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Quantifying Periphyton Responses to Phosphorus in the Florida Everglades: A Synoptic-Experimental Approach

Water quality and periphyton were sampled along a nutrient-enrichment gradient in the northern Everglades to develop correlative relationships between periphyton taxonomic composition and changes in water chemistry. A controlled phosphorus-enrichment experiment was conducted concurrently to develop causal relationships between periphyton changes and phosphorus enrichment. Periphyton changes along the gradient were most strongly related to changes in total phosphorus (TP), but also were correlated with other potentially limiting nutrients (e.g., nitrogen, iron). Interior marsh stations (water column TP = 5-7 μg/L) were dominated by calcareous periphyton mats composed of the cyanobacteria Scytonema hofmanii and Shizothrix calcicola (27-70% of total biovolume) and an abundance of diatom epiphytes (11-49% of total biovolume). This assemblage was replaced at stations having slightly elevated phosphorus concentrations (TP ≥ 10 μg/L) with a periphyton assemblage dominated by filamentous green algae including Spirogyra and Mougeotia (80-99% of total biovolume) and containing few diatom epiphytes (<6% of total biovolume). These same taxonomic shifts occurred in response to experimental enrichment (0-12.8 g P m-2 yr-1) of interior marsh plots with phosphorus, thus confirming that these periphyton changes were due to phosphorus enrichment by human activities. Periphyton assemblages at marsh stations where phosphorus concentrations were highest (TP = 42-134 μg/L) were not reproduced experimentally, suggesting that environmental factors other than phosphorus may determine periphyton taxonomic composition in highly enriched areas of the marsh. The periphyton assemblage is a critical component of the Everglades ecosystem and changes documented here in response to phosphorus enrichment constitute a significant ecological impact on this ecosystem.