Ocean liming in eutrophic water: a mesocosm scale approach

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:

- to monitor and model the slaked lime dissolution kinetics and the carbonate equilibrium (alkalinity, pH spikes/alteration, Dissolved Inorganic Carbon);

- to assess Dissolved Organic Matter changes in quantity and composition;

- to assess possible changes in primary production, photosynthetic efficiency and phytoplankton abundance and associations;

- 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.

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 (~1m³).

Out of the nine mesocosms, three tanks were treated with calcium hydroxide 0.02g/L (High) and three tanks with 0.006g/L (Low) per treatment, repeated on days 1, 3 and 5 (multiple exposure). The remaining three tanks were kept as control. A record of pH, O₂, salinity, temperature and PAR was performed in the mesocosms during the experiment with a ten-minutes frequency.

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 (pre-treatment), 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.

Preliminary observations during the experiment and the first data on the plankton community suggest that the High 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.