Periphyton responses to experimental phosphorus enrichment in a subtropical wetland

A field experiment was conducted to determine the effects of increased phos phorus (P) loading on periphyton abundance, productivity, and taxonomic com position in an oligotrophic Everglades slough characterized by abundant met aphyton and epiphyton. Mesocosm enclosures were dosed weekly with different orthophosphate loads (0-12.8 g P m(-2) per year) for 5 months during the s ummer wet season (late June-November 1995). Added P was accumulated rapidly by the periphyton at a rate proportional to the load. Phosphorus accumulat ion caused the loss of the extensive mats of cyanobacteria and diatoms that were abundant in the surrounding slough. This oligotrophic assemblage was replaced by floating mats of eutrophic cyanobacteria and diatoms at the hig hest loading rates (6.4-12.8 g P m(-2) per year), and by diffuse masses of filamentous chlorophytes at intermediate loads (1.6-3.2 g P m(-2) per year) . Metaphyton and epiphyton biomass-specific productivity increased in propo rtion to the loading rate and remained elevated at higher loads until the e nd of the wet season. Respiration rates also tended to increase with P load but were never significantly higher than in unenriched mesocosms. Despite higher productivity rates, both epiphyton biomass and floating mat coverage declined at higher loads compared to controls. Periphyton changes induced by P enrichment may affect wetland function by reducing (1) periphyton domi nance, (2) the food quality of the periphyton for herbivores, and (3) the n utrient storage capacity of the wetland. Many of these changes also have be en documented in other wetlands, thereby implicating P as a principal facto r affecting wetland periphyton structure and function. (C) 2001 Elsevier Sc ience B.V. All rights reserved.