Agricultural land-use effects on the colonization dynamics of the benthic diatom assemblage of lowland streams

Various agrochemicals have been shown to affect diatom growth, physiology, and the structure and diversity of the assemblage. This research aims to study the structural and functional parameters of the benthic diatom assemblage, including indices, ecological traits, nuclear, and frustule abnormalities during the colonization period when exposed to different levels of agricultural impact in lowland streams. To achieve this objective, an experiment was conducted in the Pescado watershed (La Plata, Buenos Aires, Argentina), where the colonization process of artificial glass substrates by diatoms was followed for 75 days in two streams with different intensities of agricultural land use. The results show that the Pampean Diatom Index and diversity reflect water quality in agricultural sites, and there is an increase in diversity and richness during the colonization process, which stabilizes after 20 days. Considering the ecological guilds, the motile guild (fast-moving species) dominated the assemblages, while the low-profile guild (species of short stature, including prostate, adnate, and erect diatoms resistant to physical disturbance and low tolerance to nutrient enrichment) was more abundant in the most impacted stream. However, the variations in the size classes did not produce a consistent trend representing environmental quality. The nuclear alterations were sensitive enough to show the differences in water quality, while the deformed frustules were not significantly different between the studied sites. These results show that measuring the impact of human activities on freshwater bodies under multiple-stressor scenarios has to consider multiple endpoints of the assemblage rather than a single structural or functional metric.