Analysis of the long term consequences of sand extraction on the macrofauna communities

Transcription

1 Analysis of the long term consequences of sand extraction on the macrofauna communities Ine Moulaert, Hans Hillewaert & Kris Hostens Ecological Monitoring Section CLO - Sea Fisheries Department, Ankerstraat 1, 84 Oostende, Belgium. 1. Compilation of historical data concerning the macrobenthos of the Kwintebank and surroundings (from 1979 onwards): density, diversity and community structure Figure 3.1 shows the position of the different stations on the Kwintebank sampled by the Sea Fisheries Department from 1979 onwards. Figure 3.1 Location of the main stations on the Kwintebank sampled by SFD 25

2 Table 3.1 gives an overview of the samples from the Kwintebank that have been gathered by the Sea Fisheries Department, with an indication of the samples (ZG1-ZG4) which have already been processed (data from the period ) and those that have not been processed yet (data from the period ). Table 3.1 Overview of the Kwintebank samples gathered by SFD Station ZG1 np Np np np np np p p p p p p p P np ZG2 np Np np p p p p p p p P np ZG3 np Np np p p p p p p p P np ZG4 p p p p p P np 315 np Np np np np np np p p np np np np Np np ZG5- ZG1 P np For all samples at least 3 replicates for the macrobenthos and 1 sample for the sediment are available. All data are assembled in Excel data files. Station 315 is included for comparison at a later stage, while the stations ZG5 to ZG1 are specifically taken in to investigate the recovery processes of the macrobenthic fauna after cessation of sand extraction. 2. Evaluation of the long term consequences of sand extraction on the structural characteristics of the macrofauna communities 2.1 Introduction Several studies have been investigating the effects of marine aggregate extraction on the benthic fauna (see Boyd et al., 24). However, most of these studies are limited to the initial effects of extraction on species richness, abundance and biomass of the benthos (de Groot, 1979, 1986; Poiner & Kennedy, 1984; Pagliai et al., 1985; Jones, 1986; Van Moorsel, 1994; Kenny & Rees, 1994; Desprez, 2; Sarda et al., 2; Van Dalfsen et al, 2; Newell et al., 21; Van Dalfsen & Essink, 21; Boyd & Rees, 23; Boyd et al., 23; Guerra-Garcia et al., 23; Sanchez-Moyano et al., 23: Boyd et al., 24; Newell et al., 24a, 24b). Almost all studies are conducted over a relatively short time-scale, up to a period of 1 year. Thus by their nature, 26

3 these studies do not address the effects of dredging over the life-time of a typical commercial extraction licence. On the other hand, a number of studies deal with the recovery potential of the area or the rates and processes of macrobenthic re-colonisation upon cessation of the dredging activities. (Kenny et al., 1996, 1998; Desprez, 2; van Dalfsen et al., 2; Sardá et al., 2; van Dalfsen and Essink, 21 Boyd et al., 23; Guerra-Garcia et al., 23; Boyd et al., 24). An overview of the latter will be given in the next SPEEK report where re-colonisation will be handled. Most serious physical impacts of sand extraction are related to substratum removal, alteration of the bottom topography and sediment composition, changes in depth and current strength and the creation of plumes through the disturbance by the drag head and from screening (De Groot, 1996; Newell et al, 1998, Hacking, N., 23; Newell et al., 24b). Next to the indirect physical effects of sand extraction on the macrofaunal community, the direct removal of species and individuals is considered as the main biological impact. Dredging causes an initial reduction in abundance, species diversity and biomass of the benthic community in the extraction area (Poiner & Kennedy, 1984; Van Moorsel, 1994; Kenny et al., 1998; Desprez, 2; van Dalfsen et al., 2; Sardá et al., 2; van Dalfsen and Essink, 21; Guerra-Garcia et al., 23; Newell et al.,24b; Sanchez-Moyano et al., 24). But also outside the boundaries of the extraction area, an effect has been shown (Poiner & Kennedy, 1984; Desprez, 2; Newell et al., 24a, b). The effect of sedimentation from plume fall-out or from screening operations on the benthic fauna (and on the recolonization rate) is an issue which has been receiving increased attention (Poiner and Kennedy, 1984; Desprez, 2; Newell et al., 22; Boyd and Rees, 23). In a study on a fine sediment site in Moreton Bay (Australia), Poiner and Kennedy (1984) found higher abundances of benthic invertebrates adjacent to the dredged subtidal sandbanks and attributed this outside effect to the release of organic matter from the dredged plume material. More recently, Newell et al. (24a,b) found evidence that the impact of dredging extended beyond the margins of licensed extraction sites in the U.K, mainly in terms of the suppression of benthic biomass. They suggested that this was the result of the remobilisation of sediments through screening activities. Increased sedimentation and resuspension due to dredging of clean (mobile) sand are generally thought to be of less concern, as the fauna inhabiting such deposit areas tend to be adapted to naturally high levels of suspended sediments caused by wave and tidal current action (Newell et al., 24b). Several models try to describe the effects of disturbance on ecological communities, but only few are applicable to predict the responses of the benthic fauna to sand and gravel extraction. Two models, adapted from Gray (1976) and Pearson and Rosenberg (1978), can be used in the case of 27

4 sand extraction on the Kwintebank. The global stability model of Gray (1977) suggests that climax benthic communities could be viewed as a 'ball' occupying a 'basin of attraction' (Figure 3.2). Within this model the system always returns to the same stable equilibrium point with the same species dominating. A more useful model in the context of dredging related disturbance is the 'neighbourhood stability' model. This presupposes a local stable community in state 1 which can be altered into state 2 with a different species dominating, only by means of a slightly increased disturbance. To switch to state 3 (or to return to state 1) exposure to bigger perturbations would be required. A) Global Stability B) Neighbourhood Stability Perturbation Figure 3.2 Two models after Gray (1977) showing stability of the community represented by the ball which can be perturbed from stable equilibrium With respect to long-term marine sand and gravel extraction, the movement from one steady state community to another could be related to a change in sediment composition, e.g.. a shift from a sandy gravel to a gravely sand. However, it might be difficult to determine at which part of the curve the benthic community is located. For example, the non-return of a community to baseline conditions can be interpreted in two ways (1) the community has become truly unstable or (2) the community has shifted to another stable point. Complementary to the Gray (1977) model, the Pearson and Rosenberg (1978) model describes the community response to a series of events following organic enrichment. The model has been shown to be applicable to stable substrata at many localities. The Pearson and Rosenberg (1978) model predicts that as enrichment of the substratum takes place there is an initial increase in number of species and in abundance of most species (Figure 3.3). At this point, organic matter is sufficient to provide a rich food source, but does not significantly deplete oxygen levels or have any other adverse consequences (e.g. smothering). The initial phase is followed by a phase whereby the more sensitive members of the community disappear in response to physicochemical changes in the sediment, especially the rise of the redox discontinuity layer, while the 28

5 Abundance Taxa Numbers/Weight Biomass Intensity of effect Figure.3.3 Simplified models of change in the benthos in response to disturbance (after Pearson and Rosenberg, 1978) few remaining species (mainly small polychaetes) show a population explosion with high biomasses. In the next progression stage of the Pearson and Rosenberg (1978) model the number of species is significantly reduced. This can be seen as a generic 'disturbance' response. 2.2 Results and comparisons The initial change in the community as a consequence of sand extraction can not be studied for the Kwintebank as no baseline data are available. But the stability of the community during 25 years of sand extraction on the Kwintebank can be studied. As mentioned above the historical data from the Kwintebank sampled by the Sea Fisheries Department during the late 197s and 198s are not yet fully processed due to shortage of time and budget. Therefore data from other studies were used for comparison. Waeterschoot (198) used an 87 µm sieve, so the comparison of the biological data needs to be done with caution. Because of the differences in sieving methodology (25µm mesh size), only the sedimentological data from Vanosmael et al. (1982) will be used. Vanosmael & Heip (1986) used a 1 mm sieve after fixation, producing biological data comparable to the data available from the Sea Fisheries Department. Finally, the data gathered by the Sea Fisheries Department from the last 9 years ( ) also give an idea about the stability, either global or neighbourhood stability, of the macrobenthic community of the Kwintebank. Waeterschoot, station 46, vs. SFD, station ZG4,

6 Table 3.2 Comparison of data taken from Waetershoot (198) and SFD Macrobenthos Year Season med grain (µm) mud % gravel % density ind/m² # species Waeterschoot (198) SFD (own data) Spring Autumn Spring Autumn Spring Autumn Spring Autumn Spring Autumn Spring Autumn* Spring Autumn Spring Autumn S A S A S A S A S A S A S A S A S A S A S A S A S A S A S A S A Density # species median grainsize mudfractie Figure 3.4 Comparison of data taken from Waetershoot (198) and SFD For this comparison samples from almost the same location could be used, although Waeterschoot (198) used a smaller sieving mesh size (87 µm). The density and the species richness were lower in compared to (Table 3.2, Figure 3.4). The difference in density and species richness might be due to a different location of the sampling point. For the sediment small differences could be noticed in the median grain size (on average 29 µm in 79-3

7 8 compared to 25 µm in 98-3). On the other hand the mud fraction was lower (on average.2 % in the former vs. 1.5 % in the latter period). Vanosmael et al., station SB5, 1978 vs. SFD, station ZG9, 23 (only sediment data) Table 3.3 Comparison of data taken from Vanosmael et al (1982) and SFD 1978 (SB5) 23 (ZG9) depth (m) median grain size (µm) mud content (%).79 gravel content (%) organic material (%) skewness SB5 and ZG9 are located close to each other in an area of the Kwintebank where intensive extraction has taken place, but which has been closed for extraction since 23. Although it is only one sample and the locations of the two sampling points are not exactly corresponding, there was a large difference in median grain size between the sample taken in 1978 and the one taken in 23 (Table 3.3). Vanosmael & Heip, station 44, vs. SFD, station ZG6, 23 Table 3.4 Comparison of data taken from Vanosmael & Heip (1986) and SFD (44) 23 (ZG6) depth (m) median grain size (µm) mud content (%) gravel content (%) organic material (%) skewness macrobenthic density (ind/m²) species richness 2 17 diversity % polychaetes

8 The location of stations 44 and ZG6 is not exactly corresponding but they are located in the same central area of the Kwintebank. Median grain size has decreased in the last 2 years (Table 3.4). Although some interstitial species still occurred in the 23 samples at lower densities (Hesionura elongate or H. augeneri, Polygordius appendiculatus, Thia scutellata), several interstitial species were no longer found (Pisione remota, Streptosyllis arenae, Shaerosyllis bulbosa). The reduction in interstitial species due to the changes to sandy sediments might explain the reduced average densities of the macrobenthic fauna. SFD ZG1 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q ZG2 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q ZG3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q ZG4 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q3 Q1 Q Figure 3.5 Comparison of density (bars) en species (line) data from SFD period Comparing data from the last 9 years from different sampling stations in sand extraction zone II gives us an idea of the stability during that period (Figure 3.5). Differences in sampling techniques have to be kept in mind when comparing the data (day/night, different people determining the species, etc.). These definitely introduce variability which can not be excluded. However, it can be concluded that no real changes in species richness, abundance or macrobenthic community structure have occurred since

9 2.3 Discussion The impact of sand extraction depends on numerous factors, including extraction method and intensity, sediment type and mobility, bottom topography and current strength. The effects are also dependent of the type of macrofaunal community present and are thus site specific. As the Southern North Sea is a dynamic area and no screening of the sediment is needed during the dredging operation, some of the effects of sand and gravel extraction mentioned above will not be as important. Both the short term studies and the models emphasize the importance of the initial period in relation to the main changes in the community structure of the macrobenthos. As no base line data (before sandext raction took place, i.e. before 1976) are available, the initial impact of sand extraction on the Kwintebank will remain unknown. The comparison with older data ( ) does not show big differences. Samples from Waeterschoot (198) and Vanosmael & Heip (1986), as well as the recent data ( ) from the SFD, are characteristic for sandbank systems in which the dominant ecotypes are mobile and quickly burrowing organisms such as the genera Hesionura, Microphthalmus and Nephtys. These species are able to withstand extreme physical disturbances of the sediment caused by strong tidal currents or sand extraction activities. So far we can conclude that in the last 9 years of sand extraction on the Kwintebank, no major changes can be detected in density, species richness, diversity or community structure. Therefore we assume that the macrobenthic community of the Kwintebank is currently in a stage of relative stability. References Boyd, S.E. & Rees, H.L., 2. The effects of dredging intensity on the macrobenthos in commercial aggregate extraction sites in the English Channel. ICES BEWG Report, Annex 6. Boyd, S.E. & Rees, H.L., 23. En examination of the spatial scale of impact on the marine benthos arising from marine aggregate extraction in the central English Channel. Estuarine, Coastal and Shelf Science 57,

10 Boyd, S.E., Limpenny, D.S., Rees, H.L., Cooper, K.M. & Campbell, S., 23. Preliminary observations of the effects of dredging intensity on the re-colonisation of dredged sediments off the southeast coast of England (Area 222). Estuarine, Coastal and Shelf Science 57, Boyd, S.E., Cooper, K.M., Limpenny, D.S., Kilbride, R., Rees, H.L., Dearnaley, M.P., Stevenson, J., Meadows, W.J. & Morris, C.D., 24. Assessment of the rehabilitation of the seabed following marine aggregate dredging. Sci. Ser. Tech. Rep., CEFAS Lowestoft, 121, 154pp. De Groot, S.J., An assessment of the potential environmental impact of large-scale sand-dredging for the building of artificial islands in the North Sea. Ocean Management, 5, De Groot, S.J., Marine sand and gravel extraction in the North Atlantic and its potential environmental impact, with emphasis on the North Sea. Ocean Management, 1, De Groot, The physical impact of marine aggregate extraction in the North Sea. ICES Journal of Marine Science, 53, Desprez, M., 2. Physical and biological impact of marine aggregate extraction along the French coast of the Eastern English Channel: short- and long-term post-dredging restoration. ICES Journal of Marine Science, 57, Gray, J.S., The stability of benthic ecosystems. Helgol Mar Res, 3, Guerra-Garcia, J.M., Corzo, J. & Garcia-Gomez, J.C., 23. Short-term benthic re-colonisation after dredging in the harbour of Ceuta, North Africa. P.S.Z.N.: Marine Ecology, 24(3), Hacking, N., 23. A review of the ecology of offshore ocean sediments with particular reference to marine aggregate resources for beach nourishment in New South Wales. Centre for Natural Resources. Department of Infrastructure, Planning and Natural Resources, Newcastle. Jones, A.R.,1986. The effects of dredging an spoil disposal on macrobenthos, Hawkesbury Estuary, NSW. Marine Pollution Bulletin, 17,

11 Kenny, A.J. & Rees, H.L., The effects of marine gravel extraction on the macrobenthos: early post dredging re-colonisation. Marine Pollution Bulletin, 28, Kenny, A.J. & Rees, H.L., The effects of marine gravel extraction on the macrobenthos: results 2 years post-dredging. Marine Pollution Bulletin, 32, Kenny, A.J., Rees, H.L., Greening, J. & Campbell, S., The effects of gravel extraction on the macrobenthos at an experimental dredge site off North Norfolk, UK (results 3 years post-dredging). ICES CM, 1998:V:14,1-7. Newell, R.C., Seiderer, L.J. & Hitchcock, D.R.,1998. The impact of dredging works in coastal waters: a review of the sensitivity to disturbance and subsequent recovery of biological resources on the sea bed. Oceanography and Marine Biology 36, Newell, R.C., Seiderer, L.J., Robinson, J.G. & Simpson, N.M., 21. Impact of marine aggregate dredging on benthic biological resources. Marine Ecological Surveys Ltd. St. Ives. Cornwall TR26 2PF. Technical Report for Coastline Surveys Limited, Stonehouse, Gloucestershire & U.S. Department of the Interior, Minerals Management Service. 96 pp. Newell, R.C., Seiderer, L.J., Robinson, J.E., Simpson, N.M., Pearce, B. & Reeds, K.A., 24a. Impacts of overboard screening on seabed and associated benthic biological community structure in relation to marine aggregate extraction. Technical Report to the Office of the Deputy Prime Minister (ODPM) and Minerals Industry Research Organisation (MIRO). Project No. SAMP Marine Ecological Surveys Limited, St. Ives. Cornwall. 152 pp. Newell, R.C., Seiderer, L.J., Simpson, N.M. & Robinson, J.E., 24b. Impacts of marine aggregate dredging on benthic macrofauna off the south coast of the U.K. Journal of coastal research, 2, Pagliai AMB, et al., Environmental impact of extensive dredging in a coastal marine area. Mar Poll Bull. 16(12), Pearson, T.H. & Rosenberg, R., Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanography and Marine Biology: an annual review 16,

12 Poiner, R. & Kennedy, R.,1984. Complex patterns of change in the macrobenthos of a large sandbank following dredging. Marine Biology 78, Sanchez-Moyano, J.E., Estacio, F.J., Garcia-Adiego, E.M. & Garcia-Gomez, J.C., 24. Dredging impact on the benthic community of an unaltered inlet in southern Spain. Helgol Mar Res, 58, Sardá, R., Pinedo, S., Gremare, A. & Taboada, S., 2. Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea. ICES Journal of Marine Science, 57, van Dalfsen, J.A., Essink, K., Toxvig Madsen, H., Birklund, J., Romero, J. & Manzanera, M., 2. Differential response of macrozoobenthos to marine sand extraction in the North Sea and the Western Mediterranean. ICES Journal of Marine Science, 57, van Dalfsen, J.A. & Essink, K., 21. Benthic community response to sand dredging and shore face nourishment in Dutch coastal waters. Senckenbergia Maritima, 31, Van Moorsel, G.W.N.M., The Klaverbank North Sea, geomorphology, macrobenthic ecology and the effect of gravel extraction (pp. 1-65). Report n Culemborg, The Netherlands, Bureau Waardenburg bv. Vanosmael, C., Willems, K.A., Claeys, D., Vincx, M. & Heip, C., Macrobenthos of a sublittoral sandbank in the southern bight of the north sea. J. mar. biol. Ass. U.K. 62, Vanosmael, C. & Heip, C A comparative study of the macrobenthos of three sandbanks in the Belgian coastal waters in Has sand exploitation an influence on the macrobenthos? in: Heip, C.; Coomans, A. (Ed.) (1986). Ecology, ecotoxicology and systematics of marine benthos. pp Waeterschoot H., 198. Macrobenthos van de Kwintebank ( ): studie in het kader van de zand- en grindexploitatie voor de Belgische kust. Lic.Thesis, University of Ghent, Belgium. 36