Skeletonema potamos (Bacillariophyta) en la Laguna dos Patos, sur del Brasil: taxonomia y distribucion.
Skeletonema potamos was first described as Microsiphona potamos by Weber (1970), from the Miami River, USA. The species was distinguished by its small "siphon" (strutted process) and by occurring in a lotic system. The transfer of this species to the genus Skeletonema, and the establishment of its synonym Stephanodiscus subsalsus (A. Cleve) Husted were made later by Hasle & Evensen (1976).
This species is common in the rivers and lakes of North America, Europe (England, France, Spain, Germany, Poland, Finland, Hungary, Ukrany and Russia) and Australia (Weber 1970, Belcher & Swale 1978, Nicholls et al. 1983, Chessman 1985, Kiss 1986, Chang & Steinberg 1988, Sabater & Klee 1990, Genkal & Ivanov 1990, Descy & Willems 1991, Kiss et al. 1994, Turkia & Lepisto 1997), and its abundance is related to eutrophication. In Brazil the species was first reported by Torgan (1997) in Patos Lagoon (30[grados]23'-32[grados]07'S, 50[grados]41'-52[grados]12'W) on the Coastal Plain of the state of Rio Grande do Sul. Recently it was found in the freshwater Lagoa Mirim (32[grados]10'-33[degrees]37'S, 52[degrees]38'-53[degrees]40'W), also in Rio Grande do Sul, on the BrazilUruguay border (Perez & Odebrecht 2005).
We analyzed the morphogical features of the population of Skeletonema potamos in Patos Lagoon. We discuss the abundance and distribution of the species along the salinity gradient in this subtropical coastal lagoon.
Material and methods
The Patos Lagoon is a large (250 km long), shallow (average depth 6,0 to 8,0 m), polymictic, circumneutral, mesotrophic to eutrophic system. It is connected to the Atlantic Ocean by a single narrow canal. The water retention time in the lagoon is relatively long, because of the low tidal oscillation from the ocean.
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Samples were taken monthly at eight stations along the longitudinal axis of Patos Lagoon, from December 1987 to December 1988 (Fig. 1). The samples were collected from the water surface and fixed with Lugol's iodine solution. The species were counted by the method of Utermohl (1958), and the density (cells m[L.sup.-1]) was estimated based on live cells. A minimum of 100 individuals of the phytoplankton were counted in an inverted microscope, giving a counting accuracy of [+ or -]20% (95% confidence limits). The temperature and salinity were measured using a Yellow Springs Instruments Model 33. Transparency was measured with a Secchi disc, and silicate according to the method of Mullin & Riley (Aminot & Chaussepied 1983). The Spearman's correlation analyses were performed using the SYSTAT Program.
The taxonomic study of the species was based on examination of the cells and frustules. The material was cleaned according to the method of Simonsen (1974). Light micrographs were taken with phase-contrast illumination, and SEM electron micrographs were taken using a Jeol JSM-5200 at a voltage of 25 KV. The samples are preserved in the Herbarium of the Museu de Ciencias Naturais - Herbario Alarich Schultz (HAS 25088, 25094, 25095, 25097, 25101, 25111, 25112, 25115, 25203, 25212).
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Results and discussion
Skeletonema potamos (Weber) Hasle, J. Phycol. v. 12, p. 74, figs. 1-17.
Frustules cylindrical in girdle view, joined in short chains, frequently of two cells and rarely of three, four or eight cells, separated by short strutted processes. There are 1-2 parietal chloroplast in each cell. There are two refractive, small spherical granules (libroblast or oil drops), one towards each end of the cell (Figs 2, 3). Short marginal processes on the end of the chain are usually visible in light microscopy, but other details of valvar features can be only resolved with electron microscopy.
The length of the strutted processes can vary with the salinity. At a salinity of 0% the processes are extremely short (Fig. 4), and at a salinity of 7,28% the processes are longer (Fig. 5). The influence of salinity on the length of the strutted process of S. potamos was first observed by Hasle & Evensen (1976).
Electron micrographs reveal a rounded valve face, convex in the middle (Fig. 6). The valvar surface is provided with radial rows of elongate areolae. These areolae extend the full length of the mantle. Small granules are present in the middle of the valvar surface. One excentric rimoportula is present on the valve (Fig. 7). There are 5-7 strutted processes located at the junction of the mantle and the valve face. These processes are tubular, with a cleft at the distal tip (Figs 8, 9).
The morphology of the specimens agrees with the type, from the Little Miami River, Ohio, USA (Weber, 1970), except for the convexity and the pattern of granules on the valve face. The small granules of the specimens from the Patos Lagoon are limited to a middle area of the valve face. While such granules are rare in the Skeletonemataceae, granulations are usually interpreted as an ecophenotypic variation, probably caused by differences in the availability of silica (Tuchman et al. 1984). Other features observed in S. potamos were similar to the specimens described in the literature (Table 1).
The frustule of S. potamos is thin, weakly silicified, and breaks easily in oxidation. It may be confused with some species of Aulacoseira, because of the filamentous habit and the narrow space separating the cells in the chain. These features make the correct identification of S. potamos difficult, and it is possible that the species may sometimes be overlooked. This may be the reason that this species is not more widely reported.
Spatial and temporal distribution
Skeletonema potamos was found in the winter and spring of 1988 in the Patos Lagoon, and occurred in the limnetic (stations 3, 5, 9) oligohaline (station 11) and mesohaline (station 16) regions.
The population density ranged between 1 and 442 cells.m[L.sup.-1] and the species reached its highest concentration in August, at station 3, where the salinity was 0 % and the temperature was 15,8 [degrees]C (Fig. 10). On this occasion, S. potamos shared high abundance with Aulacoseira ambigua (Grunow) Simonsen (566 cells. m[L.sup.-1]) both reached 22% of total phytoplankton density. According to Perez & Odebrecht (2005), S. potamos was also mainly observed in August in the Mirim Lagoon (> 20 ind m[L.sup.-1]).
The concentration of cells of S. potamos did not show any correlation with silicate concentration or temperature (Figs 10, 11). On the other hand, it showed a significant negative correlation with salinity (r= -0,690; p= 0,05) (Fig. 12). In consequence, the population of S. potamos appears to be controlled by the salinity in the Patos Lagoon. The influence of salinity on the growth of this species was studied in culture experiments. The cells grew at salinity of 2-24%, but when they were inoculated into a medium devoid of the major seawater salts, unexpectedly they failed to grow (Paasche 1975).
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S. potamos usually appears together with S. subsalsum in the River Wumme (Germany), according to Hasle & Evensen (1976). In the Patos Lagoon, the species was also found with S. subsalsum (July, station 9; August, stations 9 and 16; and October, station 11) and .frenquently with Aulacoseira ambigua e A. granulata (Ehrenberg) Simonsen (Fig. 13).
It is interesting to observe that S. potamos has not been found in high density in Patos lagoon, which is impacted by organic matter and eutrophication, although it is considered a pollution-tolerant species. We suppose that the main factors influencing the development of the populations are probably light and/or competition. The Patos Lagoon has low transparency (< 0,50 m), and S. potamos has high light demand, as demonstrated by Kiss et al. (1994) in their investigation of the diurnal pattern of this species in the Danube River. Competition with other chain-forming centric diatoms, particularly Aulacoseira granulata and A. ambigua, also may be possible, because these species are abundant in the phytoplankton of the Patos lagoon.
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We thank the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) for financial support for the authors (Grants Proc. 302102/2007-8; 501961/2008-9) and Dr. Janet W. Reid (JWR Associates) for the revised the English text.
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Presentado: 13/05/2009 Aceptado: 23/06/2009 Publicado online: 28/08/2009
Lezilda Carvalho Torgan (1), Vanessa Becker (2) and Cristiane Bahi dos Santos (1)
(1) Fundacao Zoobotanica do Rio Grande do Sul, Museu de Ciencias Naturais. Rua Salvador Franca 1427, Porto Alegre, 90690-000, RS, Brazil. Email Lezilda Carvalho Torgan: firstname.lastname@example.org
(2) Universidade Federal do Rio Grande do Sul. Instituto de Pesquisas Hidraulicas. Av. Bento Goncalves 9500, Porto Alegre, 91501-970, RS, Brazil. Email Vanessa Becker: email@example.com
Table 1. Features of Skeletonema potamos observed in the population in Patos Lagoon, and previously reported in the literature. Features Observed Weber (1970) Chloroplast 1-2 several Frustule diameter ([micro]m) 3-4,5 3-4 Pervalvar axis ([micro]m) 6-10 4-8 Areolae in 10 [micro]m 8 - Number of processes 5 -7 5-8 Rimoportula 1 - Features Hasle & Evensen Belcher & Swale (1976) (1978) Chloroplast Frustule diameter ([micro]m) 1-2 (4) 1-2 Pervalvar axis ([micro]m) 3-4 3-4 Areolae in 10 [micro]m - 6-10 Number of processes - 8 Rimoportula 6-8 5-6 1 1
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|Author:||Carvalho Torgan, Lezilda; Becker, Vanessa; Bahi dos Santos, Cristiane|
|Publication:||Revista peruana de biologia|
|Date:||Aug 1, 2009|
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