Topical Area: Nutrient Dynamics

D.T. Rudnick, S.P. Kelly, F.H. Sklar and C.J. Madden, Everglades Research Division, SFWMD, West Palm Beach, FL; D.L. Childers, S.E. Davis and N.J. Oehm, Southeastern Environmental Research Program, FIU, Miami, FL; C. Coronado-Molina, E. Reyes, J.W. Day, Jr. and B.C. Perez, Coastal Ecology Institute, Louisiana State University, Baton Rouge, LA; M.S. Koch, Department of Biology, FAU, Boca Raton, FL

Efforts to restore the Everglades and Florida Bay largely entail changing the supply of freshwater and reducing the nutrient loads to these ecosystems. Changing fresh water inflow to the Bay may affect its ecological structure and function via several mechanisms. Our research is focused on quantifying how changing fresh water inflow affects the cycling of nutrients within the mangrove dominated ecotone between Florida Bay and the Everglades and also affects the net transport of nutrients through this ecotone. Understanding nutrient dynamics in this ecotone is important because this region contains a large pool of nutrients and its importance as a source or sink of nutrients may change with changing fresh water flow. Furthermore, salinity in this ecotone has a wide range and high variability; effects of changing salinity on nutrient biogeochemical cycles should be evident in this region.

During the past two years, we have measured the net exchange of water and nutrients between Florida Bay and a major creek flowing from Taylor Slough (see abstract by Sutula et al. in this volume). Concurrently, we seasonally measured some nutrient fluxes within this region of the mangrove ecotone. These nutrient fluxes include benthic-pelagic exchange in coastal ponds, mangrove island-pelagic exchange in the scrub mangrove zone, creek bank-creek exchange in the mangrove fringe zone, and mangrove prop root-creek exchange (see abstracts by Davis et al. and Rudnick et al. in this volume). Additionally, we measured mangrove tree growth, litter production, litter decomposition, soil respiration (carbon dioxide production, sulfate reduction, and methanogenesis), and net soil accretion in the fringe forest and scrub mangrove wetland (see abstracts by Coronado-Molina et al. and Oehm et al. in this volume).

Seasonal patterns of nutrient cycling, coincident with strong seasonality of fresh water inputs, were evident in the wetland adjacent to the Taylor River and Taylor Ponds. Soil respiration rates were higher in the wet season than during the dry season. The C:N ratio of mangrove leaves was lower during the wet season than during the dry season. In contrast, seasonal nutrient flux patterns were not evident within the creek and pond system. Different portions of this system, however, had distinct flux patterns, particularly with regard to nitrogen cycling. From August 1996 through January 1998, mangrove islands (about 4 m diameter) within the creek system were a net source of inorganic N. The release of nitrate +nitrite was consistent year round. These islands also released total P, probably as dissolved organic P. In contrast, a nearby coastal pond that is downstream of the mangrove island site was a consistent sink of inorganic N (nitrate + nitrite and ammonium). Total P fluxes between the pond's sediment and water were not detectable.

Some evidence for the importance of episodic nutrient uptake by mangrove roots was found. During the dry season, when mangroves were acclimated to salinity of about 30 ppt, mangrove roots were experimentally exposed to fresh water from Taylor Slough in 2 liter bags. Over a 48 hour period, 50% to 80% of the water was absorbed from the bag and almost all dissolved inorganic nutrients were also removed by the roots and associated epibionts.

The mangrove creek and pond system of southern Taylor Slough is a region that processes nutrients that are derived both from the Everglades and Florida Bay. Phosphorus concentrations in the waters of this ecotone tend to be higher than in either the fresh water slough or the bay, but net exchanges of P between the waters of this creek and pond system and its mangrove islands and sediments were of a small magnitude (< 1 umol m^-2h^-1). Net N exchanges had a higher magnitude, with DIN release from islands as high as 30 umol m^-2h^-1 and uptake by sediments as high as 60 umol m^-2h^-1. The fate of nutrients in the Everglades-Florida Bay landscape may be strongly influenced by the accumulation of P and the loss of N via nitrification and denitrification within the mangrove ecotone.