Catchment Partnerships What palaeoecology tells us about the — history of Poole Harbour © Lorraine© Igar Introduction July 2019. Final version This note provides a non-technical summary of the research carried out by Dr Laura Crossley for her PhD at Southampton University and is re-produced here with the kind approval of; Laura, her supervisors; Professors Peter Langdon, David Sear and John Dearing; and Wessex Water. The original PhD report can be obtained online (under licence) from; https://eprints.soton.ac.uk/429023/ Poole Harbour Poole Harbour estuary is one of the largest and have been made but based on infrequent samples or shallowest natural harbours in the world. It is fed by two over short timescales. So, Wessex Water co-funded a 3- major rivers, the Frome and Piddle, two smaller rivers, year PhD study in 2014 at the University of Southampton the Sherford and Corfe, and several small streams. The that used sediment cores and palaeoecology to; western side is largely rural, with the town of Poole Reconstruct the historic water quality and sediment draining to the eastern shore, and the main sewage changes within Poole Harbour. treatment works drain into Holes Bay, in the north of the harbour. The estuary connects to the English Channel via Determine where key ‘tipping points’ in water a narrow entrance, approximately 100 m wide (Figure 1). quality had occurred and why. The Harbour is protected for its European importance to This study would indicate what level of nutrient and birds, knownWho as awill Special deliver Protection projects? Area, and is an sediment accumulation offers a ‘safe operating space’ internationally recognised wetland, or Ramsar site, with for the protection and improvement of Poole Harbour saltmarsh fringing mudflats that are exposed at low tide. for wildlife and people. It supports a significant shellfishery, as well as being a key recreation area for boating and fishing. How the work was done Sediment records from estuary deposits can provide The rivers Frome and Piddle have been subject to valuable archives with which to reconstruct many key extensive research leading to the multi-partner ‘Strategy ecosystem variables over hundreds of years, including for Managing Nitrogen in Poole Harbour Catchment to nutrients, sediment loads and ecological responses of 2035’. This strategy was based on analysis of river water plants and animals. quality since the 1960s. Levels of nitrogen and Sediment cores, between 124 and 275 cm in length, phosphorus have risen from inputs from agriculture and were collected from four sites in 2015 and 2016 at the treated sewage. This study aimed to see whether the sites shown in Figure 1. Core sites from the channels of historic trends within the harbour itself reflect those freshwater studies. Figure 1. Survey map of Poole Harbour (core sample sites Background to the project are shown in red) It is known that Poole Harbour has been subject to eutrophication, or over enrichment by nutrients, since the 1980s when mats of green algae were seen to be spreading over its mud flats. The levels of nitrogen and phosphorus have been well documented by scientists in the freshwaters that supply it; the rivers Frome and Piddle, and the Sewage treatment works (STW) of the Poole catchment. However, data on the harbour waters is less reliable due to the changing conditions with each tide. Historic estimates of sediment input from the rivers Lorraine Isgar Catchment Partnerships (cont.) the Rivers Frome and Piddle were selected to reflect the major freshwater sources, whilst Holes Bay receives the treated sewage from Poole. Arne was selected as a control site, but is also influenced by other local freshwater sources (Corfe and Sherford rivers), the sea and indirectly the larger freshwater sites. The cores were preserved intact in cold storage to maintain their chronology, until the analyses were carried out for a range of parameters, along their lengths. The parameters were chosen to indicate one of three areas; • Chronology To determine the timeframe of the core through its length. Spheroidal Carbonaceous Particles and radionuclides can be related to known dates, such as the burning of fossil fuels in the industrial revolution or the release of caesium from nuclear weapons. Both of these techniques were applied to the sediment cores to obtain a more reliable age-depth model. • Sedimentology To determine the nature of the sediment throughout the core, its makeup of mineral and organic matter. These can indicate the source of sediment and nutrients. The ratio of carbon and nitrogen in the sediment also provides indications of algal activity. • Palaeoecology To detect the remains of plants and animals through their chemical pigments, such as Chlorophyll, and the silica shells of diatoms (microalgae). The four cores collected were dated back to between 1860 and 1880. From laboratory and statistical analysis, a picture of the past accumulation of sediment, water quality and algae was developed for each site and cross- referenced against historic local data on rainfall, sea level, temperature, river flows and records of population and agricultural practice. This study also compared the results, and sediment accretion rates, with three similar studies in Poole Harbour. What the results show Sediment accumulation rates Typically, estuaries accumulate sediment until the mudflats rise above the tidal range. The rates vary with sediment availability, sea level changes, erosion and urban construction. Four cores provide a very small representation of the whole harbour but sediment accumulation rates at the Arne control site lie within the range found in neighbouring south coast estuaries. All four showed a rise in sediment accumulation rates between 1940 and 1970s (Figure 2) but show differences in initial level and rates post 1970. This period of sediment change coincides with intensification of agriculture following the Agriculture Act 1947, government grants for drainage and the Common Agriculture Policy in 1962, as well as land reclamation in Holes Bay. The increasing calcium levels, changes in Figure 2. Graph of the different sediment accumulation rates for the organic matter and magnetic susceptibility (which reflects four cores sites (Frome- FRM4, Piddle-PID1, Holes Bay-HB1, Arne- erosion of top soils) in the cores at this time supports the ARNE5). Note the different scales for each site. The grey shading indicates the period of higher sediment accumulation rates ca. 1940s conclusion that agriculture intensified. -1970s. Lorraine Isgar Catchment Partnerships Sediment accumulation rates (cont.) Additional changes in magnetic susceptibility also appear in the Frome estuary core between 1910-1930 which may reflect an increased soil loss from the lower catchment, but no rise was evident in the core sediment accumulation rates until 1935. Other research suggests that the River Frome initially absorbed some of the fine sediments, narrowing its channel. There was a flooding incident (1909), increased ploughing and development of the Bovington army ranges that was offset afterwards by the new forestry plantations from the 1920s. Sediment accumulation rates within the harbour will also be affected by vegetation. The saltmarsh changed to Spartina (cordgrass) after 1885, which then fell into widespread decline in 1930s, releasing sediment. At Holes Bay, sediment input would have been affected by land erosion on the adjacent heathlands (after WWII) and import of chalk for reclamation of land for the new power station. This reclamation reduced the area available for deposition of the mobilised sediment load. The sediment quality also reflects changes in air and water quality (lead), antifouling (copper) and chalk levels. At the control site, Arne, a sediment rate rise was also influenced by sea level rises between 1945 and 1960, with associated salt and bromine rises. Although these are key, there is also evidence of the riverine sediment load increase, as seen from the River Frome core too. However, there is little sign though of the urban influence that was seen in the Holes Bay core. Post 1970 the rate of sediment accumulation rates declined, as heathland erosion and agricultural soil erosion stabilised. However, whilst the control site at Arne reduced to pre-1940s accumulation rates, the other three did not. The Piddle site alone shows a reduction followed by a further raised sediment accumulation rate from 2000, from fine grained sediment. Although this may be linked to high flows during 2000, it does not follow the usual flow peak pattern so either the flood disturbed previously deposited fine silt (from agriculture) that has continued to be released from the channel, or there may be some other local farming changes in this smaller catchment. Nutrient patterns These were determined by comparing the ratios of carbon and nitrogen isotopes, which reflect terrestrial and freshwater sources of productivity. The cores also show Silica, pigments and diatom shells from deposited algae, both micro and macro. The earliest record was from the Frome core which showed a shift from external production (e.g. from the catchment) to in-river productivity from 1840-1880 which will represent anthropogenic changes. The geochemical record shows that the nitrogen isotope 15N increased through the 1940-1970 period in Frome and Piddle Cores and continued afterwards, reflecting the time lag in nitrogen being released from groundwater after the farming practices had changed (as evidenced by sediment record above). Silica, usually from algae and land wash-off also increased from 1940 onwards so here, with the high N15 fraction, the source is probably increased agricultural nutrient inputs causing increased algae, rather than sewage derived nutrients despite the gradually rising rural population. In Holes Bay, early records pre-1900, show a relatively more marine than freshwater response. However, it shows increased terrestrial influence, or productivity, until 1920, after which a decline occurred. This may reflect the impact of the growing population before the new treatment plant was built in 1922 at Poole STW.