Key words: Phytoplankton, freshwater zone, ecology, large rivers, Argentina. Abstract .... of the Argentine Navy) in October 1994; sampling was carried out at low ...
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Hydrobiologia 380: 1–8, 1998. c 1998 Kluwer Academic Publishers. Printed in Belgium.
Phytoplankton from the Southern Coastal Fringe of the R´ıo de la Plata (Buenos Aires, Argentina) Nora G´omez1 & Delia E. Bauer1 1
Instituto de Limnolog´ıa ‘Dr. Ra´ul A. Ringuelet’, CONICET-UNLP, C.C. 712, 1900 La Plata, Argentina, Scientific Contribution No 651 Received 16 January 1997; in revised form 15 November 1997; accepted 18 November 1997
Key words: Phytoplankton, freshwater zone, ecology, large rivers, Argentina
Abstract Structural characteristics of spring phytoplankton from the Southern Coastal Fringe of the R´ıo de la Plata between 58˚320 and 57˚410 W, up to 10 km from the coast line are analysed. From 126 recorded taxa, 69 had a relative frequency above 5%. Main frequencies coincided with main quantitative abundances, corresponding to centric diatoms of the genus Aulacoseira – A. granulata var. augustissima especially – Stephanodiscus hantzschii and the coccal green Dictyosphaerium pulchellum and D. subsolitarium; the exception was the Cyanophyceae Microcystis aeruginosa, which showed the greatest quantitative abundance, but was not among the most frequent species. Centric diatoms accounted for most of the biovolume. Average phytoplankton density was 259 cells ml,1 (11–3700 cells ml,1 ). Diversity index (H0 ) varied from 0.51 to 2.78 bits ind,1 . Phytoplankton size structure was: 43% picoplankton, 56% nanoplankton and 1% microplankton. Quantitative data on selected species were assessed for similarity among stations, 74% are joined in a group dominated by Aulacoseira sp. pl. and codominated by coccal green algae. The groups dominated by Stephanodiscus hantzschii and Microcystis aeruginosa included stations affected by pollution. Algal patchiness was greatest following low tide river currents. The assessed phytoplankton was eutrophic, oligohalophilous and mesosaprobic, a composition in agreement with some characteristics of a true potamoplankton. Introduction The del Plata basin, with an area of 3 170 000 km2 (Figure 1A), drains to the Atlantic Ocean through the R´ıo de la Plata. This is a large river under temperate subhumid climate. Its main tributaries are Paran´a and Uruguay rivers, with a mean discharge that totals 26 300 m3 s,1 (Urien, 1972). Three major zones can be distinguished in the R´ıo de la Plata (Figure 1B) based on geological hydrological and biological criteria (CARP-INIDEP-INAPE, 1990): – Interior, which comprises the area from its rise up to the line that joins Buenos Aires and Colonia (salinity: 0.2–0.5‰). – Intermediate, extending to the line that joins Punta Piedras and Punta Brava (salinity: 0.3–5‰).
– Exterior, whose outer boundary is the a joining Punta Rasa and Punta del Este (salinity: 5–25‰). Close to the seaward limit of the intermediate zone there is a continuous enhancement of salinity that marks the beginning of the proper estuary. The zones upstream are considered riverine but with an influence of tides (Urien, 1972). In this study we consider the phytoplankton from the coastal fringe between 58˚320 and 57˚410 W, up to a distance of 10 km from the coast line. This area lies within the geomorphological unit known as Southern Coastal Fringe, extending along the Argentine side of the river (CARP-SHN- SOHMA, 1989). Previous studies on the phytoplankton from this area have been few and mostly focused on zones close to drinking water intakes (Guarrera, 1950; Guarrera & K¨uhneman, 1951–1952; Roggiero, 1988). In recent years, multidisciplinary studies have assessed the phys-
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Figure 1. A – Del Plata drainage basin. B – R´ıo de la Plata: 1 – Interior zone; 2 – Intermediate zone; 3 – Exterior zone. C – Study area, showing sampling stations.
ical and chemical variables at 33 sampling stations distributed along 12 transects between the Luj´an river and Magdalena City, and up to a distance of three km from the coast line (AGOSBA-OSN-SIHN, 1994). Within the framework of this research project (still in progress), the study of phytoplankton was begun in 1993 (AA-AGOSBA-SIHN-ILPLA, in press). An extensive sampling, adding new stations, was carried out during spring 1994. G´omez & Bauer (in press) provide partial results from that sampling. The objective of this work is to extend the analysis of the sampling, providing further results on the taxonomic composition of the phytoplankton, and its abundance, dominance, size, biovolume, spatial distribution and diversity.
Table 1. Main physical and chemical characteristics of the Southern Coastal Fringe of the R´ıo de la Plata according to AA-AGOSBA-SHN-ILPLA (1997). pH: Conductivity (S cm,1 ): Suspended solid matter (mg l,1 ): Transparency (cm): P/PO4 (mg l,1 ): N/NH4 + (mg l,1 ): N/NO3 , (mg l,1 ): N/NO2 , (mg l,1 ):
6.7–8.2 164–473 12–150 20–50 0.05–0.35 0.05–1.68 0.1–1.1 0.01–0.13
Study area The Southern Coastal Fringe of the R´ıo de la Plata is, in the study area, on average 4 m (0.8–8.5 m) in depth.
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Figure 2. A – Low tide currents according to Balay (1961) and quantitative spatial distribution of spring phytoplankton. B – Phytoplankton cell distribution in size classes.
Changes in water level depend on the influence of tides and prevailing surface winds. Tidal amplitude in the interior zone is 0.35 m. Currents are the result of the interaction between tidal movement and downstream flow of the river, following, at low tide, the natural form of the riverbed (Balay, 1961) (Figure 2A). Current velocity is between 28 to 56 cm s,1 (AGOSBA- OSNSIHN, 1994), and the mean hydraulic residence time has been calculated to be 36.3 days (Pizarro & Orlando, 1984). Average salinity in the study area ranges from 0.2 to 0.4‰ (Baz´an & Arraga, 1993). Table 1 shows other physical and chemical properties, according to AAAGOSBA-SHN-ILPLA (1997). About 11 milion inhabitants live in the numerous urban centers of the study area, among which Buenos Aires and La Plata cities. The river provides their main drinking water supply. Moreover, many important activities related to industry (48% production val-
ue of the country), agriculture, cattle raising, harbours, turism, and sports, take place throughout the region.
Material and methods Samples were taken during a three days cruise on board of the ‘A.R.A. Cormor´an’ (Naval Hydrography Service of the Argentine Navy) in October 1994; sampling was carried out at low tide. A total of 53 samples was taken along fifteen transects perpendicular to the coast, at 0.5, 1.5, 3, 5 and 10 km from the coast (Figure 1C). Samples for qualitative analysis were taken with plankton nets of 10 and 32 m mesh, and for quantitative analysis with a plastic tube, integrating the first 2 m water column (Lund et al., 1958). They were fixed in formalin in situ. Taxonomic analysis was carried out under an Olympus BH microscope with phase contrast. Counts were
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4 Table 2. Phytoplankton species present in more than 5% of the samples, inside parentheses its percentage of relative frequency in the sampling. Cyanophyceae (35.8) Microcystis aeruginosa K¨utz. (7.5) Mixosarcina burmensis Skuja Chlorophyceae (9.4) Actinastrum hantzschii var. subtile Wolosz (17.0) Closterium aciculare West (9.4) Coelastrum microporum N¨ag. (39.6) Crucigenia quadrata Morr. (5.7) C. tetrapedia (Kirchn.) W. & G. S. West (7.5) Crucigeniella rectangularis (N¨ag.) Kom. (68.0) Dictyosphaerium pulchellum Wood (68.0) D. subsolitarim Van Goor (9.4) Franceia ovalis (Franc´e) Lemm. (26.4) Kirchneriella contorta (Schmidle) Bohl. (41.5) K. obesa (W. West) Schmidle (32.1) Monoraphidium arcuatum (Kors.) Hind. (17.0) M. contortum (Thur.) Kom.-Legn. (26.4) M. griffithii (Berk.) Kom.-Legn. (28.3) M. komarkovae Nyg. (13.2) M. tortile (W. & G. S. West) Kom.-Legn. (13.2) Pandorina morum Bory (7.5) Pediastrum simplex Meyen (11.3) P. tetras (Ehr.) Ralfs (43.4) Scenedesmus acuminatus (Lagerh.) Chod. (22.6) S. acutus Meyen (11.3) S. cf. aldavei Hegew. & Schnepf (5.7) S. bicaudatus Dedus. (35.8) S. intermedius Chod. (17.0) S. magnus Meyen (22.6) S. nanus Chod. (15.1) S. opoliensis P. Richt. (20.8) S. quadricauda (Turp.) Br´ev. sensu Chod. (18.9) Schroederia setigera (Schr¨od.) Lemm. (5.7) Staurastrum leptocladum Nordst (17.0) Tetraedron trigonum (N¨ag.) Hansg. sensu Skuja (5.7) T. trigonum var. gracile (Reinsch) de Toni (18.9) Tetrastrum glabrum (Roll) Ahlstr. & Tiff. (32.1) T. heteracanthum (Nordst.) Chod. (32.1) T. komarekii Hind. Bacillariophyceae (54.7) Actinocyclus normanii (Greg.) Hust. (96.2) Aulacoseira ambigua (Grun.) Simon. (96.2) A. distans (Ehr.) Simon. (98.1) A. granulata (Ehr.) Simon. (100) A. granulata var. angustissima (O. M¨ull) Simon (54.7) A. granulata var. angustissima f. spiralis (O. M¨ull.) Czarnecki (13.2) A. muzzanensis (Meist.) Kram. (5.7) Cocconeis placentula Ehr. (20.8) Cyclotella meneghiniana K¨utz. (13.2) C. stelligera Cl. & Grun.
Table 2. Continued. (17.0) (20.8) (5.7) (15.1) (13.2) (7.5) (7.5) (9.4) (15.1) (20.8) (11.3) (13.2) (17.0 (13.2) (5.7) (13.2) (13.2) (11.3) (41.5) (26.4) (5.7) (62.3)
Cymbella silesiaca Bleisch Eunotia monodon Ehr. E. rabenhorsti Cl. & Grun. Fragilaria construens (Ehr.) Grun. F. heidenii Oestrup F. ulna (Nitzsch) Lang.-Bertal. Gomphonema clavatum Ehr. G. parvulum (K¨utz.) K¨utz. Melosira varians Ag. Navicula accomoda Hust. N. capitata Ehr. N. capitata var. hungarica (Grun.) Ross N. cuspidata (K¨utz.) K¨utz. N. cryptocephala K¨utz. N. pupula K¨utz. Nitzschia acicularis (K¨utz.) W. Sm. N. brevissima Grun. N. hungarica Grun. N. levidensis (W. Sm.) Grun. N. palea (K¨utz.) W. Sm. Rhopalodia gibba (Ehr.) O. M¨ull. Stephanodiscus hantzschii Grun.
Table 3. Morphometric characteristics of most abundant species: greatest axial linear dimension (GALD), standard deviation (SD), volume (V) and their percentages of total density. % of the GALD SD Volume total (m) (m3 ) density Microcystis aeruginosa Dictyosphaerium pulchellum D. subsolitarium D. pulchellum + D. subsolitarium Aulacoseira ambigua A. distans A. granulata A. granulata var. angustissima Stephanodiscus hantzschii
26 – – 8 3 5 9 14 6
2 5 2 – 24 7 26 30 8
0.2 4 0.6 80 0.6 4 – – 1.9 1202 1.4 216 8.2 1583 6.7 271 1.4 429
done with an inverted microscope, 5 to 10 ml subsamples were sedimented. Estimated counting error was 20% (Lund et al., op. cit). Species occurring in more than 5% of the samples are shown in the floristic list (Table 2) preceded by its percentage of relative frequency. In view of difficulties in differentiating Dictyosphaerium pulchellum from D. subsolitarium poaled data were recorded, indicated as D. pulchellum + D. subsolitarium in the floristic list.
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5 Phytoplankton was classified by size according to Margalef (1955) in the following classes: < 5 m picoplankton, 5–50 m nanoplankton, 50–500 m microplankton, 500–1000 m mesoplankton and > 1000 m macroplankton (Figure 2B). Biovolume of species representing more than 3% of total phytoplankton density was calculated by approximating each species’ shape to a geometrical one (Vollenweider, 1969); to do this, 20 cells were measured at a magnification of 1250 (Table 3). Specific diversity was estimated after Shannon & Weaver’s (1963). A cluster analysis of the sampling stations was based on logarithmically transformed and reduced matrix of 69 selected species (Table 2). Pearson’s correlation index and UPGMA linkage procedure were used (Sneath & Sokal, 1973).
Results and discussion Phytoplankton composition We recorded 126 specific and infraspecific taxa distributed in Cyanophyceae (6%), Chlorophyceae (48%) and Bacillariophyceae (46%). Sixty nine taxa had a relative frequency above 5% (Table 2). The centric diatoms Aulacoseira granulata var. angustissima, A. granulata, A. distans, A. ambigua and Stephanodiscus hantzschii were the most frequent. These centric diatoms are selected for their capability for exploiting this low light environment owing to their efficient light-harvesting mechanisms. Moreover, they depend on the high turbulence for avoiding passive sinking losses (Reynolds, 1987, 1992, 1995). Although pennate diatoms were well represented by frequency, their share of total abundance was low and they concentrated close to the Delta of the Paran´a and the mouth of the Riachuelo river. Chlorococcales was the second group by frequency (Table 2), represented by species of Scenedesmus, Tetrastrum, Kirchneriella, Crucigenia, Monoraphidium and principally Dictyosphaerium. Microcystis aeruginosa was the most frequent species among the Cyanophyceae (Table 2). The high percentage of common taxa in the Paran´a and Uruguay rivers (Guarrera, 1950) as well as in minor tributaries of the R´ıo de la Plata, leads to the view that the phytoplankton in the Southern Coastal Fringe is mainly of an extrinsic origin type. The composition of the spring planktonic algae of the R´ıo de la Plata is in agreement with that consigned by Reynolds (1984) for
temperate rivers between vernal and late spring period. The species composition suggests selection towards species with rapid intrinsic rates of growth and tolerant of frequent hydraulic disturbances, substantially pre-adapted to rivers, following Reynolds (1988). The assessed phytoplankton is an indicator of an eutrophic, oligohaline and mesosaprobic environment. Density, size structure and biovolume The most abundant groups were Bacillariophyceae (43%), Chlorophyceae (28%) and Cyanophyceae (29%). Euglenophyceae, Dinophyceae and Silicoflagellata were groups with too low abundances to take into account. Microcystis aeruginosa, Dictyosphaerium pulchellum + D. subsolitarium, Aulacoseira ambigua, A. distans, A. granulata, A. granulata var. angustissima and Stephanodiscus hantzschii were the most abundant species (Table 3), adding the 71% of total density. Dominance levels of centric diatoms are similar to those recorded from the Paran´a and other large rivers (Emiliani, 1990; Bonetto et al., 1994; Sabater & Klee, 1990). The rest of the species had densities below 2%. Among these, the highest were contributed by Aulacoseira granulata var. angustissima f. spiralis, Scenedesmus acuminatus, S. intermedius, S. quadricauda, Tetrastrum komarekii, Kirchneriella contorta, Pandorina morum and Coelastrum microporum. Average phytoplankton density was 261 cells ml,1 , lower than that from the lower Paran´a river, recorded during spring by Bonetto et al. (1994) and Izaguirre et al. (1995). There were two maxima of total density (Figure 2A) at station 111 (3600 cells ml,1 ) and 551 (3700 cells ml,1 ). The minimum (11 cells ml,1 ) was recorded at station 302. About the cell size of the species of the floristic list, the distribution in size classes of their total density was: 43% picoplankton, 56% nanoplankton and 1% microplankton (Figure 2B). Regarding biovolume of cells, there was a clear predominance of centric diatoms, with Microcystis aeruginosa and Dictyosphaerium pulchellum + D. subsolitarium having a minor contribution. The top biovolume values of Aulacoseira granulata and Stephanodiscus hantzschii were at 0.5 km from the coast whereas those of A. granulata var. angustissima and A. ambigua were more regularly distributed at different distances from the coast (Figure 3).
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Figure 3. Biovolume of most abundant species (mm3 m,3 ) according to distance from the coast.
Spatial distribution and diversity The spatial distribution of planktonic algae showed a patchy pattern that tended to follow the river currents during low tide. Up to transect 700 the highest concentrations were found close to the coast whereas beyond
this transect they deviated, following a natural channel located at 3–4 km from the coast (Figure 2A). H0 values were in the range of 2.81 bits ind,1 (sample 551) to 0.51 bits ind,1 (sample 111). A trend toward decreasing diversity with distance from the coast was found. This can be related to the fact that in open waters far from the coast, few centric diatom
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Figure 4. Dendrogram produced by the cluster analysis of samples based on log-transformed abundance data of the species present in more than 5% of the samples. Cluster-groups are marked with a code and explained in the text.
species predominate, in contrast with areas close to the coast where a species input from channels, streams and rivers of variable discharge takes place (G´omez & Bauer, 1998). Diversity values were lower than those from the Paran´a river, 1.38 to 3.07 bits ind,1 (Izaguirre et al., 1995). Clasification of samples The cluster analysis between stations is shown in Figure 4. Five groups of samples appear. Group I comprised sample 205 only, with the presence of centric diatoms and coccal green algae, without predominance of any taxa in particular. Group II included stations dominated by Stephanodiscus hantzschii and codominated by A. granulata var. angustissima, Dictyosphaerium pulchellum + D. subsolitarium, Scenedesmus acuminatus and S. acutus. Group III included stations dominated by the coccal green D. pulchellum + D. subsolitarium, Kirchneriella contorta, Monoraphidium contortum, M. komarkovae and Scenedesmus quadricauda. Group IV joins 74% of stations and was similar to group II, dominated by centric diatoms, but in this case with predominance of Aulacoseira sp. pl. and codominated by coccal green algae (Dictyosphaerium sp. pl., Monoraphidium sp.
pl., Scenedesmus sp.) and/or by Microcystis aeruginosa. Group V was dominated by M. aeruginosa. Groups II and V included stations subject to polluted discharge from tributaries i.e. the Luj´an and Riachuelo rivers, Santo Domingo stream, as well as from the Berazategui’s sewage outlet (AGOSBA-OSNSIHN, 1994; Boschi, 1988; Baz´an & Arraga, 1993). According to del Giorgio et al. (1991) the impact of pollution results in the replacement of one association by another. Here, the effect was to alter the distribution and relative proportions of the algae while retaining the basic species composition.
Acknowledgements We are indebted to our colleagues of the ‘Southern Coastal Fringe Group’, and the crew of A.R.A. ‘Cormor´an’. Thanks are also to anonymous referee for comments on the manuscript and for linguistic improvements in the text. This work was founded by the University of La Plata.
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