Diatom motility and nuclear alterations are affected by sediment elutriates of agricultural streams.

Diatoms respond to toxicants in lotic systems, and they are commonly considered to be sensitive indicators in environmental safety assessment. In addition to the structural characteristics of the algal populations, recent studies have shown that endpoints such as nuclear anomalies or diatom motility measures can be affected quickly by environmental changes. We sought to determine if cell density, cell size, nuclear anomalies and motility of the diatom Nitzschia palea were useful indicators of sediment quality from agricultural streams. For this purpose, we exposed cultures of the diatom to elutriates from sediments of a stream that flows through an intensive agricultural area, and measured the responses of the populations for 7 days in laboratory tests. The bioassays showed that motility measures in Nitzschia palea and the condition of their nuclear membranes rapidly reflected the effects of sediment quality after only 48 h of exposure; mean cell density and length were affected by day 7. The sediment elutriates affected cell movements by shortening the total path length and decreasing cell velocity; they also increased the number of cells with nuclear membrane breakage. Our results from these bioassays show that diatom motility measurements and the condition of the nuclei might be indicators that respond faster to impacts than the traditional structural parameters, such as cell density, specific composition of the assemblage or diversity metrics of the algal communities more often used in biomonitoring.

Seasonal and spatial distribution of the microbenthic communities of the Rio de la Plata estuary (Argentina) and possible environmental controls.

The relationship between microbenthic communities, the habitat characteristics, and physical and chemical gradients was studied in the Rio de la Plata estuary. Five replicates of the surface layer were collected seasonally, in 10 sampling sites influenced by different land uses. The distribution of microbenthic communities was governed by two gradients, the first one determined by anthropic factors, related to pollution, and the second one to conductivity and turbidity. The higher densities of producers were observed in sites characterized by fine sediments. During winter, spring, and summer cyanophytes were abundant, while in autumn the diatoms, particularly birraphideans, dominated. The ciliates were the most abundant group among consumers, particularly in winter, and their spatial distribution was influenced by the turbidity. The whole study area has an eutrophic condition. Turbidity and the enrichment with nutrients and organic matter explained 50% of the variability in the species' distribution.