Influence of hydrodynamics on controlling algal growth in the Ythan estuary, Scotland

[摘要] The estuary of the River Ythan, North-East Scotland, UK, is a Site of Special Scientific Interest (SSSI) located in Forvie National Nature Reserve (NNR) and listed under the Ramsar Convention on Wetlands of International Importance. This site has long been affected by eutrophication despite designation as a nitrate vulnerable zone (NVZ) under the EU Nitrates Directive (ND) in 2000. Annual proliferation of macroalgal mats in the estuary results in decreases in the number of the invertebrates, major food sources for terns, waders and wildfowl, which causes a serious ecological problem in the estuary. It is critical to understand water circulation in the estuary in response to river flows and tides, to assess the role of hydrodynamics on controlling nutrient movement and the distribution and density of annual macroalgal blooms. This understanding will contribute a deeper knowledge of estuarine eutrophication for use in developing management strategies and mitigation measures. This study aims to understand the role of hydrodynamics on controlling annual macroalgal mat formation in the Ythan estuary by simulating water circulation, nutrient concentrations and pathways in the estuary using the Delft3D model. In addition, the study aims to evaluate potential of using remote sensing data for aiding model calibration and validation. The results from the study reveal that hydrodynamics play a key role in controlling macroalgal growth in the estuary. Interaction between seasonal river flows and tides not only helps to create optimal water quality conditions which facilitate algal growth, but also influences nutrient movements across the estuary resulting in high nutrient availability in particular areas. In addition to hydrodynamics, tributary inflows and the timing of nutrient release from these tributaries impact nutrient concentrations and transport directions in the areas surrounding their input to the estuary. Areas of high nutrient availability coupled with low flow velocities (lower than 0.06 m/s) particularly on mudflats along the main channel, driven by interactions of river flow, tides and bed characteristics, have long durations of nutrient enrichment, and so much of the Ythan estuary is prone to eutrophication. As hydrodynamics along with the timing of nutrient inputs play key roles in controlling macroalgal growth, strategies and mitigation measures to modify hydrodynamics to prevent long residence time are required, as well as guidelines on the timing of release of wastewater from tributaries. The study also demonstrates advantages of using remote sensing in conjunction with modelling studies. Remote sensing data are very valuable for model calibration and validation as well as for estuarine ecological management to study impacts of eutrophication and other disturbances.

[发布机构] University:University of Glasgow;Department:School of Geographical and Earth Sciences

[关键词] Eutrophication, River Ythan estuary, algal mats, Delft3D hydraulic model, hydrodynamics, remote sensing.