Seasonal and interannual variability in population abundances of the intertidal macroinfauna of Queule river estuary , south-central Chile

Sediment samples were monthly collected at Queule river estuary (ca. 39° S), south-central Chile, from October 1990 to April1992, and from September 1995 to November 1997 to study temporal variability in population abundances of the macroinfauna inhabiting sandy and muddy-sand intertidal substrates. Sandy sediments had higher percentages of sand particles and lower percentages of mud particles, biogenic aggregates and total organic matter than muddy-sand sediments. The same macroinfaunal species were found at both sites. That macroinfauna was dominated by polychaetes: the spionid Prionospio (Minuspio) patagonica Augener 1923, the capitellid Capitella sp. and the nereid Perinereis gualpensis Jeldes 1963. Other common organisms were the amp hi pod Paracorophium hartamannorum Andres 1975 and the small bivalve Kingiella chilenica Soot-Ryen 1959. The highest abundances of the total macroinfauna usually ocurred during summer months (January-February). The most abundant species was P. (M.) patagonica (up to 130140,000 ind m-2 in the muddy-sand sediments). During some months, this species had significantly higher abundances at the muddy-sand sediments. A similar trend is that shown by P. hartmannorum; i.e., significantly higher abundances at the muddy-sand sediments (up to 75,000 ind m-2). During many months, the population abundances of Capitella sp. and K. chilenica were significantly higher at the sandy site. The highest population abundances of Capitella sp. were close to 3 7,800 ind m 2 (February 1991 and February 1996), while the maximum values for K. chilenica ranged from 13,000 to 14,000 ind m-2 (February 1991 and November 1995, respectively). The population abundances of P. gualpensis (with the exception of the period October 1995-January 1996) were similar at both sites. Interannual comparisons of macroinfaunal abundances carried out for the sandy site showed no significant differences among years for the total macroinfauna and for all the species, but P. gualpensis. No significant differences among years were found for the total macroinfauna, P. (M.) patagonica, P. gualpensis and P. hartmannorum at the muddy-sand site. On the other hand, the abundances of Capitella sp. and K. chilenica differed significantly among years. Significant differences among months within annual periods were found for the total macroinfauna and species population abundances at the sandy and muddy-sand sites. The temporal variability ofthe macroinfauna did not have any significant relationship with the temporal variability in sediment characteristics.

sitio arenoso no mostraron diferencias para Ia macroinfauna total y todas las especies con excepci6n de P. gualpensis. No se encontraron diferencias interanuales significativas para Ia macroinfauna total y las abundancias de P. (M.) patagonica, P. gualpensis y P. hartmannorum en el sitio areno-fangoso. Por otra parte, las abundancias de Capitella sp. y K. chilenica mostraron diferencias significativas entre afios. Se encontraron diferencias significativas entre meses dentro del mismo periodo anual, tanto para Ia macroinfauna total como para las abundancias especificas en el sitio arenoso y areno-fangoso. La variabilidad temporal de Ia macroinfauna no tuvo ninguna relaci6n significativa con Ia variabilidad temporal de las caracterfsticas del sedimento. INTRODUCTION Microtidal estuaries (1-2 m of tidal range) are well represented along the coast of south central Chile (ca. 35-41° S, Pino 1994 1 ). Most of them can be classified as plain river estuaries, with ages of about 6,000 years (Pino 1994 1 ). The majority of these estuaries get waters from the coastal range, and just a few of them originate in rivers which drain water from lakes located in the foothill of Andean mountains (Campos & Moreno 1985). According to water movement, most of these estuaries can be classified as partially mixed (Pino 1994 1 ). Water currents and tectonic movement (Pino 1995) have created extensive intertidal flats with sandy and muddy-sand sediments as the most representative substrates. The highest content of organic matter is associated to that muddy-sand sediments (Pino & Mulsow 1983, Bertran 1984, 1989, Turner 1984, Jaramillo eta!. 1985b, Pino 1994 Most of the faunistic studies carried out on that intertidal estuarine flats deal primarily with the benthic macroinfauna, which is numerically dominated by polychaetes (Jaramillo et a!. 1985a, Quij6n & Jaramillo 1993. The community structure of the intertidal estuarine macroinfauna has been mainly analyzed in relation to physical factors, such as water salinity gradients and substrate characteristics (Bertran 1984, 1989, Turner 1984, Jaramillo eta!. 1985b, Donoso 1991, Quij6n & Jaramillo 1993, 1996. As found in other estuarine temperate areas (e.g., McLusky 1971), finer sediments and richer in organic matter content support the highest macroinfaunal abundances and biomasses, while that sandy bottoms located closer to the estuarine mouths support higher species richness (e.g., Bertran 1984, Jaramillo et a!. 1985b. There are no studies on the temporal variability of the estuarine intertidal macroinfauna of south-1 PINO M (1994) Geomorfologfa, sedimentologfa y di mica de Ia circulaci6n en estuarios micromareales del centro sur de Chile. Resumen XIV Jornadas de Ciencias del Mar, Puerto Montt, Chile: 106-107.
central Chile. Thus, the main objective of this study was to characterize the intertidal macroinfauna of Queule river estuary in relation to sediment characteristics and temporal variability. Results from this study will provide a baseline data for future studies aimed for example, to examine the role of anthropogenic use of this estuary on the benthic macroinfauna.

The study area
Queule river estuary is located in south central Chile (39° 24' S, 73° 13' W, Fig. 1). The intertidal flat studied is located in the middle reach of the estuary, about 1,500 m from the mouth. It is roughly triangular with an approximate surface of 10 ha. The two study sites were located at the sandy and muddy-sand areas of the flat. The tides are semi-diurnal with maximum tidal differences of 1.5 m. The highest salinities (25-29 ppm) in the adjacent waters to the flat occur during the high waters of summer and autumn months (February-May), while the lowest values (1-7 ppm) have been measured during the low tides of winter months (July-early September) (Quij6n & Jaramillo 1993).

Sampling procedures and analyses
Monthly sediment samples were randomly collected at each site (n = 5 replicates) during two periods: October 1990 to April1992, and September 1995 to November 1997. A plastic tube, 75 mm in diameter, was buried to a depth of 30 mm to collect the samples. Samples were sieved through a 250 m mesh sieve and the residue preserved in 10 % formalin for later sorting and counting of the ,macroinfauna in the laboratory. Only repeatedly recorded species during the whole study were considered for further analyses. Monthly samples were also collected for textural analyses carried out to analyze the proportion of sand (63-1,000 m), mud(< 63 m) and biogenic The location of Queule river estuary is indicated by the square close to 40° S. (b) outline of Queule river estuary. The location of the study area at the middle reach of the estuary is indicated by the box; (c) location of the sandy (A) and muddy-sand sites (B). Shaded area indicates tidal flats, hatched area indicates salt marshes, and dotted line marks the low tide level.
aggregates (Anderson et al. 1981). The content of organic matter was determined as the loss in weight of dried samples after combustion (during 4 h at 550 °C).
Monthly comparisons of sediment characteristics and macroinfaunal abundances between sites were carried out with the use of analysis of variance (ANOV A). Assumptions of normality and homocedasticity were tested with the Kolmogorov-Smirnov' s and Bartlett's tests, respectively (Sokal & Rohlf 1995). These tests revealed some significant departures from that basic assumptions; thus, the sediment data (percentages) were arcsin transformed while abundance data were transformed to log (n + 1).
Sedimentological and biological relationships were examined using non-metric multidimensional scaling (MDS). MDS was based upon a similarity matrix calculated with the Bray Curtis similarity coefficient after double root transformation of data as run by the PRIMER (Plymouth Routines in Multivariate Ecological Research) program (Carr 1997). MDS was used to graphically display two-dimensional ordination plots of the inter-relationships between sites based on the sediment characteristics and mean abundance of the major taxa found at each site. Thus, the closer the points through these plots the more similar they were. The usefulness of the MDS analyses (i.e., display of relationships between sites) was ******** ******** *** ** * * * * * * * * * * * * * * * * * , D F A J A D F A N J MMJ N J M M J N * * * * * * * * ****** * * * ******** * * * * * 1991 1995 1996 1997 Sandy sediments Muddy sand sediments Fig. 2: Temporal variability in sediment characteristics at the sandy and muddy-sand sites. Asterisks indicate statistically significant differences between means (P < 0.05) after using ANOV A.
Variabilidad temporal en las caracterfsticas sedimentol6gicas en el sitio arenoso y areno-fangoso. Los asterfscos indican diferencias estadfsticamente significativas entre medias (P < 0,05) luego de usar ANOV A. evaluated with the stress statistics: values smaller than 0.1 indicate that the depiction of relationships is good, while stress values over 0.2 indicate a poor depiction (Clarke 1993). The matching of macroinfaunal abundances and sediment data was analyzed with the program BIOENV included in the PRIMER package. Basically, this program search for the best single variable or combination of variables that best explain the grouping of samples belonging to a site (Clarke & Warwick 1994).
Temporal comparisons of sediment characteristics and macroinfaunal abundances were made among spring-summer months (October to March) of the periods 1990-1991, 1991-1992, 1995-1996, and 1996-1997. Such comparisons were carried out with the use of nested ANOVA (on data transformed as mentioned earlier), and with annual period, month, and replicate as factors (Sokal & Rohlf 1995). A similar comparison for autumnwinter months was not carried out as these months were not sampled enough. Probability levels lower than 0.05 were regarded as statistically significant.

The sediments
As expected, the sediments of the sandy site had significantly (P < 0.05) higher percentages of sand particles (63-2,000 ), and significantly lower percentages of mud particles ( < 63 m), biogenic aggregates and total organic matter. Thus, sand particles were close to 90 % at the sandy site, and most of the time below 70% at the muddy-sand site (Fig. 2). Percentages of mud particles were lower than 5 % at the sandy site, and about 30-60 % at the muddy-sand site. Biogenic aggregates were represented by less than 5 % at the sandy site and by a percentage varying from 5 to 25 %at the muddy-sand site. A seasonal trend of higher concentrations of these aggregates was found during spring-summer months at the muddy-sand site (Fig. 2). Total organic matter content was lower than 5 % at the sandy site, while at the muddy-sand sediments it varied between 5 and 10 % (Fig. 2). During all the study period, the percentages of sand, mud, and biogenic aggregates were significantly different between sites (Fig. 2). The total organic matter content was significantly different between sites during all the study period, but September 1996 (Fig. 2). Differences in sediment characteristics at both sites are depicted by means of MDS in Fig.  3. Sediment samples collected from the sandy site are separated from that collected at the muddysand site. Interannual comparisons of sediment characteristics carried out at both sites showed no significant differences among years. Significant differences among months within annual periods were found in the percentages of sand and mud particles, biogenic aggregates and total organic matter at the sandy and muddy-sand sediments (Table 1 and 2).

The macroinfauna
The macroinfaunal assemblage of the intertidal of Queule river estuary was dominated by poly- Thurston. Due to the low abundances of the last two species, they were excluded from further analyses. The highest abundance of the total macroinfauna usually ocurred during summer months (January-February) (Fig. 4 ). The most abundant species was P. ( M.) patagonica (up to 130-140,000 ind m 2 in the muddy-sand sediments). During some months, this species had significant higher abundances at the muddy-sand sediments (Fig. 4 ). A similar trend is that shown by P. hartmannorum; i.e., significantly higher abundances at the muddysand sediments (up to 75,000 ind m 2 ). During many months, the population abundances of Capitella sp. and K. chilenica were significantly higher at the sandy site (Fig. 4 ). The highest population Variabilidad temporal en las abundancias poblacionales de Ia macroinfauna total yen las de las especies m s comunes de Ia macroinfauna en el sitio arenoso y areno-fangoso. Los asteriscos indican diferencias estadisticamente significativas entre medias (P < 0,05) luego de usar ANOVA.
abundance of Capitella sp. was close to 37,800 ind m (February 1991 andFebruary 1996), while the maximum values for K. chilenica ranged between 13,000 and 14,000 ind m (February 1991 andNovember 1995, respectively). The abundance of P. gualpensis (with the exception of the period October 1995-J anuary 1996) was similar at both sites (Fig. 4). The macroinfaunal similarities between sites are depicted by means of the MDS plot, which revealed a gradient of macroinfaunal abundances from sandy to muddysand sediments (Fig. 5).
Interannual comparisons of macroinfaunal abundances carried out for the sandy site showed no significant differences among years for the total macroinfauna and for all the species, but P. gualpensis (Table 3, Fig. 6). The use of LSD posthoc tests showed that the abundance of this species was significantly higher during the springsummer months of the periods 1991-1992 and of 1995-1996, as compared with that of the periods 1990-1991 and 1996-1997 (Fig. 6)  sand site (Table 4, Fig. 6). On the other' hand, the abundances of Capitella sp. and K. chilenica differed significantly among years (Table 4). The use of LSD tests showed that the abundance of Capitella sp. was significantly higher during the spring-summer months of the period 1990-1991 as compared with that of the other spring-summer periods (Fig. 6). During the same spring-summer period (1990)(1991), the abundance of K. chilenica was significantly lower as compared with other periods. Significant differences among months within annual periods were found for the total macroinfauna and species population abundances at the sandy and muddy-sand sites (Table 3 and 4). Table 5 displays the outcome of the BIO-ENV procedure included in the PRIMER package. The inclusion of either a single or a combination of environmental variables rendered no significant correlations between macroinfaunal abundances and physical variables.

DISCUSSION
This study revealed significant differences in the sedimentary characteristics at two closed sites of the intertidal flats of the middle reach of Queule river estuary. Such differences persisted through years, a trend similar to that of subtidal soft bottoms recorded before in the same estuary (Jaramillo et al. 1985a, Quij6n et al. 1996. The muddy-sand site had significantly higher concentrations of mud sized particles, biogenic aggregates and total organic combustion matter. Since biogenic aggregates are easily broken by physical disturbances such as tidal currents (Haven & Morales-Alamo 1968), the most sheltered position of the muddy-sand site may well explain the higher concentration of such aggregates, as well as a higher sedimentation rate of organic matter at that site. Our data also showed some sort of seasonal trend in biogenic aggregates, with higher percentages during the warmer months at the muddy-sand site. This trend may be related to the higher population abundance of the macroinfauna, which would result in a higher production of biogenic aggregates (Rhoads et al. 1978). Ovoid fecal pellets of Capitella sp. and P. (M.) patagonica are quite distinctive features of the study area during summer months. Also, the cohesive feature of the muddy-sand sediments may probably account for the higher concentration of biogenic aggregates due to the low erodability of such sediments (Rhoads & Boyer 1982).
Despite the notorious differences in sediment characteristics, the same macroinfaunal species were collected at both sites, suggesting that sediment type does not affect the taxonomic composition of the intertidal macroinfauna studied. The macroinfauna of the intertidal sandy and muddysand sediments of Queule river estuary is quite stable in terms of dominant species, a similar feature recorded for the benthic communities of the subtidal soft bottoms of the same estuary (Jaramillo et al. 1985a, Quij6n et al. 1996. At both sites, the community was dominated by polychaetes which accounted for an average of 80.1 and 76.7 % of the total macroinfauna at the sandy and muddy-sand site, respectively. High dominance of polychaetes have been recorded for other estuarine coasts, such as for example intertidal areas in South America (Bertran 1984, 1989, Donoso 1991, Ieno & Bastida 1998 1991-1992 1995-1996 1996-1997 1991-1992 1995-1996 1996-1997 Sandy sedim nts Muddy-sand sedim nts Fig. 6: Interannual variability in population abundances of the most common species of the macroinfauna at the sandy and muddy-sand sites. Analyses are based upon sets of data collected during spring-summer months (see Material and Methods). Statistically significant interannual differences in population abundances were detected for P. gualpensis in the sandy site, and for Capitella sp. and K. chilenica in the muddy-sand site as indicated by different capital letters.

TABLE4
Result of the nested analyses of variance carried out to test for differences among years in macroinfaunal abundances of the muddy-sand site (see Material and Methods) Resultados de los an isis de varianza anidados llevados a cabo para evaluar las diferencias entre afios en Ia abundancia de Ia macroinfauna en el sitio areno-fangoso (ver Materiales y Metodos)

Source of variation
Total macroinfauna Among groups (annual periods) Among subgroups within groups (among months within annual periods) Within subgroups (error among replicates collected at each month) Total

Prionospio ( Minuspio) patagonica
Among groups (annual periods) Among subgroups within groups (among months within annual periods) Within subgroups (error among replicates collected at each month) Total Capitella sp. Among groups (annual periods) Among subgroups within groups (among months within annual periods) Within subgroups (error among replicates collected at each month) Total

Perinereis gualpensis
Among groups (annual periods) Among subgroups within groups (among months within annual periods) Within subgroups (error among replicates collected at each month) Total

Paracorophium hartmannorum
Among groups (annual periods) Among subgroups within groups (among months within annual periods) Within subgroups (error among replicates collected at each month) Total

Kingiella chilenica
Among groups (annual periods) Among subgroups within groups (among months within annual periods) Within subgroups (error among replicates collected at each month) Total muddy-sand sediments of the intertidal of Queule river estuary, population abundance of most species differed at both substrates. While the abundance of the polychaete P. (M.) patagonica and the amphipod P. hartmannorum were usually significantly higher at the muddy-sand sediments, the polychaete Capitella sp. and the small bivalve K. chilenica were more abundant at the sandy sediments, suggesting preferences for different types of sediment. The opposite trend shown by the animal-sediment relationships of the polychaetes P. (M.) patagonica and Capitella sp. was also observed in earlier studies. In an across shore transect at Queule river estuary, Turner (1984) found that these polychaetes divided the intertidal with maximum abundances of P worthwhile to get deeper in the factors involved in community organization of this macroinfauna. Most macroinfaunal communities exhibit seasonal and interannual variability in population abundances (Rachor et al. 1982, Dfaz 1984, L6pez-J amar et al. 1986, Beukema 1989, Gaston et al. 1995, Bone & Klein 2000, McCarthy et al. 2000. Densities of the temperate intertidal soft-bottom macroinfauna peak during the spring, then decline throughout the summer, and usually have a second smaller peak during the autumn to decline again in winter (Levin 1984, Frid & James 1989, Marsh & Tenore 1990, Sarda et al. 1995. The results of this study generally agreed with such trend as the intertidal macroinfauna of Queule river estuary usually peaked during spring-summer months. However, population abundances of some species such as P. (M.) patagonica varied between sites. During some months, this polychaete presented similar abundances at both sites. However, subsequently higher abundance at the muddy-sand site suggests higher mortality of juveniles at the sandy site (Fig. 4). The interannual variability observed in the population abundances of some species was probably related to recruitment variability since the sediment samples were sieved through a 250 sieve; thus, many small animals were indeed collected. This interpretation would explain, for example, the high population abundance of the macroinfauna during some specific periods: P. (M.) patagonica in the muddysand sediments during March through April1992, Capitella sp. in the sandy site during February 1991 and 1996, P. gualpensis in the same site during October through November 1995, and P. hartmannorum in the muddy-sand site during March 1997 and K. chilenica in the sandy site during January-February 1991 and November 1995. Thus, such higher abundances were probably the result of high recruitment pulses. Since the inspection of macroinfaunal population abundances and sediment variability did not show any significant relationship (results of the BIO-ENV procedure of PRIMER), it can be said that variability in recruitment and subsequent population abundance is apparently not related to the variability of that environmental factor. If so, it can be hypothesized that this variability is probably related to nearshore events such as wind and propagation of internal waves in the water column (Shanks & Wright 1987, Eckman 1996 that, when occurring out of the estuary, it may indeed extend their effects to the middle reaches of this estuary. Temporal variability in population abundances of the estuarine macroinfauna has been also linked to biological interactions such as predation by fishes and birds (Virnstein 1977). According to the studies of Turner ( 1988), the macroinfauna of this estuary contributes significantly to the food of demersal fishes Eleginops maclovinus (Valenciennes) and Cauque mauleanum (Steindachner). The same can be said for the brown pintail duckAnas georgica Gmelin and the whimbrell Numenius phaeopus (Linne) (Velasquez 1987). Both fishes and birds were particularly abundant during late spring-summer months. Venegas (1992) showed that the experimental exclusion of predators (demersal fishes and birds) during those months resulted in increased macroinfaunal population abundances, which suggest that seasonal variability in macroinfaunal abundances may well be related to seasonal variability in population abundance of these vertebrate predators. Nonetheless, and since most of the macroinfauna of the intertidal flats of Queule river estuary peaked at the same time of the highest abundance of predators, vertebrate predation does not control population abundances of this macroinfauna.
The results of this study emphasize the fact that basic to the understanding of inter-annual variability in population abundances of the intertidal estuarine macroinfauna, is the awareness of factors which may cause long-term changes in the estuarine environment. Contemporaneous records of water temperature, river runoff, nutrient and oxygen water concentrations, changing sea level and seasonal variability in biological interactions, should probably shed light on their importance for macroinfaunal communities, specially if for example, changes in water properties are expected to occur in the small sized estuaries of south central Chile from the continued increasing human influence around the intertidal flats (Reise 1982). That would help in making predictions on long-term variability of the intertidal estuarine macroinfauna in this area of the Chilean coast or elsewhere. In the meantime, the results of this study stands as the most complete baseline data on the population abundances of the intertidal estuarine macroinfauna of south central Chile.

ACKNOWLEDGMENTS
To Marcia Gonzalez, Carlos Gonz lez, Claudia Afiazco, Sonia Fuentealba, and Cesar Cuevas for help during field and laboratory work. To FONDECYT grants 0304-90 and 1951168 for financial support. The final data analyses and manuscript writing benefited from the financial support given to E.J. and P.Q. by