Since November of 2015, when ore tailings from a dam rupture reached the Atlantic Ocean, researchers are trying to assess the degree of impact across the Doce River and adjacent coastal area. This study aims to use the zooplankton dynamics as a tool to evaluate the environmental impact in the coastal region, five years after the rupture, during periods of low and high river flow. Doce River flow varied from 49 to 5179 m ³/s and structured the zooplankton community between periods of low and high river flow, but salinity and chlorophyll-a had stronger correlation with depth (r = 0.40 and − 0.40 respectively) than with the Doce River discharge variation along the sampling period (r < 0.2). On the other hand, inorganic particles in the water and total metal concentration (dissolved + particulate), used as tracers of the iron enriched tailing (Al, Cd, Cr, Cu, Fe, V), were correlated with fluvial discharge and showed to be the main factor driving the zooplankton community dynamics. For assessing the degree of environmental impact, we tested the ecological indexes for the zooplankton community. Margalef Richness, Pielou Evenness and Shannon-Wiener Diversity varied from 2.52, 0.40 and 1.39 (all registered during high river flow period) to 9.02, 0.85 and 3.44 (all registered during low river flow period), respectively. Along with those community indicators, we evaluated the response of representative taxonomical genera such as Paracalanus, Oikopleura and Temora, regarding the Doce River flow, and found population patterns that established a baseline for future monitoring in the region. Our results showed that the zooplankton community is more fragile when the river discharge is stronger, and this pattern is confirmed by all indicators tested.

In the past several decades, anthropogenic changes throughout drainage basins have threatened physical, hydrological and ecological river-to-ocean continuum balances. Dams are one of these anthropogenic influences that can seriously affect sediment fluxes to the ocean. Here, we investigate the formation and dispersion of a high-concentration mud suspension (HCMS) at a river mouth due to a tailing dam failure. In November 2015, the Fundão dam (Mariana-MG, Southeast Brazil) failure released approximately 39.2 million m ³ of tailings into the Doce River system. The slurry travelled 660 km down to the river mouth, reaching the Atlantic Ocean. Pre- and Post-dam failure surveys were used to investigate temporal and spatial changes in Suspended Particulate Matter (SPM) concentration, seabed sediment grain size and seabed sediment bulk density along the inner shelf. Results indicate that the combination of the tailing dam failure with an extremely dry seasonal condition led to the formation of a HCMS, altering, at least in a short term, the sedimentary processes along the inner shelf, leading to high sedimentation rates and low seabed sediment bulk density. Changes in particle size defined by an increase in clay content, a significant increase in SPM concentration in the inner shelf and a decrease in seabed sediment bulk density were observed as a result of the dam failure. SPM concentrations were higher near the seabed than at the surface, reaching up to 9000 mg/l. The highest SPM concentration measured during a major flood in January 2014 reached around 100 mg/l, The combination of a HCMS with tailings high clay content resulted in low values of seabed sediment bulk density and the formation of fluid mud along the inner shelf. In the first week after the tailings arrival, a 5 cm thick tailing fluid mud was observed. In this stage of bed formation, bed strength is lower, easing particle reentrainment by waves or currents. Thus, tailings could be advected, spreading the impact along the coast.