PARENT SESSION
Poster Session # 17: Aquatic Systems.

Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Photosynthetic characterization of Rhizophora mangroves at Twin Cays, Belize and responses to fertilization.

Cheeseman, John^*,1, Lovelock, Catherine², ¹ Department of Plant Biology, Urbana, IL, USA² Smithsonian Environmental Research Center, Edgewater, MD, USA

ABSTRACT- Rhizophora mangle (red mangrove) is the dominant tree on the mangrove cays in Belize. These cays are composed of mangrove peat which has accreted over the past 10,000 years in association with rising sea level. Twin Cays is further characterized by a zone of taller trees around the fringes of the island and creeks, and a zone of dwarf trees in interior areas less accessible to tidal exchange. Previous studies have shown that dwarf trees are phosphorous limited, while tall trees are nitrogen limited. As part of a larger biocomplexity study, we have analyzed the photosynthetic characteristics of red mangrove, using in situ gas exchange, chlorophyll fluorescence, and stomatal porometry, and biochemical determination of the activity of Rubisco. Light saturated photosynthesis in tall trees was approximately 40% higher than that in dwarf trees. In both cases, photosynthesis light saturated at about 500 umol m^-2 s^-1. Mean conductances were similarly greater in tall trees (0.11 vs 0.7 mol m^-2 s^-1), however, and comparison of photosynthetic conductance responses showed no difference between the populations. Elevation of external CO₂ to 1400 umol mol^-1 doubled maximal net assimilation, but without completely overcoming stomatal limitations. Nitrogen fertilization of dwarf trees increased stomatal conductance and mean net assimilation rates without noticeably stimulating growth; phosphorous fertilization rapidly led to increased growth, based on increased stem hydraulic conductivity, stomatal conductance and net assimilation. Rubisco activity, in vitro, was similar in controls, and N- and P-fertilized trees, but showed a statistically significant dependence on irradiance at the time of leaf harvest. The results will be discussed with attention both to photosynthesis per se and to the integration of photosynthesis into an overall strategy of carbon gain and allocation in a harsh, oligotrophic environment.

Key words: gas exchange, Rhizophora mangle, fluorescence, Rubisco