Nutrient enrichment modulates the structure and temporal assembly of riverine benthic algal and macroinvertebrate assemblage.

Nutrient enrichment has become one of the most widespread anthropogenic forces impacting freshwater biodiversity worldwide. Stream communities are frequently limited by nutrient availability, and enrichment over natural concentrations can impact both the distribution of species and the temporal assembly of natural communities. By studying benthic algal and macroinvertebrate assemblages from 35 streams over two years that spanned a wide gradient of nutrient enrichment, I found that the temporal assembly patterns of both assemblages were heavily influenced by novel nutrient concentrations. Eutrophication reduced seasonally driven variation in macroinvertebrate assemblages, which resulted in more temporally homogenous communities. Declines in seasonal macroinvertebrate variation suggests that enrichment decreases the competitive advantage that seasonal specialists gain by occupying particular temporal niches, which degrades an important facet of stream biodiversity. In assemblages of periphytic algae, temporal niche partitioning has evolved in response to seasonal oscillations in environmental conditions. Compensatory dynamics of algae under natural nutrient regimes broke down at relatively low levels of total phosphorus (TP) enrichment (~ 25 μg L-1). More species were able to coexist at any given time, and the fitness of a subset of species was increased by enrichment which led to seasonal variation characterized by synchronous swings in species biovolumes. Also, assemblage biovolumes during the study were much more unstable with greater enrichment, which indicates that anthropogenic alteration of nutrient regimes can affect community stability by changing the dominant mode of seasonal succession. I examined how enrichment changed the distribution of species among study sites, and found algal assemblage structure displayed sharp, non-linear changes in response to TP enrichment. Many algal species (57) synchronously increased in frequency and biovolumes with modest increases in TP concentrations of ~ 16 μg L-1, and 17 species declined at TP enrichment of ~ 20 μg L-1. Macroinvertebrate assemblage responses to TP concentrations were generally weak when compared to the algal assemblages, but there was some evidence of synchronous changes of taxa in response to periphyton nutrient content. These findings provide insight into how nutrient enrichment alters natural communities and leads to serious structural alterations to natural patterns in community composition.