Mangrove Ecology

1996 Abstracts

Biogeochemistry and Forest Development of Mangrove Wetlands in Southwest Florida: Implications to Nutrient Dynamics of Florida Bay

Robert R. Twilley and Ronghua Chen, University of Southwestern Louisiana, Lafayette, LA; Margueritte Koch, Department of Biology, Florida Atlantic University, Boca Raton, FL.

The biogeochemical properties of mangroves are the least understood of ecological processes along the transition from upland to coastal margin ecosystems. Thus the specific nature as to how the distribution of nutrients influences mangrove structure and productivity, and the role of mangroves in the fate of nutrients in sub-tropical estuaries, are poorly understood. There has been much investigation related to the issues of water management in south Florida including projected impacts of nutrient enrichment on the Everglades and associated wetland ecosystems. However, the responses of mangroves to changes in water quantity and quality and the role of these land-margin ecosystems to mitigating nutrient enrichment in the coastal zone have received little attention. We have been investigating the biogeochemical properties of mangroves along longitudinal gradients of the estuary and the response of these processes to Hurricane Andrew. Together with an initial effort to develop an ecological model of these processes, we have determined the significance of phosphorus and nitrogen to the structure and productivity of mangroves in this region. Continued efforts monitoring these biogeochemical processes, together with further development of ecological models, will provide important information on the projected response of mangroves to changes in water managment in this coastal watershed.

Nutrient, light, salinity, soil redox potentials, and other soil characteristics were determined across a range of mangrove forest heights across the South Florida peninsula encompassing large stature trees (10 m) on the west coast and dwarf forms (<1 m) in the southeast Everglades. Relationships were established between abiotic factors and Rhizophora mangle seedling growth rates in four height (cm) classes, 25-40, 40-55, 55-70, and 70-85. Light in open canopy gaps was an important determinant of seedling growth. Growth rates in gaps (0.32 ñ 0.04 to 1.89 ñ 0.18 mm d-1) were two- to five-fold greater than adjacent closed canopy forests (0.14 ñ 0.01 to 0.40 ñ 0.07 mm d-1). Among open canopy sites, labile soil phosphorus and soil redox potentials were significantly correlated to growth (P < 0.05; r = 0.98 and 0.89, respectively). Interstitial salinity ranged from 0 to 27 ppt across sites, well below hypersaline conditions. Thus, under low salinity stress and high light availability, soil fertility is proposed to be the dominant factor controlling R. mangle seedling development to a sapling stage (85 cm) in South Florida mangrove forests. In addition, soil anoxia is hypothesized to be an important stressor in lagoonal-bay estuaries and marsh-mangrove ecotones with minimal tidal exchange.

Patterns of biomass allocation and C-fixation rates in Rhizophora mangle seedlings were investigated in the Florida Everglades at sites with variable light and soil nutrients. Soil nutrient levels and light availability significantly influenced light-saturated C-fixation rates. Photosynthetic rates at light saturation decreased from 7.8 to 5.5 æmol CO2 m-2 s-1 with decreasing soil P. Low light under the canopy suppressed average daily photosynthetic rates 64% relative to an adjacent gap. Biomass allocation to photosynthetic tissue was also dependent on high light and soil nutrients. Photosynthetic tissue comprised approximately 50% of the total above-ground biomass in open canopied, high nutrient sites, whereas under low soil P, only 12-13% of the total biomass was allocated to leaf tissues. A general trend was found across size classes in open and closed canopied sites: biomass partitioning to leaves increased with size class in the gap, while under the canopy, leaf biomass increased initially, but subsequently decreased with increasing size. The inability of Rhizophora mangle to allocate increasing biomass to photosynthetic tissues to compensate for low C-fixation rates under closed canopies, identifies this species as shade intolerant. Whole plant C-fixation rates were strongly correlated to seedling growth rates in southwest Everglades sites. These results indicate the importance of both C-production and foliar biomass partitioning for early mangrove development. However, further study is needed to reconcile limited growth in response to C-production in dwarf mangrove forests in the southeast Everglades.

The spatial gradients of soil nitrogen (N) and phosphorus (P) mineralization rates, extractable N and P pools, and P fractionation were examined in a mangrove land-margin ecosystem along the Shark River estuary in the Everglades National Park. The mean tree height of mangrove forests linearly declined with distance from the mouth near the Gulf of Mexico to the oligohaline regions of the estuary. Mangrove forests along the estuary represented a landscape gradient of soil N and P availabilities. Higher amounts of extractable ammonium were found in the marine site of the estuary near the Gulf of Mexico. Mean rates of net ammonification and relative ammonification per unit of total N based on 45 d incubation periods decreased exponentially with distance from the estuary mouth to inland sites. N mineralization was positively correlated with total P and available P, but negatively with N:P ratio. Total P and available P increased from the oligohaline to marine sites. There was no net P mineralization in the marine sites based on 45 d incubation periods. Residual P was the largest P fraction in all soils and decreased from 70% in the oligohaline site to 45% in the marine site along the estuary. The percentage of Ca-bound-P increased from 3.3% at the oligohaline site to 32.3% at marine sites. Ca bound-P pool was larger than Fe-Al-bound P pool in the marine sites, while the reverse was found in the mesohaline and oligohaline sites. The landscape gradient of P in the study area indicates that the Gulf of Mexico is the major source of P in this mangrove land margin ecosystem rather than direct terrigenous input from inland watershed.

Mangrove forest structure, growth, soil nutrient resources and pore water stressors were examined along the longitudinal gradient of the Shark River estuary in the Everglades National Park. Patterns of forest structure were based on 34 yrs of growth since Hurricane Donna destroyed most of the mangrove resources in this region in 1960. Forest basal area increased from lower values at the upstream and intermediate locations of the estuary (19.61 and 20.72 m2/ha, respectively) to nearly double these values at two downstream locations (40.36 and 39.67 m2/ha) near the mouth of the estuary. Tree density with dbh 10 cm was 1900/ha in the downstream location, which indicated a more mature stand than in the upstream location where only 334 trees/ha were 10 cm dbh. Laguncularia racemosa had a higher importance value in the downstream location, whereas Rhizophora mangle dominated in the intermediate and upstream locations. Biomass increments were higher at the downstream sites (10.73 and 11.96 Mgúha-1úyr-1) than at the lower stature forest in the upstream and intermediate sites (3.23 and 4.20 Mgúha-1úyr-1). Pore water salinities in mangrove soil were <35 g/kg at all three sampling seasons and sulfide concentrations were generally < 0.15 mmol/L. There was no spatial trend of total C and N contents per unit soil volume, nor in C:N ratios among the sites. However, total P, extractable P and extractable ammonium contents increased from the upstream sites to the sites at the mouth of the estuary. Soil properties in the downstream marine region of the Shark River estuary include high levels of nutrient resources and low concentrations of toxic stressors supporting a region of optimum growth of mangroves. A regression of mangrove basal area on concentrations of soil total P per unit volume suggest that the landscape gradient of mangrove forest development is associated with soil P resources along this subtropical estuary.

The vertical distributions of organic matter, nitrogen and phosphorus in mangrove soils were investigated with a mechanistic and process based model. The nutrient mangrove model (NUMAN) was developed from the SEMIDEC and CENTURY soil organic matter models and parameterized with data from four mangrove sites along the Shark River estuary. The soil characteristics in the four mangrove sites varied greatly in both concentrations and profiles of soil C, N and P. Organic matter, expressed as percentage of ash free dry weight, decreased from 82% in the upstream locations to 30% in the marine sites. Comparisons of the simulated and observed results demonstrated that the landscape gradient of soil characteristics can be adequately modeled by accounting for plant production, litter decomposition and export, and allochthonous input of inorganic materials. The results illustrated that the increase in organic matter content and decrease in soil bulk density from mangrove sites at the mouth of the estuary to those at upstream locations was controlled mainly by variation in allochthonous inputs of inorganic material and mangrove root production. Model simulations showed that the greatest change in organic matter, N, and P occurred in the soil surface, but changes were also evident at depths ranging from 1 cm to 5 cm at some of the sites. The rapid decomposition of labile organic matter was responsible for this decrease in organic matter. Simulated mineralization rates of nitrogen decreased quickly with depth, which corresponded with the decrease of labile organic matter. Fitting the simulation results of the model to observed nutrient profiles indicated that N and P dynamics are tightly linked to organic matter retention. In the future, NUMAN will be modified and linked to a mangrove forest gap model to simulate the feedback mechanisms of soil nutrient recycling, which controls development of the mangrove wetlands.

A computer model (MANGAL) derived from an individual-based gap model (FORMAN) and a soil organic matter model (NUMAN) was developed to investigate mangrove forest dynamics in relation to soil characteristics. Simulated forest structure was compared to that of three mangrove forests along the Shark River estuary, Florida, with a known disturbance history. Simulations of total and species specific basal area of each mangrove site fit well with data from field surveys of the forests. Low nutrient availability in the intermediate site of the estuary limited the forest development. The measured size-class distribution of Rhizophora mangle showed higher frequencies for the smaller size classes, however, that of Laguncularia racemosa was bell-shaped. The model accurately tracked this pattern for each species. Mangrove succession was projected using the present stand condition of the downstream marine site in the estuary. Without disturbance, dominance ofL. racemosa was eventually replaced by R. mangle or Avicennia germinans, depending on the recruitment rate of A. germinans. Simulated basal area of the three mangrove species along gradients of soil nutrient resource and salinity illustrated a change in competitive balance among the three species over time. L. racemosa dominated in fertile soils with low stress at early stages of recovery, but its abundance decreased while A. germinans increased in this region. The dominance of R. mangle was limited to regions with low nutrient availability and low salinity.

The relative ecological significance of nutrient availability varies among fringe, basin and dwarf mangroves according to tidal inundation frequency from lower to upper regions of the intertidal zone. Fertility may also vary for each ecological type of mangrove depending on allochthonous inputs from riverine environments, compared to the lack of allochthonous inputs in lagoons and carbonate environments. Nutrient cycling in mangrove ecosystems in areas with high tidal or riverine forcing would be considered 'open' with high rates of material exchange across the mangrove-estuarine boundary. Whereas, less frequently inundated systems, such as inland basin mangroves, have a greater accumulation of leaf litter resulting in higher remineralization within the forest. Thus the ecological types of mangroves across geomorphological regions with contrasting fertility may have different plant and ecosystem strategies of nutrient conservation. We propose that the biogeochemical properties of mangroves such as accumulation, mineralization, exchange, and nutrient-use efficiency will vary among sites along the nutrient gradients of the south Florida terrace. Thus the implications to regional water quality are very distinct across these fertility gradients from peat to marl sediments.

Last updated: 2/26/98
by: Monika Gurnée
gurnee@aoml.noaa.gov