Excess of nitrogen reduces temporal variability of stream diatom assemblages

Nutrient enrichment degrades water quality and threatens aquatic biota. However, our knowledge on (dis)similarities in temporal patterns of biota among sites of varying level of nutrient stress is limited. We addressed this gap by assessing temporal (among seasons) variation in algal biomass, species diversity and composition of diatom assemblages in three streams that differ in nutrient stress, but are otherwise similar and share the same regional species pool. We monitored three riffle sections in each stream bi-weekly from May to October in 2014. Temporal variation in water chemistry and other environmental variables was mainly synchronous among riffles within streams and often also among streams, indicating shared environmental forcing through time. We found significant differences in diatom assemblage composition among streams and, albeit less so, also among riffles within streams. Diatom assemblages in the two nutrient-enriched streams were more similar to each other than to those in the nutrient-poor stream. Taxa richness did not differ consistently among the streams, and did not vary synchronously at any spatial scale. Temporal variation in diatom assemblage composition decreased with increasing DIN:TotP ratio, likely via a negative effect on sensitive taxa while maintaining favorable conditions for certain tolerant taxa, irrespective of season. This relationship weakened but remained significant even after controlling for stochastic effects, suggesting deterministic mechanisms between nutrient levels and diatom assemblage stability. After controlling for stochastic effects temporal variability was best explained by DIN suggesting that excess of nitrogen reduces temporal variability(intra-annual beta diversity) of diatom assemblages. The high temporal variation, and especially the lack of temporal synchrony at the within streams scale, suggests that single sampling at a single site may be insufficient to reliably assess and monitor a complete stream water body. Our results also showed that measures including species identity outperform traditional diversity metrics in detecting nutrient stress in streams.