Investigating microplastic behaviour in a well-mixed estuary

<p>Rivers and estuaries act as conduits of microplastic transport, linking terrestrial and marine environments: however, it is unclear to what extent estuaries act as sources or sinks for marine plastic waste. In densely populated catchments, microplastic pollution could impact human populations and natural ecosystems including through industry, domestic activities or direct exposure. An investigation into the physical behaviour of microplastic within estuarine systems will allow for a greater understanding of plastic retention and exportation to coastal and offshore environments. A high resolution 3D model (Delft D-Flow FM) of the Conwy Estuary (UK) is under development, with current and future projections of microplastic concentrations used to determine local exposure levels, residence times and temporal variability.</p><p>The Conwy Estuary (UK) is a well-mixed macro-tidal, embayment type system connecting the Conwy catchment to the North Wales coast and Irish Sea – where waters are used for leisure and aquaculture. Microplastics derived from the catchment population, industry and agriculture are thought to flow into the estuary primarily from the Conwy river network. Because of this, this study will incorporate in-situ samples of microplastic concentrations in river water to be able to predict microplastic levels in the estuary with greater accuracy. Plastic dispersal simulations through particle tracking and water quality monitoring will be undertaken using known concentrations and future projections of microplastic.</p><p>The results of the model validation as well as application to plastic dispersal simulations will be presented.</p>

Existence and Stability of Morphodynamic Equilibria in Double Inlet Systems

<p>Tidal inlet systems, ubiquitous along sandy coasts, are very valuable areas in terms of ecology<br>(breeding and feeding areas), economy (gas–mining and dredging) and recreation, and important<br>for coastal safety. To properly manage these systems, good insight into their morphodynamic<br>behaviour is essential.<br>In this presentation, we focus on morphodynamic equilibria of so-called double inlet systems,<br>i.e., systems in which the tidal basin is connected to the open sea by two tidal inlets. In our model,<br>the water motion is described by the cross-sectionally averaged shallow water equations, and forced<br>by prescribed tidal elevations at both seaward sides. The sediment transport is modeled by an<br>advection–diffusion equation with source and sink terms, while the bed evolution is governed by the<br>convergences and divergences of sediment transports. The sediment transport consists of various<br>contributions, a diffusive contribution, a transport term related to the variations in topography<br>and an advective contribution (ter Brake and Schuttelaars, 2010).<br>To directly identify morphodynamic equilibria, we employ continuation methods and bifurcation<br>techniques. By systematically varing the amplitude φ<sub>M2</sub> at one of the inlets, while keeping all other<br>parameters fixed, a region in the φ<sub>M2</sub> parameter space is found where the bed level reaches the<br>water surface, resulting in two single inlet systems. Outside this region, morphodynamic equilibria<br>exist. These equilibria are characterized by their minimum water depth and location. There are<br>branches of stable equilibria, while there are also branches of unstable equilibria, coinciding with<br>the stable equilibria at so-called limit points. Varying both the amplitude and phase of the M2 tide<br>at one of the inlets while keeping the other parameters fixed, results in limit points in A<sub>M2</sub> − φ<sub>M2</sub><br>space that form an ellipse.<br>In our presentation, we will systematically discuss the number and stability of morphodynamic<br>equilibria and compare our results to observations in the Marsdiep-Vlie system, a double inlet<br>system in the Nothern Dutch Wadden Sea.<br>References<br>ter Brake, M. C. and Schuttelaars, H. M. (2010). Modeling equilibrium bed profiles of short tidal embayment. on<br>the effect of the vertical distribution of suspended sediment and the influence of the boundary conditions. Ocean<br>Dynamics, 60:183–204.</p>

Pushing the Limits of Seagrass Remote Sensing in the Turbid Waters of Elkhorn Slough, California

Remote sensing imagery has been successfully used to map seagrass in clear waters, but here we evaluate the advantages and limitations of different remote sensing techniques to detect eelgrass in the tidal embayment of Elkhorn Slough, CA. Pseudo true-color imagery from Google Earth and broadband satellite imagery from Sentinel-2 allowed for detection of the various beds, but retrievals particularly in the deeper Vierra bed proved unreliable over time due to variable image quality and environmental conditions. Calibrated water-leaving reflectance spectrum from airborne hyperspectral imagery at 1-m resolution from the Portable Remote Imaging SpectroMeter (PRISM) revealed the extent of both shallow and deep eelgrass beds using the HOPE semi-analytical inversion model. The model was able to reveal subtle differences in spectral shape, even when remote sensing reflectance over the Vierra bed was not visibly distinguishable. Empirical methods exploiting the red edge of reflectance to differentiate submerged vegetation only retrieved the extent of shallow alongshore beds. The HOPE model also accurately retrieved the water column absorption properties, chlorophyll-a, and bathymetry but underestimated the particulate backscattering and suspended matter when benthic reflectance was represented as a horizontal eelgrass leaf. More accurate water column backscattering could be achieved by the use of a darker bottom spectrum representing an eelgrass canopy. These results illustrate how high quality atmospherically-corrected hyperspectral imagery can be used to map eelgrass beds, even in regions prone to sediment resuspension, and to quantify bathymetry and water quality.

Successes of Restoration and Its Effect on the Fish Community in a Freshwater Tidal Embayment of the Potomac River, USA

After a local pollution control plant significantly reduced phosphorus loading into a phytoplankton-dominated tributary of the Potomac River in the early 1980’s, water quality and biological communities were monitored bi-weekly from April-September. After a 10-year time-lag, submerged aquatic vegetation (SAV), once abundant in this freshwater tidal embayment, returned to the area in 1993. After additional reductions in nitrogen load starting in 2000, the system completely switched to a SAV-dominated state in 2005. Fish abundance didn’t change during these distinct phase changes, but the fish community structure did. Increases in SAV provided refuge and additional spawning substrate for species with adhesive eggs such as Banded Killifish (Fundulus diaphanus), which is now the most abundant species in the embayment. Other changes seen were a decrease in the relative contribution of open water dwelling species such as White Perch (Morone americana), and an increase of visual predators such as Largemouth Bass (Micropterus salmoides). The 30-year record of data from this Potomac River tributary has revealed many important long-term trends that validate the effectiveness of initiatives to improve water treatment, and will aid in the continued management of the watershed and point source inputs.