Use of Marine Sponges as Stress Indicators in Marine Ecosystems at Algeciras Bay (Southern Iberian Peninsula)

Vol. 135: 109-122, 1996 ' MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser 1 Published May 17 Use of marine sponges as stress indicators in marine ecosystems at Algeciras Bay (southern Iberian Peninsula) J. L. Carballo*, S. A. Naranjo, J. C. Garcia-Gomez Laboratorio de Biologia Marina, Dpto de Fisiologia y Biologia Animal, Facultad de Biologia, Apdo 1095, E-41080 Sevilla, Spain ABSTRACT Infrallttoral sponge fauna was studied as part of a multidisciplinary investigation of benthic communities in Algeciras Bay On a monthly basis over 1 year, a senes of environmental vanables were measured (hydrodynamlsm, silting, suspended solids, dissolved organic matter % orgamc matter In silt) The only abiotic vanable that was significantly correlated wlth beta diversity was hydrodynamism, with a linear iegression model between the 2 variables showing a correlatlon coefficient of 0 66 The distributional pattern of the sponges (based on the relative abundance matrix) was correlated with the environmental vanables by matchlng sample simllanties uslng the Spearman rank correlatlon thus showing that the variable combination that best explalns the patterns of distribution is hydrodynamlsm/organlc matter ~n silt (p, = 0 6) Of the species considered, Phorbas fictit~us, Cliona celata, Cl~ona v~ndis, Crella elegans. Oscarelld lobular~s, Dysidea fragilis were among those show~ng the greatest adaptive plast~city in their relatlonshlp to environmental variables, depth, and selection by substrate, and are categorized as eurytoplc specles present in areas subject to great environmental stress Other species such as Phorbas tenacior, Ren~era fulva Renjera mucosa Cliona rhodensis proved to be much more sensitive to these vanables and were categonzed as stenotop~c species lndicators of normal conditions Due to the part~cular environmental cond~tions where it is located, A4ycale rnicracanthoxea was categonzed as a good ind~cator species in port environments Others such as Dysidea avara, Halichondna bower bank^ 01 Crella elegans presented morphological differentiations which have permitted them to adapt to sedimentary environments KEY WORDS. Sponge communit~es . Abiotic environment . Bioindicator species. Stress. Port environments INTRODUCTION Sponges are dominant members of many benthic communities and, from the autoecological point of view, individual sponge species can have very specific requirements for substrate quality, food particles, light, current regime (Reiswig 1971, Wilkinson & Vacelet 1979). Moreover, sponges are strongly associated with the abiotic environment and are therefore very sensitive to environmental stress, which is why some authors have suggested them as useful tools for environmental monitoring (Alcolado 1978, Alcolado & Herrera 1987, Muricy 1989, 1991, Carballo et al. 1994). There is now widespread recognition that chemical monitoring alone is not enough and that pollution IS essentially a biological phenomenon because its impact is on hving organisms (Mlright et al. 1994). Stress operates on individual organisms, but its effects may be recognised at population and ecosystem levels. Stress induces qualitative and quantitative changes in the structure of communities which can usually be assessed by analysing diversity, relative abundance (Hawkins et al. 1994). Sponge communities can provide us with important information about the temporal behaviour of the abiotic factors which affect them; hence, it is feasible to interpret the environment by studying this ecological O Inter-Research 1996 Resale of full article not permitted Mar Ecol Prog Ser 135: 109-122, 1996 structure (Alcolado & Herrera 1987). However, the use of sponges in monitoring studies is still limited by the little knowledge available on biotic and abiotic factors affecting the structure of these communities. Algeciras Bay, Spain, is characterised by a great patchwork of small homogeneous environments, and along its 30 km coastline there are important industrial areas with chemical industries, refineries, thermal power plants, ironworks, paper mills, and shipyards, along with a major port. All of this activity may in some way influence the sessile benthic organisms found in the bay. The amount of urban sewage corresponds to a population of approximately 205000 inhabitants; industrial contamination corresponds to 150000 inhabitants (Wait et al. 1990). Into the bay's center flow 2 rivers, the Palmones and the Guadarranque, each with an amount of sewage corresponding to 15000 inhabitants, and industrial contamination corresponding to 15000 inhabitants for the former and 2500 for the latter (Wait et al. 1990). The composition and abundance of the benthic communities situated in Algeciras Bay are conditioned by their level of adaptation to general environmental factors and to factors of a more local character, such as organic material, contamination, and deep water movement. These physico-chemical factors generally have a great influence on the distribution of the majority of filtering sessile invertebrates. In this sense, we have related the distnbution patterns of sponges to a series of abiotic variables (water movement, suspended solids), which are considered by the majority of authors as responsible, along with a series of other biotic factors (type of substrate, epibiotic relationships, symbiosis, etc.), for the presence/absence of a species in a given habitat (Sara & Vacelet 1973, Wilkinson & Cheshire 1989). codes which permitted us to objectively compare the information obtained at the various sampling stations (Table 1). Coding usually tends to have the effect of normalising data, and subsequent transformation is then unnecessary (Field et al. 1982). In order to monitor water quality we analysed hydrodynamism, dissolved organic matter, suspended solids, silting and % organlc matter in the silt gathered in collecting bottles (SOM]. These parameters were measured on a monthly basis for l yr at 8 of the 12 stations. To measure silting and hydrodynamism a set of bottom-mounted frames was placed along the Algeciras Bay littoral, each one having 6 sediment collecting bottles (Moore 1972), which remained for a month at each sampling station (Carballo et al. 1994). To estimate the water movement we used the method of plaster dissolution described by Gambi et al. (1989). The hydrodynamism was calculated in water speed equivalents (V) according to the formula suggested by Bailey-Brock (1979). The suspended solids, dissolved organic matter and SOM (silt which remained in the sediment traps) were measured using methods described by Strickland & Parsons (1960). All environmental variables were log transformed to normality, the latter confirmed by means of a chi-squared test. Similarities among the sampling stations were established by means of a classification analysis, using as variables the species present in the established zones. The similarity matrix for the classification was calculated by means of the Bray-Curtis index (Bray & Curtis 1957). This index was chosen as it does not consider MATERIAL AND METHODS The sampling stations inside Algeciras Bay were chosen on the basis of the heterogeneity of the environment at depths between 5 and 30 m. In this way, stations were set up at the main port constructions, .,,,,,,-, shipyards, piers, natural rock formations, etc. Fig. 1 ,!?:E;=:) shows the various sampling areas. The specimens were D (R.&S-C-J E (%awl d vrlfi~l r& collected by SCUBA diving along permanent transects wide he pan d ,q~rru) D in order to ensure sampling reproducibility. Each tran- ~,~u~,~~~ioal,b,nhehmour R sect (approximately 50 m long) was sampled at least ~.dmd.lbrthrm~mpr~~) L ISaluml ad .ntTmrl nxks m rhr rd* hubour) 4 times a year with a similar immersion time at all the N (c,,, ~b,~, stations (there were no replicates on each occasion). All g :~~~y-&~"~&ludr~~,huhvhovl sponges sighted at a distance of 1 m at either side of the R Eu-) transects were collected and/or photographed. We used semi-quantitative sampling, which is more appropriate for our subject, and we established abundance _)B Fig. 1. Locatlon of the sampling stations in the study area Carballo et al.. Marine sponges as stress indicators 111 double absences, frequently found in our data base, in its calculations. The results were then graphically described using dendrogranls with the UPGMA ('unweighted pair-group method using centroids') aggregation algorithm (Sneath & Sokal 1973). The species mainly responsible for the dissimilarity between sampling stations were determined using the computer program SIMPER (Clarke 1993). The ordination analyses were carried out by means of an MDS ('non-metric multi-dimensional scaling program') based on the similarity matrix between stations. In order to individually relate environmental variables to sampling stations, the letters of the stations, proportional in size to the numerical value of each variable, were superimposed on a 2-dimensional MDS configuration to provide a 'visual correlation' (Field et al. 1982). In this manner we were able to study the influence of different environmental variables on the various areas of study. In addition we carried out a factorial analysis in order to explore the relationship patterns which group the various species together. The relationship between the abiotic and the biotic variables was studied by means of a correlation and regression analysis, as well as the BIO-ENV program which establishes the relationships between the data and the way in which the biotic and environmental data are linked (Clarke & Ainsworth 1993). The software used was PRIMER from Plymouth Marine Laboratory for MDS, SIMPER and BIO-ENV analysis, and Statgraphics for multiple 'box-and-whisker' plots, regressions and factorial analysis. RESULTS Complete data matrix: species composition Fig 2 shows the beta diversity (P, = - ) (~ilson U-l & Shmida 1984) for each of the stations sampled, where S is the total number of species recorded in the sampling station, and E is the average number of species found within the community samples. The total number of species varied considerably among the different areas. Generally, the main differences occurred between the group of stations situated at the innernlost parts of the bay (Stns E to 0) and the group of stations most externally situated (Stns A, B, C, D, Q and R). The maximum values occurred at Stn A (37 species), and the minimum value was at Stn I (16 species) (Table 1). Similarly, the beta diversity (P,") values were greatest at Stns A (1.510) and D (1.469). Table 1 shows the bathymetric distribution range, the substrate type, and the frequency of appearance in a determined enclave. The study of the local distribu- Fig 2. Beta diversity (P,,) of sponge comlnunities at sampling Stns A to R tion of the common species showed 3 basic distiibutional patterns: - Species found throughout the Bay: Cliona celata, Cliona viridis, Crambe crarnbe, Crella elegans, Dysidea fragilis, Oscarella lobularis, Phorbas fictitius, Terpios fugax, Suberites domuncula, etc. - Species distributed mainly at the extremes of the Bay: Acan thella acuta, Cliona rhodensis, Haliclona mucosa, Haliclona fulva, Dyctionella incisa, etc. - Species found mainly in the centre: Mycale micracanthoxea, Hy~nedesmia senegalensis, Stylopus dujardini, Haliclona cinerea, etc. Cluster analysis and MDS Fig. 3 shows the result of clustering the stations using the Bray-Curtis index for similarities. Cluster analysis distinguished 2 main groups of sites, which were considered as representing different communities on the basis of semi-quantitative species composition. In the first group (Gl) the sites are clearly at the exterior of the bay (Stns A, B, D, Q and R). Within the G1 group the strongest similarity (76%) occurs between Stns A and B, which are geographically very close and have a large number of common species. Moreover, there is approximately 68% similarity between these 2 stations and Stn R. We can observe 2 subgroups within group G2, one formed by Stns I, L, and N, corresponding to the most internal areas in Algeciras Bay (power plant, refinery and shipyards, respectively), and the other formed by Stns F, E, and C, corresponding to the Port of Algeciras and its zone of influence (E and F respectively), as well as a small cove located at the west margin (C). The species found at these 6 stations can generally be considered as cosmopolitan and abundant throughout the bay (Phorbas fictitius, Cliona celata, C. viridis, Crambe Mar Ecol Prog Ser 135: 109-122, 1996 Table 1. Complete data matrix. Coded abundances: 0 = absent; 1 =present (1 or 2 specimens on average per ~mmersion); 2 = common (between 3 and 5 specimens per immersion); 3 = very common (an average of more than 6 specimens per immersion). No.: total number of stations at which a species appears; 6: average number of species found within the community samples; V: vertical surfaces; 0: overhangs; H: horizontal surfaces; U: under rocks; CC: crevices and cavities; C: small caves; E: epibiont Taxa Relative abundance at Stn: ABCDEFILNOQR No. Bathymetric distribution and substrate onentatlon 0-5m 5-10m 10-20m 20-30m 1 Acanthella acuta 2 Ciocalapata almae 3 Antho ~nvolvens 4 AplysllIa rosea 5 Aplysllla sulphurea 6 Axinella dam~cornis 7 Batzella inops 8 Cacospongla mollior 9 Cacospongis scalaris 10 Chalinula fertilis 1 1 Chalunula nigra 12 Chondrosia reniformis 13 Clathria coralloides 14 Cliona celata 15 Cliona rhodensis 16 Cliona vastifica 17 Cliona viridis 18 Crambe crambe 19 CreUa elegans 20 Darwinella corneostellata 2 1 Dendroxea lenis 22 Dictyonella lncisa 23 Dys~dea avara 24 Dys~dea fragilis 25 Dysidea tupha 26 Erylus discophorus 27 Espenopsis fucorum 28 Gellius angulatus 29 Halichondna bowerbanki 30 Haliclona cinerea 3 1 Haliclona fulva 32 HaLicIona indistincta 33 Haliclona mediterranea 34 Haliclona mucosa 35 HaLiclona neens 36 Haliclona palmonensis 37 Hemim ycale columella 38 Hymedesmia pansa 39 Hymedesmia peachi 40 Hymedesmia senegalensis 4 1 Hymedesmia versicolor 42 Hymeniacidon sanguinea 43 Ircinia faciculata 44 Ircin~a oros 45 lrcin~a variablis 46 Leptolabis brunnea 47 Microciona strepxitosa 48 Mycale macilenta 49 Mycale massa 50 Mycale micracanthoxea 51 Mycale rotalis 52 Myxllla iotrochotina 53 Myxilla rosacea 54 Oscarella lobularis 55 Pellina semitubulosa 56 Petrosia ficdormis Carballo et al.: Marine sponges as stress indicators 113 - Table l (continued) Taxa Relative abundance at Stn: No. Bathymetric distribut~on and substrate orientation ABCDEFILNOQR 0-5m 5-10m 10-20 m 20-30m 57 Phorbas fictitus 2 2 2 2 2 1 2 2 1 3 2 1 12 H.V.0 H,V,E H.V.0 H,V 58 Phorbas paupertas 000100000000 1 U 59 Phorbas tenacjor 110000000012 4 0 60 Plakina rnonolopha 001000000000 1 U U 61 Pleraplysilla spinifera 111121120002 9 C,V,O H,O 0, c 62 Polymast~a mamillaris 001000000000 1 U.H 63 Pronax dives 000000000001 1 CC 64 Pronax lieberkijhni 000001000000 1 H 65 Pronax plurnosum 000010000200 2 H,E H,E 66 Phyteras fuslfera 100100000000 2 0 67 Raspaciona aculeata 100000000001 2 0 0 68 Sarcoti-agusmuscarum 1 1 1 3 2 0 0 0 0 O 1 0 6 0 0,v 0,V H 69 Sarcotragus spinosula 010100000001 3 0 0 70 Scopalinalophiropoda 1 2 2 1 3 1 0 0 0 0 1 2 8 V V, 0 H,CC 71 Scopalinamadeirensis 0 1 0 0 0 0 0 0 0 0 0 0 1 H 72 Spongia agaricana 120120000012 6 0 0 H 73 Spongia officinalis 110110000001 5 V O,H,V E 74 Spongia vilgurtosa 110000000001 3 E 75 Stylopus dujardini 000231322110 8 H,V,U H,V,U E,V,O H,V 76 Suberitis domuncula 11200111111110 E E E,O 77 Tedania anhelans 000100000010 2 CC.H 78 Terpios fugax 011011111101 9 U V. E V,H 79 Tethya aurantjum 110011000001 5 C,CC 0 0 80 Timea unistellata 111100000001 5 U 0 0 81 Ulosa stuposa 000100000000 1 E Total specles 37 31 21 36 24 18 16 21 18 19 30 32 U= 22.66 RBAODONLI FEC CI I G i Fig. 3. Dendrogram showing classification of 12 sampling stations using the Bray-Curtis index crambe, Crella elegans, Oscarella lobulans), or as species typical of harbour areas (Mycale micracanthoxea, Hymedesmia senegalensis). Ten species were responsible for approximately 50 % of the dissimilarity between group G1 and group G2 (Table 2). Table 2 Breakdown of average dissimilarity between groups G1 and G2 into contributions from each specles; species are ordered In decreasing contnbution (only partly given). 6,: contribution of the ith species to the average Bray-Curtis dissimilarity between the groups, L6,"t: cumulative percentage. Average dissimilarity between groups = 42.16 (SD = 9.076) Spongla agancinad Chondrosla reniform~s Antho involvens Phorbas tenacior" Cliona vastificaa Aplysilla rosea Sarcotragus muscarurn Stylopus dujardini Scopalina lophiropodaa Dysidea a vara 5pecies likely to be good discriminators of groups I 114 Mar Ecol Prog Ser 135: 109-122. 1996 Suspended solids I I Dissolved organic mrller 0 Hydrodynamism Silting r Fig. 4. Two-dimensional non-metric multi-dimensional scaling (MDS) plot of stations based on a similarity matrix species established by Bray-Curt~s ~ndex with each abiotic variable superimposed as letters of size varying in proportion to the abiotic value (stress = 0.124) The results of the MDS generally confirm the results of the cluster analysis (Fig. 4) and permit us to distinguish between the group of stations at the extremes of the bay (A and B on the west, and R on the east) to which the stations at the west (D) and east (Q) margins are closely situated, and the group of stations corresponding to the internal areas (I, L and N). Environmental variables A set of 5 environmental variables was recorded for 8 sites: suspended solids, dissolved organic matter, hydrodynamism, silting, and % organic matter (SOM) in the silt. Fig. 5 shows a multiple 'box-and-whisker' plot for each abiotic vanable analysed. The magnitude of the various parameters generally shows an inverse relationship between the group of stations located at the inner parts of the bay and the group of stations at the extremes. The highest average annual values for suspended solids were registered at Stn 1 (17.35 mg I-'), which also had the maximum annual values (52.35 mg 1-l). Silting tended to be higher at inner areas, where, as in the previous case, the maximum annual values (29.78 g m-2 mo-') and the highest average annual values (15.83 g m-2 mo-') were registered at Stn I. Hydrodynamism seems to act inversely to silting; the lowest values were registered at Stn I (4.39 V), and at a small partly enclosed port area (0) (2.85 V). The maximum annual values (17.6 V) were registered at Stn N, and the highest average annual values (7.62) occurred at Stn A. The annual average of dissolved organic matter was very similar at the various stations, with the highest average values registering at Stns E (3.77 mg I-'), L (3.53 mg 1-l) and I (4.16 mg I-'), in the interior of the bay, and at Q (3.47 mg 1- l), more externally situated; the lowest average values were registered at Stns A (3.13 mg 1-l) and D (3.15 mg I-'). In order to establish significant correlatlons between variables, we carried out a regression analysis and obtained the results shown in Table 3. The correlations established between the different parameters mainly present a linear-type regression. In the majority of the cases a fairly logical relationship was established In accordance with the function of the different variables. A clear positive linear relationship was found between suspended solids and silting, which indicates that an increase in suspended sol~ds translates into an increase in silting. On the other hand, silting and SOM follow a negative multiplica- Table 3. Summary of the abiotic variables which showed significant correlation (I) Model r Solids vs dissolved organlc matter Linear 0.747 Solids vs silting Linear 0.719 Solids vs g organic matter in silt Linear 0.805 Silting vs dissolved organic matter Linear 0.747 Silting vs SOM Multiplicative Y = aXb -0.888 SOM vs g organic matter in silt Multiplicative Y = aXb -0.843 Hydrodynamism vs beta diversity Linear 0.666 Carballo et al.: Marine sponges as stress indicators 115 tive-type model, which seems to indicate that small variations in silting, around 1 to 60 5 g m-' mo-', translate into elevated SOM values, and starting from certain silting val- 48 ues (>6 g m-* mo-l), SOM is maintamed at fairly stable levels. As is shown in Fig. 6, Q \ 3 0 SOM tended to be greater at the stations lo- D cated at the extremes of the bay than at the central areas. However, the real quantity of organic matter which settles (g) is much greater in the central areas of the bay, due to A the fact that silting tends to be greater in ADEILNOQ these areas (Fig. 5B). The relationship of SOM versus g organic matter follows a negative multiplicative regression model which clearly indicates that when SOM is low, the real quantity of organic matter (g) which settles is high (internal stations), and that when the SOM is high, the real quantity of organic matter which settles (g) is less (external stations) (Fig. 7). This relationship seems fairly 8 logical, given that not only are the internal parts of the bay withstanding wastes from a large industrial area, but there is also a con- RDEILNOQ FIOEILNOQ tinuous flow of urban and fluvial discharges which increase silting (composed of organic 28; D ' I ' ' I ' and inorganic matter), while at the extremes of the bay silting seems less and is mainly composed of organic matter from the same environment (Fig. 5B, D). Hydrodynamism is inversely related to silting but the relationship is non-significant (Fig. 6). Nevertheless, Fig. 6 suggests that with more data there may have been a relation, with the lowest values for silting being 6: registered when hydrodynamism is highest B- , and vice versa. The only abiotic variable FIDEILNOQ FIOEILNOQ (average annual values) which shows a significant correlation to beta diversity is Fig. 5. Multiple 'box-and-whisker' plot for each abiotic variable at 8 sampllngstations. (A) Suspendedsolids, nlg I-'; (B) silting, g m-2mo-'; (C) hydrohydrodynamism: a linear regression dynamism, V; (D) % organic matter (SOM), %; (E) dissolved organlc matter, exists between the 2 variables with a cormg l-' The small black or open diamonds indicate outliers relation coefficient (r) equal to 0.67 (Table 3). Stn N, which had an elevated hydrodynamism (6.4 V), Table 4 shows the combinations of variables which had lower beta diversity values than expected in give rise to the largest rank correlation (p,) between relation to the rest of the stations. If we exclude the biotic and abiotic sample similarities. The variable value of this station in the regression with beta diverwhich best groups the sites, in a manner consistent sity, we obtain a very significant new model (r = 0.83; with the fauna1 pattern, is hydrodynamism (p, = 0.48); p = 0.05). next best is SOM (p, = 0.47). The best 2-variable combinations are hydrodynamism and SOM (p, = 0.6), and hydrodynamism and silting (p, = 0.53). The best 3-vari- BIO-ENV procedure: Spearman rank correlation able combination is hydrodynamism-silting-SOM (p, = 0.57). The relationship that hydrodynamism shows The BIO-ENV procedure selects which subset of abiwith SOM can be explained because the areas where a otic variables maximises a correlation between the greater diversity was found are areas with good water biotic and abiotic similarity matrices. renewal and generally less organic matter (total g). 116 Mar Ecol Prog Ser 135: 109-122, 1996 ADtll.NOQ Sampling stations 1 8 Silting *p Orp*nic mamr + SOM +- Hflrodyrumkn I Fig. 6. Average value of 4 abiotic variables in the 8 sampling stations Factorial analysis With the data from Table 1, we carried out a factorial analysis in order to try to simplify the relationships formed between different species in terms of their distribution and relative abundance in Algeciras Bay (Fig. 8). Factor I clearly discriminates between 2 groups of species: a group formed by species which generally have a very precise distribution (Acanthella acuta, Tedania anhelans. Cliona rhodensis, Reniera Eulva, Reniera m ucosa, Pe frosia ficiformis, Haliclona neens, Dictyonella incisa) (negative side of axis), and a group we can categorise as composed of eurytopic species widely present in the different habitats and enclaves Table 4. Combinations of variables, taken K at a time, giving the largest rank correlations (p,) between biotic and environmental similarity, measured by standard Spearman coefficient. Bold type indicates the combination with the greatest p overall All variables are log-transformed K Best vanable combinations (p,) 1 Hydrodynamism Silting Organic Matter (0.486) (0.471) 2 Hydrodpamism - SOM Hydrodynamism -Silting (0.600) (0.537) 3 Hydrodynamism - Hydrodynamism - Dissolved Silting - SOM (0.575) Organic Matter - SOM (0.507) 4 Silting - Hydrodynamism - Dissolved Organic Matter - SOM (0.500) studied, such as Stylopus dujardini, Crella elegans, Phorbas fictitius, Cliona celata, C. viridis, Dysidea Eragilis, Suberites dom uncula, Terpios Eugax, Hymenia - cidon sanguinea, Scopalina lophiropoda, Cliona vastifica, Pleraplysilla spinifera, Dysidea a vara (positive side of axis). On the other hand, Factor I1 seems to correspond to the tolerance level of the species. The positive side of the axis shows a group of species which are distributed throughout the areas of the bay taken into account: Stylopus dujardini, Crella elegans, Crambe crambe, Phorbas fictitius, Cliona viridis, C. celata, Oscarella lobularis, Dysidea fragilis, Suberites domuncula, Terpios fugax, or throughout zones subject to non-optimal environmental conditions: Mycale micracanthoxea, Haliclona cinerea, Hymeniacidon sanguinea. The negative side of the axis shows a group of species which are distributed throughout practically all of the bay, except for the most internal areas (piers, shipyards), and, which can be considered to have a medium level of tolerance between the most eurytopic and the strictly stenotopic species: C. vastifica, Cacospongia scalaris, Aplysilla rosea. DISCUSSION AND CONCLUSIONS Description of the sponge communities and characterization of species groups The various harbour works and civil constructions along the bay's littoral, along with different types of urban and industrial wastes, condition the heterogene- 9 8.4 0.8 1.2 1.6 ity of the environment under research. Factors which Gr. Organic matter in silt regulate specific richness are directly related to the Fig. 7, Multiplicative regression between % organic matter in heterogeneity of the environment. In Algeciras Bay we silt (SOM) and g orqanic matter (the dotted lines are the showed that this heterogencity has conditioned the -conf~dence limits at 95%) number of species present, and we generally obtained Carballo et al.: Marine sponges as stress indicators 117 a clear differentiation between areas subject to environmental stress (internal) and areas with more normal conditions (external). We obtained the same differentiation by means of classification and ordination analyses. In this way, specific richness and beta diversity provide a quick and efficient method of obtaining information about the sponge community. Therefore, in the description of the communities we observed 2 major groups: the first one is composed of the stations located at the margins and external parts of the bay, and the second one is composed of the stations located in the inner areas of the bay. The external areas include species like Reniera mucosa, R. fulva, Ircinia oros, Petrosia ficiformis, Phorbas tenacior, Cliona rhodensis, Dendroxea lenis (negative side of Factor I of the factorial analysis). The first two, R. fulva and R. mucosa, are frequent species in the outer regions of the bay and seem very sensitive to light and extreme values of rate of sedimentation. They are found in protected areas like crevices, bottoms of small cavities and overhangs. In general they are absent from stations with a rate of sedimentation higher than 2.8 g m-2 mo-l, and hydrodynamism below 7.6 V. These species, though, are absent from Stn 0 which has semi-dark conditions, with levels of sedimentation (2.6 g mo-l) and suspended solids (9.8 mg 1-') similar to the ones found at the previous stations. Nevertheless, their absence seems to be explained by the levels of hydrodynamism which are the lowest of all those registered (2.8 V). At Stn N, which apparently has values somewhat similar to those found at other stations where higher values of diversity have been found (A, D, etc.), their absence can be justified in view of the elevated hydrodynamisni values which sometimes reach 17.6 V, and which in some way can hinder the larval emplacement of some species. Other species, like Spongia agaricina and Scopalina lophiropoda, show a less restricted distribution than the ones previously cited, with both these species being found from the FACTOR 11 Fig. 8. Factorial analysis in a 2-dimensional ordination of species on plane created by first 2 factorial axes. Data were standardized. The ::, variation explained by factor I is 76.4 and by Factor 11 is 15 2 (% variance explained = 91 6%). Numbers refer to species as in Table 1