While macroalgae provide habitat and trophic support in many marine ecosystems, excessive proliferation is often considered an indicator of an impacted system, particularly in tropical reef ecosystems. How the processes structuring these macroalgal communities are affected by anthropogenic impacts, particularly within the context of spatial and temporal environmental heterogeneity, remains unclear. I conducted a series of short-term and long-term experiments in both tropical and intertidal temperate ecosystems examining the role of nutrient enrichment and its interactions with other anthropogenic stressors (reduction of herbivores, sediments) in regulating macroalgal populations and structuring macroalgal communities.

On an impacted tropical reef, I manipulated nutrient availability on the dominant reef flat macroalgal species at various times after rainfall. Nutrient limitation rapidly switched from nitrogen to phosphorous to no limitation over very short time scales, highlighting the dynamic relationship with environmental context. Additionally, field and lab experiments examined how terrestrial sediment loads on these algal thalli disturbed nutrient and herbivory control. I found that environmental conditions created by sediment loads had variable effects on algal biomass accumulation of different species, suggesting effects by different mechanisms. On an intertidal temperate reef, I manipulated nutrient availability and herbivory on macroalgal communities for two years, encompassing the heterogeneous nature of rocky reefs. Grazers had more dramatic and immediate effects, increasing cover by >10x in the first year. However, nutrients influenced the community in nearly all metrics in the second year.

Overall, my results indicated that nutrient control of tropical reef macroalgae is more complex than previously recognized and depends on both the species and context under consideration. Moreover, sediment loads may strongly modulate controls on macroalgal dynamics by altering, among other things, nutrient availability and herbivory. Finally, on intertidal temperate reefs, where nutrient control remains a matter of debate, my results showed that nutrient addition and herbivore reduction have complex effects on algal diversity and structure that changed over time and depended on habitat complexity. Together, these results indicate the importance of considering timescales and environmental context when determining the consequences of anthropogenic alteration to controls of macroalgal dynamics on both tropical and temperate reefs.