. There are substantial variations in the surface ocean carbonate system in different environments and on various time scales, resulting from the interactions of various physical and biogeochemical processes. In this study, the use of Ships of Opportunity (SOO, as carriers of automatic underway measuring systems and platforms for sample collections) was promoted to enhance the surface ocean observing capacity to provide in-situ observations with better temporal resolution and spatial coverage in a cost- effective way (Chapter 2). The functionality, reliability and accuracy of an automatic pCO2 sensor were assessed extensively under various field and laboratory conditions (Chapter 3). The sensor proved to be suitable for long-term onboard and in-situ measurements, while its uncertainty is largely determined by the reference used for calibration. Observations made from two SOOs were examined to better understand the variability and control of the surface ocean carbonate system (Chapter 4, 5). The spatial variability of alkalinity in different marine environmental settings were investigated focusing on the influences of physical and biogeochemical processes on the alkalinity-salinity relationship (Chapter 4). By using salinity-normalized alkalinity as an indicator, the TA addition or removal processes were examined in the open ocean regime in the Atlantic, Pacific and Indian Ocean (mainly controlled by precipitation and evaporation), in the western North Atlantic margin, eastern North Pacific and Mediterranean Sea (additional alkalinity inputs from rivers, currents or the Black Sea), and in the Red Sea (alkalinity removal by CaCO3 precipitation). In coastal regions, a regional-specific term for zero salinity end member should be considered in salinity normalization practice, and care should be taken when use the abstract intercept of alkalinity-salini