Long term analyses of the microbial loop, centred on the picoplankton dynamics, were carried out over a five-year (1998 to 2002)

No abstract

Free-living amoebae (FLA) are protozoa that are widely distributed in nature. They are important in the cycling of nutrients in aquatic food chains, but their distribution in natural aquatic environments is not well known. We conducted a survey to determine the presence and distribution of FLA and their relation to some physicochemical parameters in streams of the Mexico Basin in Central Mexico. Thirty-two sites from 18 streams were sampled. Samples were centrifuged and cultured onto NNA-media to isolate amoebae. Identifications were based on morphology. The pathogenicity of Acanthamoeba isolates was tested. Oxygen saturation, temperature, pH, specific conductance, water flow, dissolved reactive phosphorus, nitrites, nitrates, ammonia, dissolved inorganic nitrogen and coliforms were determined. One hundred-and-twenty FLA representing 18 genera were identified. The most frequent genera were Vannella, Rosculus and Acanthamoeba. The frequency of potentially pathogenic FLA was low and only 3 Acanthamoeba isolates were invasive in mice. The highest species richness of FLA was found in streams located into agriculture activity areas and those close to small villages that discharge wastewater into them. Water temperatures were always below 17°C. Oxygen saturation and pH were within the limits for the growth of most FLA. The presence of Acanthamoeba and Naegleria was low; nevertheless, they include potentially pathogenic species and can act as vectors and reservoirs for microbial pathogens and can produce human infections.

Testate amoebae are unicellular organisms characterized by a shell-like test. Due to their potential use as bioindicators (and paleoindicators), these organisms have been increasingly studied in the last decade, particularly in temperate latitudes. This study's objective was two-fold: to identify the testate amoeba communities sampled from 29 water bodies in Mexico and to determine if their presence and distribution also made them suitable bioindicators for tropical latitudes. A total of 40 taxa were recorded within 12 genera, and six significant variables -oxygen, pH, depth, temperature, conductivity, and total alkalinity -that explained testate amoeba distribution within and among the water bodies were identified through a canonical correspondence analysis. The Q-mode clusters rendered five assemblages, each named after their respective dominant species: 1) Centropyxis aculeata strain "aculeata" assemblage, 2) Difflugia oblonga strain "bryophila" assemblage, 3) diverse assemblage, 4) Cucurbitella tricuspis assemblage, and 5) Difflugia protaeiformis strain "acuminata" assemblage. We found that Cucurbitella tricuspis and the Difflugia protaeiformis strain "acuminata" have similar ecological preferences to those reported previously for temperate lakes, with the former identified as an indicator of eutrophic environments and the latter as an indicator of low oxygen levels. On the other hand, Centropyxis aculeata strain "aculeata" and Arcella vulgaris seem to indicate adverse conditions, but the source of this environmental stress apparently differs from that reported in temperate latitudes. Although this stress source could not be identified in all cases, our study nonetheless demonstrates that testate amoebae in the water bodies of central Mexico could reveal the presence of environmental stress.

Due to their dimensions, small and shallow water bodies are more sensitive to changes in nutrient load, water flow, and human management. The four water bodies studied are small (area <0.01 km2), constantly supplied by a non-anthropogenic source of nutrients, and these water bodies present different trophic states: mesotrophic, eutrophic, and hyper-eutrophic. The objective of this study was to identify the key environmental factors that created differences in the trophic state of these adjacent shallow urban lakes by modeling chlorophyll-a (Chl a) through the application of the Partial Least Squares Regression (PLSR). The models (n = 36) explain 45.8–60.6% (R2), and predicts 39–52.9% (Q2) of the variance. Environmental variables were identified in the water bodies as critical factors of trophic state determination, water residence time (WRT), ions (e.g., Ca2+), and minerals as hydroxyapatite (HAP). These variables were related to processes that could improve trophic conditions, such as flushing and phosphorous precipitation. Conversely, N-NH3 concentration was associated with nutrient recycling, and found to be able to promote eutrophication.