Aquatic Ecology 34: 355–367, 2000. 355 © 2000 Kluwer Academic Publishers. Printed in the Netherlands. Effect of environmental factors on the spatial distribution of the epifauna of the alga Halopteris scoparia in Algeciras Bay, Southern Spain J.E. Sanchez-Moyano,´ E.M. Garc´ıa-Adiego, F.J. Estacio and J.C. Garc´ıa-Gomez´ Laboratorio de Biolog´ıa Marina, Dpto. Fisiolog´ıa y Biolog´ıa Animal, Facultad de Biolog´ıa, Universidad de Sevilla, Apdo. 1095, 41080 Sevilla, Spain (E-mail: [email protected].) Accepted 17 January 2001 Key words: algal epifauna, Halopteris scoparia, southern Spain Abstract The physical characteristics and environmental versatility of the alga Halopteris scoparia (Phaeophyta, Sphacelari- ales) make it a suitable substrate for development of epiphytic communities. Spatial variation of the epifauna on this alga in Algeciras Bay (southern Spain) in response to different environmental conditions is investigated. There is a clear difference in community composition between external and internal areas of the bay, with an important group of species present in only one of the areas (e.g., in outer areas crustaceans such as Tanais dulongii or Amphilochus neapolitanus or the polychaete Nicolea venustula; and species from inner areas such as the crustacean Jassa marmorata and the mollusc Alvania montagui or Rissoa similis). The external zone shows high hydrodynamics and low sedimentation rates, whereas in the internal one, there is a high sedimentation rate (as a result of two main rivers, a less strong current regime, and the presence of urban and industrial wastes). The conditions prevailing in the internal zone of the bay are unfavourable for most of the epifaunal species in the external bay areas. Introduction of waters from the Atlantic and Mediterranean seas, reduce the possible negative effects. Algeciras Bay (southern Spain) located in the Strait Such has been demonstrated previously by our re- of Gibraltar, provides a non-uniform medium with di- search team in the distribution of particular zoological verse environmental conditions that should give rise to groups such as sponges and ascidians; certain condi- differing composition of the benthic communities. It is tions (e.g., lesser regimen of currents, urban and indus- a submarine canyon of pronounced bathymetry (with trial effluents, river mouth, ...) were found to be more depths of more than 500 m) and includes a narrow restricting for growth of the organisms towards the in- platform limited by the 30-m contour, with a width terior of the bay (Carballo et al., 1994, 1996; Naranjo not exceeding 2 km. Its coast (30 km in perimeter et al., 1996). However, in these cases, quantification and 8 km wide at the mouth) houses an important in- is complicated, above all for colonial organisms. This, dustrial complex, including petrochemical companies, together with the problems derived from a great spatial two power stations, iron and steel works, paper mills, non-uniformity, makes comparisons between rocky ar- etc., together with a busy port whose seawalls, piers eas difficult. Other studies carried out by us on the and fills cause alterations in the natural currents, with, communities of the sediment and the epiphytes of the in some cases, a low water renovation rate. This is Bryozoan Bugula neritina have enabled us to evalu- worsened by the lack of urban sewage treatment and ate the environmental state of some of the bottoms a considerable load of nutrients and sediments from (Conradi et al., 1997; Estacio et al., 1997). The study the rivers – due to increasing agricultural activity and of the structure of animal communities associated to the effect of erosion. Nevertheless, its great water macroalgae of wide spatial distribution, relatively easy mass and intense hydrologic regimen, with circulation to collect and quantify, could reveal the effect of the 356 different environmental characteristics throughout the The samples were sieved through a 0.5-mm mesh. bay. The different species were separated, classified, and In order to discover the effect of environmental quantified. The vagile epifauna was selected, although factors on the epifauna of macroalgae, the variations solitary sessile organisms such as polychaetes and caused by the algal configuration must be elimi- bivalves were also included. The remaining sessile an- nated. Thus, we selected a single algal species of imals were discarded, in particular the colonial ones, high environmental versatility, and assumed that the because they were impossible to quantify, at least at a intraspecific variations produced in the alga by the level different to that of the vagile organisms. different environmental conditions will be less than The data of abundance were expressed as number between different substrates. The macroalga selected of individuals per 100 g (dry weight) of alga. was Halopteris scoparia (L.) Sauvageau (Phaeophyta, A series of parameters was calculated for each al- Sphacelariales). This species is erect and flexible, gal specimen: maximum height, diameter, volume, perennial, and can be found from the midlittoral, in and dry and fresh weight. Volume was estimated by intertidal pools, to the infralittoral, especially on rocks displacement of water in test-tube. The theoretical vol- of sandy bottoms. It is common in the whole area of ume (‘canopy volume’) was calculated assuming that, Algeciras Bay, and is one of the most abundant algal in the environment, H. scoparia adopts a geometric species throughout the year. Its physical characteris- form akin to a parabola. Deducting the real volume tics (dense ramification, large number of interstices, from the theoretical one gives the interstitial volume, ...) make it a suitable alga for supporting an interesting which represents the space useful for organisms. Fur- epiphyte community (Sánchez-Moyano, 1996). thermore, the ratio between the theoretical and real volumes gives an idea of the level of compactness of the alga (compactness index) (Sánchez-Moyano, Materials and methods 1996; Sánchez-Moyano & García-Gómez, 1998). This compactness can be considered a measurement of Thirteen stations were selected, grouped in five areas habitat complexity. along the coast of Algeciras Bay in order to encom- Species richness and Shannon–Wiener diversity pass the greatest range of environmental conditions (loge) were calculated from the data of species abun- (Figure 1). The Isla de las Palomas (IP), located at the dance. western lip of the bay, is a photophil rock zone. Punta The following environmental variables were con- de San García (SG) has similar characteristics to the sidered: maximum and minimum temperature, hydro- former, although with a sciophilous environment at dynamics, sedimentation rate, % of organic matter lesser depth. In both areas, stations were established of the sediment, and solids and % of organic mat- along a transect (200 m long) and at 5, 8, and 10 m ter in suspension. Sampling was carried out monthly in depth. The Cucareo inlet (CU), close to the port from November 1992 to November 1993 from con- of Algeciras, is a wide platform between 3 and 5 m crete structures sunk throughout the bay between 5 and in depth; its three stations were located on a transect 10 m in depth (Figure 1: sites S1 to S9). 200 m long. Los Rocadillos (Guadarranque) (GU) is Water movement was estimated using the ‘plas- in the internal zone of the bay, close to the mouth ter dissolution’ method modified by Gambi et al. of the River Guadarranque on a strip of natural rock (1989) with six replicates and an exposure time of running along the coast between 3 and 5 m in depth. 48 hours. Hydrodynamics is expressed as ‘equiva- Two stations were set up at different distances from lent water speed’ (V). The sedimentation rate was the mouth: GU1 (the further) and GU2. Crinavis (CR), measured using sediment traps (six 1-l bottles) which also located in the internal zone, is a disused shipyard. were removed monthly. The data are expressed as The two stations were situated at 5 m in depth. kg/m2/month. Part of the sediment was used to calcu- Four samples were taken at each station in five late the percentage of organic matter by combustion samplings during the year (September 92, Decem- at 500 ◦C. Solids and organic matter in suspension ber 92, March 93, June 93, and September 93). Each were determined by the method of Strickland & Par- sample consisted of one algal specimen, which was son (1969). The median was employed for statisti- bagged in situ and extracted from the substrate by cal analysis and to show the annual trend for each SCUBA diving. variable. 357 Figure 1. Location of the sampling stations in Algeciras Bay. IP: Las Palomas Island; SG: San Garc´ıa Point; CU: Cucareo inlet; GU: Guadarranque; CR: Crinavis; S1 to S9: physical data sites. The space and time differences in diversity, species in grouping of the different stations. This analysis, richness, total abundance, and environmental variables based on the matrix of similarity in species abundance were analyzed by one-way ANOVA, after verifying obtained from the Bray–Curtis index, calculates the normality (Kolmogorov–Smirnov test) and uniformity contribution of each species to either the dissimilarity of variances (Barlett test). Homogenous groups were between groups of stations (discriminatory species) or separatedbyaTukeytest. the similarity within a group (typifying species). Affinities between stations were established us- ing an MDS (non-metric multidimensional scaling) analysis with the annual average species abundance Results (transformed by the fourth root). Canonical correspondence analysis (CCA) was ap- Environmental variables plied to determine whether the environmental vari- ables affected community composition, and to estab- Most of the physical data of the sites were selected lish affinities between stations. This technique is one to show the general variability of these parameters of direct gradient, where the resulting ranking is di- throughout the bay, although, on the other hand, some rectly related with the values of the environmental were selected as representative areas of the sites where factors (Ter Braak, 1986).