Coastal lagoons and their evolution: A hydromorphological perspective

Estuarine, Coastal and Shelf Science 110 (2012) 2e14

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Coastal lagoons and their evolution: A hydromorphological perspective Robert W. Duck a, *, José Figueiredo da Silva b a b

School of the Environment, University of Dundee, Nethergate, Dundee DD1 4HN, Scotland, United Kingdom Departamento de Ambiente e Ordenamento, Universidade de Aveiro, 3810-193 Aveiro, Portugal

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2011 Accepted 4 March 2012 Available online 16 March 2012

From a geoscience perspective, coastal lagoons are essentially ephemeral features which are part of a continuum of coastal environments. Their natural hydromorphological evolution is seldom if ever permitted to take place as a consequence of human action; either directly by engineering interventions, to maintain or create navigable inlets, or indirectly due to activities within their catchment areas. The hydromorphological impacts of historical, contemporary and proposed engineering activities in coastal lagoons around in the world are reviewed and from these a powerful exemplar is that of the Aveiro system in Portugal. Here, two centuries of channelization, jetty and breakwater construction and progressive dredging have transformed a then fluvially dominant system into one that is today tidally dominant. Both the tidal range and tidal prism have increased along with the extent of saline intrusion. The associated stresses imposed by increased tidal currents have, in turn, led to important changes in the sedimentary regime and to the loss of almost all seagrass species which were once abundant in the system. This, along with observations from other related case studies, raises important questions regarding the concept of lagoon ecosystem ‘health’ and the baseline or reference conditions to which it is assessed. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: lagoon types evolution physical processes hydromorphology human intervention review

1. Introduction Several definitions of coastal lagoons have been proposed and the reader is referred to Tagliapietra et al. (2009) for a full review. However, the one that has been most widely adopted in the literature is that of Kjerfve (1994) who defined a coastal lagoon as: “a shallow coastal water body separated from the ocean by a barrier, connected at least intermittently to the ocean by one or more restricted inlets, and usually oriented shore-parallel.” A subsequent modification to this by Bird (2008) provides some additional insight as to the most likely mode of lagoon formation and the typical nature of the enclosing barrier: “Coastal lagoons are areas of relatively shallow water that have been partly or wholly sealed off from the sea by the deposition of spits or barriers, usually of sand or shingle, built up above high tide level by wave action.” Most coastal lagoons formed during the Holocene as a result of rising relative sea level and the construction of such barriers by marine processes (e.g. Dias et al., 2000). Since barrier islands form during rises in relative sea level, lagoons are a common feature along coasts experiencing such conditions. By contrast, in regions where isostatic uplift

* Corresponding author. E-mail addresses: [email protected] (R.W. Duck), [email protected] (J.F. da Silva). 0272-7714/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2012.03.007

exceeds eustatic sea level rise, lagoons are relatively rare (Martin and Dominguez, 1994). A fundamental distinction exists in terms of the nature of the sedimentary infill, siliciclastic or carbonate, of a lagoon. Terrigenous silicates are the predominant sediment infilling temperate lagoons whereas those of tropical regions are typically characterised by the accumulation of calcium carbonate remains or organisms. Bird (2008) noted, however, that lagoons enclosed by coral reefs, either between barrier reefs and the land or within atolls, are excluded from his definition since these are best regarded as marginal marine environments linked with the open sea at high tide. As such, they are not considered further in this paper. The primary controls on coastal hydromorphology are the climatic conditions, hydrodynamic setting, pre-existing topography and substrate materials, and sea level history (Wanless, 1976). In situations where river discharge has a strong influence, estuarine or deltaic systems tend to develop. However, where marine influences are dominant, lagoons may evolve. Coastal geomorphology, together with the sediment stabilising effect of some organisms, exert a secondary control on the hydromorphological evolution of lagoons. Barrier enclosed lagoons can be further sub-divided (Table 1) on the basis of the degree of isolation from the coastal ocean provided by the barrier (Kjerfve, 1994; Isla, 1995). This sub-division is

R.W. Duck, J.F. da Silva / Estuarine, Coastal and Shelf Science 110 (2012) 2e14 Table 1 Classification of coastal lagoons according to the degree of isolation provided by the barrier. Degree of isolation provided by the barrier

Dominant hydromorphological conditions

Dominant geomorphological features

No barrier e coastal bay Leaky lagoon e many wide tidal inlets Restricted lagoon e several tidal inlets

Macrotidal Mesotidal e tidal action similar to open ocean Mesotidal e tidal action through inlets restricted by wave action Microtidal e small tidal action through inlets; barrier washover by waves

Offshore sand ridges Tidal flats dominate

Choked lagoon e narrow tidal inlets

Salt marshes dominate Open water and intra-lagoon deltas

strongly related to the dominant hydromorphological conditions observed in the lagoon. Kjerfve (1994) used the term ‘choked’ to refer to lagoons with a single entrance channel which functions as a ‘dynamic filter’ largely damping out tidal currents and water level fluctuations inside the lagoon. In such water bodies, tidal oscillations are typically reduced to 5% or less than those of the adjacent ocean. By contrast, Kjerfve (1994) used the terms ‘restricted’ and ‘leaky’ to describe coastal lagoons with two to many entrance channels and thus well-defined tidal circulation and unimpaired water exchange with the ocean, respectively. Thus leaky lagoons are at the opposite end of the spectrum from choked lagoons and this classification underlines the important link between the inlet characteristics of a lagoon and its ability to exchange water with the open sea (e.g. Guyondet and Koutitonsky, 2008). It is important to note that, within the European Union (EU), the term ‘transitional waters’ has come into being during the past decade as a consequence of the Water Framework Directive (WFD) that has established a typology of water bodies on the basis of physical, hydromorphological and biological criteria (McLusky and Elliott, 2007). It refers to those water bodies like estuaries that are ‘transitional’ between wholly freshwater and wholly seawater and is now becoming used as a scientific term worldwide to embrace, inter alia, coastal lagoons (e.g. Tagliapietra et al., 2009). Whilst many coastal lagoons are characterised by the strong physicoechemical gradients that are prerequisite of the ‘transitional’ descriptor, it can be a problematic term as it has been noted, for example, that many coastal lagoons around the periphery of the Mediterranean Sea lie strictly outside this EU definition as they do not receive any significant freshwater influence (Pérez-Ruzafa et al., 2010; Joint Nature Conservation Committee, Undated). Similarly, several of the rock-bound, silled lagoons, that are characteristic of the Atlantic coast of the Outer Hebrides of the Western Isles of Scotland, display freshwater conditions (Joint Nature Conservation Committee, Undated). ‘Hydromorphology’ is also a term that has been widely attributed to the WFD but it has actually been ‘appropriated’ from soil science (Vogel, 2011). However, it has been given a new and specific meaning by the WFD: ‘the hydrological and geomorphological elements and processes of water body systems’ (i.e. lakes, rivers, transitional waters and coasts). Such physical characteristics of the shape, the boundaries, the connectivity with the ocean and the content of coastal lagoons are, as will be discussed later, highly susceptible and vulnerable to human impact. The hydromorphological classification proposed for transitional waters can be applied both to coastal lagoons and to estuaries (Edgar et al., 2000; Galván et al., 2010). The hydrological and morphological parameters used for this classification have been related to ecological parameters and have the advantage of being easier to measure and more objective (Elliott and McLusky, 2002).

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By comparing 12 small transitional water bodies (rias) along the southern Bay of Biscay coast of northern Spain, Galván et al. (2010) verified that the highest values of species diversity were found in the water bodies with the most complex morphologies and the highest tidal influence. Definitions aside, coastal lagoons are present on all coasts worldwide and collectively occupy 13% of the coastal areas with individual water bodies ranging in surface area from less than 1 ha up to 10,200 km2, in the case of the Lagoa dos Patos (“Lagoon of Ducks”) in southern Brazil (Kjerfve, 1994). The latter has experienced an average of 6 m of sediment accumulation, mostly muds, during the Holocene (Toldo et al., 2000). The ‘shallow’ water columns referred to in the Kjerfve (1994) definition are typically in the range of 1e3 m, with maximum depths of 5 m occurring in channels or restricted relict hollows. As alluded to above, the salinity of coastal lagoons can vary from that of freshwater to hypersaline depending on local climatic conditions and the extent of hydrological connectivity (Kjerfve, 1986). However, within a single lagoon system, as in estuaries, there can be three salinity zones that will vary in spatial extent according to seasonal conditions. These comprise relatively freshwater close to the mouths of influent rivers, brackish water within the central reaches of a lagoon and fully marine salinities at the entrance channel or channels. From an ecological perspective, they represent both dynamic and highly productive ecosystems, typically 10e15 times more so than continental shelves (Valiela, 1995), that yield a broad range of natural services (e.g. Anthony et al., 2009). Large numbers have been designated as Ramsar sites (Schuyt and Brander, 2004). In consequence, much attention has been focussed on their biodiversity, functioning, resource exploitation, conservation and sustainability (e.g. Silva et al., 2002; Dahdouh-Guebas et al., 2005; Danovaro and Pusceddu, 2007; Iwassaki and Shaw, 2008; Micheletti et al., 2011; Pérez-Ruzafa et al., 2011). However, from a geoscience perspective, i.e. geological and geomorphological, it is important to emphasise that such water bodies are considered as essentially ephemeral features at the landesea interface, which are one of a complex variety of inter-linked types of coastal inlet: “There are three broad classes of coastal inlet e lagoons, estuaries and deltas e which form a continuum and are difficult to define as entities. At one end of the continuum are lagoons produced solely by marine action and lying between some sort of barrier feature and the original coast. Then come estuarine lagoons where a river emerges into a lagoon which still owes most of its form to the sea. In the middle of the continuum are estuaries which are essentially the lower courses of rivers more or less invaded by the sea and which may or may not be partly blocked by marine barriers. Further along still are estuarine deltas in which there has been appreciable infilling of the estuary and the river bifurcates around the fill. Finally at the other end of the continuum come deltas in which river action is so strong that it causes progradation in one of many forms” (Davies, 1980). The above quotation carries two important messages that are prerequisite to understanding coastal lagoons, their form and development and the physical processes operating therein. The first is that, in the geological and geomorphological sense, they are at the beginning of a continuum of coastal environments that, given no human intervention of any sort will naturally evolve through time, one to the next as a continuum of events in succession. The second message is a consequence of the first; that the divisions between each of the successive five coastal environments within this continuum are arbitrary and there are not necessarily clear-cut distinctions between inlet types or a single sequence resulting from their evolution. There is a fundamental difficulty in trying to define parts of a continuum (see Elliott and McLusky, 2002). This is

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exemplified in a morphogenetic classification of estuaries in which Perillo (1995) considers that coastal lagoons are varieties of ‘secondary estuaries’, the observed form of which is the product of marine processes being relatively more influential than river discharge at the present day sea level. However, as will be described later, the natural evolution of these coastal forms is seldom if ever permitted to take place as a consequence of human endeavours, engineered typically to improve navigability in the coastal zone or in attempts to maintain environmental status quo. Thus human intervention at the coast can characteristically induce discontinuities, breaks or even reversals in the natural space-time continuum of coastal evolution. This raises important questions regarding the concept of lagoon ecosystem ‘health’ (e.g. Elliott, 2011; Marotta et al., 2011) and the baseline or reference conditions to which it is assessed, since many such water bodies are in a state of arrested, punctuated or substantially modified hydromorphological evolution (e.g. Silva and Duck, 2001; Dinis et al., 2006; Morris and Turner, 2010). The aim of this paper is to review the hydromorphological framework supporting lagoon ecosystems, by means of examples of various coastal lagoon conditions, with emphasis on the important influence that human action has had on these systems. 2. The natural evolution of coastal lagoons From the two diverging models for lagoon evolution (Oertel et al., 1992), the most commonly accepted considers that lagoons evolve naturally by infilling with sediment (Fig. 1). So, in general, coastal lagoons are naturally net sediment sinks (Boyd et al., 1992; Nichols and Boon, 1994). However, no two coastal lagoons are alike. This is a consequence of wide variations in the interplay of geological, hydrological, climatic forcing and ecological factors that influence their inception and subsequent evolution (Wanless, 1976). Moreover, there is a substantial body of literature on their physical setting and geomorphology that has been synthesised by Bird (1994). The initial form of a coastal lagoon is strongly dependent on the ‘primordial character’ (Oertel et al., 1992), or morphology, of the inlet or embayment that has become enclosed and the shape of the inner shores of the barrier that developed across its mouth (Bird, 2008). Subsequent development of a lagoon depends, inter alia, upon tidal action, relative sea level fluctuation, sediment influx and local tectonic activity. The equilibrium volume and longitudinal profile of tidedominated lagoons depend on the depth and cross-sectional area of the connections with the ocean (O’Brien, 1969; Lanzoni and Seminara, 2002). Lagoons that have developed along tidedominated coasts typically have floors that are irregular, reflecting the antecedent bottom topography that characterises such environments. By contrast, lagoons developed along wavedominated coasts tend to have relatively simple topography (Oertel et al., 1992). In a regime of rising relative sea level, as for example following the Pleistocene glaciation, an offshore accumulation of sediment, such as a gravel bank, can become pushed gradually landward by wave and current action. The largest tidal lagoon in Britain, known as The Fleet and located on the English Channel coast of Dorset, has been formed in this way with the major shingle ridge of Chesil Beach forming the barrier that is periodically breached during storms (Bennett et al., 2009). Along wave-dominated coastlines where relative sea level is essentially stable, the principal agency of barrier formation is that of wave induced long shore sediment transport promoting the formation and growth of a spit that, through time, progressively encloses an inlet or embayment. The same process leads to a progressive choking of the lagoon, decreasing its equilibrium volume. The morphology of the lagoon bed is thus inherited from the sloping surface of the shore face; deepest adjacent to the

enclosing barrier and shallowest along the shoreline of the mainland. All processes acting on the benthic compartment, beyond the hydrodynamic and geodynamic referred to above, impact on the natural evolution of coastal lagoons. Some organisms, especially microphytobenthos, submerged aquatic vegetation and salt marsh vegetation, may exert a strong influence on the hydromorphology of coastal lagoons by biostabilising the sediment (Widdows and Brinsley, 2002). A prime example of lagoon formation on a wave-dominated coastline is that of the lagoon of the Ria de Aveiro (Fig. 2). This is the most remarkable geomorphological feature of the Atlantic coast of northern Portugal, whose ecosystem provides all of the 17 categories of services recognised by Costanza et al. (1997) and was thus a fitting backdrop to the 5th EuroLag, European Coastal Lagoons Symposium hosted by the Universidade de Aveiro in 2011. This coast has evolved as sea levels have risen since the Last Glacial Maximum by the accumulation of sand deposits derived principally from the north (Dias et al., 2000). This has formed an essentially linear coast, aligned at an angle of c. 15
to the east of north; while the present sea level was reached c. 3500 years BP, the coast became linear more recently. Analysis of maps dating back to the 13th century AD has suggested that the present lagoon has developed from an embayment, c. 70 km in length and 20 km in width, at the mouth of the River Vouga, the principal influent to the modern day lagoon, separated from the ocean only in the north by an embryonic sand spit. By the end of the 16th century, rapid southerly accretion of this spit had almost completely enclosed the lagoon behind it to the east. The natural inlet from the ocean to the enclosing lagoon thus became both progressively smaller in width and migrated to the south as the spit grew. By the beginning of the 19th century there was no navigable connection with the Atlantic other than for very small boats; the Aveiro lagoon was choked and almost completely isolated (Silva et al., 2001), as shown in Fig. 3. Had natural evolution been permitted to take place (see later), the water body would have become progressively infilled by landderived sediments, introduced by the River Vouga and other influents, to ultimately take the form of a deltaic wetland. A similar example, but of actual unabated evolution, is provided by the central part of the Gulf of Lions in the south of France. Here the shoreline is characterised by many coastal plain wetlands that have resulted from the infilling of a lagoon by a combination of the migration of sandy littoral barriers and the influx of fine grained fluvial sediments during the late Holocene. The final closure has been dated at around 730 þ/ 120 yr cal BP. The early stages of lagoon isolation from the Mediterranean Sea, together with the progradation of the coastal plain, are believed to have been the cause of the decline in economic activity of the harbour of Lattara (today Lattes) during the earlier Roman period (Sabatier et al., 2010). This isolation from the ocean and associated decrease in tidal dynamics within a lagoon typically results in the growth of seagrasses, benthic macroalgae and microphytobenthos, which contribute to an increase in the overall concentration of organic material within the infilling sedimentary sequence (e.g. Woszczyk and Rotnicki, 2009). 3. The impact of human intervention on coastal lagoons Close to two decades ago, Carter and Woodroffe (1994) noted that, “To ignore the role of humans and their impact on coastal evolution would be fallacious.” This statement is especially true of coastal lagoons today, which have been heavily modified by anthropogenic intervention in large numbers worldwide (see, e.g. Evans, 2008). According to Bruun (1994), canals were built with primitive tools to serve navigation between lagoons in both India

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Fig. 1. Schematic maps and cross-sections illustrating two models of coastal lagoon generation and evolution based on Oertel et al. (1992). (A) A coastal embayment becomes enclosed by wave built barriers and the newly formed lagoon evolves by internal sedimentation (B) producing inter-tidal flats and salt marshes (yellow). (C) Geomorphological features near a transgressive coastline subject to strong tidal action are converted into (D) Inter-tidal flats and salt marshes (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 2. Google Earth image of the ria de Aveiro system, taken in 2010 (north is to the top of the page). Note that all of the central area of the lagoon, dominated by salt marshes, is submerged at high water. Image reproduced courtesy of Google EarthÓ, 2011 Google.

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Fig. 3. Maps showing the area of the Ria de Aveiro covered at high water on a spring tide. The panel for 1802 represents the summer condition before the opening of the new inlet. The panel for 1865 shows that part of the central area of the ria initially flooded at high water. The panel for 1975 shows the increase of the area flooded at high water, due to the greater tidal range (from Silva et al., 2001).

and China thousands of years ago. Indeed very few such systems can today be described as natural or pristine. Over the centuries, coastal lagoons have become foci of settlement, industry, leisure and recreation and have characteristically a rich cultural heritage. In parallel with offering shelter to navigation, they are natural providers of seafood and sea salt (e.g. the Aveiro lagoon), are often sites of aquaculture and, in general, their landscapes give aesthetic pleasure. Thus coastal lagoons contribute significantly to economic, ecological and human welfare. The development of sheltered and safe harbour facilities at what are essentially fixed locations within or close to the entrance channels of such water bodies is, however, at odds with the natural evolution of these mobile coastal systems. The entrances to coastal lagoons control the exchange of water and are especially complex, dynamic and sensitive to changes in relative sea level, tides, storm waves and fluvial floods (e.g. Moreno et al., 2010; Morris and Turner, 2010). In consequence, there are numerous examples worldwide of historical ports and harbours having had to be abandoned due to natural processes of sedimentation, and the migration of sand banks and channels (e.g. Sabatier et al., 2010). Human use of lagoons may change the internal balance between accretion and erosion, schematically represented in Fig. 1, either by accelerating the rate of infill or by inducing erosion. Historically, inter-tidal areas in lagoons have been claimed for saltpans, for agriculture and for harbour or urban construction. These activities will cause a reduction in the tidal prism of the lagoon, leading to a decrease in the cross-sectional area of tidal channels and a more rapid infilling of the lagoon. By contrast, reducing sediment supply or increasing the hydraulic connectivity with the coastal ocean may reverse an accretion trend to an erosive regime.

With advances in engineering technology, the increasing tendency was to attempt to control channel migration by the construction of training walls and jetties along with the removal of sediment from harbours and their access channels by dredging to create deeper and straighter waterways (e.g. Stevenson, 1845, 1858, 1872). Thus, in many cases the natural evolution of coastal lagoons worldwide has been modified as their entrances have become essentially ‘fixed’ by engineering interventions. Moreover, to continue to maintain a lagoon entrance in a fixed location and to keep navigation routes open, typically requires ongoing dredging and the progressive installation of training walls and jetties. Rerouting of influent rivers to allow a more direct discharge of sediment load into the ocean was an even more radical change adopted in the Venice Lagoon (Molinaroli et al., 2007). It is somewhat ironic that coastal and oceanographic engineers assert that the ecological and economic value of coastal lagoons is dependent upon such human interventions (e.g. Mehta, 1996) and that engineering works are a panacea to the safeguarding of good ecological status. The maintenance of a fixed, navigable entrance channel has, however, more than simply local consequences; a whole lagoon system can be affected. This is nowhere better epitomised than in the Aveiro lagoon which provides a powerful case study of the many unforeseen impacts that have been triggered by a single cause. In the 16th century Aveiro was a prosperous port that had free access to the Atlantic Ocean directly from the front of the town. However, the continued southward growth of the spit, referred to above, along with infilling of the lagoon began to progressively hamper navigation. Local sea salt production, which was exported to northern Europe, was hampered and the urban population engaged in its production and selling decreased significantly over

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the 17th and 18th centuries (Dias et al., 2000). By the close of the 17th century, as noted above, the lagoon was choked, almost completely isolated from the Atlantic Ocean and in 1802 had a small microtidal range of between 0.07 and 0.13 m over most of the water mass (Carvalho, 1947). It was thus naturally fluvially dominant by that time, the water level being controlled mainly by river flow with winter levels up to 2 m higher than the summer conditions. Unlike at Lattara in the Gulf of Lions (see above), this trend in the natural evolution of the lagoon was, however, halted in 1808 when a new, artificial inlet, with a width of c. 350 m and a cross-sectional area of c. 1000 m2, was constructed 18 km to the north of the natural channel (Fig. 3). The immediate impact was to increase the tidal range in the lagoon to >1 m, leading to the re-establishment of salt production, of harbour facilities and to the civic renaissance of Aveiro. In addition, the course of the River Vouga was diverted to the west into an engineered channel that connected it more directly to the lagoon’s new outlet. This diversion contributed to the economic fortunes of the area; to the expanding port, to salt production in the lagoon, and to agriculture e which became far less susceptible to the damaging effects of river floods. Works to maintain the channel network wide and deep enough to permit safe navigation, including the construction of training walls and breakwaters, together with extensive dredging, commenced in 1936 e and continue to this day (for full details see Silva and Duck, 2001). Since the pre-1936 inlet channel was shallow, tidal flows involved a large loss of energy and the bed stresses caused the tides in the lagoon to have the characteristics of a progressive wave, propagating at different speeds at high and low tide. Deepening of channels by continual dredging and ongoing canalisation has lessened this effect and, as a result, there has been a progressive increase in tidal ranges, which today are >3 m on spring tides (mesotidal). The cross-sectional area at the inlet is now some five times that in 1808 and the tidal prism volume is up to 106  106 m3 (Silva and Duck, 2006). Thus, tidal currents, together with windgenerated waves, are today the major forcing agents in the Ria de Aveiro. Seaward breakwater extension to define the inlet channel has had the additional effect of starving the natural southerly long shore transport of sand (Fig. 4), thus contributing to enhanced coastal erosion in several localities in the south. Most of the old saltpans are today open to tidal oscillation and mud dykes that had not been reinforced have now been partially destroyed due to the increasing stresses imposed by the larger tidal ranges. Channels in the lagoon are widening and increasing in depth, leading to the regression of salt marshes (Silva and Duck, 2001; Silva et al., 2004). The resulting changes in bathymetry have increased the ‘channelization’ effect of the tidal flow in the lagoon and its bottom topography is not of the simple form that would have been characteristic of such systems had it been permitted to evolve naturally (Oertel et al., 1992). As a result of the progressive increase in the tidal prism that has taken place since 1808 (Silva and Duck, 2001), there has been a corresponding change in the spatial distribution of salinity. In 1910, waters with salinities >30 psu intruded inland for no more than 4 km (Nobre et al., 1915) but by 2000 fully marine waters were recorded intruding the major channels over 15 km inland from the Atlantic Ocean (Silva et al., 2004), a phenomenon that can lead to the erosion of formerly freshwater marshes (Bird, 2008). Since the 1960s, the steady increase in tidal action, triggered by the interventions described above, has led to a major ecosystem change within the Aveiro lagoon, resulting today in a large decrease in the areas covered by seagrasses (Zostera and Ruppia sp.) which formerly covered large sub-tidal flat areas (Silva et al., 2004, 2009). Correspondingly, there has been an increase in the areas of bare sediment supporting the growth of sparse macroalgae species

(Ulva, sp. and Gracilaria sp.) only. Today the fully marine Zostera noltii is the only species of seagrass found in the lagoon (Silva et al., 2009; Cunha et al., 2011), growing only in the inter-tidal zone. This is principally due to the stresses imposed by the increased tidal flows, induced by the increased tidal prism causing transport and redistribution of sandy sediment along with increased resuspension and turbidity of the water column. Increased recreational use of the lagoon, in particular motor boating, along with bait digging from the tidal flats for recreational fishing, has exacerbated this decline in both species numbers and abundance of seagrasses (Silva et al., 2005). The change occurring in the sediments of the extensive tidal flats, from finer cohesive material supporting very intense biological and chemical activity to coarser mobile sand with reduced biological activity, corresponds to a reduction in the importance of the benthic compartment of the lagoon ecosystem. Since this compartment has an important role in filtering the pollutants reaching the lagoon, it is anticipated that the ecosystem will become more susceptible to the impacts of catchment-derived pollutant loads (Silva et al., 2002), potentially further reducing its biological activity (Silva et al., 2005, 2009). Overall, human action since the start of the 19th century has transformed the Aveiro lagoon from a naturally fluvially dominant system to one that is today tidally dominant and has been the major factor controlling its hydromorphology (Dias and Picado, 2011; Lopes et al., 2011). A strikingly similar example of anthropogenically controlled evolution took place in the Venice Lagoon, which is the most important example of how coastal lagoons can have multiple uses and so generate high economic value. The origin of the Venice Lagoon has been related to the formation of a littoral bar formed from the sediment transported by rivers flowing into the northern Adriatic Sea. By the 15th century, as the sediment load transported by the rivers into the lagoon was impeding the navigation to Venice harbour, human action was taken to divert their flow to the outside of the lagoon. The three inlets to the Adriatic Sea (Fig. 5) were thus widened and protected by breakwaters and the depth of navigation channels was increased up to 20 m (Molinaroli et al., 2007). Carniello et al. (2009) have reported that repeated bathymetric surveys indicate that the water depth within the Venice Lagoon is apparently increasing through time and thereby permitting larger waves to develop and erode the bed. Cavazzoni (1983) previously attributed such a trend to tectonic subsidence deepening the water column. During the last century, this water body has been experiencing a general degradation comprising the progressive deepening of inter-tidal flats and the reduction of areas of salt marsh (Carniello et al., 2009). The extent of salt marshes has been reduced from 68 to 32 km2 between 1927 and 2002, whilst the area of subtidal flats between 0.75 and 2.0 m in depth has increased from 88 to 206 km2 during the same period (Sarretta et al., 2010). Rather than a tectonic effect, this recent study has shown that the large changes in lagoon morphology observed since the 1927 bathymetric survey are, however, the direct and indirect results of human activities. These include the dredging of docks and channels during the period 1927e1970 for improved navigation, urban settlement and groundwater extraction for industrial uses, which has induced subsidence. The infilling of wetland areas and the exploitation of natural resources, in particular the harvesting of Manila clams with mechanical dredges, which disturbs and re-entrains the bottom sediments, have also contributed to the progressive deepening of the water column. Sediment budget calculations have shown that the net loss of sediment from the Venice Lagoon to the sea has increased greatly from 0.3  106 m3 a1 during the period 1927e1970 to 0.8  106 m3 a1 in 1970e2002. This transformed the lagoon from a well-developed microtidal system in the 1930s to a subsidence-dominated and sediment starved water body in the 1970s. Since that time, it has become simpler in morphology, i.e.

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Fig. 4. The heavily engineered entrance channel to the Ria de Aveiro. Note the build up of sand on the updrift side of the northern breakwater (foreground) causing the ‘step’ in the coastline that is seen in Fig. 2. Rapid coastline recession to the south results in storm waves overtopping the barrier and flowing into the lagoon.

more flat-bottomed, and the increase in water depth has stimulated the propensity for exchange with the Adriatic Sea, thereby increasing the wave energy dynamics of the system (Sarretta et al., 2010). In other parts of the world, engineering interventions similar to those that began in 1808 in the Aveiro lagoon are still being planned, but stimulated by a rather different motivation. The 90 km long Vistula Spit separates a delta plain and the Vistula Lagoon from  sk Bay in the Baltic Sea (Fig. 6). Composed largely of sand, the Gdan spit is capped by an extensive veneer of aeolian dunes (Kobelyanskaya et al., 2009; Badyukova et al., 2011). Not only is this a major feature associated with the intense building construction activity, owing to its growing popularity and potential as a tourist destination, it straddles the border between Poland and the Russian exclave of Kaliningrad Oblast. The single entrance to this very large lagoon is through the Pilawa Strait, which is located within Russian territorial waters. In consequence, politically-driven plans have been made to excavate a second navigable connection between the lagoon and the Baltic Sea, to provide direct access through the Polish sector of the spit (Kaczmarek et al., 2010). This will involve the dredging of an approach channel on the seaward side together with the construction of breakwaters. A site has been chosen that is

about 3 km from the ‘root’ of the spit from which it has evolved progressively in a northeeasterly direction through the Holocene. On the basis of theoretical analysis and numerical modelling it has been predicted that, should this enterprise be carried out, the minimum length of the breakwaters defining the 5.5 m deep mouth of the entrance channel should be 350 m. It is proposed that these should be arc shaped rather than linear to facilitate sediment bypassing. It is estimated that 18,500 m3 of sand will accumulate at the channel mouth per annum, which will result in the need for dredging at least once per year. At this locality, the total long shore sediment transport rates are c. 84,000 m3 a1 in an easterly direction and c. 68,000 m3 a1 to the west, yielding a net long shore transport rate of only c. 16,000 m3 a1. As a result of this relatively small net long shore transfer of sand, it is predicted that, after ten years, the shoreline will have accreted by some 10e40 m on the updrift side of the breakwaters and receded by a similar distance range downdrift. The lateral extent of influence of the breakwaters is anticipated to be c. 1 km to either side after a period of ten years (Kaczmarek et al., 2010). In other areas where there has been extensive tourism development, such as Hurghada on Egypt’s Red Sea coastline, engineering enterprise has been such that completely artificial lagoons have been excavated (Ismail and Khalail, 2010).

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Fig. 5. NASA Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image of the Venice Lagoon. Note the three inlets to the Adriatic Sea. North is to the top of the page.

What has become the ‘traditional’ hard engineering means of stabilising the inlets to coastal lagoons by jetties not only interrupts long shore sediment transport causing downdrift erosion, but also has negative visual impacts. Coastal engineers have thus sought to develop other ‘solutions’ to promote lagoon inlet stabilisation and minimise maintenance dredging, as at the Óbidos Lagoon in western Portugal (Fortunato and Oliviera, 2007). This water body has a long history of accelerated sedimentation due to agricultural activities in its catchment (Dias et al., 2000) along with flood tidal

ingress of materials from the sea. The lower reach is connected to the wave-dominated coast of the Atlantic Ocean via several channels that dissect large sand banks. The proposed solution at this locality, incorporating both soft and hard components, involves initially the dredging of transverse channels over the inter-tidal sand flats outside the lagoon entrance with the aim of promoting ebb tide dominance. Modelling suggests that this will increase the ability of the lagoon to flush out or export marine-derived sediments. Subsequently, a partially submerged training wall is

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Fig. 6. NASA Landsat image of the Vistula lagoon and Spit. Note that the single entrance channel to the lagoon is in Russian territorial waters. North is to the top of the page.

proposed near the southern shoreline. Along with the existing guiding wall on the northern shore, this is designed to limit the migration of the inlet channel and to concentrate the ebb flow within the channel. Together, these techniques have been predicted by analytical, empirical and numerical models to reduce accretion at the inlet (Fortunato and Oliviera, 2007). It is noteworthy that in the nearby Alfezeirão Lagoon, which hosted an important Portuguese port in the 16th century, the sediment influx that ultimately put an end to harbour activities was apparently exacerbated by the dumping of ships’ ballast (Dias et al., 2000). Although coastal lagoons may be in an overall state of net sediment infill, they can still be exporters of sediment to the coastal ocean on the ebb tide as, for example, observed by airborne remote sensing of the outlet channel from the Ria de Aveiro (Anderson et al., 1999; Silva et al., 2001). The consequences of preventing the natural export of materials from a lagoon have been highlighted by a study of Tacumshin Lake on the southeeast coast of Ireland (Orford et al., 1988). This lagoon was isolated from the Irish Sea by a sandy gravel barrier and naturally supplied fine sand to the beach on the seaward side by means of an outlet stream through the barrier. Long shore transport moved this material along the shoreline creating a terrace in front of the coarse clastic barrier that served to dissipate the energy of breaking waves. However, in an attempt to claim the lagoon for agricultural purposes, the natural outlet from Tacumshin Lake was dammed in 1974 following the construction of a sub-barrier drainage culvert. As a result, the sand supply from lagoon to beach through the barrier was cut off and the deposits that had previously accumulated from this source became

progressively removed by long shore currents thereby steepening the angle of the beach face and thus reducing the natural level of protection afforded to the gravel barrier behind. This, in turn, led to increased wave attack on the seaward face of the barrier itself, to overtopping during storms which redistributed sediment on the landward side as washover fans and to an overall degradation of the integrity of the barrier itself (Orford et al., 1988). The so-called Cacela Peninsula is a long, thin, vulnerable spit that forms a component of the Ria Formosa barrier island system of Portugal’s Algarve coast. To landward is a slender lagoon, one of many along this coastline. During the winter of 1995/96, a series of washovers associated with storm events induced dune crest destruction along several parts of the peninsula and increased sedimentation in the lagoon behind. In addition, a new, 35 m wide inlet channel to the lagoon was created that made the village of Fábrica vulnerable to direct wave attack during storms (Matias et al., 2005). In an attempt to prevent future washover events and further breaches, sediments were dredged from the lagoon and used to construct artificial dunes along the western 2 km of the peninsula to a height of c. 5.5 m above mean sea level, thereby nourishing the peninsula. The new inlet channel was also closed off with dredged materials (Matias et al., 2005). Subsequent storms have, however, caused dune erosion, further breaches and washover events on what is a highly mobile and naturally dynamic barrier island coast. In addition to direct engineering interventions e employing both hard and soft technologies e that have prevented or interrupted the natural hydromorphological evolution of coastal

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lagoons, it should also be noted that activities within the catchment area of a lagoon, can lead to changes in the natural influent sedimentary regime. These include the construction of dams, deforestation, the intensification of agricultural practices and industrial activities. Either decreased or increased sediment loads may thus be introduced which, along with land-derived pollutants from point source or diffuse origins, can lead to the preservation of physical (e.g. changes in particle size distributions) or geochemical signatures (e.g. increases in heavy metal concentrations) in the stratigraphic record of the bottom deposits, thus preserving a historical record of human impact in a drainage basin (e.g. Dinis et al., 2006; Avramidis et al., 2008; Patchineelam et al., 2011). The construction of the Aswan High Dam on the River Nile in Egypt in the 1960s has, for instance, reduced the water discharge to the lower Nile Delta plain, from 84  109 m3 a1 prior to impoundment to 18e21  109 m3 a1 thereafter. This has resulted in a corresponding reduction in the sediment load into the Mediterranean Sea from 160 to 178  106 t a1 before the 1960s to almost zero at the present day. Lagoons within the Nile Delta, such as the Edku, Burullus and Manzala water bodies, have thus been starved of their natural fluvial sediment supply since the dam became operational (Gu et al., 2011). The principal source of sediments to the Kotychi lagoon, located on the northewestern, wave-dominated, microtidal, Peloponnese coast of Greece, is from fluvial sources. Marine-derived shell fragments are a minor contribution, along with autochthonous biogenic production. A Ramsar site, it is one of the most important ecological areas of Greece. On the basis of a reference surface area of the lagoon in 1945, Kalivas et al. (2003) demonstrated, using a GIS, an almost 60% reduction in the four decades to 1987. More recent studies have similarly shown that this rate of loss of area covered by water has continued at least until the close of the 20th century. During the decade 1990e2000, siltation reduced the area of the lagoon by some 800,000 m3, which represents c. 13% of the area covered by water (Avramidis et al., 2008). Sedimentation rates are greatest close to the landward margins of the water body and related to river discharges, with agricultural activities and deforestation in the catchment thought to be contributing to the pattern of sediment accumulation. Thus, although this rate of lagoon sedimentation, estimated to have been around 5.2 mm a1 for the last c. 150 years (Kontopoulos and Koutsios, 2010), is believed to be influenced by human activities in its catchment and is likely greater than the sediment yield under pristine conditions of erosion, the management response is to propose measures to prevent siltation, “for the protection of the lagoon ecosystem” (Avramidis et al., 2008). Tuzla Lake, an important recreational facility for Istanbul, is located 60 km to the east of the city on Turkey’s Marmara coast. This is a region that has experienced two to three decades of rapidly expanding metropolitan development and many buildings have been constructed around the lake directly onto its fringing marshes, despite heavy criticism from environmental groups. Naturally, this was a coastal lagoon with a free connection with the sea. This connection was, however, severed by the construction of a dockyard in 1978. A short time thereafter, the sea established a new connection to the lake as it breached the sandy barrier, thus restoring lagoon characteristics (Öztürk, 2005). Following the establishment of the leather industry in the region, the creeks flowing into the lagoon became progressively polluted by chemical wastes, some of which were exported via the newly created inlet channel onto nearby tourist beaches. Beach users were not surprisingly unhappy with this situation and demanded action. The response of the municipality was to cut off the route by which pollution was discharged to the beaches and thus the lagoon’s inlet channel was closed again in 1987. Thus, for a second time, Tuzla

Lake became a freshwater system. However, it was necessary to prevent the accumulation of pollutants in the lake and to this end a system of channels and an underwater tunnel were constructed to divert water from the polluted creeks into the sea away from recreational beaches. These engineering works, accompanied by over-abstraction due to pumping which has induced saline intrusion, caused a 3 m fall in the groundwater level resulting in the almost complete desiccation of the lake in the summer of 2001, the season of highest evaporation. Dredging of the bed was considered but deemed to be too costly. Since that time, summer desiccation of Tuzla Lake has become a recurrent problem. The installation of two pipelines, to facilitate seawater entry to the lake, was a failure as these became blocked rapidly with marine-derived shell debris and despite the construction of rain water channels around the lake as a further rehabilitation measure, it continues to be filled with water only during the spring and winter seasons (Öztürk, 2005). Metropolitan expansion and uncontrolled development have, in effect, all but completely destroyed this coastal lagoon system.

4. Conclusions The profound impact of human intervention on the hydromorphology of a coastal lagoon and the ecological consequences that result is nowhere better illustrated than in the Aveiro system. This, and other examples, of the impacts of historical, contemporary and proposed engineering activities from elsewhere in the world, serves to emphasise the vulnerability of natural coastal lagoon evolution to anthropogenic effects over protracted time periods. This raises important questions regarding the concept of lagoon ecosystem ‘health’ and the baseline or reference conditions to which it is assessed. To restore a lagoon to full, natural ecosystem health would demand the restoration of the hydromorphological status quo ante, involving the removal of structures, the relocation of settlements and decontamination to reactivate natural evolution, which is clearly impracticable. From a geological and geomorphological perspective it is evident that lagoon ecosystem heath should ideally be measured relative to a moving baseline that is in step with its natural hydromorphological evolution. Pragmatism, however, dictates that this is not possible. However, it should be recognised that a so-called ‘healthy’ lagoon ecosystem in the 21st century is likely to be far removed from that which would have existed under conditions of unabated hydromorphological evolution.

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