ABUNDANCE AND SPECIES COMPOSITION OF PLANKTONIC CILIOPHORA FROM THE WASTEWATER DISCHARGE ZONE IN THE BAHÍA BLANCA ESTUARY , ARGENTINA

The specific composition and abundance variation of the ciliate community from a wastewater discharge zone in the Bahía Blanca estuary, Argentina, were studied all throughout a year, from June 1995 to May 1996. The polluted area exhibited high values of particulate organic matter and nutrients, particularly phosphates. Aloricate ciliates were represented by 15 species belonging to the genera Strombidium Claparède & Lachmann, 1859; Strombidinopsis Kent, 1881; Cyrtostrombidium Lynn & Gilron, 1993; Strobilidium Schewiakoff, 1983; Lohmmanniella Leegaard, 1915 and Tontonia Fauré-Fremiet, 1914. Tintinnids were represented by nine species belonging to the genera Tintinnidium Kent, 1881, Tintinnopsis Stein, 1867 and Codonellopsis Jörgensen, 1924. The total abundance of aloricate ciliates reached a peak of 1,800 ind. l-1 and the total abundance of tintinnids reached a peak of 9,400 ind. l-1. Tintinnidium balechi Barría de Cao, 1981 was the most abundant ciliate in the community. Considerations on the presence and abundance of ciliates are made in relation to physicochemical and biochemical parameters.


INTRODUCTION
The major source of organic pollution in the world is domestic sewage (CURDS, 1982).In coastal areas, it is common for untreated sewage to be passed directly to the sea.Such is the case in Bahía Blanca City, Argentina.Sewage contains large amounts of organic matter, which, when utilized by bacteria, reduces the dissolved oxygen levels in the aquatic environment.This fact can cause great damage to marine organisms.The disposal of sewage in the sea is considered to be the major source of addition of nitrogen and phosphorous compounds to the aquatic environment.Wastes discharged into the sea cause eutrophication, which modifies the planktonic populations in coastal waters BARRÍA DE CAO et al. both quantitatively and qualitatively.Some planktonic ciliates can live in highly eutrophic waters; hence, they can be used as indicators of organic pollution.Bacterivorous ciliates can reach the highest standing stocks among the pelagic ciliates in some estuarine zones, and should be able to consume one third of annual bacterial production (ARNDT et al., 1990), so they play an important role in regulating the growth of bacteria populations (RIVIER et al., 1985).
The subject of planktonic ciliates in relation to organic pollution has not been studied in the coasts of Argentina, with the exception of the Río de La Plata coast (RIVES, 1997).
The aim is to analyze the composition and abundance variation of the planktonic ciliate community in the wastewater discharge zone of the Bahía Blanca estuary in relation to physicochemical and biochemical parameters.

MATERIAL AND METHODS
The studied area is located in the inner part of the Bahía Blanca estuary (38,8°S, 62,2°W) (fig.1).To compare results, sampling was carried out at two fixed stations, one in the wastewater discharge zone (WWDS) and the other (Boya 31) in the principal channel of navigation.The wastewater discharge zone is affected by untreated sewage from Bahía Blanca, a city with a population of approximately 350,000 inhabitants (80% of them within the sanitation network).
Samples were collected from 16 June 1995 to 20 May 1996 at two fixed stations approximately every 15 days with a 30 µm mesh plankton net and a Van-Dorn bottle and then fixed with 4 % formaldehyde solution and Lugol's solution, respectively.Observations were made with a contrastphase microscope, and ciliates were counted with an inverted microscope following the Utermöhl method (HASLE, 1978).
Nutrients were determined using a Technicon Auto Analyzer II, according to TREGUER & LE CORRE (1975), EBERLEIN &KATTNER (1987) andGRASSHOFF (1983).Chlorophyll "a" and phaeopigments were measured after LORENZEN (1967).Other determinations were performed according to STRICKLAND & PARSONS (1968).A correlation test was done between the total abundance of tintinnids, the total abundance of aloricate ciliates, the abundance of Tintinnidium balechi at WWDS, and the physicochemical and biochemical parameters.
The estimated total abundance of aloricate ciliates varied between a minimum of 100 ind.l -1 and a maximum of 3,700 ind.l -1 , the latter being registered at the Boya 31 Station, where the total number of individuals was always higher than that at WWDS.Values registered at WWDS varied from 100 ind.l -1 to 1,800 ind.l -1 (fig.12).
Strombidinopsis and Strombidium were the most widely represented genera, and both were present at the two stations during the sampling period.Strombidium reached the highest abundance levels at both stations, representing 37% of the total abundance, while Strombidinopsis reached 30%.The other genera showed relative abundances that were slightly higher at WWDS, with the exception of Cyrtostrombidium, which was found to have twice the number of individuals at Boya 31 (fig.13).
Eleven species of tintinnids were encountered in the principal channel of navigation (Boya 31) of the Bahía Blanca estuary: Tintinnidium balechi Barría de Cao, 1981; T. aff.semiciliatum; Tintinnopsis baltica Brandt, 1896; T. beroidea Stein, 1867; T. brasiliensis Kofoid & Campbell, 1929;T. glans Meunier, 1919;T. gracilis Kofoid & Campbel, 1929;T. levigata Kofoid & Campbell, 1929;T. parva Merkle, 1909;T. parvula Jörgensen, 1912 andCodonellopsis lusitanica Jörgensen, 1924.Nine species were sporadically represented at the wastewater discharge station (figs.9-11): Tintinnidium balechi, T. aff.semiciliatum, Tintinnopsis baltica, T. beroidea, T. brasiliensis, T. glans, T. gracilis, T. parva and Codonellopsis lusitanica.The total amount of tintinnids reached a peak of 9,400 ind.l -1 at WWDS, and the highest value observed was of 8,200 ind.l -1 at Boya 31 in spring (fig.14).These peaks were almost exclusively due to T. balechi, which was the most conspicuous component of the tintinnid community and was present throughout the sampling period at WWDS.Tintinnidium balechi reached percentages over 90% of the total abundance of tintinnids at this station (fig.14) while it never exceeded 51% of the total abundance of tintinnids at Boya 31.Chlorophyll "a" Phaeopigments S i g n i f i c a n t correlation values were found between the abundance of tintinnids, the abundance of the aloricate ciliates, and the abundance of Tintinnidium balechi, and some of the p h y s i c o c h e m i c a l parameters (tab.II).

DISCUSSION
In general, the total abundance values of aloricate ciliates registered at Boya 31 Station were similar to the values observed in other areas of the estuary (PETTIGROSSO et al., 1997).
Most of the species found at both stations belonged to Tintinnidium and Tintinnopsis, which is a characteristic feature of the species composition of the tintinnids community in the Bahía Blanca estuary (BARRÍA DE CAO, 1992).The total abundance of tintinnids was nearly always higher at Boya 31 than at WWDS.Positive correlation values were observed between the abundance and salinity values at WWDS, but their abundance was negatively correlated with POM and phosphate.The peak of tintinnids observed on 27 September 1995 at WWDS was due nearly exclusively to Tintinnidium balechi.The most remarkable characteristic of the tintinnid community at WWDS was the constant presence and high abundance values of T. balechi, although its abundance was negatively correlated with POM and phosphate.One explanation for the remarkable occurrence of T. balechi at WWDS could be its specific food requirements.Tintinnidium balechi is a small species with a mean oral lorica diameter of 17.3 µm and a mean peristome diameter of only 12 µm (BARRÍA DE CAO et al., 1997); therefore, its specific diet must consist of very tiny preys.The peaks of chlorophyll "a" at both stations were found during a phytoplankton bloom during the winter.Phytoplankton bloom in the Bahía Blanca estuary is chiefly composed of nanophytoplankton species, but these species, which are chain forming and projection bearing diatoms, could represent only a part of the diet of T. balechi, as they are not suitable food for tintinnids in general (BARRÍA DE CAO et al., 1997).
The physicochemical and biochemical characteristics at WWDS would favor the existence of an adequate source of food such as bacteria and heterotrophic microflagellates.Both terrestrial and marine heterotrophic aerobic bacteria develop well in this area (CABEZALÍ & BURGOS, 1988), and coliform bacteria are also very abundant (BALDINI & CABEZALÍ, 1988).

Table I .
Physicochemical and biochemical parameters measured at the wastewater discharge zone (WWDS) and a reference station (Boya 31).Mean values and S.D. (in brackets).