Impact of phosphorus enrichment patterns on stream periphyton microbial communities

Abstract

Delivery of excess nutrients, such as phosphorus (P), to nearby surface waters due to intensified agricultural and urban land use activities has resulted in the degradation of aquatic ecosystems such as lakes, coastal areas, and flowing waters. In stream ecosystems, excess P can influence the diversity and composition of periphyton communities. However, P enrichment can vary over time and in intensity according to the source of P and there is limited knowledge of the impacts of event-based loadings on the microbial community composition of stream periphyton. To address this knowledge gap, two related stream mesocosm experiments were conducted. The first objective of my research was to evaluate whether P delivery patterns that are commonly associated with human land use practices influence the microbial community composition of stream periphyton. I achieved this objective by conducting a 25-day mesocosm experiment using nine artificial streams subjected to one of three P enrichment loading patterns in triplicate: unenriched, continuously enriched, or event-based enriched. Periphyton samples were collected at six time points and underwent high throughput sequencing of 16S and 18S rRNA genes to profile prokaryotic and eukaryotic microbial assemblages. Results showed that microbial community composition could not be differentiated among enriched delivery patterns, indicating the P load magnitude was more important than the P loading pattern. Thus, management of P enrichment in streams should prioritize targeting the most efficient way to reduce P loads, regardless of source. The second research objective was to assess how stream periphyton microbial community composition responded along an increasing gradient of P event concentrations and test whether antecedent P availability influenced this response. Two artificial stream experiments were conducted under low or high antecedent P conditions, with each stream receiving one of nine different P event concentrations. Periphyton were collected one day before and 10 days after the event and high throughput sequencing of the 16S and 18S rRNA genes were used to characterize prokaryotic and eukaryotic members of stream periphyton communities. The results indicate that small levels of P enrichment from P events can lead to a common enriched stream periphyton community. Thus, bioassessments of periphytic communities in anthropogenic landscapes may underestimate the extent of enrichment in streams with high baseflow P concentrations.