View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Sapientia J. Mar. Biol. Ass. U.K. (2003), 83,4180/1^11 Printed in the United Kingdom Changes in benthic community structure due to clam dredging on the Algarve coast and the importance of seasonal analysis P O Filipe Alves* ,Lu|¤ s Ch|¤charo*, Anto¤ nio Nogueira and Joa‹ oRegala* *Centro de Cie“ ncias do Mar, Universidade do Algarve, Faculdade de Cie“ ncias do Mar e do Ambiente, Campus de Gambelas, O 8000-810 Faro, Portugal. Departamento de Biologia, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal. P Corresponding author, e-mail: ¢[email protected] Patterns in community structure of meiofauna and macrofauna in relation to Portuguese clam dredging were compared during a 2-y-period o¡ Lagos and Vilamoura, south Portugal. SCUBA divers randomly sampled corer and quadrat samples before and immediately after simulating commercial dredge ¢shery. Univariate measures (abundance, number of taxa, evenness, diversity and biomass) and multivariate ana- lyses (Cluster, MDS and SIMPER) revealed changes in the meio- and macro-benthic community structure caused by dredge disturbance (short-term e¡ects), with a general decrease in all measures. Macrofauna were found to be more sensitive to dredge disturbance, and aggressive predatory behaviour was observed after disturbance in the continuously dredged area. Nevertheless, higher signi¢cant natural changes were found between seasonal periods, indicating that dredging may only cause a small-scale and short-term impact. INTRODUCTION Parry, 1996; Kaiser & Spencer, 1996; Pranovi & Giovanardi, 2000). Such studies can provide reliable Commercial ¢shing can a¡ect benthic communities by results; however, long-term e¡ects are more di⁄cult to directly removing and/or injuring surface-living organ- evaluate and are consequently scarce in the literature isms in the path of the equipment, or by digging into the (Witbaard & Klein, 1994; Tuck et al., 1998). sediment and thus a¡ecting both infauna and the epi- The analysis of seasonal changes in benthic assemblages benthos (Bergman & Hup, 1992; Tuck et al., 1998). The is also of major importance in ecological studies. Seasonal environmental e¡ects of shell¢sh dredging have thus variations among several marine coastal benthic commu- received particular recent attention worldwide (see Caddy, nities of the north-east Atlantic have been studied in the 1968, 1973; Meyer et al., 1981; Hall et al., 1990; Kaiser & North Sea (Buhl-Jensen & Fossa, 1991; Hamerlynck & Spencer, 1996; Jennings et al., 2001). Mees, 1991), the Baltic Sea (O¤ lafsson & Elmgren, 1997), In Portugal, bivalve dredging has played important the Celtic Sea (Bottrell & Robins, 1984), the Bay of economic and social roles since 1969. Today, the industry Biscay (Sorbe, 1999) and the west coast of Portugal (R|¤a targets the species Spisula solida, Donax trunculus, Venus de Aveiro; see Cunha et al., 1999). However, there has striatula, Pharus legumen and Ensis siliqua (Gaspar, 1996). been no analysis of the seasonal variations of the meio- Bivalves are caught with a dredge that can penetrate and macro-fauna benthic communities of the south into the sediment up to 50 cm, depending on the target Portuguese soft-bottom coastal waters. species and sediment type. However, only recently has The results presented here concern sampling in two areas concern developed about the possible impact of dredging of the Algarve coast during a 2-y-period, to investigate the on the fauna of Portuguese waters, as shown by studies of short-term changes in the meiofauna and macrofauna damage to E. siliqua (Gaspar et al., 1994, 1998, 2001), benthic communities structure caused by clam dredging, predatory aggregation (Ch|¤charo et al., 2003) and diver- and to compare them within seasonal variations. The sity along dredge track sections (Ch|¤charo et al., 2002a). results are discussed in relation to overall ¢shing impact In general, dredges are designed to dig into the surface for the areas, and highlight the importance of seasonal £uc- of the sediment and can cause extensive mortality to the tuations for an accurate assessment of short-term changes. benthic organisms, particularly in areas with an estab- lished sessile epifauna (Holme, 1983; Robinson & Rowell, 1990; Kaiser & Spencer, 1994). The dredge ¢shery’s MATERIALS AND METHODS impact upon benthic communities is responsible for Sampling design changes in biodiversity, taxonomic composition, feeding habits and community structure (Currie & Parry, 1996, The research vessel ‘Donax’ undertook dredging at a site 1999; Jennings et al., 2001). These changes in the commu- 7^9 m deep (plummet data) o¡ Fale¤ sia Beach, Vilamoura, nity structure caused by ¢shing disturbance can be and o¡ Lagos Bay, both in the Algarve region of South detected on a short-term scale (de Groot, 1984; Currie & Portugal (Figure 1) in April and October 1999, and in Journal of the Marine Biological Association of the United Kingdom (2003) 4180.2 F. Alves et al. Seasonal analysis for short-term impact studies Figure 1. The South Portugal (Algarve) coast, showing location of the studied areas: Lagos and Vilamoura (according to Ch|¤ charo et al., 2002b). Table 1. Signi¢cance levels from two-way ANOVA tests for di¡erences in various univariate measures of community structure (see text) between: (1) before/after dredging; (2) seasons; and (3) interaction between both, for meiofauna and macrofauna at Lagos and Vilamoura. Meiofauna Macrofauna Lagos Vilamoura Lagos Vilamoura Univariate measure 121Â21 21Â21 21Â21 21Â2 pp A (pp) 50.01 50.01 50.01 0.07 50.01 0.05 50.01 50.01 50.01 0.43 50.01 0.09 T ( ) 50.01 0.04 50.01 50.01 0.12 0.92 50.01 50.01 50.01 0.87 50.01 0.02 J 0.06 50.01 0.46 0.12 50.01 0.13 0.04 50.01 0.16 0.17 0.6 0.31 0 H pp 0.19 50.01 0.76 50.01 0.04 0.08 50.01 50.01 0.12 0.56 50.01 50.01 B ( ) 50.01 50.01 0.66 0.5 50.01 0.24 0.37 50.01 0.34 0.66 50.01 0.05 Table 2. Mean and standard deviations of the meiofauna abundance (m2) before and after dredging, at Lagos and Vilamoura, using data from all seasonal periods. n. id., non identi¢ed. Blanks represent zeros. Lagos Vilamoura Meiofauna taxa Before After Before After Phylum NEMATODA 293730 Æ263584 127677 Æ99623 122450 Æ125925 84000 Æ86434 Class TURBELLARIA 29230 Æ175552 435 Æ861 450 Æ605 219 Æ608 Class OLIGOCHAETA 11932 Æ34576 2565 Æ6232 300 Æ571 250 Æ718 Order COPEPODA 9378 Æ13223 5516 Æ12117 18600 Æ22993 12031 Æ25555 Class POLYCHAETA 6554 Æ6194 3694 Æ3405 2400 Æ6012 844 Æ1568 Order ISOPODA 1770 Æ8921 65 Æ248 300 Æ571 94 Æ390 Phylum NEMERTINEA 1203 Æ3351 1645 Æ8667 1400 Æ2741 969 Æ2845 Class BIVALVIA 1122 Æ3515 1113 Æ3398 400 Æ754 1594 Æ3826 Order AMPHIPODA 797 Æ3701 210 Æ547 100 Æ308 31 Æ177 Order OSTRACODA 608 Æ1004 242 Æ645 550 Æ1395 406 Æ798 Order TANAIDACEA 446 Æ846 484 Æ1036 Phylum TARDIGRADA 243 Æ857 32 Æ254 Phylum CNIDARIA 189 Æ734 16 Æ127 31 Æ177 Class GASTROPODA 135 Æ448 31 Æ177 Ordern CUMACEA 68 Æ302 48 Æ216 50 Æ224 63 Æ246 Class HALACAROIDEA 68 Æ253 950 Æ1356 469 Æ915 Order CLADOCERA 16 Æ127 Phylum SIPUNCULO 250 Æ1107 n. id. Taxa 176 Æ866 97 Æ534 100 Æ308 31 Æ177 July and October 2000. On each occasion, a commercial can be up to 100 cm wide, with teeth up to 10 cm in length, Portuguese clam dredge (Figure 2) was used. The dredge which act as a rake when the dredge is dragged over the was towed for 1 min at a mean speed of 2 knots; this ¢shing sea bottom (Gaspar, 1996). speed is similar to that used by commercial dredge £eets. Six replicate quadrats (0.0625 m2Â0.15m depth) for Our survey was done with the type of dredge used by macrofauna and four replicate corers (0.001m2Â0.15 m commercial ¢shers for the clam Spisula solida. The dredge depth) for meiofauna were randomly sampled by SCUBA is a large, heavy iron structure, with a 25-mm mesh net bag divers from the dredge track immediately after dredging. and a toothed lower bar at the mouth. The gear’s mouth The control samples were collected from the study area Journal of the Marine Biological Association of the United Kingdom (2003) Seasonal analysis for short-term impact studies F. Alves et al. 4180.3 before dredging, using the same method and number of replicates. This procedure was performed in triplicate on each occasion and location. Macrofauna were sieved in situ using 1-mm mesh bags. Laboratory procedure All the residues were preserved in 70% ethanol. Meiofauna were sieved using a 500-mm (upper limit) and 15 0 -mm (lower limit) mesh sieve. Identi¢cation of all organ- isms in a community to species level is a major time and cost constraint (Warwick, 1993; Clarke & Warwick, 1994; Somer¢eld & Clarke, 1995). Moreover, many studies on Figure 2. Representative view of a Portuguese clam dredge meio- and macro-benthic disturbance have shown that (according to Monteiro, 1985). Tooth bar, 100 cm; tooth size very little information is lost by working at a taxonomic for Spisula solida, 10 cm; space between tooth, 1.5 cm; net bag, level higher than species (e.g. family or even phylum; see 25-mm mesh size. Heip et al., 1988; Herman & Heip, 1988; Warwick, 1988; Figure 3. Means and standard deviations of univariate measures: abundance (A), number of taxa (T), Pielou’s J,Shannon^ Wiener diversity (H0)andbiomass(B)before(^)andafter(&) dredging for: (A) meiofauna from Lagos station; (B) meiofauna from Vilamoura station; (C) macrofauna from Lagos station; (D) macrofauna from Vilamoura station.