Ecological Surveys of Saline Lagoons on the Benacre NNR and Suffolk Coast NNR February 2012
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Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 1 Ecological Surveys of Saline Lagoons on the Benacre NNR and Suffolk Coast NNR February 2012. An ecological survey including floral and fauna observations undertaken for Natural England by Abrehart Ecology. Defra License number:----------- abrehart ecology Carried out by Toby R. Abrehart FLS Dr Simon J. Forster Pound Farm Low Road Great Glemham Suffolk IP17 2DQ Tel: 01728 663282 - 07798 941555 Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 2 C O N T E N T S Page Summary 4 1. Introduction and background 8 2. Materials and methods 9 2.1 The field survey and sample collection 9 2.2 Laboratory procedures and analyses 11 3. Site-by-site results 12 3.1 Benacre Pit 12 3.2 Benacre Broad 15 3.3 Covehithe Broad 26 3.4 Easton Broad 28 3.5 Reedland Marsh 30 4. General discussion 59 4.1 Benacre Pit 59 4.2 Benacre Broad 60 4.3 Covehithe Broad 62 4.4 Easton Broad 63 4.5 Reedland Marsh 64 5. Acknowledgements 65 6. References cited in text 65 Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 3 Appendices: Appendix A Taxonomic list for the macrofaunal species recorded in the sweep net samples collected during the Suffolk saline lagoon survey, February 2012. Appendix B Site-by-site results for the macrofaunal species recorded in the sweep net samples collected during the Suffolk saline lagoon survey, February 2012. Appendix C Location figures and photographs of the lagoons and sampling sites assessed during the Suffolk saline lagoon survey, February 2012. Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 4 Summary Background Abrehart Ecology was requested by Natural England to undertake an ecological assessment survey of selected saline lagoons on the Suffolk coast. Previous studies had been undertaken in 1996 (Bamber, 1997) and 1998 (Bamber, 1999). The aim of the survey was to provide an update as to the ecological status of the selected lagoons and to provide Natural England with relevant data to enable informed managerial decisions to be made in the future. The survey was undertaken between 16 - 18 February 2012. The findings Benacre Pit Benacre Pit supported the most diverse and abundant aquatic plant and animal communities recorded in the lagoons that were assessed during the February 2012 survey. A range of archetypal lagoon species were documented included the protected species Nematostella vectensis (Starlet Sea Anemone) and Gammarus insensibilis (Lagoon Sand Shrimp). To determine the full ecological and conservational value of Benacre Pit, it is recommended that a detailed survey of the N. vectensis and G. insensibilis population as well as the aquatic plants is undertaken at a more appropriate time of year (for example late summer 2012). The principal threat to the Benacre Pit saline lagoon habitat appeared to be associated with the encroachment and/or breaching of the shingle embankment on the eastern (seaward) side of the site. Benacre Broad To mitigate coastal erosion, a number of the small pools were created in 1996 to provide additional lagoon habitat. Prior to November 2011, Benacre Broad Lagoon had been a stable, near freshwater system for four years. In November Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 5 2011 the shingle bank at the eastern end of Benacre Broad was over-topped and this, ostensibly freshwater, lagoon was filled with sea water. The 2011 inundation event caused the main broad to briefly become fully saline, this was then diluted with freshwater inputs from the sluices at the western end of the broad. The previously shallow “lagoon” pools had been submerged under deep water and in-filled with fine sediment for over four years. Within the main body of water, the floral and faunal assemblages were poor. The estuarine amphipod Corophium volutator had become dominant and was recorded at very high densities at a number of sites. Conversely, the low salinity amphipod, Corophium multisetosum, previously recorded by Bamber (1999), was not observed. No Nematostella vectensis was recorded. It is considered that the habitat within the eastern section of the main broad is no longer suitable for this species. The western-most sampling sites were separated from the main broad by a clay bund. At the time of sampling, the conditions at these sites were freshwater. In the freshwater site there was evidence of eutrophication of the water, including the presence of large quantities of the cyanobacteria (blue-green algae) Nostoc species. It appears that nutrient-rich run-off from farmland may cause the apparent eutrophication and that this may represent an immediate threat to the water quality, and hence the ecological value, of the Benacre Broad aquatic system. Covehithe Broad Over recent years, Covehithe Broad has been reduced in size due to coastal erosion and the encroachment of sand and gravel. Evidence indicated this process was continuing. This is a significant threat to the lagoon habitat. The flora and fauna of Covehithe Broad was very poor. No previous data was available for the invertebrates’ communities. However, a demonstrable decline of the aquatic floral community had occurred over the past decade. There were indications that the water was eutrophic. It appeared that a potential source of nutrient-rich waters was surface run-off from the Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 6 adjacent free-range pig farms. Further investigations to determine the current water quality status of Covehithe lagoon and the source and pathway of any nutrient-rich materials entering the water body is recommended. Easton Broad Easton Broad was a low salinity lagoon system, at the time of the survey the salinity was 0 ‰. The extent of the water body had been reduced by sand encroachment. This process was continuing. The flora and fauna at Easton Broad was generally poor in terms of abundance and number of species. There was also evidence of eutrophication. The endangered (RDB 1) snail, Mercuria (=Pseudamnicola) confusa, was recorded from the edge of the Phragmites bed at the south-eastern end of the lagoon. The continued existence of M. confusa is threatened by (i) the continued encroachment of the sand and (ii) the potential for persistent salinity increases due to regular over-topping of the sand dune coastal embankment. It is recommended that targeted assessment of M. confusa and suitable habitats within the local area is undertaken to enable appropriate mitigation and/or habitat enhancement measures to be undertaken. Reedland Marshes The Reedland Marshes lagoons continue to support a relatively wide range of lagoon and estuarine invertebrates. A very similar array of species was recorded during the previous survey of this area during 1998. The species of particular conservational importance was the scheduled species Nematostella vectensis and Gammarus insensibilis. The occurrence and abundance of the Starlet Sea Anemone, Nematostella vectensis, was significantly reduced at the time of the February 2012 survey compared to the results of the previous 1998 survey. However, it is likely that this was due to the time of year at which the surveys were undertaken. Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 7 The encroachment of the shingle embankment had reduced the number and area of saline lagoons. This was particularly apparent at the northern end of the Reedland Marshes survey area. It was noted that encroaching shingle provided a suitable substrata for coastal vegetated shingle, a Biodiversity Action Plan priority habitat. Evidence of tidal variations in the water depth within the lagoons in the mid- and southern sections of the Reedland Marsh survey area was noted. At the time of the survey a number of sites had up to 90% of the lagoon substrate exposed. It was suggested that tidal conditions would diminish the potential of the sites to support specialist lagoon floral and fauna species. Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 8 1. Introduction and background Abrehart Ecology was requested by Adam Burrows (Senior Reserves Manager, Suffolk Coast National Nature Reserves) of Natural England to undertake an ecological assessment survey of selected saline lagoons on the Suffolk coast. Previous studies had been undertaken in 1996 (Bamber, 1997) and 1998 (Bamber, 1999). The saline lagoons of the Suffolk coast are, in the absence of expensive coastal defence measures, threatened by coastal erosion and the relentless encroachment of the sand and shingle into the marshland environment. In addition, inundation events, such as breaches in, and over-topping of, the coastal defence embankments and have caused dramatic, rapid changes in the physical conditions of coastal water bodies. The objectives of the 2012 saline lagoon survey were: to provide an indication of the diversity and species structure of the invertebrate and plant communities that currently exist in the selected saline lagoons, to determine and document any changes in the conditions of the lagoon and their associated floral and faunal communities since the previous survey was undertaken, to provide baseline information against which (i) any future changes in the lagoon conditions could be compared and (ii) the success of any mitigation and/or habitat enhancement measure could be evaluated, and to provide Natural England with valuable information to enable informed decisions regarding the future management of the coastal lagoon systems to be made. Abrehart Ecology Natural England Saline Lagoon Survey, Benacre NNR and Suffolk Coast NNR, 2012 9 2. Materials and methods 2.1 The field survey and sample collection The saline lagoon survey was undertaken by Abrehart Ecology between 16 - 18 February 2012.