Exudates by the diatom Phaeodactylum tricornutum were incubated with a natural community of marine heterotrophic prokaryotes for 24 d in order to investigate the link between the biological lability and the molecular weight, fluorescence, and polarity of phytoplankton dissolved organic matter (DOM). Dissolved organic carbon (DOC) removal, changes in fluorescence and in the heterotrophic prokaryote abundance were followed over time both in the total exudates and in the low‐ and high‐molecular‐weight fractions. To detect changes in the polarity of proteins, reverse‐phase high‐performance liquid chromatography (HPLC) was applied to the high‐molecular‐weight fraction. Our results indicate that freshly produced phytoplankton DOM exhibits a dynamic pattern of degradation that is accompanied by large changes in the growth efficiency of the bacterial community that are likely related to changes in DOM quality. Approximately 20% of high‐molecular‐weight DOM and 40% of fluorescence attributed to protein‐like DOM were degraded over the first days of the incubation indicating that protein‐like DOM is likely a labile component of phytoplankton exudates. In contrast, fluorescence measurements suggest that humic‐like substances are resistant to bacterial degradation over the 24 d of the experiment. Despite fluctuations in the short‐term rates of high‐molecular‐weight and low‐molecular‐weight DOM removal, the relative contributions of these fractions to DOM pool were similar in the fresh exudates and at the end of our incubation experiments.

Abstract. The last few decades have seen dramatic changes in the hydrography and biogeochemistry of the Mediterranean Sea. The complex bathymetry and highly variable spatial and temporal scales of atmospheric forcing, convective and ventilation processes contribute to generate complex and unsteady circulation patterns and significant variability in biogeochemical systems. Part of the variability of this system can be influenced by anthropogenic contributions. Consequently, it is necessary to document details and to understand trends in place to better relate the observed processes and to possibly predict the consequences of these changes. In this context we report data from an oceanographic cruise in the Mediterranean Sea on the German research vessel Maria S. Merian (MSM72) in March 2018. The main objective of the cruise was to contribute to the understanding of long-term changes and trends in physical and biogeochemical parameters, such as the anthropogenic carbon uptake and to further assess the hydrographical situation after the major climatological shifts in the eastern and western part of the basin, known as the Eastern and Western Mediterranean Transients. During the cruise, multidisciplinary measurements were conducted on a predominantly zonal section throughout the Mediterranean Sea, contributing to the Med-SHIP and GO-SHIP long-term repeat cruise section that is conducted at regular intervals in the Mediterranean Sea to observe changes and impacts on physical and biogeochemical variables. The data can be accessed at https://doi.org/10.1594/PANGAEA.905902 (Hainbucher et al., 2019), https://doi.org/10.1594/PANGAEA.913512 (Hainbucher, 2020a) https://doi.org/10.1594/PANGAEA.913608, (Hainbucher, 2020b) https://doi.org/10.1594/PANGAEA.913505, (Hainbucher, 2020c) https://doi.org/10.1594/PANGAEA.905887 (Tanhua et al., 2019) and https://doi.org/10.25921/z7en-hn85 (Tanhua et al, 2020).

Abstract. The last decades have seen dramatic changes in the hydrography and biogeochemistry of the Mediterranean Sea. The complex bathymetry, highly variable spatial and temporal scales of atmospheric forcing and internal processes contribute to generate complex and unsteady circulation patterns and significant variability in biogeochemical systems. Part of this variability can be influenced by anthropogenic contributions. Consequently, it is necessary to document details and to understand trends in place to better relate the observed processes and to possibly predict the consequences of these changes. In this context we report on data from an oceanographic cruise in the Mediterranean Sea on the German research vessel MARIA S. MERIAN (MSM72) in March 2018. The main objective of the cruise was to contribute to the understanding of long-term changes and trends in physical and biogeochemical parameters, such as the anthropogenic carbon uptake and to further assess the hydrographical situation after the major climatological shifts in the eastern and western part of the basin, known as the Eastern and Western Mediterranean Transients. During the cruise, multidisciplinary measurements were conducted on a predominantly zonal section throughout the Mediterranean Sea, contributing to the global GO-SHIP repeating hydrography program and adhering to the GO-SHIP requirements.

No abstract

<p>The project OLCAPP was aimed at exploring the response of a natural, eutrophic system to ocean liming by slaked lime (calcium hydroxide) dispersal in the wake of ships. The main objectives were:</p> <p>- to monitor and model the slaked lime dissolution kinetics and the carbonate equilibrium (alkalinity, pH spikes/alteration, Dissolved Inorganic Carbon);</p> <p>- to assess Dissolved Organic Matter changes in quantity and composition;</p> <p>- to assess possible changes in primary production, photosynthetic efficiency and phytoplankton abundance and associations;</p> <p>- to assess the short and medium-term response of planktonic and benthic calcareous primary producers (calcareous red algae = maerl) to alkalinization and potential precipitation of carbonate crystals induced by the treatments.</p> <p>In the framework of the Transnational Access provided by AQUACOSM-plus, at the ECIMAT-UVIGO facility (Vigo, Spain) we had the opportunity to test ocean liming in nine mesocosm tanks. A sediment trap and a basket of calcareous algae (maerl) were positioned at the bottom of each tank. Mearl was previously collected in the Ria de Vigo at 7m depth and prepared for the experiment. Successively, each tank was filled with natural coastal seawater (~1m<sup>3</sup>).</p> <p>Out of the nine mesocosms, three tanks were treated with calcium hydroxide 0.02g/L (<em>High</em>) and three tanks with 0.006g/L (<em>Low</em>) per treatment, repeated on days 1, 3 and 5 (multiple exposure). The remaining three tanks were kept as control. A record of pH, O<sub>2</sub>, salinity, temperature and PAR was performed in the mesocosms during the experiment with a ten-minutes frequency.</p> <p>Nutrient concentration is monitored on a long-term basis in the Ria de Vigo, and was also tested on days 1, 3 and 5 for all treatments and controls. Seawater samples were collected from the mesocosms before (<em>pre-treatment</em>), and after 1h, 4h, and 24h from each treatment, with a 5L Niskin bottle. Dissolved Organic Matter as Dissolved Organic Carbon (DOC) and Chromophoric Dissolved Organic Matter (CDOM) were analysed, along with the Dissolved Inorganic Carbon (DIC). Moreover, samples collected after 1h from treatments were used for assessing also the bacterial association, the size-fractionated Chl-a concentration and the plankton primary production and photosynthetic efficiency. Gross primary production, community respiration and net community production were measured by changes in oxygen concentrations after 24 h light-dark bottle incubations. Dissolved oxygen was measured by Winkler titration. A total of 165 samples were obtained from filtering 2L of mesocosm water from pre-treatments, 4h and 24h samples, for the collection of the phytoplankton community, to be analysed under optical and scanning electron microscope.</p> <p>Preliminary observations during the experiment and the first data on the plankton community suggest that the <em>High </em>treatment leads to important flocculation and sedimentation affecting both the transparency of the water and the bottom environment, with significant and stable pH increase and decrease in phytoplankton production and efficiency. The mineralogical nature of the flocculation, the response of benthic calcareous algae and phytoplankton community, in term of composition and abundance of the major components, is here discussed.</p>