Superficial sediments and their relation to polyehaete families in a subtropieal embayment , Mexieo

The soft bottoms in front of Mazatlan Bay and "Isla de la Piedra" Peninsula were studied to produce maps (depth, grain size and organic matter content in sediments). Sixty samples were óbtained with a 30 x 30 van Veen grab (4-21 m depth) and polychaetes were extracted from 21 subsamples. Grain size ranged from -1.05 phi (gravel) to 3 .81 phi (very fi ne sand), with fme sand predominl\ting. Organic matte r content in sediment was 0.91-3.06 % (most values = 1-2 %). Thirty polychaete families (905 individuals/m2 in mean) were found, and Cirratulidae, Spionidae, Onuphidae, and Pilargiidae were domiuant. Pearson's correlation of grain size, organic matter and depth proved significant (p<O.01; p<0.02), confirming the observed relationships of distribution pattems in the area. A Principal Component Analysis showed the association of several families to depth (Factor 1), to grain size (Factor 2), and to organic matter (Factor 3) . Nevertheless, the distribution of polychaete families in the area may he govemed by the combination of the three abiotic variables. Results indicated that abiotic and biotic variables have not changed considerably since 1980.

The benthic fauna inhabiting soft bottoms presents a strong relationship with the size and texture of sediments.Several authors (Wieser 1959, Fresi et al. 1983, lunoy and Viéitez 1989) have demonstrated that the type of sediment plays an important role in the distribution of benthic fauna.Additionally, organic matter is an important parameter, since it represents the major food source of many detritivores (Rodríguez 1972).Many organisms may modify the texture of sediments during movement and feeding (Rhoads 1974).According to Gambi and Giangrande (1985a), the feeding guilds of many polychaetes inhabiting soft bottoms are strongly related to environmental features• such as granulometry and organic content in sediments.Each polychaete farnily includes several feeding guilds (Fauchald and lumars, 1979); nevertheless, such relationship can be assessed taking into account the farnily level, which is applicable to other faunistic groups.This strong relationship places sediment as a major component in any faunistic study.
Few studies on sediments have been undertaken in the Mazatlan Bay.In 1979-80, a multidisciplinary group from the Instituto de Ciencias del Mar y Limnología, UNAM, analysed sediments obtained in the depth range of 6-25 m.The results, however; were never published.Orozco-Romo (1980) studied sediments in 68 collection sites chosen at random.He also produced a bathymetric map to a depth of 30 m. Later, López-Avilés (1986) studied sediments from the continental slope off the coast of Sinaloa, including Mazatlan Bay.Unfortunately, recent data concerning grain size of sediments, organic matter content and bathymetry do not exit for shallow zones in the bay.Taking into account that such parameters are the basis of several sedimentological as well as benthic macrofaunal studies, a description of the whole area by means of cartographic maps representing their spatial distribution becomes necessary.
Only a few polychaete studies have been carr ied out in shallow bottoms inside the Mazatlan Bay.Rioja revised material collected between 1941 and 1962, but no data of depth have been reported in the majority of cases (see Salazar-Vall e jo 1989).Chan-León (1983) analysed the families of polychaetes associated with rocky bottoms in the intertidal zone of the bay.The most recent studies conceming the polychaete fauna inhabiting shallow soft bottoms of Mazatlan Bay correspond to Solís Weiss (1983) and Arias-González (1984), who revised material collected at depths between 3.5 and 33 m.The latter related the most frequent species with sediments grain size and concluded that this parameter may affect, among others, the polychaete distribution.
Because of the scarcity of studies related to the nature of soft bottorns in front of Mazatlan, the purpose of this study is to characterize the zone by means of depth and the spatial distribution of sediments according to their grain size and organic matter content.Moreover, the distribution of the polychaete families related to such characterization is analyzed.

MATERIAL AND METHODS
The city of Mazatlan is located in the Southeastetn Gulf of California, on the Pacific coast of Mexico.Mazatlan Bay is a semi enclosed e embayment located between 23°1O'and 23°16' N and 106�5' and 106�9' W, and ineludes a coastline of about 17 Km.It has an area of about 35 km2 along the continental platform.The northem limit of the bay is Pajaros Island and its most southerly point is Creston Hill, which forms a peninsula with the Neveria and the Vigia Hills.The "Isla de la Piedra" Peninsula, about 16 Km long, is a sandy bar located to the south of Mazatlan Bay.It is bordered by the Urias Estuary which runs in a northeast direction, by the Presidio River in a southeast direction and by the open sea in a southwest direction (Montaño-Ley 1985).Sandy beaches predominate along the coast, except at Punta Camaron and in the southem portion of the bay, where rocky shore and cliffs are found.Sorne volcanic islands are located to the north (Pajaros, Venados, and Lobos Islands) and to the south (Hermanas Islands; Pig. 1).The spatial variation of the types of substrate is mainly related to the presence of currents and tides, as well as to the geomorphological features of each zone (Orozco-Romo 1980).The marine limit has been considered to be between O and 25 m depth (Arias-González 1984).
According to Orozco-Romo (1980), Mazatlan Bay comprises a typical marine ecosystem characterized by an homogeneous salinity (34 to 35 ppm) and by surface water temperature ranging from 20 to 30 oC, depending upon the period of the year.Unfortunately, there are no detailed studies conceming marine currents.Sorne authors have observed seasonal changes in current direction depending mainly on wind direction and coastal physiography.Cabrera-Dueñas (1988) proposed a model describing how different situations of wind may affect currents in Maztlan Bay (e.g.predominant winds from the northwest produce currents in the same direction).Roden (1958)  the Secretaría de Marina (Anonymous 1974) detected the presence of a main coastal current running parallel to the study area, with a westerly direction at "Isla de la Piedra" and changing its direction towards the northwest inside the bay.
The study area comprises an extensive area covering the entire Mazatlan Bay and extending about 6 km along the "Isla de la Piedra" Península.Sediment samples were coUected using a grid of 30" of latitude by 30" of longitude.A total of 60 sampling sites were visited and an equal number of samples were obtained from a depth range of 4-21 m (Fig. 1).Sampling was performed from 3rd to 10th December, 1996.
Collection of samples was performed with a 30 x 30 cm van Veen grabo To insure that only the superficial layer of sediments was collected, sub-samples for granulometric analyses and measurements of organic matter cantent were taken with a 2.5 cm diameter core intraduced 3 cm inside the sediment contained in the grabo In each collection site, five sub-samples were taken in order to provide enough sediment for laboratory analyses.Depth was measured using a sounding lineo Grain síze of sediments was obtained by wet sieving on a 2 mm (-1 phi) mesh width screen to sort gravel from sand and a 0.0625 mm (4 phi) mesh width screen to sort sand from mud (clay and silt) , respectively.Analyses of mud were performed following the Pipette technique (Folk 1968).The mean size diameter of samples was calculated by the Moment statistic method (Krumbein and Pettijohn 1938).The granulometric Nomenclature of Wentworth (1922) was then applied.Organic matter was ca1culated by the loss of weight by ignition at 500°C to obtain percentages of organic carbon per sample (Dean 1974).
Samples of polychaetes were taken in 21 localities (of the 60 collection sites).These sites were chosen according to their spatial distribution, allowing a good representation of the whole study area.Once the five sub samples needed for sediments analyses were taken, the remaining sediment in the grab was sieved through a net with a mesh width of 0.5 mm and fixed with a 10% formaldehyde sea water soIution on board.Specimens were laler sorted and preserved in 70% ethyI alcohol.The specimens were identified to family level (Fauchald 1977) and counted.In order to allow further comparisons, a conversion factor of 12.47 was calculated to express density values as individuals per square meter (taking into account the substracted sub-samples for analyses).
Pearson's Correlation (Microsoft Excel for windows, version 7) was obtained for the abiotic variables (60 sites) and also for the abiotic versus biotic variables (21 sites).Moreover, a Principal Component Analysis (MINITAB for windows, release 10.2) was applied on data from the 21 sites containing polychaete information.The abiotic variables considered were depth, grain size of sediments, and organic matter content; the biotic variables were density (individuals/m2) and number of families of polychaetes.

RESULTS
Depth ranged from 4 (site 11) to 21 m depth (site 14; TabJe 1).The spatial distribution presents a very clear pattern (Fig. 2a) and a general bathymetric gradient is observed, where depth increases with the distan ce from the coastline.In front of the Hacienda hotel, the slope ¡ncreases faster forming a small depression.
Mean size di ame ter of sediments ranged from -l.05 phi (grave}; sÍte 56) to 3.81 phi (very fine sand; site 33; Table 1).The spatial distribution of sediments showed that fine sand was wel1 represented in almost the whole study area (Fig. 2b).A well defined patch containing very fine sand was observed.It begins in ffont of the Hacienda hotel and deviates to the northeast on the outer si de of Lobos and Venados Islands.Only three sítes contained gravelly bioclastic sediments composed mainly of fragments of mollusc and barnacle shells (site 10: 31.17%;site 53: 30.0%; site 56: 61.94%).
Organic matter contents in sediments were low [from 0.91 % (site 1) to 3.06% (only in sÍte 56)], and the majority of samples presented values ranging from 1 to 2% (Table 1).
A total mean density of 905 individuals/m2 from 30 families of polychaetes were collected in the 21 collection sites (Table 2).According to mean density, Cirratulidae (239 indivíduals/m2), Spionidae (199 individuals/m2), Onuphidae (144 individuals/m2), and Pilargiídae (56 individuals/m2) were the most represented throughout the 21 sites.These four families were frequently found.showed that the highest posltlVe correlation was found between grain size and organic matter (r=0.403;p<O.Ol; n=60).Finer sediments (size diameter íncreases with decreasing phi values) contain more amounts of organic matter and vice-versa.Depth was found to correJate significantly with organíc matter (r=0.326;p<O.01; n=60) and grain size (r=0.308;p<O.02; n=60) , Relative to abiotic variables (21 sites containing polychaete data), only the number of families was significantly correJated with grain size (r=0.431;p<0.05; n=21).Correlation between the other pairs of biotic versus abiotic variables was found to be non significant.A Principal Component Analysis was performed to gain greater insight into the functioning of the ecosystem.The three fl fst factors extracted 22.4%, 17.5% and 11.0% of the variance, respectively (total variance extracted by the three factors = 50.8%).Although the first factor shows better correlation with depth (r=0.154), the correlation with grain size (r=-0.103)and organic matter (r=-0.089)demonstrate the same order of magnitude.The families weighting on this factor are Amphinomidae, Dorvilleidae, Lumbrineridae,  l'
The second factor is mainly reflected by graín size (r= 0.301), which decreases as posítive values of the factor increase (Fig. 3a).• PI Na 0 q, Ca -0.1

PI ,GS
:Pi • MI , Po Ah  shown to be associated to sediments eontaining the higher amounts of organic matter (Fig. 3b), espeeially those located in the enriched patch around the islands (Fig. 2e).

Magelonidae, Chaetopteridae,• and Nephtyidae
resulted associated with sediments containing low amounts of organic matt er:

DISCUSSION
The very fine sand patch detected in front of the Hacienda hotel (Fig. 2b) is associated wíth the deepest area (Fig. 2a) Relationship between bionc and abiotic variables: The high correlation between grain size and orgarnc matter can be explained because fine sediments present a larger available surface for adhesion of organic particles.This correlation was also observed by Fresi et al. (1983), Junoy and Viéitez (1989) and Méndez et al. (In press).The correlation values found between 5uch abiotic variables with .depth is probablydue to the higher energy oear the eoastline does not allow• deposition.•of•finer materials.Relative to biotic va riables, the corre lation between the numberof farnilies with grain size has been observed by several authors such as Rhoads (l974), Gray(1981), Fresi etal.(1983) and Méndez etal. (1986), among others.
According to Jumars and Fauchald (1977), Fauchald and Jumars (1979) and Gambi and Giangrande's (l985b) classifications, most of the feeding strategies and mobility categories of polychaetes are very well represented in the group of families associated with the frrst factor of the Principal Component Analysis (Fig. 3a): herbívorous, carnivorous, surface deposít feeders, subsurface deposit feeders, omnivorous, filtering and motile, discretely motile, burro wing, and sessile.These eight families were found in site 12, and most of them were found exclusively in this site, which is located in the mouth of the Urías Estuary (Fig. 1).This is a very special area where water renewal allows the interchange of nutrients, detritus and larvae, increasing the number of feeding and mobility categories of such famílies of polychaetes.This suggests that the presence of these famílies in site 12 (5.5 m depth) is controlled by the combinatíon of the three abiotic variables and the water interchange and not only by depth.
The majority of the eight farnilies weighing positively in the second factor (Fig. 3a) can be discretely motile or burrowers, and their mobility may be favoured in fine compact sand for the construction of burrows or galeries, as has been pointed out by Wieser (1959).For the contrary, Cirratulidae, Glyceridae, Syllidae, and Opheliidae may be motile, discretely motile and burr owers (Jumars and Fauchald 1977, Fauchald and Jumars 1979, Gambi and Giangrande 1985b) and specimens found hefe were small in size.It is probable that they can move inside the sediments through the interstitial spaces produced among coarse grains.
Except for Goniadidae, the other famiJies associated with the third factor (Aphroditidae, Maldanidae, and Pectinariidae; Fig. 3b) can be detritivorous and burrowers (Jumars and Fauchald 1977, Fauchald and Jumars 1979, Gambi and Giangrande 1985b), and tbis supports tbeir association with enriched sediments.On the contrary, Ampharetidae, Onuphidae, Poecilochaetidae, Magelonidae, ChaetopteIidae, and Nephtyidae demonstrate association with sediments containing low amounts of organic matter, and this is reflected by their different diet guilds (detritivores were not dominant; Gambi and Giangrande 1985b), which indicates that they do not depend on food accumulated in sediments for survival.
Although several assoclatlOns were observed through the Principal Component Analysis, the zonation of families seems to be not dependent on any exclusive abiotic variable analyzed.For instance, Maldanidae and Goniadidae sbowed a stronger affiníty for fine sediments (factor 2) and for high amounts of organic matter (factor 3), and tbis coincides with the results of Pearson's correlation.Neptbyidae, Magelonidae and Chaetopteridae showed association with fine sediments (factor 2) and with low contents of organic matter (factor 3), in disagreement with the correlation between grain size and organic matter.The establishment of a clear zonation pattem is complicated if only the family level is taken into account, sínce this group is cosmopolitan at such taxonomic level (Fauchald 1977).Moreover, differences found in depth, grain size and organic matter values are not high enough to determine with precision tbe distribution of polychaete families in the study area.Results presented here, however, offer some pointers to the knowledge of sediment features of the polychaetes habitat, thus establishing the basis for further studies in the same area.
Results indicate that the Mazatlan Bay area might not have evolved significantly in the last 17 years, and that pollution related to the increase of the city size has nol have a major impact on sediments composition due to organic matter deposition.,;.

Fig. 1 .
Fig. 1.Study area and !he 60 collection sites (polychaete samples were obtained al the 21 sites rnarket with an asterisk).

TABLE 2
Density (individua/slm 2 ) l!lthefamilies ofpolychaetes al the 21 sites and mean va/l/es (individua/slm 2 ) obtainedj(JY the etuire system which may funetion as a trap of finer sediments.The deviatipn to the northeast may also be prgduced by a predominant current in the same dírection.A similar pattern was reported by Orozco:Rorno duri ng 1980, which suggests that depth and sediment distribution in the bay have not considerably ehanged with time.The three sires eharaeterized by the presenee• of gravelly bioclasticsediments (sites 10, 53 and 56) are located close to rocky bottoms subjected to vigorous agitatíon by waves which cause the accumulation oí fragrnented sheUs in neighbour zones.In relation with organic matter content, according to Parker (1939) and Hily,(1983: p.   41), typical beach and normal sands .cancontainfromOS to 5% of organic matter,• which.agreeswithresultsfoundhere., the spatíal distribution of organic matter content (Fig.2c) showed two zones containing higher values: 1) in front of the mouth of the Urías Estuar)', a natural ecosystem enriched with .mangrovedetritUsandorgarucmatte r, and polluted by shrimp fann s and food industrial areas, and also by terrigenous inputs; 2) in the middle of the bay, where a similar pattem to that of grain size is observed.The enrichment ofthese sediments with high amounts of silt and clay (Table1) Their recent elimination during the nineues could have diminish organic content in such sediments.Unfortunately, there are no previousstudies of organic rnatter contents to compare with our results.Faunistic results have indicated that• the highest density (3 404 individuals/m2) was found in site 12, just inside the Urías Estuary mouth.This is a confluence zone where marine and brackishwater mixing process may allow for the presence of high faunal variety.The estuary provide larvae of typical estuarine species .and also nutrients and detritus (produced by mangrove trees) available for food, which may increase the number of individuals and families.Comparisons were made with data obtained during 1979"80 in MazatIan Bay by Arias-González (1984), who colIected polychaetes in .the same .kind of substrate and depth range.Of the 30 families reported in this .study,21 were found in common.Arias-Gonzáles (1984)indicated that Onuphidae, Spionidae and Lumbrineridae were the three most represented families .in the bay, which coincides with out results.