Climate change is projected to increase the frequency and intensity of extreme rainfall events in the Mediterranean region, increasing runoffs of terrestrial matter into coastal waters. To evaluate the consequences of terrestrial runoff for plankton key processes, an in situ mesocosm experiment was conducted for 18 d in the spring of 2021 in the coastal Mediterranean Thau Lagoon. Terrestrial runoff was simulated in replicate mesocosms by adding soil from an adjacent oak forest that had matured in water from the main tributary river of the lagoon. Automated high-frequency monitoring of dissolved oxygen, chlorophyll a fluorescence, salinity, light, and temperature was combined with manual sampling of organic and inorganic nutrient pools, pH, carbonate chemistry, and maximum quantum yield (Fv:Fm) of photosystem II (PSII). High-frequency data were used to estimate the gross primary production (GPP) of oxygen, community respiration (CR), and phytoplankton growth (μ) and loss (L) rates. During the first half of the experiment (d2–d11), the simulated runoff reduced light availability (−52 %), chlorophyll a concentrations (−70 %), and phytoplankton growth rates (−53 %). However, phytoplankton maintained a certain level of primary production by increasing its photosynthetic efficiency. Meanwhile, the runoff enhanced CR (+53 %), shifting the metabolic status (GPP : CR) of the system toward heterotrophy and increasing the partial pressure of carbon dioxide (pCO2), potentially switching the direction of the air–sea CO2 exchange. However, during the second part of the experiment (d11–d17), remineralized nutrients boosted phytoplankton growth (+299 %) in the terrestrial runoff treatment but not its loss rates, leading to phytoplankton biomass accumulation and suggesting a mismatch between phytoplankton and its predators. Our study showed that a simulated terrestrial runoff significantly affected key plankton processes, suggesting that climate-change-related increases in runoff frequency and intensity can shift the metabolic balance of Mediterranean coastal lagoons towards heterotrophy.

The Mediterranean region is undergoing an increase in the frequency and intensity of extreme rainfall events, resulting in terrestrial runoffs that can affect aquatic environments in coastal regions. The goal of this study was to investigate the effects of terrestrial runoff on natural coastal planktonic assemblages. For this purpose, an in situ mesocosm experiment was conducted in May 2021 in the Mediterranean Thau Lagoon. A terrestrial runoff event was simulated in duplicate mesocosms by adding natural forest soil that was left to maturate naturally for two weeks in river water. After the addition of maturated soil, the abundance and diversity within the planktonic food web, from viruses to metazooplankton, were monitored for 18 days. The addition of maturated soil to the terrestrial runoff treatment greatly depressed the light availability in the mesocosms and potentially enhanced flocculation and sedimentation in the mesocosms, resulting in an immediate negative effect on phytoplankton, decreasing the chlorophyll- a (Chl- a ) concentration by 70% for 12 days. Afterward, remineralized nutrient in the terrestrial runoff treatment induced a subsequent positive effect on phytoplankton, which resulted in a diatom bloom and an increase in picophytoplankton and cyanobacteria abundance toward the end of the experiment. Overall, the Chl- a concentration was 30% lower in the terrestrial runoff treatment over the 18 days of experiment, whereas bacteria were 15% more abundant than in the control. This suggests that over the course of the experiment, the addition of maturated soil favoured bacteria instead of phytoplankton at the base of the planktonic food web. The addition of the maturated soil was detrimental for all protozooplankton groups and mixotrophic dinoflagellates, but seemed to favour metazooplankton, notably mollusk larvae, copepod nauplii, and rotifers. This implies that in the terrestrial runoff treatment, the preferential pathway for biomass transfer was through the direct consumption of bacteria and/or phytoplankton by metazooplankton. Therefore, in Thau Lagoon, after a terrestrial runoff, the transfer of biomass within the planktonic food web would potentially be more efficient by promoting direct transfer from the base to the top of the food web, subsiding intermediate trophic levels such as protozooplankton.

Characterizing the different chemical forms of trace elements in surface inland waters of mining regions is very important to understand the mechanisms underlying metal(loid) transport, their accumulation and toxicity, especially when waste storage areas are closely linked to the hydrographic network. This study focuses on the oldmining district of Salsigne in the Orbiel Valley (France), which was at one time the largest gold mine in Europe and the world’s largest arsenic mine. Our objective was to better understand the factors governing the transport of As and to investigate the role of colloidal matter as vector of major (Al, Ca, Fe, Mg) and trace (As, Pb, REE) elements in waters. A field sampling campaign was carried out during high water season to collect samples from the Orbiel River and its tributaries. In situ (ultra)filtrations at different cut-off thresholds (0.22 μm, 0.025 μm, 100 kDa and 3 kDa) were performed and physicochemical parameters (conductivity, pH, alkalinity, Dissolved Organic Carbon: DOC, anions, major cations, and metal(loid)s concentrations) of river water were assessed in all fractions. Results showed decreasing concentrations of DOC, Fe, Al and REE in the conventionally dissolved fraction (< 0.22 μm) from upstream to downstream. Arsenic concentration increased from upstream to downstream, especially at the exit of the old mining zone, and was mainly present in the truly dissolved fraction. However, arsenic in the colloid fraction was about 40% upstream and showed a clear decrease when going downstream. Upstream, Fe and Al were mostly present in the colloidal fraction (around 90%, but decreased to 30% downstream for Al). REE patterns suggested that colloidal organic matter played a significant role in metal transport. This was also inferred from the greater enrichment in middle REE as compared to light and heavy REE; this difference in enrichment progressively disappeared with decreasing cut-off filter size. The concentration variations along the length of the river were found to be a combined effect of the changes in lithology and input of contaminants from storage areas or alluvial table.