Molluscan Diversity Along a Mediterranean Soft Bottom Sublittoral Ecotone
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SCIENTIA MARINA 70(4) December 2006, 573-583, Barcelona (Spain) ISSN: 0214-8358 Molluscan diversity along a Mediterranean soft bottom sublittoral ecotone PANAYOTA KOULOURI 1, COSTAS DOUNAS 1, CHRISTOS ARVANITIDIS 1, DROSOS KOUTSOUBAS 2 and ANASTASIOS ELEFTHERIOU 3 1 Hellenic Centre for Marine Research, Institute of Marine Biology and Genetics, Gournes Pediados, P.O. Box 2214, 71003, Heraklion, Crete, Greece. E-mail: [email protected] 2 University of the Aegean, Department of Marine Science, Sapfous 5, 81100, Mytilene, Greece. 3 University of Crete, Department of Biology, P.O. Box 2208, Vasilika Vouton, 71409, Heraklion, Crete, Greece. SUMMARY: Molluscan diversity associated with sublittoral soft-bottoms was studied in Heraklion Bay as part of a long- term multidisciplinary research programme in the coastal environment of northern Crete (eastern Mediterranean). Quantitative benthic samples were collected in a pilot survey from a grid of 28 stations, followed by a seasonal sampling car- ried out over a seven- station transect located from 10 to 40 m depth. Univariate and multivariate analyses indicated the pres- ence of a clear zonation along the depth gradient for the molluscan fauna and three main associations were identified in bathymetric zones parallel to the coastline. The structure of the local molluscan communities revealed the existence of an extended transition zone (ecotone) between a shallow sandy association and a deeper muddy one and a functional pattern, based on the feeding guilds, was evident along the environmental gradient. Application of graphical techniques, by using molluscan taxocoenosis, indicated a rather naturally disturbed habitat. Beta-diversity measures indicated different seasonal community adaptations to the prevailing environmental conditions. Results derived from this study revealed that beta-diver- sity may be a good descriptor for the assessment of the strength and breadth of sublittoral transition zones along environ- mental gradients. Keywords: mollusc diversity, ecotone, sublittoral, soft bottom, beta-diversity, perturbation, environmental assessment, east- ern Mediterranean. RESUMEN: DIVERSIDAD MALACOLÓGICA EN UN ECOTONO SUBLITORAL MEDITERRÁNEO DE FONDOS BLANDOS. – La diversidad malacológica asociada a la zona infralitoral de fondos blandos se ha estudiado en la bahía de Heraklion como parte de un programa de investigación multidisciplinar a largo plazo en la costa norte de Creta (Mediterráneo Oriental). Muestras cuan- titativas del bentos fueron recogidas durante un muestreo piloto realizado en una parrilla de 28 estaciones. A continuación se realizó un muestreo estacional en un transecto de siete estaciones situadas entre 10 y 40 m de profundidad. Los análisis univariables y multivariables indicaron la presencia de una clara zonación malacológica a lo largo del gradiente de profun- didad, identificándose tres asociaciones principales en zonas batimétricas paralelas a la línea de costa. La estructura de las comunidades locales de moluscos puso de manifiesto la existencia de una extensa zona de transición (ecotono) entre la aso- ciación de arena somera y la de fango profundo y evidenció un patrón funcional basado en los tipos de alimentación a lo largo del gradiente ambiental. La aplicación de técnicas gráficas, empleando la taxocenosis de moluscos, indicó un hábitat perturbado. Lla diversidad beta indicó diferentes adaptaciones de las comunidades estacionales a las condiciones ambien- tales reinantes. Los resultados obtenidos en este estudio muestran que la diversidad beta puede ser un buen descriptor para la evaluación de la capacidad y la amplitud de las zonas de transición sublitoral a lo largo de gradientes ambientales. Palabras clave: diversidad malacológica, ecotono, sublitoral, fondos blandos, diversidad beta, perturbación, evaluación ambiental, Mediterráneo oriental. 574 • P. KOULOURI et al. INTRODUCTION factors which are instrumental in determining the molluscan community structure; and (c) to investi- The European Water Framework Directive gate the applicability of the turnover diversity meas- (WFD; Directive 2000/60/EC) gives emphasis to urements as a potential descriptor of the molluscan the assessment of the Ecological Quality Status community responses to natural or anthropogenic- (EcoQ) of coastal and transitional waters, as these induced environmental perturbations in this specific highly dynamic systems are usually exposed to coastal environment. anthropogenic forcing. Although macrozoobenthos is among the basic biological elements in the WFD, the benthic community approach has received crit- MATERIALS AND METHODS icism because the procedures involved, such as sampling, sample analysis and computing, are too Study area time-consuming and labour-intensive to be cost- effective in environmental assessments (Warwick The present study was conducted in the sublit- et al., 1990). The use of different sieve mesh sizes toral zone of Heraklion Bay at depths of 10 to 40 (Thompson et al., 2003) and the application of sub- metres, covering an area exposed to wave action sampling techniques (Carey and Keough, 2002) along a coastline stretching more than 18 km (Fig. have been proposed for the establishment of more 1). The coastal zone is affected by small-scale fresh- cost-effective methods. Additionally multivariate water inputs and urban waste discharges, while the techniques (Olsgard et al., 1997) and recently offshore system is characterised by marked oligotro- developed sample-size/sample-effort free biodiver- phy (Karakassis and Eleftheriou, 1997; Psarra et al., sity indices have also been used as an alternative to 2000). The study area is located outside the local species-level identification of the total fauna (e.g. bottom trawling fishing grounds. Warwick and Clarke, 2001). Therefore, it has been A detailed description of the environmental vari- proposed that the diversity of single taxonomic ables measured seasonally in the surface sediments groups, e.g. amphipods (Gómez-Gesteira and of the study area is given in Dounas and Dauvin, 2000), cumaceans (Corbera and Cardell, Papadopoulou (1993). In summary, sediment tem- 1995) and polychaetes (Samuelson, 2001), could perature, taken 2 cm below the sediment surface, be used as an indicator of environmental stress and shows strong seasonality, ranging from 14.2°C in pollution. winter to 24.0°C in summer. Overall temperature Although benthic molluscs have been used as gradually decreases with depth, with a particular descriptors of sublittoral soft-bottom benthic com- sharp decrease observed at the 40 m station in sum- munities (Zenetos, 1996; Rueda et al., 2001), there mer and autumn indicating the depth of the seasonal is no information concerning their use as an appro- thermocline. Surface sediment redox potential (Eh) priate taxonomic group for the detection and dis- values range from 86 mV in summer to 454 mV in crimination of community response to natural or winter, indicating moderately to well-oxygenated anthropogenic-induced forcing. The present study is sediments throughout the year. Lower Eh values are part of a long-term multidisciplinary research pro- generally observed in the deeper muddy areas. gramme of the Hellenic Centre for Marine Research Chlorophyll-a concentrations in the sediment range in the coastal environment of northern Crete from 0.14 µg g-1 in winter to 1.83 µg g-1 in spring, (Eastern Mediterranean), and especially in with higher values recorded at the intermediate Heraklion Bay, with the main objective of investi- depths. Phaeopigment concentrations gradually gating the structure and function of the benthic increase with depth and range from 0.10 µg g-1 in ecosystem in this area (Dounas and Papadopoulou, autumn to 4.71 µg g-1 in spring. Total organic carbon 1993; Karakassis and Eleftheriou, 1997; Koulouri et (TOC) values range from 0.21 mg g-1 in summer to al., 2000; Tselepides et al., 2000). The detailed 8.43 mg g-1 in winter, with the highest values in the objectives of the present study were: (a) to describe deeper areas throughout the year. Sediment particle the bathymetric distribution and seasonal variability size decreases with depth in all seasons. Therefore, of the sublittoral soft bottom molluscan taxocoeno- while sediments deeper than 20 m contain higher sis by performing a series of uni-, multivariate and percentages of mud (60.85 to 90.97%), shallower graphical methods; (b) to identify the environmental depths consist mainly of sand (89.29 to 96.47%). It SCI. MAR., 70(4), December 2006, 573-583. ISSN: 0214-8358 MOLLUSC DIVERSITY ALONG A SOFT BOTTOM SUBLITTORAL ECOTONE • 575 FIG. 1. – Map of Heraklion Bay showing the sampling stations (10, 20, 30 and 40 m) of the seven transects sampled during the pilot survey (July 1990) and the location of the seven stations (10, 15, 20, 25, 30, 35 and 40 m) along the transect H2 sampled during the following four surveys (October 1990, February, May and July 1991). is worth mentioning that a seasonal shift in sediment series of reference manuals were used for the classi- type observed at about 20 m depth changes from fication of the specimens and can be found in mud (83.12% silt) in winter to sandy mud (14.35 to Koutsoubas (1992), Zenetos (1996), Delamotte and 33.23% silt) throughout the rest of the year. Vardala-Theodorou (2001), while the nomenclature followed was that adopted by Sabelli et al. (1990). Sampling techniques Shell-free dry weight (SFDW) calculations were made using the mean values of conversion factors A pilot field survey was carried