Methodologies for Measuring and Modelling Change in Coastal Saline Lagoons Under Historic and Accelerated Sea-Level Rise, Suffolk Coast, Eastern England
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Hydrobiologia (2012) 693:99–115 DOI 10.1007/s10750-012-1089-x PRIMARY RESEARCH PAPER Methodologies for measuring and modelling change in coastal saline lagoons under historic and accelerated sea-level rise, Suffolk coast, eastern England Thomas Spencer • Susan M. Brooks Received: 17 December 2011 / Revised: 12 March 2012 / Accepted: 16 March 2012 / Published online: 5 April 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Thorough assessment of vulnerable ‘time to extinction’. Loss rates are likely to accelerate coastal habitats, impacted by sea-level rise and considerably after 2015 and a fundamental revision of anthropogenic pressures, requires both the accurate UK saline lagoon creation targets is urgently required. establishment of the evidence base for current status The approach is generic and could be used to assess the and scientifically-informed forward planning of evolutionary trajectories for other vulnerable coastal expected future status. Coastal saline lagoons are habitats, under a range of near-future environmental transitional, ephemeral habitats of considerable con- change scenarios. servation interest; under European legislation their status requires on-going maintenance of ‘favourable Keywords Lagoon biology Á Reedbeds Á Barrier status’. Over decadal timescales, the seaward barriers dynamics Á Shoreline response models Á Shoreline that enclose saline lagoons migrate progressively Management Plans Á Southern North Sea landwards. Geo-referenced and digitised historic maps and aerial photographs are used to create a detailed trajectory of barrier migration and loss of lagoon area Introduction for three saline ‘broads’ on the rapidly retreating coastline of Suffolk, eastern England. The SCAPE Coastal habitats provide considerable challenges for shoreline response model is then employed to extend conservation and management which have become this trajectory, under a range of sea-level rise scenar- more complex and insistent over the last two decades ios, to 2050 and 2095 and to predict saline lagoon (1990–2010). After centuries of low valuation of coastal habitats—accompanied by habitat degrada- tion, fragmentation and loss, from both agricultural Handling editor: Pierluigi Viaroli and industrial land claim (e.g. Bakker et al., 1997; Wolters et al., 2005) and changed estuarine and T. Spencer (&) riverine hydrodynamics (e.g. van der Wal & Pye, Cambridge Coastal Research Unit, Department of 2004)—has come the realisation that coastal ecosys- Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK tems provide a wide range of ecosystem services of e-mail: [email protected] considerable direct and indirect benefit to human populations (Zedler & Kercher, 2005). It is perhaps S. M. Brooks not surprising, therefore, that ‘in the relatively short Department of Geography, Environment and Development Studies, Birkbeck, University of London, space of 20 years we have moved from a position Malet Street, London WC1E 7HX, UK where maintaining the existing line of defence was a 123 100 Hydrobiologia (2012) 693:99–115 first priority to one where it is only one of a number of consequent upon accelerated sea-level rise and possi- options. Today not only has the enclosure of tidal land ble changes in storminess (Lowe et al., 2009). Such for agriculture ceased … but also habitat restoration environmental futures suggest the need for proactive, and re-creation for conservation and sea defence long-term conservation planning rather than reactive, purposes has become much more acceptable’ [Doody short-term responses to events such as windstorms, (2004, pp. 136–137) in discussing UK coastal policy]. river floods and coastal surges. However, as Leafe et al. (1998, p. 288) point out, this It is clear that better-informed decision-making on shift in thinking has generated dilemmas for nature coastal habitat futures requires improved scientific conservation interests where ‘the issues of sustain- data on status. In addition, there is a need for new ability and biodiversity tend to intertwine and can methodological tools, both to improve near-future easily become confused’. If an engineered structure forecasting of habitat areal change and to better currently protects an area of conservation importance constrain the envelope of uncertainty around such should the defence be removed to create a more forecasts. In this paper, we demonstrate how (i) the use environmentally robust shoreline of likely value in of geo-referenced and digitised historic maps and reducing flood risk or should the conservation interest aerial photographs can be used to create a detailed prevail, particularly if the reason for the designation is trajectory of coastal lagoon change and (ii) environ- of national or international significance? mental modelling can be used to extend such a In the USA, such debates have been sharpened by trajectory into the near-term future, under a range of the contexts of ‘no-net-wetland-loss’ policies and the sea-level rise scenarios. Such an approach provides, instrument of the Clean Water Act (e.g. Zedler, 2004). amongst other benefits, improved input into the In Europe, legislative pressure has been applied by necessity for, and timing of, compensatory habitat European Union directives, most notably by the provision. We illustrate this approach using the Habitats Directive (Council Directive 92/43/EEC). example of the history and likely fate of the saline The purpose of the Directive is to maintain or restore lagoons of the Suffolk coastline, eastern England. at ‘favourable status’ a representative network of Here, the shoreline has been displaced between 300 natural habitats (Annex I) and species of wild fauna and 600 m landwards since the 1880s. This research and flora of community interest. ‘Favourable status’ is thus goes some way towards refuting, for example, defined as a position where the range and area of statements such as ‘although coastal lagoons face habitats are stable or increasing and where sufficient severe threats from global warming (with sea-level areas of habitats exist to maintain viable long-term rise) and associated coastal squeeze in the longer-term, populations of key indicator species (Council of the there are currently insufficient data to quantify these European Communities, 1992). Maintenance of hab- threats’ (JNCC, 2007, p. 14). We stress, however, that itat area is a key measure of maintenance of status and the approach is generic and could be used to assess the thus generates a need for regular monitoring within a evolutionary trajectories for other vulnerable coastal delineated spatial unit. Furthermore, status needs to be habitats, under a range of near-future environmental measured against a benchmark condition recorded at a change scenarios. particular point in time. These demands run counter to the notion of a dynamic coast where both extent and Characteristics of saline lagoons condition oscillate within an envelope of possible natural environmental states. As Pethick (2002, Coastal saline lagoons are areas of comparatively p. 433) notes, this is equivalent to ‘a preservation shallow water, defined by the combination of three policy, immobilizing the undeveloped areas of the characteristics: ‘(i) the presence of an isolating barrier [British] coast by legislation, in the same way that beach, spit or chain of barrier islands; (ii) the retention inappropriate infrastructures have immobilized the of all or most of the water mass within the system developed coast’. Finally, these challenges are likely during periods of low tide in the adjacent sea; and (iii) to intensify in the near future with changing physical the persistence of natural water exchange between the energetics in the coastal system. Greater wave lagoon and the parent sea—by percolation through and tidal activity are to be expected (Nicholls et al., and/or overtopping of the barrier, through inlet out- 2007), enhanced by increased nearshore water depths, flow channels, etc.—permitting the lagoonal water to 123 Hydrobiologia (2012) 693:99–115 101 remain saline or brackish’ (Barnes, 1989, p. 296). On macro-tidal coasts ([4 m tidal range), coastal lagoons are largely restricted to locations where gravel barriers have accumulated under postglacial sea-level rise, migrating landwards and alongshore to enclose river valleys. In such settings, with lagoons at relatively high elevations above sea level, continued freshwater inflows and terrestrial sediment additions lead either to evolution towards freshwater lakes or to continued barrier retreat ‘squeezing out’ the lagoon between the seaward barrier and higher ground to landward. A further subset of lagoon type is where the gravel structures themselves incorporate saline lagoons. The unusual and ephemeral nature of all of these sys- tems—which are typical of the North Atlantic region—thus makes them of considerable conserva- tion interest, not least in the face of near-future accelerated sea-level rise. For gravel barriers at the mesoscale (\102 year), it is decadal scale sea-level rise that is the driver of barrier migration (Orford et al., 1995). Barnes (1989) identifies 41 saline lagoons around the UK coastline. A significant proportion of this resource is found in 11 areas around the coastline of eastern England. In this paper, we focus on a set of three saline lagoons of the Suffolk coast, between Benacre Cliffs and Southwold (Fig. 1). Known locally as ‘broads’, they comprise a series of discrete brackish water lagoons, fringed by reedbeds of Phragmites australis (Cav.)Trin.