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

ABSTRACT

 

Summary

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. ³²PO₄ was added to the water column in six 1-m² 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 μm) incorporated radiotracer within the first minute after dosing and stored 95–99% of total water column ³²P activity throughout the study. Soluble (<0.45 μm) ³²P in the water column, in contrast, was always <5% of the ³²P in surface water. Periphyton, both floating and attached to emergent macrophytes, had the highest specific activity of ³²P (Bq g⁻¹³¹P) 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 ³²P uptake was associated with Ca, but most of this ³²P entered and remained within the organic pool (Ca-associated = 14% of total) after 1 day. In the floc layer, ³²P rapidly entered the microbial pool and the labile fraction was negligible for most of the study.

4. Budgeting of the radiotracer indicated that ³²P moved from particulates in the water column to periphyton and floc and then to the floc and soil over the course of the 18 day incubations. Floc (35% of total) and soil (27%) dominated ³²P storage after 18 days, with floating periphyton (12%) and surface water (10%) holding smaller proportions of total ecosystem ³²P.

5. To summarise, oligotrophic Everglades marshes exhibited rapid uptake and retention of labile ³²P. Components dominated by microbes appear to control short-term P cycling in this oligotrophic ecosystem.