The seasonal variability of porewater nutrient concentrations, metabolism, nitrogen cycling (denitrification and nitrogen fixation), and primary production (oxygenic and anoxygenic photosynthesis and chemoautotrophy) was examined in benthic mangrove environments on Twin Cays, Belize. Twin Cays mangroves exhibit a tree-height gradient from tall seaward fringe trees, through a transition of intermediate tree heights to short dwarf trees surrounding interior ponds and lagoons. Detailed investigations of steady state porewater profiles of nutrients and terminal metabolic products in dwarf mangrove soils revealed strong seasonal differences in salinity, organic carbon and nitrogen inventories, redox conditions and reduced manganese concentrations. Substantial rates of organic matter remineralization were coupled primarily to sulfate reduction. Redox conditions contributed to variability in mat nitrogen fixation and denitrification response to nutrient addition, while dissolved organic carbon did not. Nitrogen fixation was controlled primarily by the sensitivity of nitrogenase to oxygen inhibition, whereas denitrification was limited by nitrate availability.

Community composition of photosynthetic organisms appeared to be controlled by light fluctuations due to mangrove canopy light gaps and by differential tolerance to environmental stresses such as desiccation or nitrogen limitation. Dwarf mangrove cyanobacteria-dominated microbial mats achieved a high biomass of photopigments in well-illuminated soils. Transition and fringe soils were more shaded and contained diatoms and green algae and less cyanobacteria and anoxygenic photosynthetic bacteria than in dwarf soils. Oxygenic photosynthesis was the primary mode of carbon fixation (56%) in all habitats under full sun, with a lesser contribution by anoxygenic photosynthesis (32%) and chemoautotrophy (12%). In situ light conditions underscore the gradient from highest rates of carbon fixation in dwarf mangrove habitat mats (0.21 g C m^-1 d^-1) to diminished rates in shaded transition and fringe mangrove habitat mats (0.08 and 0.05 g C m^-1 d^-1, respectively). Well-lit mats associated with dwarf mangrove habitats fix 18-20% of the net primary productivity of Twin Cays’ dwarf mangrove trees and can supply 5-28% of the nitrogen requirement of Twin Cays’ dwarf mangrove trees via nitrogen fixation. Light limitation restricts the fixation of carbon and nitrogen in transition and fringe mangrove habitat mats which account for only <0.3% of the net production and <2% of the nitrogen requirement of the respective mangrove trees.

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