L-31E Surface Water Rediversion Pilot Project

Throughout the southern Biscayne Bay watershed, existing coastal wetland communities have been cut off from sheet flow for decades. With the expectation that reconnection of these wetlands to upstream water sources would alter existing hydrologic conditions and recreate a more natural sheet flow to Biscayne National Park, a demonstration project on freshwater rediversion was undertaken. The objectives of the project were to document the effects of freshwater diversion on: (a) swamp and nearshore water chemistry and hydrology; (b) soil development processes; (c) macrophyte and benthic algal community composition, structure, and production; (d) abundance of epiphytic and epibenthic invertebrates; (e) zonation, production, and phenology of primary producers in the nearshore environment, and (f) exchanges of nutrients and particulates between nearshore and mangrove ecosystems.

The study area is comprised of the coastal mangrove ecosystem bounded by the Mowry, L-31E, and Military Canals on the south, west, and north, respectively, as well as adjacent portions of Biscayne Bay. For this study, a BACI (Before-After Control-Impact) research and monitoring design was chosen. In this design, the interaction of Site with Time was examined, substituting temporal replication for the spatial replication. Among five hydrologically-independent blocks divided by East-West oriented drainage ditches or canals, Blocks 1 and 4 were chosen as Treatment and Control Blocks, respectively. Both blocks were then divided into sub-blocks representing the coastal Fringe and interior Dwarf forests. Within each of the four sub-blocks, hydrologic monitoring and biotic monitoring were performed at one or four locations, respectively. The periodic freshwater delivery from the L-31E Canal into the Treatment sub-blocks started on 28 Aug 1997.

Water quality in Delivery Canal, Dwarf and Fringe mangrove forests, and adjacent Biscayne Bay was monitored between June 1995 and October 2001. Surface waters were analyzed for specific conductivity, NH3-N, NO3 + NO2-N, NO2-N, soluble reactive phosphorus, total phosphorus, total nitrogen, total organic carbon, copper, lead, zinc, iron, alkaline phosphatase and chlorophyll-A, and pore waters were analyzed for all parameters except the last two.

In mangrove forests, leaf production and survival were monitored on a number of branches of selected red mangrove trees for 4-6 years. Sampling for leaf tissue analysis began in March 1996 and continued at about three-month intervals thereafter. Every sampling period, ten mature leaves were collected from five individual R. mangle stems in each of the four sub-blocks. Samples collected through October 2001 were analyzed for carbon, nitrogen, phosphorus, δ15N, and δ13C. During the pre-treatment period, selected samples were analyzed for δ15N on a few occasions. However, the analysis for δ15N became regular beginning in January 1998. To address the question of whether the mangrove ecosystem is nitrogen or phosphorus limited, molar N:P ratios were calculated. Concentrations of trace elements, i.e., copper, iron, lead and zinc were determined in the samples collected through July 2000. nasmuch as red mangrove leaves are initiated a minimum of 3-6 months prior to reaching maturity, no measurable effect of the August 1997 water delivery on the composition of mature leaves was considered likely before January 1998. Therefore, unlike the water data for which all samples collected after August 1997 were considered as post-treatment samples, leaf samples collected through October 1997 were treated as pre-treatment, and the samples collected after that time were considered as post-treatment samples. With the view that changes in salinity regime due to freshwater delivery would also affect the competitive balance between mangrove species, several techniques were used to examine the water relations of two major species, R. mangle and L. racemosa