SEASONAL CHAETOGNATH ABUNDANCE AND DISTRIBUTION IN A TROPICAL ESTUARY ( SOUTHEASTERN , BRAZIL )

This study focuses on the seasonal variation of the chaetognath species in the Vitória Bay/Passage Channel estuarine system, Espírito Santo, Brazil, in terms of their abundance and distribution. Specimens of chaetognaths were collected between July 1997 and April 1998 at 10 sampling stations, with a cylindrical-conical plankton net of 200 μm mesh and 30 cm mouth, fitted with a mechanical flowmeter. Five chaetognath species were identified: Sagitta enflata, Sagitta decipiens, Sagitta hispida, Sagitta friderici and Sagitta minima. Most of them were distributed in areas of high salinity (e.g. at the stations closest to the outer estuary). The dominant species, S. enflata and S. friderici, were more frequent in the outer estuary where salinities varied from 32 (wet season summer) to 28 (dry season winter). S. friderici was the only species found right in the middle of the Passage Channel, at a station close to the main freshwater input into the estuary. Results showed that chaetognaths only enter the estuary due to the tidal effect, and that they are not typical residents of this system. This is to be expected because the group normally inhabits only truly marine regions.


INTRODUCTION
Chaetognaths play an important role in marine food webs, being active predators.Their diet includes a variety of pelagic organisms, consisting mainly of copepods, but they may also prey on fish larvae thus impacting the zooplankton and ictioplankton communities (Casanova, 1999).These organisms live in various marine habitats from polar to tropical waters and at all depths.Thirty-nine species have so far been reported in the South Atlantic between 0 and 60°S (Casanova, 1999).These species belong to both oceanic and coastal environments, but a few have also been found in estuarine regions.Some chaetognaths are well known as indicators of certain water masses due to their close relationship with the environmental variables (Casanova, 1999); their occurrence and distribution being determined by hydrological conditions (Liang & Vega-Pérez, 1994).
The main purpose of this paper was to study chaetognath distribution and abundance in the Vitória Bay/Passage Channel estuarine system, a tropical estuary in Southeastern Brazil, during an annual seasonal cycle.

Study Area
The Vitória Bay/Passage Channel estuarine system (20º23'S -40º20'W) is located in the Vitória metropolitan area, Espírito Santo, Brazil.With a mangrove area of 2,051 hectares and an extension of approximately 25 kilometers, this system has a horseshoe shape with two coastal water entrances, one in the Vitória Bay and the other in the Passage Channel (Fig. 1).Local depths vary from 1.5 to 10 meters (outside the main port channel), tidal currents entering the estuary by both entrances.The system is influenced by coastal waters from the Espírito Santo Bay and several rivers that flow into this system (e.g.Aribiri, Marinho, Bubu and Santa Maria rivers -this latter being the main freshwater input).The Passage Channel serves also for drainage as it is narrow and shallow, but the main tidal flow enters the system through the Vitória Bay.Average monthly temperatures range from 21 to 28°C and precipitation from 50 to 200 mm, with the months of December/January being the warmest and wettest and July/August the coldest and dryest.

Seasonal Sampling and Data Analysis
Sampling of the biological material and environmental parameters was done every three months at ten stations from July 1997 (winter) to April 1998 (fall).No sample was taken at station 2 in July.Zooplankton samples were collected using a cylindrical-conical plankton net with a 30-centimeter mouth opening and 200µm mesh, fitted with a mechanical flowmeter to estimate the amount of water filtered (Omori & Ikeda, 1992).Sub-surface samples were obtained with tows at approximately 1 knot during five-minute periods.The biological material collected was preserved in an aqueous solution of formaldehyde 5%, buffered with sodium tetra-borate.Along with the biological parameters, environmental parameters such as salinity and temperature were measured "in situ" at each sampling station throughout the water column using portable multi-sound equipment (YSI 85).Salinity is expressed in practical salinity units (psu).A total of 39 samples were collected during the sampling period.
Copepod abundance was also estimated in each sample for further correlation with chaetognaths.Numbers of organisms were expressed as individuals per cubic meter of filtered seawater.
Multivariate analysis of the environmental variables and copepod abundance related to chaetognath abundance and distribution was performed using the Spearman correlation (p<0.05).Group analysis was also made to evaluate the distribution by species.

RESULTS
Over the study period, water temperature varied between 22.0 and 28.8°C, with higher values during summer and fall at the sampling sites in the inner part of the Passage Channel (Fig. 2).Salinity varied between 12 and 38 psu with the highest values occurring during fall and spring, mainly at the sampling stations in the outer estuary region (Fig. 3).
During winter, S. friderici was the dominant species at the outer stations with approximately 36 ind.m -3 .The same species was also found right in the middle of the estuary (Fig. 4), where salinities were around 14 psu (Fig. 3).S. enflata and S. decipiens were found in one of the estuary entrances where salinity was higher, the former being the more abundant (36.5 ind.m -3 ) in this area (Fig. 4).S. decipiens was the second species observed in the areas with greater freshwater input in the inner part of the estuary, along with S. friderici, both with reduced abundance (2.4 ind.m -3 each) (Fig. 4).In spring, chaetognaths were also abundant in the outer region of the estuary (Fig. 5).The most abundant species for this season was S. enflata, attaining almost 40 ind.m -3 (Fig. 5) where salinity values reached their peak in the sampling period (37.5 psu -Fig.3).Other abundant species were S. friderici and S. decipiens.S. minima had its first and only occurrence (0.4 ind.m -3 at St. 2) in the estuary during the entire sampling period.However, hardly any chaetognaths were present in the estuary in the summer.In this period, S. friderici was found only in the outer estuary, though in small numbers (max. of 1.9 ind.m -3 ) (Fig. 6).
In contrast, chaetognaths showed a slight increase in abundance in the fall as compared to the summer.Sagitta friderici was the most numerons species in the system, but with no more than 10.2 ind.m -3 at the entrance to Vitória Bay.Sagitta friderici was the only chaetognath species found right in the middle of the Passage Channel (Fig. 7), with salinities around 23 (Fig. 3).Other species present during the fall were S. enflata, S. decipiens and S. hispida, with this last occurring, on the first and only occasion, at station 2, though in small numbers (0.4 ind.m -3 -Fig.7).Table 1 shows the correlation values obtained by the Spearman test between temperature, salinity and copepod abundance and the chaetognath species.Copepod data were obtained elsewhere.S. enflata had a positive correlation with salinity and a tendency to negative correlation with temperature.This same pattern was observed for S. friderici and S. decipiens, whereas S. hispida and S. minima did not show any significant correlation with either temperature or salinity.On the other hand, positive  Cluster analysis (r-mode) for the chaetognath species revealed the existence of two groups: group A consisting of S. hispida, S. minima and S. decipiens, and group B of S. friderici and S. enflata (Fig. 8).

DISCUSSION
The results obtained in this study show that most of the species present in the Vitória Bay/Passage Channel estuarine system, such as S. hispida, S. enflata, S. decipiens and S. minima, are typical inhabitants of coastal and oceanic waters (Casanova, 1999).The higher abundance observed during winter and spring are in accordance with other studies of Brazilian coastal regions (Liang & Vega-Pérez, 1994).
The first evidence of S. friderici in Espírito Santo Bay, adjacent to Vitória Bay, State of Espírito Santo, was recorded by Bonecker et al. (1987).In the present study, S. friderici was one of the most abundant chaetognath species in the Vitória Bay/Passage Channel estuarine system during the entire year.The occurrence of this species has also been observed in other Brazilian estuarine systems (Montú, 1980;Lansac Tôha &Lima, 1993 andLopes, 1996).According to Casanova (1999), S. friderici is a typical neritic species and can be found off the South American coast, this raising the possibility of its being found in many coastal systems.S. friderici is also considered an indicator of colder waters and regions with upwelling (Liang & Vega-Pérez, 2001).In the present study, S. friderici and S. decipiens were found in the middle of the estuary, where salinities were around 14 during the wet season and near an unusually low 12 psu during the dry season.
The results obtained in the present study show that the chaetognath species found in the Vitória Bay area only enter this estuarine system due to the tidal effect, not being typical residents.This was to be expected especially because this group mainly inhabits coastal and oceanic regions (Casanova, 1999).The presence of S. friderici in very low salinities might be explained either by a higher tolerance of this organism to lower salinities or just as a result of the fact that few individuals were trapped in the estuary when the tide started to ebb.Sagitta enflata, a cosmopolitan species found in regions where salinities can vary frequently and markedly (Boltovskoy, 1981), has also already been recorded in Brazilian estuaries (Montú, 1980;Lira & Magalhães, 1996: Lopes, 1996).In this study, S. enflata was abundant throughout the year at the stations closer to the entrance of the Bay and Channel, where the salinity was higher.Liang & Vega-Pérez (2001) found low densities of S. enflata in the coastal region off Vitória, and observed that this species seemed to be limited to the areas of high salinity values (>36) found in the tropical waters of the region, since this is a semi-neritic species (Liang & Vega-Pérez, 2002).
Sagitta hispida, a tropical species whose distribution extends southward with the Brazil Current (Casanova, 1999), is mainly neritic and is distributed in both oceanic and coastal waters (Alvariño, 1965).S. hispida has not so far been recorded in estuarine waters in Brazil.However, in this study, a low abundance of S. hispida was observed in one sampling station in the Vitória Bay area, probably carried in by a strong tidal current.This species is known to be able to survive in salinities varying from 20 to 45 (Boltovskoy, 1981).
Sagitta minima is a neritic species found in both coastal and oceanic areas (Alvariño, 1965).It has never before been recorded in Brazilian estuaries, this being its first recorded occurrence in this type of system, as also for the State of Espírito Santo.Some authors limit the occurrence of this species to tropicalsubtropical waters (Casanova, 1999) and costal species distributed from the equator down to 40°S, occurring mainly in subtropical waters (Casanova, 1999).Liang & Vega-Pérez (1994) mentioned that the distribution and occurrence of chaetognaths are determined by hydrological conditions.In this study, the distribution of the two dominant species S. friderici and S. enflata was associated with the sampling stations characterized by their elevated salinity values, as can be observed from the Spearman correlation test results (Table 1), indicating the entrance of coastal waters into the estuary.
The occurrence of S. enflata e S. friderici throughout the year, along with the other species observed in the study area, is likely to be due to the tidal currents that entered the estuary during the sampling period, and were trapped in this system.
Apart from hydrological conditions, food availability also seems to influence chaetognath distribution.Copepods are important food items for chaetognaths (Liang & Vega-Pérez, 1995).Pierrot-Bults (1996) mentioned that approximately 10 to 30% of the copepod biomass is transferred to the chaetognath biomass.The results obtained in this study are in accordance with those observations, since a close correlation of chaetognath occurrence and high copepod densities was observed.

Fig. 1 .
Fig. 1.Map showing the location of ten sampling stations in the Vitória Bay/Passage Channel estuarine system.

Fig. 2 .
Fig. 2. Temperature (°C) variation for the ten sampling stations between July 1997 and April 1998 in the Vitória Bay estuarine system.

Fig. 3 .
Fig. 3. Salinity (psu) variation for the ten sampling stations between July 1997 and April 1998 in the Vitória Bay estuarine system.
ET AL.: CHAETOGNATH SEASONALITY IN A TROPICAL ESTUARY correlation was observed between copepod abundance and S. enflata, S. friderici and S. decipiens.

Fig. 8 .
Fig. 8. Cluster analysis of chaetognath species in the Vitória Bay/Passage Channel estuarine system.

Table 1 .
Spearman correlation among the environmental variables, copepod abundance and chaetognath abundance in the Vitória Bay/Passage Channel estuarine system.