Responses of benthic diatoms to water browning and nutrients: a field experiment

Browning and eutrophication threaten freshwater biodiversity. However, the independent and joint effects of browning and nutrient enrichment on algal communities remain poorly known. Our aim was to investigate such effects on benthic diatoms using a field experiment with freshwater microcosms including 5 levels of water color (clear to humic) and 2 levels of nutrients (control and nutrient enrichment). We also compared diatom communities in the experimental pools to those of surrounding natural ecosystems. We found that the experimental pools had low species richness, most probably due to dispersal limitation, showing no clear response to the independent or combined effects of water color and nutrients. Water color was significantly associated with community composition, but only in the nutrient control pools on the first sampling occasion. Nutrients were not significantly related to community composition. The communities in the experimental pools and surrounding natural waterbodies were highly dissimilar. The low diversity in the experimental pools suggests that environmental conditions and dispersal limitation may jointly limit the number of species. The fact that richness did not show a clear response to color or nutrients suggests that tolerant species were dominant in the pools. The results further imply that nutrient enrichment may weaken the effect of browning, potentially leading to diatom community homogenization. Our findings indicate that the diatoms colonizing new habitats may possibly originate from the surrounding terrestrial habitats, in addition to aquatic ecosystems. Our study highlights the importance of investigating multiple environmental pressures on algal communities.