Aldridge, John

Edwards, Karen

Haverson, David

English

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Conference Paper, Published VersionHaverson, David; Aldridge, John; Edwards, KarenAssessment of nutrients and macroalgae growth in PooleHarbour, UKZur Verfügung gestellt in Kooperation mit/Provided in Cooperation with:TELEMAC-MASCARET Core GroupVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/104508Vorgeschlagene Zitierweise/Suggested citation:Haverson, David; Aldridge, John; Edwards, Karen (2017): Assessment of nutrients andmacroalgae growth in Poole Harbour, UK. In: Dorfmann, Clemens; Zenz, Gerald (Hg.):Proceedings of the XXIVth TELEMAC-MASCARET User Conference, 17 to 20 October2017, Graz University of Technology, Austria. Graz: Graz University of Technology. S.175-181.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.Assessment of nutrients and macroalgae growth in Poole Harbour, UK David Haverson, John Aldridge Centre for Environment, Fisheries & Aquaculture Science Pakefield Road, Lowestoft, NR33 0HT, UK  david.haverson@cefas.co.uk Karen Edwards Environment Agency Manley House, Kestrel Way, Exeter, EX2 7LQ, UK   Abstract— A high-resolution depth-averaged hydrodynamic model was developed for Poole Harbour, UK, with the aim to test water quality scenarios for reducing nutrient levels. These scenarios were developed from a separate Combined Macroalgae and Phytoplankton (CPM) model, a simple linked box model that can be used to calculate nutrient concentrations and the biomass of phytoplankton and macroalgal communities. For the CPM model to function, exchange rates between the different parts of the water body are required. The flushing rates of Poole Harbour are calculated from the hydrodynamic model. Furthermore, as there is uncertainty in what leads to the spatial distribution of macroalgae growth, the hydrodynamic model was used to investigate any links between environmental conditions, nutrient concentrations and macroalgae growth. I. INTRODUCTION Poole Harbour is a water body, located on the south coast of the UK, whereby high nutrient concentrations have led to large growth of macroalgae along the shorelines and mud flats. Large macroalgae mats can have negative environmental impacts by reducing the total area of mudflats available to wading birds [1], in addition to reducing dissolved oxygen leading to anoxia in benthic communities within the sediments and lead to nitrogen loading within Poole Harbour waters [2]. Investigations into the feasibility of the removal of macroalgae as a mitigation measure to reduce nutrients and improve water quality is ongoing [3]. Nutrients are fed into the harbour via farm run offs into a number of rivers, notably the River Frome and River Piddle, in addition to a number of outfalls from sewage treatment works at Wareham, Lytchett and Holes Bay. Historically, very high levels of nutrients were found within the harbour. Within the last few decades water quality controls were implemented reducing nutrient inputs from point sources. However, the trend of the amount of nutrients from ground water diffuse sources is increasing as historic run off and land use is still slowly working through the surrounding water table. As such, the aim of the study was test a range of water quality scenarios to investigate how nutrient levels could be further reduced. Furthermore, there is a degree of uncertainty as to what leads to the spatial distribution of macroalgae growth within Poole Harbour. Therefore, the secondary aim was to identify possible links between environmental conditions, nutrient concentrations and macroalgae growth.  II. HYDRODYNAMIC MODEL A.  Model Domain A high-resolution depth-averaged model of the Poole Harbour, UK, was built with an unstructured triangular mesh, using the hydrodynamic software Telemac-2D (v7p1). The model domain extends between 1.646°W – 2.239°W and 50.362°N – 50.737°N. The unstructured mesh was discretised with 76,448 nodes and 145,947 elements. Along the open boundary, the mesh has a resolution ranging between 200m to 5km, reducing to 100m along the coastline. Within Poole Harbour the resolution is further refined to 30m. Bathymetry of the outer domain was sourced from the Department for Environment, Food & Rural Affair’s UKSeaMap 2010 [4]. The resolution of the bathymetry points from this dataset are 1 arc-second (~30m). Within Poole Harbour, bathymetry was provided by Environment Agency with a resolution of 20m. The hydrodynamics are forced along the open boundaries using 11 tidal constituents (M2, S2, N2, K2, K1, O1, P1, Q1, M4, MS4 and MN4) from the OSU TPXO European Shelf 1/30° regional model. After a spin up period of 5 days, the model was run for 30 days to cover a full spring-neap cycle. Four fresh water inputs are included within the model domain, representing the Rivers Piddle, Frome, Sherford and Corfe. The model uses a k-ε turbulence model with velocity diffusivity set to 1×10–6 m2/s, representing the kinematic viscosity of water. In the absence of accurate wide spread sediment data, the Nikuradse law for bottom friction was used, with a constant value of roughness length, ks =0.04, applied to the whole model domain. B. Validation Validation data have been obtained from the British Oceanographic Data Centre (BODC) for surface elevation at the Bournemouth tide gauge, whose location is shown in Fig. 1. After a spin-up period, the model was run for 30 days from 19/05/2012 00:00 to 19/06/2012 00:00. Comparisons of the modelled free surface elevation and observed tidal elevations, at Bournemouth, is shown in Fig. 2.  To validate the free surface elevations, three statistical tests have been applied: the coefficient of determination, the root mean squared error (RMSE) and the scatter index. The scatter index is the RMSE normalised by the mean of the observations. It is widely used in the validation of wave models [5-7], meaning there is a wide source of literature for 17524th Telemac-Mascaret User Conference Graz, Austria, 17-20 October, 2017  1 and 4.5 mg/l, macroalgae was located in areas with concentrations over four standard deviations from the mean. Similar conclusions were found for a study of macroalgae growth in the Medway Estuary, UK, where bed shear stress was a controlling factor and nutrient supply a limited role [15].  Further work is required to assess potential indicators of macroalgae growth as it is clear there are other controlling factors. Whilst depth and mean velocity were good indicators, it does not explain why then it is not more prevalent in the Upper Wareham region and Lytchett Bay, which has extensive intertidal regions. Factors such as sediment composition, light penetration and grazing should be considered. Furthermore, there may be climatic variables, such as temperature and rain fall, that are indicators. These might explain why there is a 50% reduction in the total area of macroalgae growth from the 2008 and 2015 field observations.  V. SUMMARY & CONCLUSION A depth-average hydrodynamic model was developed for Poole Harbour, UK, to test water quality scenarios for reducing nutrient levels. The scenarios were developed from a Combined Macroalgae and Phytoplankton model, tuned by flushing rates calculated from the Telemac model. The hydrodynamic model was a useful tool in providing insight into the dominant sources of nutrients into the water body and most effective solutions for reduction nutrient loads.  Furthermore, the hydrodynamic model was used to identify model outputs could be used as indicators for spatial distribution of macroalgae growth. Mean water depth and velocity were shown to be good indicators. However, mean winter concentrations of nutrients were less important. Further work is required to identify other controlling factors as field observations suggest water depth and velocity are not the only indicators. ACKNOWLEDGEMENTS This work was carried out on the High-Performance Computing Cluster supported by the Research and Specialist Computing Support service at the University of East Anglia.  REFERENCES [1] Lewis, L.J. and Kelly, T.C., 2001. A short-term study of the effects of algal mats on the distribution and behavioural ecology of estuarine birds. Bird Study, 48(3), pp.354-360. [2] Jones, M. and Pinn, E., 2006. The impact of a macroalgal mat on benthic biodiversity in Poole Harbour. Marine pollution bulletin, 53(1), pp.63-71. [3] Taylor, D., 2015. Review of potential to remove harmful algae and reduce nitrogen load in Poole Harbour. Commissioned by Dorset Local Nature Partnership.  [4] McBreen, F. and Joint Nature Conservation Committee, 2011. UK SeaMap 2010: predictive mapping of seabed habitats in UK waters. [5] Cox, A.T. and Swail, V.R., 2001. A global wave hindcast over the period 1958-1997- Validation and climate assessment. Journal of Geophysical Research, 106(C2), pp.2313-2329. [6] Niclasen, B.A. and Simonsen, K., 2007. Validation of the ECMWF analysis wave data for the area around the Faroe Islands. Societas Scientiarum Færoensis. [7] van Nieuwkoop, J.C., Smith, H.C., Smith, G.H. and Johanning, L., 2013. Wave resource assessment along the Cornish coast (UK) from a 23-year hindcast dataset validated against buoy measurements. Renewable energy, 58, pp.1-14. [8] CSTT, 1994. Comprehensive studies for the purposes of Article 6 of DIR 91/271 EEC, the Urban Waste Water Treatment Directive. Published for the Comprehensive Studies Task Team of Group Coordinating Sea Disposal Monitoring by the Forth River Purification Board, Edinburgh. [9] CSTT, 1997. Comprehensive studies for the purposes of Article 6 & 8.5 of DIR 91/271 EEC, the Urban Waste Water Treatment Directive, second edition. Published for the Comprehensive Studies Task Team of Group Coordinating Sea Disposal Monitoring by the Department of the Environment for Northern Ireland, the Environment Agency, the OAERRE page 40 version of July 4, 2002 Scottish Environmental Protection Agency and the Water Services Association, Edinburgh. [10] Painting, S.J., Devlin, M.J., Parker, E.R., Malcolm, S.J., Mills, C., Mills, D.K. and Winpenny, K, 2003. Establishing Practical Measures for the Assessment of Eutrophication Risks and Impacts in Estuaries: Biological Response to Nutrient Inputs in different estuary types in England and Wales. CEFAS contract for the Environment Agency, Countryside Council for Wales and English Nature. [11] Painting, S.J. Devlin, M.J. Malcolm, S.J. Parker, E.R., Mills, D.K., Mills, C., Tett, P., Wither, A., Burt, J., Jones, R. and Winpenny, K., 2007. Assessing the impact of nutrient enrichment in estuaries: susceptibility to eutrophication. Marine Pollution Bulletin, 55(1-6): 74-90. [12] Cefas, 2003. Investigation of Factors Controlling the Presence of Macroalgae in some Estuaries of South East England. Cefas contract C1642 contract for the Environment Agency. [13] Aldridge, J.N., Trimmer, M., 2009.  Modelling the distribution and growth of ‘problem’ green seaweed in the Medway Estuary, UK. Hydrobiologia. 629: 107-122. [14] Hartnett, M., Dabrowski, T. and Olbert, A.I., 2011. A new formula to calculate residence times of tidal waterbodies. Water Management, 164, pp.243-256. [15] Aldridge, J.N. and Trimmer, M., 2009. Modelling the distribution and growth of ‘problem’green seaweed in the Medway estuary, UK. Hydrobiologia, 629(1), pp.107-122                              181

Assessment of nutrients and macroalgae growth in Poole Harbour, UK

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Assessment of nutrients and macroalgae growth in Poole Harbour, UK

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