Transfer of metals and contaminants through the water column and lower trophic levels: a mesocosm experiment

Essential trace metals (e.g., Fe, Co, Cu) are vital for phytoplankton metabolism and marine biogeochemical cycles, while toxic metals and emerging contaminants (e.g., Cd, Pb, Hg, Li, microplastics) pose ecological risks. Their bioavailability and transfer through marine food webs depend partly on their chemical form (speciation between dissolved, colloidal, particulate) and biological uptake by plankton. These processes – chemical speciation and trophic transfer -  can modify phytoplankton community structure and the entire marine food web with potential consequences for ecosystem functioning.

To investigate how metals and contaminants transfer through the water column and plankton, a mesocosm experiment conditions was carried out. Mesocosms with the natural phytoplanktonic assemblage from Villefranche Bay (Mediterranean Sea, France) and copepods collected from the same location were exposed to a gradient of metals, Li and UV-degraded microplastics, ranging from present-day concentration to levels representative of plausible future environmental scenarios.

This experiment was conducted in 9 x 300 L with a 1 m high water column allowing the development of export fluxes. Major nutrients were added in all mesocosm to insure the phytoplanktonic development before metals and contaminants additions of treatment. Treatments were as follow : 1 mesocosm control; 3 mesocosms with x1.5, x2 and x5 of natural metals concentrations (Fe, Mn, Zn, Co, Cd, Cu, Ni and Li); 2 mesocosms with addition of different concentrations of UV-degraded polypropylene (10 and 160 µg/L, size distribution centred at ~50 µm); and 3 mesocosms with different concentrations of both metals and microplastics.  

During the 17-day experiment biological parameters (nutrient concentrations, biovolume, particulate organic carbon and nitrogen, pigment concentration and cell abondance) were monitored throughout the experiment and first results indicate an initial exponential growth phase, followed by nutrient limitation and a transition toward heterotrophic conditions. Metals concentrations will be analysed in colloidal (3kDa – 0.22 µm), dissolved (< 0.22 µm) and particulate (> 0.22µm) fractions. The particulate fraction, included microphytoplankton, was rinsed with EDTA/oxalate to quantify intracellular metals. Zooplankton was sampled at the beginning and the end of the experiment to assess potential bioaccumulation. Exported material was collected daily to quantify and characterize export fluxes. These data will allow determination of partition coefficients and bioaccumulation. Daily photophysiological measurements - including the maximum quantum yield (Fv/Fm), absorption cross-section (SigmaPSII) of photosystem II, photosynthesis–irradiance curves and photoprotection capacity - will enable the detection of any potential adverse effects of the treatment on the physiological status of phytoplankton. 

This presentation will report on the experiment and present preliminary results on the partition coefficients and bioaccumulation of metals and contaminants, and will explore their potential impact on phytoplankton and zooplankton communities.