The eco-hydrology of wet dune slacks: experience from Winterton Dunes, Norfolk

Transcription

1 The eco-hydrology of wet dune slacks: experience from Winterton Dunes, Norfolk William Coulet 1, Kevin Hiscock 2, Tony Davy 3, Hans Schutten 4 & Ab Grootjans 5 1 Van Hall Larenstein University of Professional Education, The Netherlands 2 School of Environmental Sciences, University of East Anglia, Norwich 3 School of Biological Sciences, University of East Anglia, Norwich 4 Marine, Coastal and Freshwater Ecosystems Evidence Team, Natural England, Norwich 5 Community and Ecology Conservation Group, University of Groningen, The Netherlands What are dune slacks? Damp or wet hollows l eft between dunes, where t he groundwater reaches or approaches the surface of the sand Seas onally fluctuating wat er table, which usually reaches a maximum in wint er and spring, and drops in summer Primar y slac ks originate fr om s andy beac hes c ut off from t he influence of the sea by new foredunes, particularl y in prograding systems Secondary sl ac ks result from bl owouts or t he landward movement of dune ridges in eroding systems Some res earch on UK s yst ems ; knowl edge depends heavil y on the Netherlands/ Wadden Sea area (see English Nature R esearch Report 696) 1

2 Conservation importance EU Habitats Directi ve Annex 1 dune habit ats t hat are s ensiti ve to wat er abstrac tion. Eur opean habitat feat ures 2190 Humid dune slac ks and 2170 Dunes with Salix repens ssp. argent ea, or Salicion arenariae (NVC SD13, 14, 15, 16, 17) Annex 2 protected species, particularly: Natterjack toad (Bufo calamita), Fen orchid (Liparis loeseliivar. ovata) Petalwort, thalloid liverwort (Petallophyllum ralfsii) Fewer than 1000 ha r emain in England and Wales Threat ened by wat er abstraction, aff orestation, recreation (golf links, t ourism) and atmospheric N deposition Disturbance and species diversity Fluctuati on i n wat er table alt ernating selection pressures of waterlogging and drought r equire opposite adaptations Physical disturbance of blowing sand (and tides in fore dunes) Successional c hanges to int ernal environment decalcification and concomitant hydrochemistry, organic matter accumulation Changing successional mosaic of dune slack hydrological types Hence di versit y is high, but f ew endemic species - many als o occ ur in c alcareous fens, fen meadows, and other wetlands 2

3 Hydrological controls on humid dune slack formation A Seaward, y oung dune slack B Rain-fed dune slack C Flow-through dune slack D Boundary dune slack E Moist dune slack Significance of the slack water table In coarsel y t extured, porous, s andy substrat es, the dept h of t he dune water t able has i mmediate consequences for plants About 28% by mass of water c an be hel d agains t gravit y in fres hl y blown sand. Substantial incor porati on of organic matter can increase it to onl y about 50% Sand remains saturated cm above the fr ee water table and capillary action carries subst antial amounts of water up to 45 cm above it The wat er t able has little infl uenc e on the moisture content of sand 1 m above it Flooded s oils bec ome anoxic and reducing, requiring anatomical and met abolic adaptation in their species 3

4 Hydrochemistry of dune slacks Princip al controls: ph of dune s and dependent on c arbonat e (shell) content gi ving acidic or calcareous conditi ons ph (alkalinit y) of local and regional groundwater fl ow s ystems creating base-rich conditi ons Flux of anoxic and Fe-rich water i mportant for vegetation successi on Regular supply of mineral-rich groundwater t o devel op microbial/ algal mats and stabilise longevity of pioneer stages Salinity distribution Expect spatial and temporal variations i n hydrochemic al conditions in res ponse to fluctuating water levels Successional change (A) Pioneer phase - small pioneer species establish on an almost bare soil, covered with a thin lay er of green algae and laminated microbial mats (B) Higher plants adapted to very low nutrient availability colonize (C) Moss lay er of pleurocarpous bryophytes dev elops, and ty pical dune-slack species become established (D) Rapid accumulation of organic matter, increase of tall grasses and shrubs appear, which leads to the decline of many typical dune-slack species (drawing by Rob Beentjes) 4

5 Location of Winterton Dunes Landscape 5

6 Winterton Dunes, Norfolk Ty pe B rain-f ed dune slack Horsey Dunes, Norfolk Ty pe D boundary dune slack Winterton Dunes Ty pe E moist dune slack Blown sand Geology Corton Formation Glacial silts Corton Formation Sand & Gravel Muddy silt and clay Peat Sand and gravel, Coastal barrier complex Situation at Sea Palling 6

7 Construction of concrete sea defence post-1953 Current morphology 7

8 Topography Winterton surface level Metres ODN pre Metres distance 8

9 Transect I cm above ODN Top pi pe LIDAR Topo 28/04/ /04/ /05/ /06/ metres Transect II cm Above OD N T op p ipe LIDAR Topo /04/ /04/ /05/ /06/ metres

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11 Hydrochemistry September 2005 (note dev elopment of f erruginous algal growth) Nov ember 2005 (note generally f resher conditions) 11

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13 Piper diagram for transects and ponds Conclusions Dune slacks are part of a larger, successional, eco-hydrological system, with a hydrological catchment Nutrient enrichment of infiltrating waters or groundwaters will accelerate successional development Slacks of most conservation interest are early successional stages that depend upon periodic disturbance for renewal They depend on int erplay bet ween infiltrating water and anoxic, calcareous groundwater. Lowering of groundwater can cause decalcification that has damaging hydrochemical consequences Dutch experience shows that the critical communities that include rare species such as Liparis loeselii have virtually no tolerance of a decrease in groundwater 13

14 Acknowledgements & References Natural England Countryside Council for Wales The Environment Agency Julie McNish Johan Schutten Sue Rees Graham Weaver Roger Meade Peter Rhind Peter Jones Kath Charles Mark Whiteman Davy, A.J., Grootjans, A.P., Hiscock, K. and Petersen, J. (2006) Development of eco-hydrological guidelines for dune habitats Phase 1. English Nature Research Report No. 696, 80 pp. Coulet, W.C.E. (2007) Managing Winterton for Natterjack Toads. Internal Report. Natural England, 73 pp. 14