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There are general guidelines and standards for measuring the microbial quality of water to prevent the incidence of disease outbreaks. Many agencies have chosen the 95th percentile; one can assess the recreational water quality, depending if the percentile value exceeds the guideline value or not. It is well known that this kind of data do not display a normal distribution and several alternatives have been proposed and are in use for estimating the percentile. A review of existing methods is given, that includes non parametric estimators as Hazen, Blom, Tukey and Weibull. We also describe transformations such as logarithmic and Box–Cox, that generate near normal data, after obtaining the normal percentile the inverse transformation is applied to obtain estimators in the original scale. A new methodology is proposed, consisting in finding the Tweedie distribution that better fits the observed data; this family has nonnegative support and can have a discrete mass at zero, making it useful to model skewed data that are a mixture of zeros and positive values. It allows working with parametric models in the original scale. We performed a Monte Carlo simulation to compare the performance of all the percentiles described above. As a result we noted that the percentile calculated from Tweedie distribution has lower mean square error than the others, which makes it the more precise estimator. All these techniques were applied to four data sets and, in all cases the Tweedie estimator was closer to the observed values than non parametric and anti transformed estimators.

We report on the biodegradation of pure hydrocarbons and chemotaxis towards these compounds by an isolated chlorophenol degrader, Pseudomonas strain H. The biochemical and phylogenetic analysis of the 16S rDNA sequence identified Pseudomonas strain H as having 99.56% similarity with P. aeruginosa PA01. This strain was able to degrade n-hexadecane, 1-undecene, 1-nonene, 1-decene, 1-dodecene and kerosene. It grew in the presence of 1-octene, while this hydrocarbons is toxic to other hydrocarbons degraders. Pseudomonas strain H was also chemotactic towards n-hexadecane, kerosene, 1-undecene and 1-dodecene. These results show that this Pseudomonas strain H is an attractive candidate for hydrocarbon-containing wastewater bioremediation in controlled environments. Since the classical standard techniques for detecting chemotaxis are not efficient at low bacterial concentrations, we demonstrate the use of the dynamic speckle laser method, which is simple and inexpensive, to confirm bacterial chemotaxis at low cell concentrations (less than 10(5) colony-forming unit per millilitre (CFU/mL)) when hydrocarbons are the attractants.

The inner shelf (5–15 m depth) off Mar del Plata (Argentina, Southwestern Atlantic) was studied to assess the influence of an intertidal sewage effluent on macrobenthic populations and to provide baseline information for a future monitoring program. Bottom samples were obtained with van Veen grabs at 49 stations. Macrobenthic organisms retained in a 0.5 mm mesh screen were used for biological analysis. Very fine sand-bottoms with a Natica–Owenia association occur in shallower sites, while patchy coarse sand bottoms plus gravel-sized shell debris inhabited by an association of patchy populations of Melita–Prionospio–Kalliapseudes–Caulleriella characterize the rest of the area. Low organic matter content in the area (from 0.78 to 2.05 %), and absence of any known indicator species (such as Prionospio spp.) in relation to distance to effluent, suggest that local benthic populations remain relatively undisturbed in spite of the heavy input of organic matter. The major factor contributing to this situ...