Typology of shallow lakes of the Salado River basin (Argentina), based on phytoplankton communities

The phytoplankton communities of eleven shallow lakes from Buenos Aires Province, Argentina, were studied seasonally from 1987 to 1989. Several physical and chemical properties were measured in each lake (pH, temperature, dissolved oxygen, transparency, nutrients), in order to interpret the structural and dynamic traits of the phytoplankton community.Important differences between the lakes studied were put in evidence by means of multivariate techniques (Cluster Analysis and PCA). The shallow lakes densely populated by macrophytes hosted lowest phytoplanktonic densities, with average values ranging from 690 to 16500 algae ml^–1. High species diversities were observed in these lakes (4.0–4.8). Lakes less colonized by macrophytes had higher phytoplankton densities. In some of them important blooms of Cyanophyceae were recorded, with between 60 000 and 179 000 algae ml^–1, and concomitant low diversities.The results of this investigation support the hypothesis that the phytoplankton community is strongly influenced by the macrophytes, by direct competition and/or by competition from periphytic algae associated with higher plants.     

Progressive changes in the structure and dynamics of the phytoplankton community along a pollution gradient in a lowland river — a multivariate approach

The phytoplankton of the River Lujan (Buenos Aires, Argentina) was studied for a period of 18 months, together with physical and chemical variables, in relation to a pollution gradient. 167 taxa were recorded within a seasonal succession characterized by dominance of diatoms with a brief summer green algae facies. A combination of several biotic indices and multivariate analysis was employed to assess the impact of pollution on the phytoplankton community. The biotic indices used were species diversity and richness, algal quotients (green algae/diatom ratio, Centrales/Pennales ratio) and the SD succession rate index. Multivariate procedures included cluster analysis and ordination by PCA of both species and samples, stepwise discriminant analysis and multiple discriminant analysis of variance (MANOVA). Results indicate that community dynamism is attenuated at the more polluted sites, concomitant with an increased predominance of a broad-tolerance algal assemblage, co-dominated by Cyclotella meneghiniana and Nitzschia stagnorum. The changes in the community structure and dynamics described herein involved alterations in the distribution and relative proportions of the algae, rather than modifications in the basic species composition. These changes may not be readily detectable by methods which over-simplify the ecological information, such as systems of indicator species and biotic indices, designed to assess the degree of pollution. The suitability of multivariate analysis and biotic indices in river phytoplankton studies is further discussed.     

Summer fluctuations of microbial planktonic communities in a eutrophic lake--Cierva Point, Antarctica

A eutrophic lake at Cierva Point, Antarctic Peninsula was surveyedduring the summers of 1997 and 1998. Phytoplankton size fractions(micro-, nano- and picoplankton) were analysed, as well as theabundance of bacterioplankton and planktonic ciliates. No permanentvertical stratification was found owing to the shallowness ofthe lake. Both nutrient concentrations and chlorophyll a valuesindicated highly eutrophic conditions, which are a consequenceof a natural enrichment by seabirds. Significant differencesin temperature between the 1997 and 1998 seasons strongly influencedmost of the biological features. The phytoplankton communityshowed a high algal species-richness, with important contributionsof epilithic, cryobiontic and soil algae. The dominant algalgroup was Chlorophyta, mainly represented by Chlamydomonas aff.celerrima, followed by Chl. aff. braunii. Some replacement ofphytoplanktonic species took place in summer and was more evidentin 1998. Picophytoplankton reached high densities, similar tothose reported from other Antarctic lakes. Bacterioplanktonabundances were typical of eutrophic and hypereutrophic lakes.There was a positive correlation between bacterial and totalphytoplankton abundance. Ciliates reached some very high peaks,with higher figures than those reported for other Antarcticsystems with similar trophic status.     

Microbial mats of twenty-six lakes from Potter Peninsula,King George Island,Antarctica

The taxonomic composition and ecology of microbial mats were investigated in 26 lentic environments of Potter Peninsula (King George Island, Antarctica) during the summer of 1995/96. These have different limnological characteristics according to the location of the basins. Among the 139 algal taxa registered, 10 are new records for Antarctica. Bacillariophyceae was the dominant class in terms of species richness, followed by the Cyanobacteria and Chlorophyceae. TWINSPAN classification was used to describe six epilithic algal assemblages; Canonical Correspondence Analysis ordination showed that soluble reactive phosphorus, phytoplanktonic chlorophyll a concentration and conductivity were the main sources for the variation in the data.     

Periphyton on artificial substrata from three lakes of different trophic status at Hope Bay (Antarctica)

The nutrient characteristics of aquatic environments at Hope Bay (Antarctica) depend on different factors, penguin enrichment being one of the most important. In this study we surveyed three water bodies of different morphometry and trophic status: Esperanza Lake (oligotrophic), Boeckella Lake (meso-eutrophic), and Pingüi Pond (hypereutrophic). This research provides a preliminary characterization of the periphyton colonization on artificial substrata (glass slides) of these water bodies from the end of January to mid-March 2000. Qualitative and quantitative analyses of the algae, chlorophyll a, dry weight, ash-free dry weight, and physical and chemical variables of the lakes were assessed. The Autotrophic Index and the Lakatos system of classification were also applied. All three lakes differed clearly in all the variables considered. The periphyton from Esperanza Lake was first dominated by Bacillariophyceae and subsequently by the chrysophyce Phaeogloea mucosa. P. mucosa was the dominant species at Boeckella Lake and Chlamydomonas subcaudata was dominant at Pingüi Pond. Highest mean maximum density values were reached at Pingüi Pond, followed by Boeckella and Esperanza Lakes. Highest biomass figures were registered with increasing trophic status. According to the Lakatos index, the periphyton of Esperanza Lake was the least developed and it was mainly organic and heterotrophic; Boeckella Lake showed intermediate development in periphyton mass, which was autotrophic and mainly inorganic whereas Pingüi Pond showed the highest development in periphytic mass, and it was mainly autotrophic.     

Periphyton flora of some lotic and lentic environments of Hope Bay (Antarctic Peninsula)

A taxonomic study was made of the periphytic algal flora of some freshwater environments at Hope Bay, Antarctic Peninsula (63°27S; 56°59W), during summer 1992–1993. Seven lakes and two pools that are characterized by different trophic levels were studied. The largest, Boeckella Lake, has an important outflow (Prasiola Stream) that was also analyzed. Ninety-seven algal taxa, including specific and infraspecific entities, are recorded. Thirty-one new records for the Antarctic continent are described and illustrated. The flora comprises 44.4% Cyanophyceae, 21.6% Bacillariophyceae, 18.5% Chlorophyceae, 9.3% Tribophyceae, 4.2% Chrysophyceae and 2% Zygophyceae. The epilithic flora of Hope Bay shows a high species richness. Phormidiumfragile and Lyngbya lagerheimii were the most abundant species among the Cyanophyceae. In the algal mats we also found Chrysophyceae (mainly Hydrurus foetidus and Chrysococcus rufescens) and the macroscopic chlorophycean Prasiola crispa. Tribonema australis sp. nov. (Tribonematales, Tribophyceae) is put forward as a new species. Hormidium fluitans (Gay) Heering is renamed Klebshormidium fluitans.     

Enhanced bud regeneration in aspen (Populus tremula L.) roots cultured in liquid media

 The regeneration potential of excised aspen (Populus tremula L.) roots cultivated in liquid medium, as affected by plant growth regulators and by the position of the isolated root explant on the main root, was investigated. The effect of various levels of benzyladenine (BA) and thidiazuron (TDZ) on bud regeneration in root explants was studied. TDZ in the medium had a marked effect on bud development as compared with BA, inducing a tenfold increase in the number of buds regenerated from various root explants. TDZ enhanced both root and root-borne shoot biomass production but reduced further shoot development and elongation. The position of the isolated root sections on the main root affected regeneration, the proximal sections further away from the root tip producing the highest number of buds per explant in both BA and TDZ treatments. Buds regenerated in close proximity to the site of lateral roots in BA-treated roots, while in TDZ-treated root sections, the buds formed all over the root regardless of the presence of lateral roots. The buds developed from inner cortical and sub-epidermal cell layers, disrupting the epidermis and the inner layers. Root biomass production and growth was greatly enhanced in well-aerated bioreactor culture in the presence of 4.5×10^–2 μM TDZ. A high number of the root-borne shoots could be rooted and converted to plantlets. However, while shoots regenerated in a medium with BA rooted well in a growth regulator-free medium, shoots formed in a medium with TDZ required auxin for rooting. Roots cultured in the presence of ancymidol, a gibberellin biosynthesis inhibitor, regenerated non-hyperhydric bud clusters and hyperhydric shoots. These were separated mechanically, subcultured to growth and rooting medium and transplanted ex vitro resulting in phenotypically true-to-type plantlets. The potential of liquid cultures for aspen shoot biomass production from roots is discussed. Received: 24 January 2000 / Revision received: 6 March 2000 / Accepted: 7 March 2000     

Cyanophyta of lakes and ponds of Hope Bay,Antarctic Peninsula

This paper presents the results of the floristic inventory of the freshwater Cyanophyceae found in plankton samples from nine lakes and ponds at Hope Bay (Antarctic Peninsula). Forty taxa were recorded, among which nine are new records for Antarctica. All the taxa are illustrated, and their distribution in Antarctica as well as the ecological characteristics of the sampling sites are given. The main morphological features and the geographic distribution are also given for the new records.