South American Lakes Research Articles

Monitoring of metal phytofiltration performance by micro-XRF methodology.

In this work, micro-XRF was considered as a possible technique for monitoring the rate of incorporation of Cu and Zn into aquatic plants of a laboratory-scale phytofiltration system. This system employed Salvinia biloba Raddi under controlled conditions of light and nutrients. This aquatic plant is being considered as an efficient hyperaccumulator of Cu and Zn and is widely spread in South American lakes and rivers. One set of plants was exposed to 40 ppm w/w of Cu and another to 40 ppm w/w of Zn. The analytical procedure was based on the periodic in vivo quantitative analysis of Cu and Zn at selected points in the plants using micro-XRF. The accuracy of this quantification was effectively improved with the assistance of the Monte Carlo XMI-MSIM simulation code. In order to establish the input parameters of this software, careful measurements of the experimental parameters necessary for the correct modeling of the micro-XRF spectrometer were performed. After that, specially manufactured standards made of tissue equivalent material were employed to validate the configuration of the simulation code and input parameters. It was fulfilled by the comparison of measured and simulated micro-XRF spectra of these standards. Once the configuration code and input parameters were verified, two strategies were considered for the application of Monte Carlo simulation for elemental quantification in plants: an iterative process and inverse method established with external virtual standards. Benefits and drawbacks of both approaches to improve the monitoring of phytofiltration systems were carefully discussed.

Changes in Planktivory and Herbivory Regimes in a Shallow South American Lake (Lake Blanca Chica, Argentina) Over the Last 250 Years

Shallow lakes are vulnerable ecosystems impacted by human activities and climate change. The Cladocera occupy a central role in food webs and are an excellent paleoecological indicator of food web structure and trophic status. We conducted a paleolimnological study in Lake Blanca Chica (Argentina) to detect changes on the planktivory and herbivory regimes over the last 250 years. Generalized additive models were fitted to the time series of fish predation indicators (ephippial abundance and size, mucrone size, fish scales, and the planktivory index) and pheophorbide a concentration. The cladoceran assemblage changed from littoral-benthic to pelagic species dominance and zooplankton switched from large-bodied (Daphnia) to small-bodied grazers (Bosmina) ca. 1900 due to increased predation. The shift in planktivory regime (ca. 1920–1930), indicated by fish scales and the planktivory index, as well as herbivory (ca. 1920–1950), was triggered by eutrophication. Changes in planktivory affected the size structure of Bosmina, reducing its body size. This study describes the baseline for the lake as well as the profound changes in the composition and size structure of the zooplankton community due to increased predation and the shift in the planktivory regime. These findings will provide a reference status for future management strategies of this ecosystem.

Evolution of the El Nino-Southern Oscillation in the late Holocene and insolation driven change in the tropical annual SST cycle

South American lake sediment records indicate that El Nino events in the eastern equatorial Pacific (EEP) became more frequent after 3000calendar years BP. The reason for this evolution of ENSO behavior remains in question. An important trigger for ocean–atmosphere state switching in the tropical ocean is the annual cycle of sea surface temperature south of the equator along the margin of South America. This annual cycle can be reconstructed from the oxygen isotope records of the surf clam Mesodesma donacium. We provide evidence that these isotope records, as preserved in archeological deposits in coastal central Peru, reflect seasonal paleo-SST. We find that the annual SST cycle in the eastern equatorial Pacific became larger over the 4500–2500calendar year BP interval. This is consistent with increased ENSO variability. The magnification of the annual SST cycle can be attributed to changing insolation, indicating that ENSO is sensitive to the intensity and seasonal timing of solar heating of the southern EEP.

Bimodal transparency as an indicator for alternative states in South American lakes

Summary1. The alternative state theory claims that shallow lakes may have either clear water, and be dominated by submerged macrophytes, or turbid water and be dominated by phytoplankton. Most evidence for this theory comes from studies in temperate or boreal regions of Europe. Because of differences in the strength of trophic interactions, such as in the pressure of zooplankton grazing on phytoplankton, this influential theory might not apply elsewhere.2. Here, we test the theory for South American lakes, combining field data and Landsat satellite data. We studied the frequency distribution of primary producers and water transparency, looking for potential bimodality separating clear and turbid lakes. A bimodal distribution might be observed if there are indeed alternative states, although would not itself be sufficient evidence for the theory. Possible shifts between alternative states were analysed by comparing satellite data from 1987 to 2005.3. In our field data, there was a bimodal pattern in phytoplankton abundance and possibly in the abundance of submerged macrophytes, but not in water transparency. Analyses of the larger satellite data set revealed bimodality in lake transparency in 2005, but less so in 1987. In 1987, the lakes were generally clearer, and the transition to higher turbidity was more gradual than in 2005. The stronger bimodality in the more recent data, and the overall lower transparency, could have been caused by an increase in fertiliser use and subsequent eutrophication but also by differences in hydrology. Further, 1987 was much wetter than 2005, which could have caused dilution of suspended particles, leading to clearer water.4. While a bimodal distribution in the abundance of primary producers and water clarity is not decisive evidence for or against the theory of alternative states, our data clearly fail to refute it.

Analysis of the reproductive strategy of Jenynsia multidentata (Cyprinodontiformes, Anablepidae) with focus on sexual differences in growth, size, and abundance

Jenynsia multidentata is one of the most abundant freshwater fishes in the subtropical region of South America and when abundant it appears to have a major impact on lake ecosystems function. We studied the life-history traits and population structure of J. multidentata in a shallow lake in Uruguay, and collected literature data along a subtropical to tropical freshwater gradient in South America. Our general focus was on reproductive strategy, particularly on sexual differences in growth, size and abundance. In Lake Blanca, we found strong reverse size dimorphism (RSD) and a feminized sex ratio. Both are attributed to sexual differences in growth patterns and longevity. RSD and a feminized sex ratio seem to characterise J. multidentata regardless of latitude, and together with other life traits such as small size, rapid growth, low age of maturity, the capacity of the females to keep sperm alive to fertilize several successive litters, high natural mortality and fertility, resistance to extreme environmental conditions and omnivory, they are indicative of high productivity to biomass ratio. The very high population growth rate helps to explain why fish removal, aiming to promote an increase in the zooplankton grazing pressure on phytoplankton, may not be a useful tool in eutrophic-turbid subtropical South American lakes. We also found that the duration of the breeding season deviated markedly with latitude, increasing towards the tropics, which may have major consequences for population growth rates and differential impact of this species on lake ecosystems in the different climate zones.

South American lake paleo-records across the Pampean Region

South American lake paleo-records across the Pampean Region

Short term effects of exposition to artificial ultraviolet radiation on Parabroteas sarsi (Copepoda, Calanoida)

The increase in the penetration of ultraviolet radiation that has been reported lately for freshwater ecosystems in southern South America would allegedly generate alterations in ecological processes. In this respect the mortality of Parabroteas sarsi, a calanoid copepod distributed in South American lakes and ponds, was studied. Specimens were reared at two different concentrations of dissolved organic carbon and subjected to 72 h exposure to artificial ultraviolet radiation. At high dissolved organic carbon concentration the mortality of P. sarsi increased at 24 h and stabilized at 48 and 72 h, whereas at low dissolved organic carbon concentration mortality increased linearly during the studied period. The results support both the description of a screen effect of dissolved organic carbon against ultraviolet radiation, and the potential photorepairing role of visible radiation that explains the increase of mortality in conditions of low dissolved organic carbon concentration.

Pronounced occurrence of long-chain alkenones and dinosterol in a 25,000-year lipid molecular fossil record from Lake Titicaca, South America

Our analysis of lipid molecular fossils from a Lake Titicaca (16° S, 69° W) sediment core reveals distinct changes in the ecology of the lake over an ∼25,000-yr period spanning latest Pleistocene to late Holocene time. Previous investigations have shown that over this time period Lake Titicaca was subject to large changes in lake level in response to regional climatic variability. Our results indicate that lake algal populations were greatly affected by the changing physical and chemical conditions in Lake Titicaca. Hydrocarbons are characterized by a combination of odd-numbered, mid- to long-chain (C 21–C 31) normal alkanes and alkenes. During periods when lake level was higher (latest Pleistocene, early Holocene, and late Holocene), the C 21 n-alkane, and the C 25 and C 27 alkenes dominate hydrocarbon distributions and indicate contribution from an algal source, potentially the freshwater alga Botryococcus braunii. The C 30 4 α-methyl sterol (dinosterol) increases sharply during the mid-Holocene, suggesting a greatly increased dinoflagellate presence at that time. Long-chain alkenones (LCAs) become significant during the early Holocene and are highly abundant in mid-Holocene samples. There are relatively few published records of LCA detection in lake sediments but their occurrence is geographically widespread (Antarctica, Asia, Europe, North America). Lake Titicaca represents the first South American lake and the first low-latitude lake in which LCAs have been reported. LCA abundance and distribution may be related to the temperature-dependent response of an unidentified algal precursor. Although the LCA unsaturation indices cannot be used to determine absolute Lake Titicaca temperatures, we suspect that the published LCA U 37 K unsaturation calibrations can be applied to infer relative temperatures for early to mid-Holocene time when LCA concentrations are high. Using these criteria, the U 37 K unsaturation indices suggest relatively warmer temperatures in the mid-Holocene. In contrast to previous speculation, lipid analysis provides little evidence of a greatly increased presence of aquatic plants during the mid-Holocene. Instead, it appears that a few algal species were dominant in the lake. Based on the dramatic rise in abundances of LCAs and dinosterol during the early to mid-Holocene, we suspect that the algal producers of these compounds rose in response to a combination of physical and chemical changes in the lake. These include temperature, salinity, and alkalinity changes that occurred as lake level dropped sharply during a multi-millennial drought affecting the Central Andean Altiplano.

Comparison of the filtration structures in South American Daphnia

The third and fourth filtering limbs were studied in 16 Daphnia populations from South American lakes using scanning electron microscopy. The mesh sizes of the juvenile and adult Daphnia ranged from 100 to 1270 nm. Daphnia pulicaria from Ecuadorian lakes had the widest intersetular distances. In contrast, D. laevis, D. peruviana and D. obtusa from eutrophic ponds exhibited the narrowest adult mesh sizes, less than 300 nm. Three species from oligothrophic lakes exhibited narrow (100-436 nm) distances between the setulae. The filter area was measured by direct and indirect methods, the largest filtration area was found in Daphnia commutata (3.33 mm 2 ). Species from oligotrophic lakes had larger filter areas but all species showed a strong correlation between the increasing body size and filter area. Two clear types of relationships between filtering area and mesh size were recognized.

Benthic community metabolism and trophic conditions of four South American lakes

Oxygen uptake rates of undisturbed sediment columns have been used as an integrative measure of the metabolic activities of benthic communities. Since the intensity of metabolic processes of profundal lake is dependent on the production of organic matter in the pelagic zone, oxygen uptake rates reflect the trophic condition of the whole lake. Four small lakes of central Chile, differing strongly in trophic conditions, provided a possibility to compare benthic oxygen uptake rates, under different oxygen conditions (Quinenco, Grande, Chica and Lleulleu). Our objective was to establish the relationship between the oxygen uptake rates and bottom characteristics of lakes with different trophic conditions. At 8 mg O2 l-1 in the overlying water of the cores studied, the oxygen uptake rates of the sediment were: Quinenco 51.2–56.0 mg O2 m2 h-1 (eutrophic), Grande 41.2–46.4 mg O2 m2 h-1 (mesotrophic), Chica 23.2–18.1 mg O2 m2 h-1 (mesotrophic) and Lleulleu 11.7–16.0 mg O2 m2 h-1 (oligotrophic). By exposing the sediments to different oxygen levels in the laboratory, it was found that benthic community metabolism decreased with oxygen concentrations. The slope of regression lines, relating oxygen uptake rates to oxygen concentrations, differed for the different sites investigated, closely related with the trophic conditions of the lakes. It was positively correlated with the organic matter content of the sediment of the cores (r2= 0.78, p<0,05) and the nutrients of the bottom waters (total-P: r2= 0.73, p<0,05; total-N: r2= 0.73, p<0,05), and negatively with the redox potential of the sediments (r2= 0.88, p<0,05).

Towards a Biogeography of North American Charophytes

The great bulk of the world’s charophyte literature of the past 100 years or more suffers from several major limitations. Much is entirely descriptive with but few attempts to ascribe any functionality to the features under consideration, or how they adapt such species to their respective ecological niches. Charophyte distributions have been attributed almost entirely to physical parameters with virtually no consideration given to the role of aquatic herbivores or other biotic environmental factors. Furthermore, most workers have focused on relatively restricted areas with little or no reference to others either near or far removed. That there is much to be gained from a wider focus (both spatially and conceptually) that incorporates greater conjecture as well as enhanced collaboration is here suggested. How are the charophyte floras of one region similar to, or different from, those of another, and, of particular significance, ‘Why?’ The authors, being North American, focus on that continent but with the firm conviction that most generalities applicable there hold equally true for other landmasses, and have done so for the previous 10, if not 100, million years. This account focuses first, if somewhat superficially, on 14 widely distinct North American charophyte communities (plus South American Lake Titicaca) and then in greater detail on four of those. Among other issues considered are how species richness relates to latitude; why some geographical entities support more charophyte species than do others; the extent to which charophyte floras reflect the availability of different habitats; the contributions of herbivory to the preceding; the stability of the North American charophyte flora; the ecological considerations most often reflected by charophyte zonation and how–or to what extent–range extensions reflect niche preferences or requirements. While the authors well appreciate just how minimal their efforts may appear a century hence, at least they hope to have placed on the table some considerations with which colleagues from other landmasses may agree, disagree or suggest modifications.

Thermal, light and oxygen characteristics in a small eutrophic warm monomictic lake (El Plateado, Valparaíso, Chile)

Table 1. Some major morphometric data on Lake EI Plateado Lake. (After DOMiNGUEZ et al. 1976). Physical processes have been recognized as important driving forces in the biology of temperate lakes (ALLANSON 1990). However, annual studies on the physical features in South American lakes have only appeared as general limnological characterizations during a year cycle. There are few data for periods greater than one year. In a study of the zooplankton of Lake EI Plateado, routine data on some physical properties were included. The results are reported here.

Brief history of nursing in Nicaragua

NICARAGUA is a Central American nation located between the Equator and the Tropic of Cancer, between 11 and 15 degrees latitude. Its geographical position is highly desirable, as it is located in the middle of the Central American isthmus, where the Atlantic and Pacific Oceans are only 220 km apart. Nicaragua is crossed from North to South by a belt of tropical jungle whose thickly intertwined branches form a vault of evergreen vegetation. The tree trunks are covered with moss and orchids. Reeds and branches nearly touch the ground, resembling curtains, where brightly feathered birds form their nests. The wildlife that inhabits this humid and warm region is formed by monkeys, wild beasts such as jaguars and coyotes, snakes, toads and frogs. Nicaragua is indeed a paradise for naturalists. The jungle divides it into two areas: the Atlantic area and the Pacific area. The territory of Nicaragua has 1300 km2 and the population is two million inhabitants. Nicaragua has two lakes. One of them, Lake Cocibolca, commonly known as ‘Lag0 de Nicaragua’, is the second largest in the continent, surpassed in size only by the famous South American Lake Titicaca. One great attraction of Lake Cocibolca is an archipelago formed by over 200 small islands covered-by exuberant tropical vegetation. The other big lake is Lake Xolotlan, commonly known as ‘Lago de Managua’, on the shores of which is located Managua, the capital city. Another special characteristic of the geography of Nicaragua is a long series of volcanoes lined up in an almost perfect parallel to the Pacific Ocean. These volcanoes are part of the volcanic system that circles the entire Pacific Ocean and is known as the ‘Circle of Fire of the Pacific’. Managua is the capital city. Recent geological studies have revealed that unfortunately this city has the greatest number of geological faults per square kilometer in the world, which makes it very susceptible to earthquakes. The last earthquake occurred on 23 December, 1972. It destroyed two-thirds of the city and claimed 10,000 victims.

Planktology of Temperate South American Lakes

Planktology of Temperate South American Lakes