Historically, wetlands along the southwestern coast of Biscayne Bay, FL exhibited a sharp discontinuity between highly productive mangrove forests close to the shoreline and sparse brackish-water grasslands farther inland. As the region has developed over the last century, sea level rise and drainage of upstream freshwater sources have encouraged the encroachment of mangroves into these interior marshes, but the unproductive nature of the ecosystem remains despite the compositional change. Growing on irregularly flooded marl sediments, these dwarf mangrove communities accumulate biomass slowly, and show signs of physiological stress (high δ13C), at least in part brought on by phosphorus limitation (high leaf N:P ratios, high soil alkaline phosphatase activity). In recent decades, efforts to reestablish sheet flow through these coastal wetlands from canal water sources have been implemented throughout the watershed, but long-term effects of restored fresh water delivery on mangrove production have not previously been reported. In this presentation, we first describe pre-treatment levels of aboveground biomass and production, plant stress, and P-availability in a basin in which freshwater diversion was initiated in 1996, as well as in an untreated control. Based on resampling of δ13C, leaf N:P, and phosphatase activity in 2016, calibration of spectral indices with measured biomass/production, and extraction of vegetation indices across a temporal sequence of LANDSAT images, we examined whether twenty years of fresh water delivery has affected these conditions.
Local relationships between vegetation index values, biomass, and productivity were established, but the time sequence of vegetation change represented by the spectral data showed no trend over the two decades. The dominant red mangrove (Rhizophora mangle) continued to exhibit significant physiological stress, and exoenzyme activity and leaf N:P indicated that phosphorus availability was still low. The results suggest that fresh water delivery has had only a minor ameliorating effect on phosphorus limitation or mangrove production. Restoration of fresh water flow through coastal wetlands may improve water quality and ecological condition in nearshore waters, but does little to alter the underlying coastal productivity gradient.