Intermittent, Double Estuarine Turbidity Maximum in a Mesotidal Estuary with Low Freshwater Input

The Estuarine Turbidity Maximum (ETM) is a biogeochemical hotspot in estuaries that regulates ecosystem health by amplifying estuarine water filtering. Convergence of elevated turbidity in an ETM is due to the intersection of repeated sediment resuspension and physical characteristics of a system. ETM formation depends on various tidal and freshwater conditions, making its ecological impact difficult to predict. Understanding the mechanisms of ETM formation has been predominantly studied in estuaries with strong tides or salt wedges. We examine how ETMs behave in a mesotidal estuary where the salt wedge is weak due to low freshwater input. Transect surveys and long-term deployments measuring salinity, suspended sediment, and currents were conducted over two summers (2024/2025) and one winter (2024) to capture spatial, tidal, and seasonal variability in ETM formation. The data revealed, for the first time, the formation of two tidally induced, intermittent ETMs. Both ETMs formed only at mid-tide, as neither gravitational circulation, tidal pumping, nor topographic trapping were strong enough to sustain cyclic/tidal sediment resuspension. The upstream ETM formed at the salt-wedge, extending further seaward in winter, whereas the downstream ETM formed due to tidal pumping, possibly assisted by topographic trapping during ebb tide. The co-occurrence of two intermittent ETMs suggests that estuaries host multiple, temporal biogeochemical hotspots. This could enhance nutrient filtering capacity but may also increase material trapping and amplify eutrophication risks. This highlights that estuarine filtering capacity may be more variable than previously assumed, with implications for water quality management within estuaries and the connected marine and freshwater systems.