Phosphorus cycling and partitioning in an oligotrophic Everglades wetland ecosystem: a radioisotope tracing study

1. Our goal was to quantify short-term phosphorus (P) partitioning and identify the ecosystem components important to P cycling in wetland ecosystems. To do this, we added P radiotracer to oligotrophic, P-limited Everglades marshes. PO4 was added to the water column in six 1-m enclosed mesocosms located in long-hydroperiod marshes of Shark River Slough, Everglades National Park. Ecosystem components were then repeatedly sampled over 18 days. 2. Water column particulates (>0.45 lm) incorporated radiotracer within the first minute after dosing and stored 95–99% of total water column P activity throughout the study. Soluble (<0.45 lm) P in the water column, in contrast, was always <5% of the P in surface water. Periphyton, both floating and attached to emergent macrophytes, had the highest specific activity of P (Bq g P) among the different ecosystem components. Fish and aquatic macroinvertebrates also had high affinity for P, whereas emergent macrophytes, soil and flocculent detrital organic matter (floc) had the lowest specific activities of radiotracer. 3. Within the calcareous, floating periphyton mats, 81% of the initial P uptake was associated with Ca, but most of this P entered and remained within the organic pool (Ca-associated 1⁄4 14% of total) after 1 day. In the floc layer, P rapidly entered the microbial pool and the labile fraction was negligible for most of the study. 4. Budgeting of the radiotracer indicated that P moved from particulates in the water column to periphyton and floc and then to the floc and soil over the course of the 18 days incubations. Floc (35% of total) and soil (27%) dominated P storage after 18 days, with floating periphyton (12%) and surface water (10%) holding smaller proportions of total ecosystem P. 5. To summarise, oligotrophic Everglades marshes exhibited rapid uptake and retention of labile P. Components dominated by microbes appear to control short-term P cycling in this oligotrophic ecosystem.