An observational assessment of the influence of mesoscale and submesoscale heterogeneity on ocean biogeochemical reactions

Numerous observations demonstrate that considerable spatial variability exists in components of the marine planktonic ecosystem at the mesoscale and submesoscale (100?km -1?km). The causes and consequences of physical processes at these scales (‘eddy advection’) influencing biogeochemistry have received much attention. Less studied, the non-linear nature of most ecological and biogeochemical interactions means that such spatial variability has consequences for regional estimates of processes including primary production and grazing, independent of the physical processes. This effect has been termed ‘eddy reactions’. Models remain our most powerful tools for extrapolating hypotheses for biogeochemistry to global scales and to permit future projections. The spatial resolution of most climate and global biogeochemical models means that processes at the mesoscale and submesoscale are poorly resolved. Modelling work has previously suggested that the neglected ‘eddy reactions’ may be almost as large as the mean field estimates in some cases. This study seeks to quantify the relative size of eddy and mean reactions observationally, using in situ and satellite data. For primary production, grazing and zooplankton mortality the eddy reactions are between 7% and 15% of the mean reactions. These should be regarded as preliminary estimates to encourage further observational estimates, and not taken as a justification for ignoring eddy reactions. Compared to modelling estimates, there are inconsistencies in the relative magnitude of eddy reactions and in correlations which are a major control on their magnitude. One possibility is that models exhibit much stronger spatial correlations than are found in reality, effectively amplifying the magnitude of eddy reactions

In search for the sources of plastic marine litter that contaminates the Easter Island Ecoregion

Subtropical gyres are the oceanic regions where plastic litter accumulates over long timescales, exposing surrounding oceanic islands to plastic contamination, with potentially severe consequences on marine life. Islands’ exposure to such contaminants, littered over long distances in marine or terrestrial habitats, is due to the ocean currents that can transport plastic over long ranges. Here, this issue is addressed for the Easter Island ecoregion (EIE). High-resolution ocean circulation models are used with a Lagrangian particle-tracking tool to identify the connectivity patterns of the EIE with industrial fishing areas and coastline regions of the Pacific basin. Connectivity patterns for “virtual” particles either floating (such as buoyant macroplastics) or neutrally-buoyant (smaller microplastics) are investigated. We find that the South American shoreline between 20°S and 40°S, and the fishing zone within international waters off Peru (20°S, 80°W) are associated with the highest probability for debris to reach the EIE, with transit times under 2 years. These regions coincide with the most-densely populated coastal region of Chile and the most-intensely fished region in the South Pacific. The findings offer potential for mitigating plastic contamination reaching the EIE through better upstream waste management. Results also highlight the need for international action plans on this important issue

Understanding the extent of universality in phytoplankton spatial properties

Phytoplankton are one of the most visible signs of life in our oceans. They also are a key component of the global carbon cycle and of the marine food web. Their complex patterns at the sea surface are routinely seen in satellite images, though the first observations go back centuries.The motivation of this thesis is to explore the spatial properties of phytoplankton. Inspired by ‘universal’ theories for the dynamics of turbulence, several ones have been proposed to explain phytoplankton patchiness as a balance between turbulent stirring by the water and biological processes involving the phytoplankton. The thesis examines the extant theories of plankton patchiness using a novel twin tracer approach, specifically using in situ simultaneous Chlorophyll-a and nitrate measurements from a cruise in the North Atlantic. A significant difference is observed between the variability spectra of the two biochemical variables, an outcome potentially explained only by one theory.More generally, although numerous observations testify to the existence of scaling behaviour of phytoplankton spatial variability, the collation of these studies indicates considerable variability, and hence uncertainty, in the power law behaviour, specifically the value of the spectral ‘slope’. The many different techniques used to evaluate the spectrum, the different sources of data, and the geographical and temporal limitations associated with the data all contribute to adding noise and uncertainties in the estimates for the slope and make a comparison between studies difficult. In this thesis, the existence of the universal scaling properties of phytoplankton are tested over a wide range of spatial (sub-regional and regional) and temporal (few days to a year) scales using in situ, satellite data and model output. For this purpose a robust method is developed that reliably evaluates the spectrum of phytoplankton. A power-law behaviour in the phytoplankton spectrum is consistently found across the sources of data used and the range of scales studied (from 10 m to 130 km). However, stronger universality for the phytoplankton spectrum, defined as constant or uniform slope, is undermined by the significant variability in spectral slope that is consistently demonstrated across the spatial and temporal scales studied.????????????????????????????????????????????????????????????????????????????????????????????????

Amazon River propagation evidenced by a CO2 decrease at 8°N, 38°W in September 2013

The surface fugacity of CO2 (fCO2) has been measured hourly at a mooring at 8°N, 38°W, using a spectrophotometric CO2 sensor, from June to October 2013. In September 2013, the fCO2 and the sea surface salinity (SSS) decrease significantly. The high precipitation due to the presence of the Intertropical Convergence Zone (ITCZ) and the propagation of low salinity waters from the Amazon River plume explain the decrease of SSS. Indeed, in fall, the retroflection of the North Brazil Current (NBC) feeds the North Equatorial Counter Current (NECC) and transports Amazon waters to the eastern part of the tropical Atlantic. Simulations from a three dimensional physical and biogeochemical model and observations at the mooring show that the Amazon plume reached the mooring in September 2013. The decrease of fCO2 is associated with a moderate peak of chlorophyll. Over the period of the CO2 observations, the site is a source of CO2 to the atmosphere of 0.65 ± 0.47 mmol m−2 day−1. Although the wind speed is at its lowest intensity in September 2013, the flux over the whole period would be about 14% higher without this month. Every month of September from 2006 to 2017, the model simulates a decrease of dissolved inorganic carbon corresponding to the SSS minimum

Assessing the Impact of the Assimilation of SWOT Observations in a Global High-Resolution Analysis and Forecasting System – Part 2: Results

A first attempt was made to quantify the impact of the assimilation of Surface Water Ocean Topography (SWOT) swath altimeter data in a global 1/12° high resolution analysis and forecasting system through a series of Observing System Simulation Experiments (OSSEs). The OSSE framework (Nature Run and Free Run) and data assimilation scheme have been described in detail in a companion article (Benkiran et al., 2021). The impact of assimilating data from SWOT and three nadir altimeters was quantified by estimating analysis and forecast error variances for sea surface height (SSH), temperature, salinity, zonal, and meridional velocities. Wave-number spectra and coherence analyses of SSH errors were also computed. SWOT data will significantly improve the quality of ocean analyses and forecasts. Adding SWOT observations to those of three nadir altimeters globally reduces the variance of SSH and surface velocities in analyses and forecasts by about 30 and 20%, respectively. Improvements are greater in high-latitude regions where space/time coverage of SWOT is much denser. The combination of SWOT data with data from three nadir altimeters provides a better resolution of wavelengths between 50 and 200 km with a more than 40% improvement outside tropical regions with respect to data from three nadir altimeters alone. The study has also highlighted that the impact of using SWOT data is likely to be very different depending on geographical areas. Constraining smaller spatial scales (wavelengths below 100 km) remains challenging as they are also associated with small time scales. Although this is only a first step, the study has demonstrated that SWOT data could be readily assimilated in a global high-resolution analysis and forecasting system with a positive impact at all latitudes and outstanding performances