Climate change and human activities impact the volume and timing of freshwater input to estuaries. These modifications in fluvial discharges are expected to influence estuarine suspended sediment dynamics, and in particular the turbidity maximum zone (TMZ). Located in southwest France, the Gironde fluvial-estuarine system has an ideal context to address this issue. It is characterized by a very pronounced TMZ, a decrease in mean annual runoff in the last decade, and it is quite unique in having a long-term and high-frequency monitoring of turbidity. The effect of tide and river flow on turbidity in the fluvial estuary is detailed, focusing on dynamics related to changes in hydrological conditions (river floods, periods of low discharge, interannual changes). Turbidity shows hysteresis loops at different timescales: during river floods and over the transitional period between the installation and expulsion of the TMZ. These hysteresis patterns, that reveal the origin of sediment, locally resuspended or transported from the watershed, may be a tool to evaluate the presence of remained mud. Statistics on turbidity data bound the range of river flow that promotes the upstream migration of TMZ in the fluvial stations. Whereas the duration of the low discharge period mainly determines the TMZ persistence, the freshwater volume during high discharge periods explains the TMZ concentration at the following dry period. The evolution of these two hydrological indicators of TMZ persistence and turbidity level since 1960 confirms the effect of discharge decrease on the intensification of the TMZ in tidal rivers; both provide a tool to evaluate future scenarios.

Understanding nonstationary tides in tidal rivers is a major contemporary challenge. In particular, the response of river tides to natural developments in the estuary remains poorly investigated. This study analyzes the evolution of tidal characteristics over the last six decades in the Garonne Tidal River (GTR, SW France), in order to explore the effect of natural and human‐induced morphological and hydrological changes on river tides. The tidal Garonne is an excellent example, as it has been subject to decreasing river discharges, natural morphological changes, and gravel extraction. Tidal range (TR), distortion (A_M4/A_M2), and asymmetry direction (2ϕ_M2‐ϕ_M4) were calculated at four locations from the water level time series of 1953, 1971, 1982, 1994, 2005, and 2014. The annual time series of M₂ and M₄ amplitudes and phases were obtained through complex demodulation. Results reveal that both TR and 2ϕ_M2‐ϕ_M4 have increased since the 1950s. River flow modulates TR and A_M4/A_M2 significantly. A long‐term decrease in summer discharges from 200 ± 50 to 100 ± 50 m³ s⁻¹ would increase TR by +11.5% in the upper GTR. Natural morphological changes amplified TR and 2ϕ_M2‐ϕ_M4 by up to +12–15% between 1953 and 2014. TR and 2ϕ_M2‐ϕ_M4 doubled in the regions affected by gravel extraction between 1953 and 1971. Further, the persistence of mobile mud in the GTR increased TR seasonally but also interannually (by up to +16% in winter and spring of dry years). The potential impact of these changes on suspended sediments is discussed, revealing complex feedback between the evolution of hydrology, morphology, tides, and sediment trapping.

Understanding and estimating the distribution and transport of microplastics in marine environments has been recognized as a major global research issue. Most of the existing research on transport modelling has focused on low-density particles floating in surface waters, using a 2-D Lagrangian approach and ignoring the vertical displacement of particles. In this work, we evaluate to what extent the vertical movement of particles within surface waters by mixing processes may affect the horizontal transport and fate of microplastics. The aim is to determine whether a 2-D approach is sufficient for the accurate modelling of neustic-microplastics transport or whether a 3-D approach is necessary. For this purpose, we compare visually and statistically the microplastics transport patterns of three simulations in a coastal system: one using a 2-D approach; and two using a 3-D approach with weak and strong vertical turbulence, respectively. The 2-D simulation roughly reproduced the transport and accumulation patterns, but accurate results required a 3-D approach. This was particularly important for strong vertical turbulence and regions characterized by strong vertical current shear. Moreover, a 2-D approach can lead to errors in the results even with negligible turbulence due to simplifications in the velocity field. A 3-D modelling approach is therefore key to an accurate estimation and prediction of microplastics distribution in coastal systems and consequently for planning mitigation and cleaning programmes.