Lauren Ross

Circulation and exchange are important factors governing material transport in all coastal areas. One such area is Northern Chilean Patagonia, a region that boasts one of the largest aquaculture industries (salmon and mussel farming) in the world and provides habitat for several protected marine mammal species. Linkages between coastline shape and forces of hydraulic conditions are thereby important considerations when understanding constituent concentrations related to water quality issues such as harmful algal blooms (HABs), which threaten aquaculture activities and endanger wildlife and humans. The research presented here characterizes circulation patterns and variability in the Guafo Mouth, the primary connection between the Pacific Ocean and the fjords and channels of Northern Chilean Patagonia, to help inform ocean-fjord circulation in this ecologically and economically important area. In situ measurements of current velocities, salinity, temperature, dissolved oxygen, and nutrients have been collected for over two years at point measurements in the channel. These data, along with companion numerical model output and reanalysis data, provide parallel evidence of subtidal flows driven by a combination of barotropic and baroclinic pressure gradients and Coriolis, indicating a geostrophic balance. Depending on the sign of the barotropic pressure gradient, the barotropic flow either augmented (when positive) or opposed (when negative) the gravitational circulation produced by horizontal gradients in density and, in certain cases, produced subtidal outflow throughout the water column. Temporal variability of the current velocities was driven by changes in sea level gradients at ~35 d and ~50 d, linked to coastal trapped waves forced by the Madden Julian Oscillation. This variability has implications for the ocean-fjord exchange of dissolved oxygen and nutrients, which are critical to the health of the Chilean Inland Seas. Near-bottom waters with high nutrient content and low dissolved oxygen are advected in-channel when the crest of a coastal trapped wave is at the coast of Northern Patagonia due to enhanced subtidal inflows. These results highlight the importance of understanding the periodicity and amplitude of remotely forced coastal trapped waves to estimate ocean-fjord exchange through the gateway to northern Patagonia.

The exchange flow in estuaries is typically characterized by an inflow of salty water near the bottom and a surface outflow of brackish water, resulting from the mixing of river and ocean water. The exchange flow has been found to be a major driver of residence times and water quality conditions in estuarine systems and has also been found to influence biogeochemical processes such as hypoxia, nutrient fluxes, and the transport of contaminants. This study focuses on the spatial and temporal variability of exchange flow in a highly turbid and tidally-driven estuary located on the southwest coast of France, The Gironde. Stratification in the Gironde varies from well-mixed to partially stratified during low to high river flow conditions, respectively. Using validated numerical simulations, the exchange flow was calculated using the Total Exchange Flow (TEF) methodology under various river regimes. A mixing ratio was also quantified to assess the relative contribution of shear production to the salinity variance dissipation. Results show that the exchange flow in the Gironde is characterized by up to four salinity layers at the mouth reducing to two layers upstream. In addition, TEF outflow transport (Qout) increases slightly with river discharge, while the most substantial variability in Qout is due to along-channel variability in bathymetry and estuary width. The mixing ratio indicates that salinity variance dissipation was more influenced by the shear production than buoyancy near both the mouth and head of the estuary during spring tide. Towards the middle of the estuary, the ratio shows a weak contribution from the shear production whereas the mixing of salinity was high. During neap tide, the contribution of shear production to the salinity variance was elevated only downstream and mixing of salinity dominated. The next steps of this work will be to assess the impact of sediments on TEF and to compare the magnitude of the exchange flow in the Gironde to other systems globally.

Over the recent decades, an apparent worldwide rise in Harmful Algae Blooms (HABs) has been observed due to the growing exploitation of the coastal environment, the exponential growth of monitoring programs, and growing global maritime transport. HAB species like Alexandrium catenella—responsible for paralytic shellfish poisoning (PSP)—Protoceratium reticulatum, and Lingulaulax polyedra (yessotoxin producers) are a major public concern due to their negative socioeconomic impacts. The significant northward geographical expansion of A. catenella into more oceanic-influenced waters from the fjords where it is usually observed needs to be studied. Currently, their northern boundary reaches the 36°S in the Biobio region where sparse vegetative cells were recently observed in the water column. Here, we describe the environment of the Biobio submarine canyon using sediment and water column variables and propose how toxic resting cyst abundance and excystment are coupled with deep-water turbulence (10⁻⁷ Watt/kg) and intense diapycnal eddy diffusivity (10⁻⁴ m² s⁻¹) processes, which could trigger a mono or multi-specific harmful event. The presence of resting cysts may not constitute an imminent risk, with these resting cysts being subject to resuspension processes, but may represent a potent indicator of the adaptation of HAB species to new environments like the anoxic Biobio canyon.

Tidal water levels and currents in the shallow water environments of the inner shelf are subject to nonlinear interactions that produce higher frequency harmonics known as overtides, which contribute to the residual transport of scalars. The primary hydrodynamic overtide generating mechanisms (advection, nonlinear continuity and bottom friction) are well known. Yet, other mechanisms may exist in coupled systems characterized by wind waves and barotropic tides. This work uses process‐oriented coupled tides‐waves numerical simulations of an idealized barrier beach domain to study the effect of varying tidal range and wave conditions (height, period, direction) to investigate new mechanisms of overtide generation. Harmonic analysis of results from 92 simulations indicates that an increase in wave height amplifies overtide current velocities in both the alongshore and cross‐shore directions, while changes in wave period and wave direction have a lesser effect. A maximum increase of ∼140% in overtide current velocity magnitudes was observed in domain‐wide averages from a tides only to a coupled tides‐waves simulation. Decomposition of the depth‐averaged momentum balance of the tides only and tides‐waves simulations identify bottom stress, pressure gradient, and bottom wave streaming enhancing overtide generation. Accelerations from wave‐enhanced pressure gradients and bottom wave streaming produce a shoreward bottom current and an offshore directed return flow that persist throughout a semidiurnal tidal cycle, interacting with tidal currents to enhance asymmetries between flood and ebb.

The applicability of the wind wave suppression model developed by Chen and Belcher (2000) to irregular wave environments is investigated in this study. Monochromatic and irregular wave environments were simulated in the W² (Wind/Wave) laboratory at the University of Maine under varying wind speeds. The Chen and Belcher (2000) model accurately predicts the reduction of the energy density of the wind waves in the presence of the monochromatic waves as a function of wave steepness, but under predicts this energy dissipation for the irregular waves. This is due to the consideration of a single wave frequency in the estimation of the growth rate and wave-induced stress of the monochromatic waves. The same formulations for the growth rate and wave-induced stress cannot be applied to irregular waves because their spectra contain energy over a wide range of frequencies. A revised version of the model is proposed to account for the energy contained within multiple wave frequencies from the power spectra for the mechanically generated irregular waves. The revised model shows improved results when applied to irregular wave environments.