A multi-faceted approach to examining the role of trace metals in freshwater phytoplankton communities

Abstract

Phytoplankton communities are complex, and much work has been done to understand how abiotic factors influence phytoplankton growth. Only until recently, trace metals have been added to this intricate equation. This dissertation explored different facets of how trace metals influence freshwater phytoplankton communities using a combination of modeling, field sampling, and mesocosm experimentation. Chapter 1 used a meta-analysis to quantify trace metal requirements for common phytoplankton. The dataset included 339 specific daily growth rates extracted from 56 research articles. A Bayesian hierarchical model for each metal showed that cyanobacteria have lower effective dose 50% values for trace metals compared to green algae, despite having higher metal requirements. Chapter 2 investigated how trace metals impact algal bloom longevity. Four-liter clear containers were dosed with either nitrogen, phosphorus, or a Micronutrient mixture in a full factorial design. This was repeated across three study sites that varied in productivity. Trace metal additions had measurable impacts on bloom longevity when compared to treatments without trace metals in the two less productive systems. Chapter 3 described a field survey of iron and manganese distribution within three study sites that varied in size, depth, and productivity. Pearson’s correlations showed variable strengths in relationships between iron/manganese and phytoplankton depending on the study site. A principal component analysis displayed that particulate iron coincides with high primary productivity, while manganese was more associated with low dissolved P. An elastic net regression identified dissolved iron, particulate iron, and particulate manganese as good predictors of cyanobacterial biovolume. Chapter 4 used four-liter containers to understand how seasonality and lake mixing influence trace metal limitation in lake phytoplankton. Containers were dosed with either single metals (iron, manganese, copper, zinc, cobalt), a Micronutrient mixture, or nitrogen, phosphorus, and the Micronutrient mixture. Experiments were conducted once when each system was mixed and again when thermally stratified. Trace metals only encouraged significant growth compared to the control in the largest study site when it was thermally stratified. Bringing the results of this dissertation together identifies the clear importance of trace metals to phytoplankton communities, while also highlighting the complexity of micronutrient dynamics within lake ecosystems.