A Quiet Tide, a Turbulent Mouth: How a Brazilian Lagoon Matches the Mixing Power of the World’s Fiercest Estuaries

Along the coast of southern Brazil, one of the world’s largest choked coastal lagoons squeezes its freshwater through a narrow, jetty-bound channel and fires it into the South Atlantic. On paper, the Patos Lagoon should be a hydrodynamic backwater: its astronomical tide rises and falls by a mere 0.4 meters, a regime oceanographers call microtidal, and tides account for less than 20 percent of the current variance in its outlet. Yet a new field study published in Ocean Science reveals that during a major flood, this supposedly gentle system generates turbulence so intense that it rivals, and in places exceeds, the mixing measured in some of the most tide-dominated estuaries on Earth. The finding challenges a long-standing assumption that vigorous estuarine mixing requires vigorous tides, and it points instead to a different engine: raw river momentum forced through a geometric bottleneck.

The research, led by Débora Barros of the Federal University of Rio Grande, together with Lauren Ross of the University of Maine and corresponding author Carlos A. F. Schettini, set out to fill a conspicuous gap in coastal oceanography. Most of what scientists know about turbulence in river plumes comes from mesotidal and macrotidal systems such as the Fraser, Merrimack, Gironde, Changjiang, and Hudson, where semidiurnal tidal reversals dominate the flow. Microtidal estuaries, where circulation is governed by river discharge and wind rather than tides, have been far less studied, and direct turbulence measurements within their plumes are notoriously difficult to obtain. Moreover, previous observations have tended to focus either on the estuarine channel or on isolated patches of the plume, rarely tracing turbulence continuously from the river’s source to the open shelf.

The team seized on a unique advantage of microtidal environments. Because inflow and outflow events can persist for days under steady meteorological forcing, rather than flipping with each tidal cycle, the estuary–plume system can approach a quasi-steady state. That stability allowed the researchers to treat a single five-hour survey as a coherent snapshot of the entire estuary-to-shelf transition, isolating the effects of discharge, wind, and channel shape from the confounding rhythm of the tide. The survey took place on 18 July 2022, during a period of sustained high discharge, and was timed to coincide with a clear-sky Sentinel-2 satellite overpass, permitting a direct match between in-situ measurements and the turbid surface signature of the plume.

The numbers behind the campaign are striking. Gauged river inflow stood at roughly 6,900 cubic meters per second, well above the July average, and the total flow measured across the channel at the lagoon mouth reached about 9,400 cubic meters per second, corresponding to a freshwater discharge of approximately 7,650 cubic meters per second. Currents flowed seaward through the entire water column without reversal, from about 0.8 meters per second near the bed to 2.2 meters per second at the surface. The fieldwork was organized into three segments: a Lagrangian drift along the channel thalweg, during which the vessel moved passively with the outflow while 108 microstructure profiles were collected over roughly 13 kilometers; a set of 17 baseline profiles on the inner shelf beyond the jetties; and a 26-profile cross-channel transect used to quantify the total outflow.

At the heart of the instrumentation was a Rockland Scientific MicroCTD, deployed 152 times in downward-profiling mode at a controlled descent of about 0.8 meters per second. Its orthogonally mounted shear probes, sampling at 512 hertz, recorded the fine-scale velocity fluctuations from which the turbulent kinetic energy dissipation rate, epsilon, is calculated under the assumption of isotropic turbulence. Concurrent velocity profiles came from a vessel-mounted 1,200-kilohertz acoustic Doppler current profiler operating in bottom-tracking mode, while salinity and temperature were captured by integrated conductivity–temperature sensors. Rigorous quality control, including vibration correction, comparison of the two independent shear probes, and rejection of profiles exceeding operational thresholds, discarded fewer than 1.3 percent of the casts.

The measurements revealed a dramatic spatial architecture of mixing. Dissipation rates spanned four orders of magnitude, from 10^-7 to 10^-3 watts per kilogram, with the highest values concentrated immediately seaward of the mouth, in the zone where the buoyant plume lifts off from the confined channel flow. This peak coincided with the plume’s lift-off region, a feature familiar from theoretical models of near-field river plumes, and decayed progressively with distance offshore as the jet spread laterally and lost momentum. In contrast, the quietest waters lay in the mid-channel between kilometers 5 and 7, where the cross-section widens and where secondary circulation patterns were weak, suggesting that both geometry and lateral flow structure modulate turbulence production along the channel.

To interpret these patterns, the team deployed a suite of dimensionless diagnostics. The internal Froude number, the ratio of inertial to buoyant forces, indicated a transition from subcritical flow in the channel to supercritical flow in the plume lift-off zone, followed by a return to subcritical conditions roughly two kilometers offshore, where hydraulic adjustment finally allowed the plume to decelerate and thicken. The buoyancy Reynolds number, which compares the Ozmidov and Kolmogorov turbulence length scales, showed that over most of the water column, particularly near the mouth, values exceeded 200, meaning stratification could not suppress the turbulence. Only in thin layers along the pycnocline did values fall below 15, marking pockets where density stratification fully killed the turbulent fluctuations.

One of the study’s most provocative results concerns the gradient Richardson number, the classical stability metric that flags shear flows as unstable below a value of 0.25. Near the mouth and along the offshore density interface, the observations recorded Richardson numbers above this threshold, which in textbook terms should imply a stable barrier to mixing. Yet the dissipation rates and buoyancy Reynolds numbers told a different story: turbulence remained vigorous enough to erode the interface dynamically. The authors note that recent work has emphasized the classical criterion applies strictly only when evaluated over sufficiently small vertical scales, and that in energetic natural flows, mixing can persist at Richardson numbers approaching unity. The buoyancy Reynolds number, they argue, offers a more robust description of turbulent efficiency in such high-energy plumes.

Placed in global context, the findings are remarkable. Peak dissipation rates near the Patos Lagoon mouth match or exceed those reported for the Gironde, Merrimack, Changjiang, Fraser, and Hudson systems, and surpass by two orders of magnitude the sill-driven turbulence of Patagonian fjords. Vertical eddy viscosity reached about 10^-1 square meters per second at the plume interface, an order of magnitude above previous measurements in the same inlet, and varied across six orders of magnitude in less than 14 kilometers, a heterogeneity that cautions against using any single mixing parameter in estuarine models. The authors conclude that the essential difference between microtidal and tide-dominated estuaries may lie not in the intensity of mixing but in what drives it: here, sustained river discharge, persistent wind-modulated outflow, and the jetty-constricted mouth acting as a morphological nozzle. Just one day after this survey, a companion study documented a salt-wedge state with suppressed turbulence in the same channel, underscoring how rapidly these systems can flip between regimes. For coastal scientists, the message is clear: quiet tides do not guarantee quiet seas, and the geometry of an outlet can turn a gentle lagoon into one of the ocean’s most energetic mixing machines.

Subject of Research: Turbulence and mixing processes along the estuary–shelf transition of the microtidal, stratified Patos Lagoon outlet in southern Brazil

Article Title: Turbulence and mixing along a microtidal and stratified estuary-shelf transition

Article References: Barros, D., Ross, L., & Schettini, C. A. F. (2026). Turbulence and mixing along a microtidal and stratified estuary-shelf transition. Ocean Science, 22(5), 3121-3144. https://doi.org/10.5194/os-22-3121-2026

Image Credits: AI Generated

DOI: 10.5194/os-22-3121-2026

Keywords: estuarine turbulence, river plume, Patos Lagoon, microtidal estuary, turbulent kinetic energy dissipation, buoyancy Reynolds number, internal Froude number, stratification, coastal mixing, oceanography, Brazil, estuary-shelf transition