Synthesis of the Upper Thurne Research and Recommendations for Management Report to the Broads Authority Holman IP and White SM July 2008 Department of Natural Resources Cranfield University Cranfield Bedfordshire MK43 0AL Telephone: +44 (0) 1234 750111 Ext. 2764 Fax: +44 (0) 1234 752970 Executive Summary The importance of the Upper Thurne (including Hickling Broad, Horsey Mere, Martham North and South Broads) for biodiversity is recognised under national and international conservation legislation. Appropriate water management (water resources, quality and flood defence) is fundamental to conservation in the broads. However, the incomplete understanding of the surface water system and their interactions with the wider catchment, particularly with respect to nutrient cycling, was recognised by the Appropriate Assessment Team (Broads Authority, 1999). The resulting workshop, held in Norwich in November 2001, developed a framework for a research and monitoring programme which intends to inform the ongoing activities which contribute towards the published 20 year Vision for the Upper Thurne water space. The ecology of the Upper Thurne broads has gone through a number of Phases in response to changing environmental conditions: Phase 1 – until the early 20th century – dominated by stoneworts (charophytes) and low- growing waterweeds, due to the low nutrient levels; Phase 2 – early to mid 20th century– luxuriant aquatic plant growth dominated by taller growing species better able to take advantage of enhanced nutrient levels; Phase 3 – mid 20th century to present – phytoplankton dominated system due to high nutrient and salinity levels. The Upper Thurne research has aimed to help understand how a range of hydrological, chemical and ecological factors contribute to achieving ‘clear water’ (Phase 1) conditions in Hickling Broad. The research has included catchment modelling of water and nutrient movement, groundwater modelling, hydrodynamic modelling of water and salt movement from the sea to the Upper Thurne, mesocosm (small experimental ponds) experiments of salinity effects, the use of remote sensing, lake sediment analyses, laboratory experiments of stonewort response to a range of factors including water temperature, cutting, establishment, pollutant concentrations. The report has 4 sections: 1. How the Upper Thurne water spaces have changed; 2. A description of the current status of the Upper Thurne waterways, and how these compare to the Favourable Condition criteria under the EC Habitats Directive; 3. A synthesis of the activities to identify the significant catchment water management issues, which is focussed around salinity and ochre, biocides and heavy metals within the sediment and water column, point and diffuse sources of nutrients (nitrate and phosphate) under current and future climate, sea level rise and coastal protection, monitoring and the population biology of charophytes; 4. Recommendations for management actions to address the significant issues previously identified and thereby achieve Favourable Conservation Status. The recommendations for management actions to achieve Favourable Conservation Status centre around: reducing salinity and ochre discharges from the land drainage pumps, principally the Brograve pump. An approach to identifying a solution is suggested based on principles of no significant change in current flood risk; compatibility with a range of farm systems; being consistent with existing agri-environment schemes; having a means of removing any seawater from a future coastal breach without discharging it through the Special Area of Conservation; and that there is recognition that the Brograve sub-catchment is a system that is ‘naturally’ brackish and which produces limited ochre; reducing diffuse source losses of nutrients from agriculture. Assuming that the farming community are following Codes of Good Agricultural Practice and Good Agricultural and Environment Condition requirements, a non-exhaustive list of practical measures to reduce nutrient losses from agricultural activities are suggested. i Key findings from the Upper Thurne studies The importance of land drainage to salinity management Much of the salinity in the Brograve drainage system enters via the coastal marshes, especially Hempstead marshes; Changes to the management of the land drainage systems have the potential to reduce the salinity entering the rivers and broads from the pumps e.g. raising water levels in the Hempstead Marshes by ~ 1 metre might lead to a 15% reduction in the amount of salt being discharged by drainage pumps; Such changes need to be considered in conjunction with flood risk and ‘knock-on’ effects to neighbouring drainage systems and the River Thurne. Sources and transport of nutrients from the catchment to the rivers and broads Total Phosphorous concentrations in the Upper Thurne broads regularly exceed the target limit for favourable conditions, but point sources appear to contribute little; Agricultural drains and water from the drainage pumps have the highest nitrogen (N) concentrations, but N concentrations in the broads are reduced through biological uptake or sedimentation; Increased rainfall and higher temperatures through climate change will increase nutrient (N and phosphorus (P) and sediment losses from the land; Erosion control measures, on susceptible soils and slopes, should be employed as part of good agricultural practice to reduce sediment and nutrient losses. The movement of water and salt within the rivers and broads Water and salt being discharged from the land drainage pumps form the main source of water and salt entering the River Thurne from its catchment; Constrictions within the river system, principally at Potter Heigham old bridge, where the narrow openings within the bridge impede both the downstream and upstream movement of water (depending upon tidal conditions); The role of the land drainage pumps changes with the tides – reducing salinities during extreme tides but increasing background salinity during normal tides. Environmental needs of the Stoneworts Increased water temperature leads to considerably higher growth rates and seed production of Stoneworts, suggesting that climate change may influence future growth patterns; While laboratory plant cutting experiments showed that cut stems had the ability to re-grow and branch, uprooting of Stoneworts, which is a risk of weed harvesting on the broad, leads to a high rate of plant mortality; Stoneworts are affected by a range of chemicals, particularly metals and boat antifouling paints, which have been found in sediment and water in Hickling Broad; Stoneworts in the broads are likely to be able to withstand a salinity increase of up to around 12.5 % of typical recent values for Hickling Broad, but such increases are likely to cause changes in community structure; Replicated small ‘pond’ experiments have shown that modest reductions in salinity, to around 1600–1800 mg Cl L-1 may have a substantial effect on Total Phosphorus and chlorophyll and hence on the potential for plant growth. Looking down from above (‘remote sensing’) ‘Remote sensing’ data can map the development of algal blooms in the broads, potentially providing early warnings for water users such as sailing clubs; It is shown to distinguish the presence of potentially toxic blue-green algae from other non- toxic species; The distribution and health of aquatic plants, both around the margins of the broads and, if the water is clear, those submerged beneath the waters surface can be mapped. ii Table of Contents Executive Summary ................................................................................................................i Key findings from the Upper Thurne studies...........................................................................ii Table of Contents.................................................................................................................. iii Introduction ........................................................................................................................... 1 How the Upper Thurne water spaces have changed ............................................................. 2 A description of the current status of the Upper Thurne waterways; ...................................... 4 Favourable Condition criteria and current condition........................................................... 4 Synthesis of the research activities to identify the significant catchment water management issues.................................................................................................................................... 4 Introduction........................................................................................................................ 4 Aquatic plant monitoring programme ................................................................................. 6 Salinity and ochre.............................................................................................................. 6 Introduction.................................................................................................................... 6 Ecological effects of the salinity ..................................................................................... 7 Causes of surface water salinity .................................................................................... 9 Solutions to the ochre and salinity problems.................................................................10 Hydrodynamic