Response of Chalk lake sediments and total Phosphorous records to External Forcing.

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1 Response of Chalk lake sediments and total Phosphorous records to External Forcing. Charlie Cuthbertson David Sear Pete Langdon, Thierry Fonville, John Dearing, Cath Langdon, Rob Scaife 24 th November 2017

2 Theory Ecosystem changes are driven by interactions between FAST and SLOW Drivers. These may be External Drivers (climate) or Internal Drivers (succession). The interaction of drivers and lake responses are often Complex. To understand these systems requires long term datasets (slow processes) and simultaneous sampling of ecosystems & drivers. Typical monitoring data is simply insufficient

3 Understanding complexity means defining long term trends Increasing pressure Decreasing pressure past future Batterbee, 2005

4 How do (Lake) ecosystems respond to change? No Threshold Change Step Change Threshold, Catastrophic bifurcation (Scheffer et al Nature)

5 What do changes in lake ecosystems look like? Water quality in Huangmei crossed threshold in ~ Water quality (sediment DI-TP mg/l) Variability of system increasing since 1960s Rising variance gives early warning signal ~30 years before threshold Dearing et al., 2010

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7 Techniques we can use to unlock the story in the sediments Biological proxies Chironomids & diatoms (insects & algae)

8 Techniques we can use to unlock the story in the sediments Biological proxies Sedimentological proxies (colour/grainsize)

9 Techniques we can use to unlock the story in the sediments Biological proxies Sedimentological proxies (Geo)chemical proxies Landnám Sn-1

10 Applying complex systems approaches to chalk headwater catchments. Using Lake sediment archives to reconstruct long term biophysical changes Using Historical data to reconstruct long term changes in drivers Using Complex systems approaches to identify thresholds, early warning signals, and Environmental canaries indicators that could be monitored in other systems.

11 Two shallow chalk lakes in S- England Alresford Pond - Historic and current watercress production (both input streams) - Studied extensively at Southampton - 2 PhD s - 3 Dissertations since 2007 The Grange - Estate - Currently no watercress production - Current dairy production land use

12 Historical change: Alresford Pond Date Event Possible consequence Source 1189 Construction of pond Dried up marshy area to West of pond Improved water flow for mills downstream ACC, 2008; Snow, 1996 Supply of fish 1700s Agricultural Revolution More meat available in winter, less reliant upon fish Snow, Little Ice Age Down-turn in agriculture Bell and Walker, Land Enclosure Increased erosion Tavener et al., s Embankment raised Disturbance in lake Environment Agency, Depression Decrease in productivity Tavener et al., Industrial revolution Increase in population and agricultural intensification Rose and Turner, 2005; Cohen, Construction of Watercress Naturally growing watercress commercially exploited ACC, 2008; Environment Railway Increase in watercress acreage Agency, Economic depression Fluctuations in arable production and growth in dairy. Tavener et al., Depression Decrease in production Tavener et al., s Downland of Hampshire converted from sheep to wheat Increased erosion Tavener et al., 1964 farming World War I Impacts upon agriculture? 1925 Code of Conduct Aim to reduce contamination entering stream feeding. Old Alresford Pond Use of modern cultivation methods ACC, 2008 Prior to 1939 Cheaper corn and meat Hit chalk farmers: shift to milk production may have increased HWAEC, 1945 production in New World soil erosion World War II Change in agriculture? Barley acreage increased Aided by fertiliser, may have entered water system Tavener et al., Dig for victory Increased erosion Boyle and Mayers, Flying fortress crashed east of Old Alresford Pond May show up in sediment record ACC, 2008 Post 1945 Increased mechanisation and Potential for an increase in soil and nutrients washed off fertiliser use catchment Tavener et al., 1964 Post 1945 Acute agricultural depression Reduction in erosion? HWAEC, Increase in fertiliser use Increased nutrient loading in pond Tavener et al., Drought in southern England Lake reported to have dried out Environment Agency, s Installation silt traps Reduced sedimentation rate in pond Environment Agency, Raised water level reported in lake May have affected secchi depth and therefore macrophytes Environment Agency, 2007

13 Alresford Pond historic maps 1870s 1890s s 1970s 1990s

14 Alresford Pond: increase in Watercress beds c

15 The Grange: Change in woodland 1870s 1890s 1910s 1960s 1970s

16 Standardised data (z-scores) Drivers of change in Itchen Headwaters Population Sheep Cattle Tot Cereals Temperature Rain Watercress beds Increased Fertiliser use on landscape Year W.Cress Ind. Switch to fertiliser & Increased demand

17 2017 Increased Rainfall Warming Cold Winters and Increased Rainfall Cold Winters 1850

18 Standardised data (z-scores) Drivers of change in Itchen Headwaters Population Sheep Cattle Tot Cereals Temperature Rain Watercress beds Increased Fertiliser use on landscape Land use Change = FAST Process Year Population / Climate = SLOW Process W.Cress Ind. Switch to fertiliser & Increased demand

19 Organic Matter Source Apportionment: Arle Catchment 2013 Watercress important 100% 90% 80% 70% 60% 50% 40% Fish Farms Septic Tanks Watercress Farms Instream Vege Road Verges FarmYard Slurries 30% 20% 10% 0% Mar Apr May Jul Aug Sep Oct Nov Dec c.40% Contribution from Watercress Farms to OM in streams. Zhang et al (2017)

20 ARL1 ALR1 ALR13 Core MagSus LOI Chiro Diatom SCP Particle Size ALR cm 0-328cm 0-306cm cm ALR cm cm GRA cm cm cm GRA13 Alresford Pond The Grange

21 Developing Transfer Functions for Total Phosphorous Lake

22 Measured TP Arlesford Pond The reconstructed TP of around 120 µg/l in the upper sediments is similar to summer-autumn TP in ,

23 Ecosystem Change in Arlesford Pond Move from Plants to sediment and higher nutrients Move from Plants to sediment and higher nutrients

24 Alresford Pond Summary Geochemistry Pre 1855 high sediment accumulation and variability in accumulation Highly variable transition to increase magnetics / minerogenic (soils?) ~ 1970 Major event minerogenic sediments increase / TP increases. Diatoms/Chironimids Pre 1885/1855 Indicate more Macrophytes in lake Peak in Blue/Green algae Post 1970 Increase in Eutrophic taxa (Response to wider catchment disturbance) Pollen Increase in Watercress Post 1985 Rapid increase in Watercress (Increase in demand/fertilisers?) Increase in Wet woodland species in top of core

25 Ecosystem Change in The Grange Lake Shallowing > Nutrients > Nutrients < Inflow

26 The Grange Summary Geochemistry Relatively stable over time and Phase of increased erosion in catchment. Diatoms/Chironimids Pollen Pre 1855 more macrophytes From 1905 Increase eutrophic taxa Increase in benthic taxa Move from clear water with macrophtes to shallow sediment and > nutrients Some watercress present pre-1915 Post 1945 Alder Grass More trees along shore or actual reduction grasslands?

27 Standardised data (z-scores) Drivers of change in Itchen Headwaters Reference state? Population Sheep Cattle Tot Cereals Temperature Rain Watercress beds Algal blooms? Critical New State? Transition Land use Change = FAST Process Year Population / Climate = SLOW Process Grange slow Transition W.Cress Ind. Switch to fertiliser & Increased demand

28 Conclusions Two Lakes behave differently so dominant driver is land use change NOT Climate. Critical transition in AP c after which ecosystem changes. Similar Nutrient response in 1970 s in both lakes CAP Agriculture change driven by Soil erosion/cattle (Grange) The Grange slow change (>TP) results in ecosystem changes towards shallow sediment filled system. Reference conditions for lakes pre 1850? AP system started to flicker prior to tipping point. Use diatoms to detect approaching state change? Watercress may be associated with AP sediment pulse.

29 Mapping Ecosystem Services on to Lake sediment records Ecosystem Service (MA 2005) Proxy Lake Sediment record Genetic resource Biodiversity Pollen (terrestrial), chironomids & diatoms (aquatic) Carbon sequestration Carbon C, Loss on ignition, terrestrial estimates from land cover Erosion regulation Sediment yield Mass accumulation (210-Pb, 137-Cs) Erosion regulation Soil erosion Magnetic susceptibility, C/N Water purification Sediment quality Geochemical,XRF, phosphorus Water purification Water quality Diatom inferred TP, TOC, δ 13 C, chironomid inferred anoxia Air quality Atmospheric particles Geochemistry, XRF, heavy metals, spherical carbonaceous particles (After Dearing et al., 2010)

30 Future Work Better data on Watercress production, land use change. Conversion of data into Ecosystem Services. Statistical analysis of trends and variance to identify system behaviour and potential Environmental canaries for chalk catchments.