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Variation in environmental conditions can profoundly influence the biology and ecology of seagrasses. Light availability, for example, is considered a primary determinant of seagrass distribution and production. During periods of low light, below-ground tissues are presumed to serve as an energy reserve that supports basal metabolic demands and maintains a positive carbon balance. In addition, seagrasses in oligotrophic conditions may allocate more carbon to roots to increase nutrient uptake from sediments. Little is known, however, about the relative allocation of photosynthetically derived resources to above- and below-ground tissues under different nutrient regimes. Altered partitioning in response to nutrient availability is likely to have implications for the morphology and shoot density of seagrasses, with subsequent effects on a broad suite of ecosystem services. For example, dense seagrass stabilizes sediment, facilitates deposition of particles, increases water clarity, and provides habitat for fish and invertebrates. To test this hypothesis, blade area, shoot height, shoot density, and above-ground and below-ground biomass were measured for seagrasses along a known phosphorus gradient off the central Gulf coast of peninsular Florida. We related these important structural characteristics of seagrasses to surface water nutrient concentrations. Water column N:P ratios were negatively related to blade area per shoot, blade biomass per shoot and above- to below-ground biomass ratios for Thalassia testudinum, but positively related to shoot density, rhizome biomass per m^2 and root biomass per m^2. In contrast, total biomass per m^2 and per shoot and blade biomass per m^2 were weakly or not significantly correlated with water column N:P ratios. These results suggest that T. testudinum maintains a relatively consistent amount of total biomass across the range of nutrient concentrations sampled during this study, but the seagrass adjusts its above-ground morphology and allocation of carbon to above- and below-ground tissues at least partially in response to surface water nutrient concentrations. |