Publications

Methodological developments in recent years have led to an increase in empirical databases on the abundance and functions of aquatic microbes, now allowing synthesis studies. Most of these studies have adopted a comparative approach, such that comparative analyses are now available for most aspects of aquatic microbial food webs (more than 50 papers published in the last 15 years). Some of these analyses apparently yield conflicting results, introducing confusion and unnecessary disputes in the field. We briefly review the comparative analyses so far produced and we highlight generalities, show that some of the perceived discrepancies largely derive from partial analyses of a general underlying trend and formulate predictions based on these general trends that provide new avenues for research.

<strong class="journal-contentHeaderColor">Abstract.</strong> Vegetated coastal habitats, including seagrass and macroalgal beds, mangrove forests and salt marshes, form highly productive ecosystems, but their contribution to the global carbon budget remains overlooked, and these forests remain <q>hidden</q> in representations of the global carbon budget. Despite being confined to a narrow belt around the shoreline of the world's oceans, where they cover less than 7 million km², vegetated coastal habitats support about 1 to 10 % of the global marine net primary production and generate a large organic carbon surplus of about 40 % of their net primary production (NPP), which is either buried in sediments within these habitats or exported away. Large, 10-fold uncertainties in the area covered by vegetated coastal habitats, along with variability about carbon flux estimates, result in a 10-fold bracket around the estimates of their contribution to organic carbon sequestration in sediments and the deep sea from 73 to 866 Tg C yr^−1, representing between 3 % and 1∕3 of oceanic CO₂ uptake. Up to 1∕2 of this carbon sequestration occurs in sink reservoirs (sediments or the deep sea) beyond these habitats. The organic carbon exported that does not reach depositional sites subsidizes the metabolism of heterotrophic organisms. In addition to a significant contribution to organic carbon production and sequestration, vegetated coastal habitats contribute as much to carbonate accumulation as coral reefs do. While globally relevant, the magnitude of global carbon fluxes supported by salt-marsh, mangrove, seagrass and macroalgal habitats is declining due to rapid habitat loss, contributing to loss of CO₂ sequestration, storage capacity and carbon subsidies. Incorporating the carbon fluxes' vegetated coastal habitats' support into depictions of the carbon budget of the global ocean and its perturbations will improve current representations of the carbon budget of the global ocean.

The variation in the concentration and the turbulent fluxes of dissolved organic nitrogen and phosphorus (DON and DOP) across the thermocline in the central Atlantic were studied along a quasi‐meridional transect from the Canary Islands to Argentina (22°N to 31°S). In general, DON concentrations were high in surface waters and declined toward the thermocline, whereas DOP concentrations were less variable with depth. Vertical, gradient‐driven fluxes of DON and DOP generally involved a downward flux, with a median DON:DOP ratio of 28:1. The downward flux of DON was closely correlated with the nitrate supply to the mixed layer and sufficed to remove an amount of nitrogen equivalent to that associated with the upward nitrate influx into the mixed layer (median = 104%). The downward flux of DON exceeded the supply of nitrate off the NW African coast, where Trichodesmium sp. was abundant. The downward flux of DOP was also very closely correlated with the phosphate supply to the mixed layer ( r = 0.87; P = 0.0007), but the downward export of phosphorus as DOP accounted for only 9% of the upward phosphate supply to the mixed layer. There was also evidence of a deep upward flux of DON and DOP from below the thermocline, which, combined with the general downward flux in the upper waters, indicates the presence of a strong sink for DON and DOP within the thermocline, tentatively identified to be microplankton respiration. Our results point to a dominant role of downward DOM fluxes as a path for the removal of nutrients, and probably carbon, from the biogenic layer of the ultraoligotrophic central Atlantic.

We measured PAR-saturated CO2 exchange rate (CER), and leaf N, P and chlorophyll (Chl) concentrations in 21 plant species, selected to encompass as broad a range in specific leaf area (SLA) as possible, and encompassing non-succulent C3 as well as succulent CAM plants. We worked with plants growing under uniform conditions in the facilities of a biological research station to ensure that any correlations found were due to inherent, genetically controlled, relationships between the measured parameters and not due to variations in resource availability in different habitats. We found CER to be strongly correlated to SLA, leaf N concentration and Chl concentration. CER increased much faster with increasing leaf N concentration (CER ≈ N3.1) than with increasing SLA (CER ≈ SLA1.2). CER also increased much faster with leaf N concentration than with increasing Chl concentration (CER ≈ Chl1.3), indicating the photosynthetic N-use efficiency (NUE) to be higher for plants with high N concentration than for plants with low N concentration (NUE ≈ N2.1). Analysis of covariance showed that these relationships exist even when comparing plants of widely different growth forms - succulent or non-succulent, and of different photosynthetic pathways, as the C3 and CAM plants compared here. Testing against scaling coefficients calculated using dimensional analysis, showed that the scaling of N, Chl and CER against SLA was not merely a result of diluting N and Chl with carbon in thicker leaves but that SLA, probably through influencing light absorptio and/or CO2 diffusion pathway, played an independent role in controlling CER.

Remote sensing is the main approach to map aquatic vegetation, and classification tree (CT) is superior to various classification methods. Based on previous studies, modified CT can be developed from traditional CT by adjusting the thresholds based on the statistical relationship between spectral features to classify different images without ground-truth data. However, no studies have yet employed this method to resolve marine vegetation. In this study, three Gao-Fen 1 satellite images obtained on 30 January 2014, 5 November 2014 and 21 January 2015 were selected, and two features were then employed to extract macroalgae farms. Results show that the overall accuracies of traditional CTs for three images are 92.0, 94.2 and 93.9%, respectively, whereas those of the two corresponding modified CTs for images obtained on 21 January 2015 and 5 November 2014 are 93.1 and 89.5%, respectively. This indicates modified CTs can map macroalgae with multi-date imagery and monitor their spatiotemporal distribution in coastal environments.