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Mangrove islands in Belize are characterized by a unique switching from nitrogen (N) to phosphorus (P) limitation to tree growth from shoreline to interior. Fertilization has previously shown that Rhizophora mangle (red mangrove) fringe trees (5-6 m tall) growing along the shoreline are N limited; dwarf trees ([less than or equal to]1.5 m tall) in the forest interior are P limited; and transition trees (2-4 m tall) are co-limited by both N and P. Growth patterns paralleled a landward decrease in soil flushing by tides and an increase in bioavailable N, but P availability remained consistently low across the gradient. Stable isotopic composition was measured in R. mangle leaves to aid in explaining this nutrient switching pattern and growth variation. Along control transects, leaf [delta] [sup.15]N decreased from +0.10% [per thousand] (fringe) to -5.38% [per thousand] (dwarf). The [delta] [sup.15]N of N-fertilized trees also varied spatially, but the values were consistently more negative (by ~3% [per thousand]) compared to control trees. Spatial variation in [delta] [sup.15]N values disappeared when the trees were fertilized with P, and values averaged +0.12% [per thousand], similar to that in control fringe trees. Neither variation in source inputs nor microbial fractionation could fully account for the observed patterns in [delta] [sup.15]N. The results instead suggest that the lower [delta] [sup.15]N values in transition and dwarf control trees were due to plant fractionation as a consequence of slower growth and lower N demand. P fertilization increased N demand and decreased fractionation. Although leaf [delta] [sup.13]C was unaffected by fertilization, values increased from fringe (-28.6% [per thousand]) to transition (-27.9% [per thousand]) to dwarf (-26.4% [per thousand]) zones, indicating spatial variation in environmental stresses affecting stomatal conductance or carboxylation. The results thus suggest an interaction of external supply, internal demand, and plant ability to acquire nutrients under different hydro-edaphic conditions that vary across this tree-height gradient. The findings not only aid in understanding mangrove discrimination of nitrogen and carbon isotopes, but also have implications for identifying nutrient loading and other stress conditions in coastal systems dominated by mangroves. Key words: carbon isotope discrimination; height forms; mangrove, red,' nitrogen isotope discrimination; nutrient limitation; Rhizophora mangle; stress tolerance.

Phytoplankton often face the dilemma of living in contrasting gradients of two essential resources: light that is supplied from above and nutrients that are often supplied from below. In poorly mixed water columns, algae can be heterogeneously distributed, with thin layers of biomass found on the surface, at depth, or on the sediment surface. Here, we show that these patterns can result from intraspecific competition for light and nutrients. First, we present numerical solutions of a reaction-diffusion-taxis model of phytoplankton, nutrients, and light. We argue that motile phytoplankton can form a thin layer under poorly mixed conditions. We then analyze a related game theoretical model that treats the depth of a thin layer of phytoplankton as the strategy. The evolutionarily stable strategy is the depth at which the phytoplankton are equally limited by both resources, as long as the layer is restricted to the water column. The layer becomes shallower with an increase in the nutrient supply and deeper with an increase in the light supply. For low nutrient levels, low background attenuation, and shallow water columns, a benthic layer occurs; for intermediate nutrient levels in deep water columns, a deep chlorophyll maximum occurs; and for high nutrient levels, a surface scum occurs. These general patterns are in agreement with field observations. Thus, this model can explain many patterns of algal distribution found in poorly mixed aquatic ecosystems.

The objectives of the study were to determine total energy intakes, distribution of energy derived from the macronutrients, and the effects of increasing altitudes on energy and macronutrient consumption during exposure to high altitudes. High fat, low carbohydrate diets (35% and 50% of energy, respectively) or low fat, high carbohydrate diets (20% and 65% of energy, respectively) were provided to two groups of subjects for a 3 wk period. Groups then consumed the alternate diet for 3 wk, followed by a return to the original diet for the remaining 3 wk of the study. Free choice of individual items and amounts within each diet was permitted. Intake of food and fluid was determined by means of monitored entries in daily food records. Five subjects remained at Base Camp (5300 m) and 10 subjects climbed to altitudes up to and including the summit of Mt. Everest (8848 m). Subjects consumed an average of 10.22 [+ or -] 4.57 MJ/d (2442 [+ or -] 1092 kcal) energy while at Base Camp, with climbers consuming significantly more than Base Camp personnel [11.89 [+ or -] 4.88 vs. 7.87 [+ or -] 2.98 MJ/d (2841 [+ or -] 1167 vs. 1881 [+ or -] 713 kcal/d), P [less than or equal to] 0.0001]. There was a significant decline in energy consumption at increasing altitudes (P = 0.022), but no shift in distribution of energy provided from fat, carbohydrate or protein (P > 0.05). Contrary to previous reports, subjects in this study did not shift their food selections away from the high fat items towards high carbohydrate items. KEY WORDS: humans, high altitude, energy intake, fat consumption, carbohydrate consumption