Mesozooplankton and Ichthyoplankton composition in two tropical estuaries of Bahia , Brazil

210 Introduction The estuaries and mangroves are currently one of the world’s most threatened ecosystems due to drastic encroachment of human activities despite their undeniable relevance (Barbier and Cox 2002; Singkran and Sudara 2005). According to Islam and Haque (2004), shrimp farming has been a great contributor to mangrove destruction, reducing biological resources such as habitats of crustaceans, mollusks and fish species of ecological and economic relevance. Zooplankton plays a key role in the ecosystem structure due to its quick response to abiotic conditions, especially in impacted environments (Levinton 1995; Neumann-Leitão et al. 1999). It is, therefore, very important to describe the taxonomic diversity in tropical estuaries since there is no published information on the composition of zooplankton communities in the state of Bahia, only some unpublished academic works which focused mainly on ecological aspects. This paper presents a description of the zooplankton and ichthyoplankton taxa density found in two similar tropical estuaries subjected to different sources of anthropogenic impact in the state of Bahia. It also highlights new records of some copepod species distribution.


Introduction
shrimp farms in the state of Bahia, which was implemented in 1993. The discharge of effluents occurs daily, after a 24 h treatment in sedimentation ponds.
In the Itapicuru River's basin, other sources of anthropogenic impacts can be found in the main course of the river, such as sewage and industrial effluents waste. However, close to the mouth of the Itapicuru River there is a small village where no industrial activities were observed. Tourism and artisanal fishery are the most important economic activities, but both are still poorly developed.
In both rivers, four sampling stations were chosen to collect data according to the decreasing salinity gradient ( Figure 1).

Data collection
The sampling strategy was carried out in four stations in each estuary during the ebbing and flooding spring tides. This data was collected in the rainy (April and August months) and dry (December month) seasons of 2007, consisting of 48 samples. Salinity and temperature were estimated through a multi-parameter probe WTW 340i/SET.
Mesozooplankton samples were collected through horizontal hauls at 0.1 m from the surface, during 3 minutes, using a conical net (200 µm mesh size) coupled to a flowmeter for filtered volume determination. Plankton samples were preserved in 4 % formaldehyde seawater solution and transported to the laboratory.
Organisms were counted and identified to their lowest practical taxonomic level through an estereoscopic microscope (Leica MZ6), an optical microscope (Olympus CH30) and the pertinent bibliography (Smith 1982;Boxshall and Halsey 2004;Boltovskoy 2005;Richards 2006). The abundance of organisms was estimated through measuring 2, 10 or 50 mL aliquots, one aliquot per sample, using a Stempel pipette. Rare taxa were counted in the whole sample and the density (D: individual per cubic meter) was calculated dividing the abundance by the total filtered volume.

Results And Discussion
The temperature was similar at all the sampling stations with smaller values in August. The salinity was a very variable parameter, which represented a decreasing gradient from station 1 to 4 in both estuaries (Tabatinga River: 5.90 to 26.30; Itapicuru River: 8.60 to 36.30). The higher values were found in December during the dry season.
The T-S diagram shows the existence of only estuarine waters in the Tabatinga River (Figure 2), while estuarine and coastal waters were present in the Itapicuru River ( Figure 3).
In the Tabatinga and Itapicuru Rivers, 65 and 73 zooplankton taxa were registered, respectively; 59 of them were found in both estuaries such as Foraminiferida, Cnidaria, Annelida, Mollusca, Echinodermata, Crustacea, Urochordata, Cephalochordata and Chaetognatha. Rotifera was solely recorded in the Tabatinga River while Thaliacea was only registered in the Itapicuru River. In relation to fish larvae, 11 and 19 species were found on these estuaries.
Considering both zooplankton and ichthyoplankton a total of 76 and 92 taxa were recorded in the Tabatinga and Itapicuru Rivers, and they are displayed on tables 1 to 4 with their respective density data. A total of 98 mesozooand 20 ichthyoplankton taxa were recorded taking into account both rivers.
The most relevant finding refers to the first register of Discoidae (Copepoda, Calanoida), represented by Disco sp. There are no previous records for Disco in the Southwestern Atlantic, nor in estuaries, being typically considered as an oceanic group (Boxshall and Halsey 2004). Currently the family is divided into three genera containing 29 species. However only two of them belong to Prodisco and four to Paradisco, the other 23 species are attributed to the genus Disco. Schulz (1993) proposed a subdivision of the Disco species according to the degree of mouth parts reduction. The specimens found in these estuaries represent a new species which is being described by the authors.
This study also represents the first record of other 7 copepod species in the state of Bahia   Ergasilidae is one of the most important families of copepods which are fish parasites. Ergasilus caraguatatubensis was first described by Amado and Rocha (1995) inhabiting the opercular cavity of Mugilidae collected in the states of Maranhão, Alagoas, São Paulo and Rio de Janeiro. Therefore the occurrence of E. caraguatatubensis and Mugil liza Valenciennes, 1836 (Mugilidae) may be linked, since both species were restricted to the Itapicuru River.
Caligus sp. (Caligidae) is also predominantly a fish parasite, including M. liza, but as it was found in both estuaries, its distribution may be also associated to other fish species.
Gonyiopsillus brasiliensis was described by Huys and Conroy-Dalton (2000) from samples collected in the state of Rio Grande do Sul, Brazil, on the outside opening of Lagoa dos Patos to the ocean. Huys and Conroy-Dalton (2000) also claimed that many South-American authors erroneously attributed this species to Clytemnestra rostrata Brady, 1883. So, our register of this species in the Itapicuru and Tabatinga Rivers confirms the hypothesis of previous misleading records along the Brazilian coast.
Among the 12 taxa exclusively reported in the Itapicuru River the distribution of some of them (M. gracilis, Mecynocera clausi Thompson, 1888, Oithona plumifera Baird, 1843, Penilia avirostris Dana, 1852 and Salpidae) was restricted to station 1, which may be explained by the higher salinity values due to the river's mouth proximity ( Figure 1). On the other hand the exclusive occurrence of Augaptilidae, Paracalanidae (Paracalanus sp.), Centropagidae (Centropages velificatus Oliveira, 1947) and P. villosa along the entire estuary could not be associated to higher salinities and may reflect the existence of environmental differences probably related to water quality parameters, once the Tabatinga River is under the influence of shrimp farm effluent discharges.
A previously unpublished study dating from 1970 which took place in Baía de Todos os Santos (BTS), a coastal marine environment located approximately 200 kilometers away from our study area, registered the following species in common with our study site: Liriope tetraphyla Chamisso and Eysenhardt, 1821, Pseudodiaptomus acutus Dahl, 1894, Calanopia americana Dahl, 1894, Acartia lilljeborgi Giesbrecht, 1889, Euterpina acutifrons Dana, 1847 and Lucifer faxoni Borradaile, 1915. Penilia avirostris and Oithona plumifera were also found in BTS but were absent from the Tabatinga river due to lower salinity values in this estuary. The BTS study also registered species belonging to the Microsetella, Centropages, Oithona, Oncaea, Temora, Labidocera and Oikopleura genera, all of which were also identified in our sampling stations.
Some taxa (Rotifera, Stomatopoda and Caprellidae) occurred exclusively in the Tabatinga River, but these represent groups rarely found in mesozooplankton surface hauls and were collected due to uncommon factors such as their small size or hyperbenthic behavior.
At both estuaries there was a strong predominance of holoplanktonic organisms in relation to meroplanktonic ones and this pattern was more evident during ebbing tides (Figure 4).
The same pattern was identified in ichthyoplankton groups. Regarding the 10 common species, 7 were more abundant in the Itapicuru River and 3 (Harengula aff. jaguana Poey, 1865, Hypsoblennius invemar Smith-Vaniz and Acero, 1980 and Trinects sp.) in the Tabatinga River (Tables 4 and 5). Anchoa sp. was one of the most abundant groups in both rivers, while Harengula aff. jaguana and Ctenogobius boleosoma Jordan and Gilbert, 1882 also presented high density values in the Tabatinga and Itapicuru Rivers, respectively. This work brings new and relevant taxonomic information on planktonic fauna of tropical estuaries. The lower number of taxa and the lower density of most taxa, concerning zooplankton and ichthyoplankton, in the Tabatinga River may reflect poor water quality conditions in this estuary due to organic pollution caused by shrimp farm effluents disposal.