Journal of Vector Ecology
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Colonization of UK coastal realignment sites by mosquitoes: implications for design, management, and public health J.M. Medlock and A.G.C. Vaux Medical Entomology and Zoonoses Ecology Group, Microbial Risk Assessment, Emergency Response Department, Health Protection Agency, Porton Down, Salisbury, Wiltshire, UK,
[email protected] Received 22 August 2012; Accepted 27 October 2012 ABSTRACT: Coastal realignment is now widely instituted in the UK as part of local flood risk management plans to compensate for the loss of European protected habitat and to mitigate the effects of sea-level rise and coastal squeeze. Coastal aquatic habitats have long been known to provide suitable habitats for brackish-water mosquitoes and historically, coastal marshes were considered to support anopheline mosquito populations that were responsible for local malaria transmission. This study surveyed the eight largest managed realignment (MRA) sites in England (Essex and the Humber) for mosquito habitats. The apparent absence of anopheline mosquitoes exploiting aquatic habitats at all of these sites suggests that the risk of malaria associated with MRA sites is currently negligible. However, three of the eight sites supported populations of two nuisance and potential arboviral vector species, Aedes detritus and Aedes caspius. The aquatic habitats that supported mosquitoes resulted from a) specific design aspects of the new sea wall (ballast to mitigate wave action and constructed saline borrow ditches) that could be designed out or managed or b) isolated pools created through silt accretion or expansion of flooded zones to neighbouring pasture. The public health risks and recommendations for management are discussed in this report. This report highlights the need for pro-active public health impact assessments prior to MRA development in consultation with the Health Protection Agency, as well as the need for a case-by-case approach to design and management to mitigate mosquito or mosquito-borne disease issues now and in the future. Journal of Vector Ecology 38 (1): 53-62. 2013. Keyword Index: Climate, mosquitoes, wetlands, ecology, control, Aedes. INTRODUCTION The rise in sea levels around the coasts of the UK is being driven in two ways. Recent studies show that the ice sheets in Greenland and Antarctica are melting faster than the snow is replacing the mass. In their fourth report (Solomon et al. 2007), the Intergovernmental Panel on Climate Change predicted a global sea level rise increase of 18-38 cm by 2100 in the most optimistic scenario and 26-59 cm in the most pessimistic. In the UK, the effect of this sea level rise is further exacerbated by the effect of glacial isostatic adjustment (or post-glacial rebound/depression). This phenomenon is related to post-glacial activity whereby northern Britain undergoes an uplift following deglaciation and southern Britain sinks, particularly in southern and eastern England. Predictions on sea level rise have been made by the UK Climate projections (UKCP09), supported by the UK Department for Environment, Food and Rural Affairs. Incorporating both these factors, the central estimates for London of sea level rise with respect to 1990 levels predict a 5.3-7.3 cm rise by 2010, 8.211.5 cm rise by 2020, 18.4-25.8 cm rise by 2050, and 30.5-43.3 cm rise by 2080 (Murphy et al. 2009). This presents challenges to coastal stability, particularly in the face of storm surges (Murphy et al. 2009). This has led to the implementation of managed realignment (MRA) schemes along the coast which essentially involve replacing artificial hard coastal defenses with natural soft defenses like coastal habitats (Leggett et al. 2004). MRA is the deliberate process of altering flood defenses to allow flooding of a presently defended area. It is now considered to offer long-term sustainable management of coasts and estuaries for a variety of stakeholders. It can reduce the pressures of coastal
squeeze and offer potential new habitat creation and re-creation opportunities (Leggett et al. 2004). An MRA scheme may take many forms, including a) retreating to higher ground, i.e., where a line of defense is breached/removed, allowing inundation up to the higher ground, producing a new intertidal area, b) constructing a set-back line of defense, thus protecting property landward of the defense, or land at lower altitude to the defense, c) shortening the overall defense length to be maintained, thus reducing costs or d) inundation through sections of defense whereby the breach allows inundation of land behind it through a defined gap, or through pipes with tidal flaps to allow an intertidal area to develop behind the defense. The flood management and environmental benefits of MRA are manifold (Leggett et al. 2004). These include a) reducing flood risk or potential for reduction in whole life costs of flood defense, b) ensuring long-term sustainability of defenses by increasing natural flood and storm-buffering capability, c) reducing the costs, particularly where existing defenses are not economically viable, d) assisting with managing the effects of sea level rise by reducing height of sea/estuary levels, and e) mitigating the effects of previous reclamations and of climate change (i.e., sea level rise), and thus offsetting the impact of coastal squeeze. Most significantly, they lead to creation of intertidal habitat, usually mudflat and salt marsh with the associated benefits for wildlife and protected species, as well as improving recreational value. MRA schemes in the UK addressing coastal squeeze are largely initiated by the Environment Agency (EA) as part of their flood defense remit. However, they may also arise as compensation sites for direct loss usually through the Environmental Impact Assessment (EIA) process and/or as part of an Appropriate
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Assessment under EU legislation. For the latter schemes, these are initiated by the prospective developer, in this case the competent authority, Associated British Ports (Mander et al. 2007, Hemingway et al. 2008). In addition to the UK, MRA is now being widely used at coastal sites in France, Belgium, the Netherlands, Germany, and Denmark. Their impact on mosquitoes is therefore a concern for a number of European countries. With respect to mosquitoes however, salt marsh habitat and coastal marshes (both favoring brackish waters) have been, and are currently, responsible for providing habitats for nuisance biting mosquitoes and, albeit historically, disease vector species. Many of the sites in coastal Essex and Kent where malaria was endemic in the 19th century were drained, and this was considered to be a major contributory factor to the decline of malaria in England (Dobson 1997). The most recent outbreaks of indigenous malaria in the UK in the 20th century were associated with coastal populations of Anopheles atroparvus (Lindsay and Willis 2006). In certain parts of the UK, the salt marsh mosquito Aedes detritus is a significant biting pest (Medlock et al. 2012) and an economic burden to local councils. Increased temperatures in southern England as a consequence of climate change, which could favor pathogen development, coupled with the increase in imported cases of mosquito-borne disease and the emergence of exotic arboviruses in Europe suggest that we should not be complacent when considering the creation of new brackish and coastal habitat in England with respect to public health (Medlock and Leach 2012). Aspects that exacerbate the potential nuisance caused by salt marsh mosquitoes are related to a) the high marsh where pools of water in mud flats or salt marsh vegetation are left by the highest tides, or alternatively are filled by rainfall/runoff or not flushed by daily tide movements, and b) the low marsh that is not well drained and where mosquitoes exploit impounded stagnant pools that are retained, usually due to siltation/blockage of tidal channels and hence not flushed (Russell 2009). Management strategies include the elimination of the potential aquatic habitat (by draining or filling), modification (with water management), and treatment with a control agent to kill the mosquito larvae. Elimination is usually not possible, but modification with strategies such as ‘Open Marsh Water Management’ (OMWM) or the use of shallow ditches (runnels) has been reported to be acceptable, practical, and effective (Russell 2009). OMWM was developed to control mosquitoes by introducing their natural predators to areas of salt marsh. With a system of pools connected by radial ditches (i.e., runnels), fish feed on mosquitoes during high tide, then retreat to sumps or reservoirs at low tide. OMWM has been found to be an effective long term method of controlling mosquito populations in salt marshes without using sprays (SWS 2009). This strategy promotes/restores ‘full tidal flushing’ by advocating the renovation of tidal channels and maintaining them in a condition which allows a) full tidal exchange and precludes the formation of impounded pools and b) ‘natural dewatering,’ whereby salt marsh pools that hold water after highest tides and rainfall are connected for tidal influence using various sized channels and with persisting ponds to support predatory fish (Russell 2009). Since 1995, there have been at least twenty coastal realignment projects on the east coast of Britain between and
June 2013
including the Thames and Humber estuaries, the largest of which covers >350 hectares (Hemingway et al. 2008). Some of these sites are in areas where malaria was previously common. Such newly created habitats have the added benefit of providing additional breeding grounds for birds, and the combination of wild birds and mosquitoes is now widely accepted as increasing the transmission risk of bird-associated viruses such as West Nile virus (Medlock et al. 2005). Despite significant biodiversity monitoring of these MRA sites post-sea incursion (Hemingway et al. 2008), there has been no assessment of the colonization by mosquitoes or the associated potential disease risks. Some sites are in remote parts fringing large estuaries, with little or no neighboring human settlement. However, new sites are being designed and planned for the south coast of England where human populations are significantly higher. A baseline understanding of the possible nuisance biting and public health risks associated with such coastal habitat creation is now required (Medlock and Vaux 2011), particularly if the design of new sea defenses are found to inadvertently create unwelcome mosquito habitats. This study surveys the largest eight MRA initiatives in Essex and the Humber to ascertain whether mosquitoes are able to colonize them. Furthermore, by comparing recent schemes with those built in the 1990s, it may be possible to semi-qualitatively assess the rate of colonization of mosquitoes. Consideration will also be given to the impact of new sea wall design on creating new aquatic mosquito habitats and an assessment of the potential public health risks, with suggestions made on design mitigation strategies to limit potential nuisance and disease risks. MATERIALS AND METHODS Field sites on the Humber Field visits were made to eight of the largest coastal realignment sites in the UK, four on the Humber Estuary in Yorkshire and North Lincolnshire and four in Essex (Table 1, Figure 1). On the banks of the Humber, Paull Holme Strays, Welwick, and Chowder Ness were all designed with the construction of a new sea wall set back from the estuary with one or more breaches in the old wall. All three sites receive highly saline water input and are flooded on each high tide and almost entirely drained at low tide. The dominant habitat created at Welwick and Chowder Ness is mudflat, with areas of salt marsh developing by the new sea wall; particularly at Welwick. The sea wall at both of these two sites is constructed with a subsidiary bank of ballast on the seaward side, presumably to mitigate the effects of the high water and wave action. Chowder Ness forms part of the south Humber Bank series of mainly freshwater wetlands close to Barton-Upon-Humber. Welwick lies adjacent to arable farmland and the silted up entrance to Patrington Haven, an area of coastal flooded grassland. Paull Holme Strays is a more complex site with a series of islands (outstrays), areas of mudflat, and developing salt marsh. It is located between the city of Hull and arable farmland, with limited neighboring coastal habitat. The fourth site on the Humber, Alkborough, is located at the head of the estuary at the confluence of the rivers Ouse and Trent, where the input is mainly freshwater. Water enters the site through
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a
b
c
Figure 1. Location of managed realignment sites (black dots) surveyed a) on the Humber estuary and b) on the Essex coast. Main towns are represented by grey dots, with size representing their human population c) locations within Great Britain.
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Table 1. Overview of managed realignment sites surveyed. Site
Location (Coordinates)
Size
Date of construction
Habitats created and driver
Paull Holme Strays
East Yorkshire (53.7N, 0.2W)
80 ha
2003 (EA)
45ha mudflat, 35ha saltmarsh; address coastal squeeze and improve flood risk management
Chowder Ness
Lincolnshire (53.7N, 0.5W)
15 ha
2006 (ABP)
Mudflat; compensation for impact of creating new harbour
Welwick
East Yorkshire (53.6N, 0.03E)
54 ha
2006 (ABP)
18-38ha mudflat, 12-28 ha saltmarsh; compensation for impact of creating new harbour
Alkborough
Lincolnshire (53.6N, 0.6W)
440 ha
2006 (EA)
375ha of estuarine habitat; flood storage and flood risk management
Wallasea Island
Essex (51.6N, 0.8E)
125 ha
2006 (RSPB)
90ha mudflats, 23ha saltmarsh, 6ha freshwater marsh; replace lost habitats for conservation
Abbotts Hall
Essex (51.8N, 0.8E)
120 ha
1999 (EWT)
120ha of estuarine habitat; replace habitats lost for conservation, ageing sea defences
Orplands
Essex (51.7N, 0.9E)
38 ha
1995 (NRA)
New habitats for wildlife, mitigate storm surges
1995 Essex Mainly mudflat habitat 21 ha (NRA) (51.7N, 0.8E) * EA (Environment Agency), ABP (Associated British Ports), RSPB (Royal Society for Protection of Birds), EWT (Essex Wildlife Trust), NRA (National Rivers Authority). Tollesbury
a single narrow breach, and the inundation area is not restrained by a sea wall, although there is a flood bank some way from the inundation zone. The neighboring habitat in the east is mainly cattle pasture, and to the south there is a new area of recently created freshwater habitat (i.e., reed-beds, ponds, and ditches) as well as inundated ash woodland. Two of the sites (Welwick, Chowder Ness) were constructed as part of a compensation package during the development of a new roll-on roll-off terminal at Immingham Harbour. The other sites (Paull Holme Strays, Alkborough) were developed as part of the Humber flood risk management plan and to mitigate coastal squeeze. Alkborough was additionally constructed to reduce water levels in the upper estuary during extreme events, being able to reduce high tide levels by 15 cm, in order to minimize flooding in the city of Hull. Field sites in Essex In Essex, two of the sites (Orplands, Tollesbury) were constructed in 1995 and were pioneering projects at the time. They were initiated to mitigate the effects of storm surges in the Blackwater Estuary and its tributaries and provide additional habitat for wildlife and recreation. Tollesbury is an extension of Tollesbury Fleet and is surrounded by extensive salt marsh and mudflat habitats. The site is almost entirely inundated by the tide. Orplands lies on the main Blackwater estuary and is largely surrounded by arable farmland with some extant salt marsh in the vicinity at St. Lawrence creek. The old sea wall at Orplands has been breached in several places with salt marsh habitat developing behind, constrained by the new sea wall.
The other sites (Abbotts Hall, Wallasea Island) were largely initiated by conservation organizations with the aim of providing habitats for birds. However, they also had additional benefits in assisting flood risk management and replacing aging sea defenses. Abbotts Hall is situated on the north bank of the Salcott Channel, a tributary of the Blackwater estuary. The old sea wall has been breached in five locations, with mainly salt marsh habitat developing behind. The MRA site at Wallasea lies on the south bank of the River Crouch estuary and forms only a small part of a much greater project to return the whole of the island to coastal habitat. In the MRA site, mudflat is dominant, with some salt marsh. However, pipes through the new sea wall allow inundation by saline water of a large 3 km borrow-ditch created by sea wall construction, lying immediately parallel with the sea wall. The grading of this ditch varies so that some areas remain permanently wet and others are flooded at each high tide or only at spring high tides. Survey strategy Two field visits were made to each of the eight sites during March/April and August, 2011 to coincide with the main activity period of the immature stages of coastal mosquitoes (principally An. atroparvus, Ae. detritus, Ae. caspius, and Cx. modestus). All aquatic habitats at each site were visited on both occasions and using standard mosquito dippers (5 x 200 ml dips per location), the occurrence of mosquito larvae was recorded. Aquatic habitats were defined as new aquatic habitat within the MRA area that is either within the inundation zone or at the margin of the inundation zone, as new aquatic habitat adjacent to the inundation
11 87 22 300 35 387 -
TOTAL
7 22 7 22 1.85±0.08 Yorkshire
Welwick B
18 Mar, 5 Aug
0 1 14 50 14 51 1.21±0.11 Yorkshire
Welwick A
16 Mar, 5 Aug
0
0 0
4 1
0 6
23 1
0 6
27 2.00±0.2
1.87±0.61
Essex Wallasea
4 Apr, 16 Aug
Essex Tollesbury
6 Apr, 16 Aug
0 2 0 56 0 58 1.10±0.06 16 Mar, 5 Aug Yorkshire Paull Holme Strays
0
0 0
0 0
0 3
33 0
0 3
33 2.44±0.2
0.13±0.03 18 Mar 8 Aug
4 Apr, 16 Aug Essex
Lincolnshire Chowder Ness
Orplands
4 53 7 79 11 132 0.016±0.005 Lincolnshire Alkborough
17 Mar, 9 Aug
2 23 0 28 2 55 1.85±0.27 Essex
15 Apr, 16 Aug
+ve sites adjacent MRA n sites adjacent MRA +ve sites within MRA n sites within MRA Total sites +ve for mosquitoes Total no. aquatic sites surveyed Salinity range (mean±SE)
Abbott’s Hall
Welwick The MRA site at Welwick (hereafter Welwick A) is largely mudflat and almost entirely inundated by saline waters at high tide (visits were made at high and low tide) and consequently was largely unsuitable for immature mosquitoes. The extant salt marsh habitat on the unbreached section of the old coastline is regularly flushed with no evidence of pooling to support mosquito aquatic habitats. However, adjacent to the new sea wall, brackish water collects in front of, among, and behind a subsidiary bank of ballast. All mosquitoes found at this site were located here and all were Ae. detritus, found in substantial numbers both in March and August. Specific aquatic habitats included water among unvegetated ballast, water in a linear depression between the ballast and the sea wall, and among flooded halophytic vegetation on the seaward side of the ballast. All these pools appear to be the result of pooling after spring tides or from spray. These were isolated from the estuarine waters even at high tide (Figure 2a).
Dates
Almost 400 aquatic habitats were sampled across the eight MRA sites, with the number of samples and number of positive mosquito samples presented in Table 2. The majority of mosquitoes were associated with the tidal-influenced aquatic habitat in the MRA (i.e., not including extant habitat or newly created permanent freshwater habitats), with the highest numbers at Alkborough and Welwick, both on the Humber. Aedes caspius dominated at Alkborough and Ae. detritus at Welwick. Aedes detritus was also found in the saline borrow ditch at Wallasea Island (Table 3). Additionally, Ae. detritus was found in extant habitat neighboring the MRA site at Abbotts Hall, with Culex pipiens and Ae. caspius in extant habitat neighboring Alkborough. Anopheles maculipennis s.l. and An. claviger were found in newly created permanent freshwater ditches and ponds adjacent to the inundated zones at Alkborough. Further details of the specific positive locations at each site are discussed briefly below.
County
RESULTS
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Location
zone within the MRA boundary (always freshwater habitats), or as extant aquatic habitat within the boundary of the MRA. All samples were returned to the laboratory for identification of mosquito species in accordance with Snow (1990) and Schaffner et al. (2001), and abundance was recorded by species and stage. Locations of aquatic habitats surveyed were recorded using a handheld GPS datalogger with further information recorded on salinity, temperature, and vegetation coverage. Each survey site was classified on the basis of its geographic location (Essex or Humber), whether the site had a subsidiary bank in front of the sea wall (lip or no lip), the salinity of the water (fresh or salt), and the impact of the tide (regular flood zone or marginal isolated pools). These constituted the explanatory variable associations between mosquito presence (the response variable) and were tested using chi-squared, Fisher’s exact and regression tests using Minitab 15 (State College, PA, U.S.A). In addition, and in order to make comparisons of new coastal habitats with extant coastal habitats, an additional extant salt marsh site at Seymer’s Marsh on Brownsea Island, Dorset (50.7N, 2.0W) was surveyed. This site was initially studied by Service (1968) and revisited by the authors in February, 2011.
Table 2. Analysis of mosquito sampling at managed realignment sites. ‘+ve sites’ refers to survey sites with mosquito immatures present; ‘within MRA’ refers to all sites that have been subject to managed realignment and therefore impacted by tidal waters; ‘adjacent MRA’ refers to additional freshwater habitat created within the construction site but not affected by tidal waters (i.e., beyond the new sea wall).
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b
a
c
d
Figure 2. Typical aquatic habitats for mosquitoes at coastal realignment sites. a). Welwick: pools above high tide adjacent to subsidiary ballast bank supporting Ae. detritus. b). Alkborough: pools in cattle pasture, left isolated by spring tides supporting Ae. caspius. c). Alkborough: pools among developing reed, at margin of mudflat supporting Ae. caspius. d). Wallasea: vegetated terrace adjacent to main saline ditch in excavated borrow-ditch supporting Ae. detritus.
Additional surveys were conducted in an adjacent area of coastal aquatic habitat (hereafter Welwick B), reported to be the silted up entrance of Patrington Haven. This area is bound by a sea wall, and includes an area of 500 m wide salt marsh and salineflooded coastal grassland. Aedes detritus was also found at this site in March but was only associated with areas of flooded grassland or vegetated pools at the spring high tide mark next to the sea wall. Alkborough Tidal waters enter the Alkborough site through a single breach and much of the inundated zone is developing mudflat. It was noticeable that accretion of the mudflat was much higher close to the entrance of the breach channel, with water collecting behind
this as more permanent shallow water at locations furthest from the breach. Salinity levels were extremely low with input largely of freshwater from the rivers Trent and Ouse. The fringes of the inundated zone are not bound by a sea wall, so there is significantly more emergent vegetated habitat at this site. Despite considerable sampling effort, immature mosquitoes were only found in two locations, each as a number of distinct aquatic pools at each location. Water from spring high tides had flooded cattle pasture on the eastern margin of the site, with a number of isolated linear open flooded sites on the grassland, which were nutrient rich with cattle dung and significantly puddled by animals. Larvae of Ae. caspius were found here in very high numbers in the drying pools (Figure 2b). On the western fringe, although mudflat water levels
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Table 3. Abundance of mosquitoes by species at managed realignment sites. Location
Inside/Adjacent MRA – site specific
Species
Stage (I-III/IV/P)
Abbott’s Hall
Extant saline pool adjacent to MRA site
Ae. detritus
22/0/0
Alkborough
Newly constructed freshwater ditches/ponds adjacent to MRA
An. claviger An. maculipennis s.l.
3/13/0 0/1/1
Alkborough
Flooded margin of ditch behind seawall, adjacent to MRA
Cx. pipiens s.l.
0/0/1
Alkborough
Flooded cattle pasture, beyond mudflat at margin of MRA
Ae. caspius Cx. pipiens s.l.
23/20/46 2/0/0
Alkborough
Reed dominated pool next to mudflat at margin of MRA
Ae. caspius Cx. pipiens s.l.
0/49/14 2/0/0
Wallasea
Intermediate bank of saline borrow ditch, behind sea wall, margin of MRA
Ae. detritus
2/2/4
Welwick A
Among vegetation and rocks in ballast lip below sea at margin of MRA
Ae. detritus
144/36/7
Welwick B
In extant saline pools with emergent reed, at margin of high tide, adjacent to MRA
Ae. detritus
41/0/0
at low tide were very low (
