Flux from Mangrove Tidal Creeks at the L-31E Freshwater Redistribution Pilot Project Site: Biscayne Bay, Florida

Meeder, John; Renshaw, Amy; Alvord, Jennifer; Ross, Michael

Flux of major nutrient species (NH3, NO, N03, TN, SRP, TP, TOC) and particulate organic carbon between two tidal creeks and Biscayne Bay was measured in December 1996 and April 1997. Two complete consecutive tidal cycles were measured (24.3 hr period) during each sampling period at two stations along both tidal creeks. Sampling stations are located near the mouths of the tidal creeks in the fringing red mangrove community and approximately 250 m inland near the boundary between the mixed mangrove and scrub mangrove communities. In all cases, there was a net movement of water into the coastal mangrove ecosystem over two tidal cycles. In one case, December TS, a single tidal cycle had more water moving towards Biscayne Bay than landward. This case is explained by a combination of three factors: 1) the TS station is a topographic basin, 2) change in wind direction, and most importantly 3) the preceding tide was very high, moving water well into the basin, which did not all drain out during the following low tide. Net movement of water landward is indicated by the coastal mangrove surface water and soil salinities. The flux of water landward is explained by several factors: 1) Evaporation of surface waters, 2) transpiration (water use by plants), 3) resaturation of soils, and 4) spring tide events. Our results are based upon four tidal cycles during or close to spring tide for the two sampling periods. In addition, the most important aspect of calculating flux is also the least precise, that is, measuring water volumes. However, our methodologies were applied uniformly and some major differences documented. The major differences between the Dec and April flux is the result of the December tide range being higher than in April and incoming waters always being higher in December, than in April, in concentrations of TP, N + N1and NH4 while TOC concentrations were higher in April than in December. Considerably more water was exchanged in the Treatment Block than in the Control Block and nutrient concentrations were usually higher in the Control Block than in the Treatment Bock. Finally, the volume of water exchanged was considerably higher at the fringe sites than the scrub sites. In summary, we have documented that a there is a net movement of Biscayne Bay water and water borne nutrients into the coastal wetlands. This is in contrast to most previous works in South Florida that report an average of 1.0 g/m2/day outwelling from coastal mangrove wetlands. We report influx of as high as O.Sg/m2/d assuming that the incoming nutrients that are left behind are evenly spatially distributed. Three major reasons for import rather than export from the wetlands exists: 1) freshwater sheet flow has been eliminated, 2) The effects of Hurricane Andrew producing extreme loads of decromass and soil destabilization, and 3) nearshore waters that move into the wetlands with tidal action are composed of nutrient rich canal and groundwater.

Flux from Mangrove Tidal Creeks at the L-31E Freshwater Redistribution Pilot Project Site: Biscayne Bay, Florida

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