An investigation of 'Thalassia testudinum' below-ground structure and productivity

A large percentage of Thalassia testudinum biomass is located below ground, yet many papers published on seagrass ecology focus on above-ground structure and productivity. This is likely due to the fact that viable estimates of below-ground biomass production are logistically difficult to obtain. In 1975 Patriquin published a paper proposing a method to estimate below-ground productivity in Thalassia testudinum. This method requires complete extraction of the plant and it may also be error prone due to variability in developmental growth. This study investigated alternative methods for estimating below-ground productivity in Thalassia testudinum. The study had two major goals: 1) Determine how changes in above-ground leaf growth correspond to changes in below-ground structure; 2) Measure and attempt to exploit structural consistencies in order to improve the estimation of below-ground productivity. During the summers of 2000 and 2001 Thalassia testudinum plants growing in Florida Bay were tagged with bird bands and leaf length repeatedly measured. At the end of the field season the plants were extracted. The extracted, tagged plants were taken to a lab in Charlottesville, Virginia where a comprehensive series of structural measurements were recorded. The record leaf dynamics was coupled with the structural measurements and the data analyzed. The amount of leaf area added per day per short shoot had a moderate correspondence (R 2 =0.42) with below-ground rhizome volume. Short shoot scar age had a high correspondence (R 2 =0.67) with the length of the rhizome runner. These results ii were an indication that there was a correspondence between above-ground growth and below-ground structure. Roots attached and growing from the short shoot were found to have three distinct patterns of growth. Roots classified as "fast growers" had an estimated elongation rate of 2.62 cm for every short shoot leaf scar (R 2 =0.63). These roots were estimated to be elongating at a rate of 0.15 cm per day. This rate translates to 0.0003 grams dry weight biomass per day. The unexplained variance in the relationship between above-ground dynamics and below-ground structure may lie in resource and energy translocation within the plant. The structural patterns of the below-ground plant components displayed natural variability, but a pattern of consistency was documented. These consistencies could be utilized to allow a refinement of methods used to estimate below-ground productivity in Thalassia testudinum. Future research needs to expand to a multivariate experimental approach. Future studies also need to explore other geographical locations to determine how the patterns observed in this study differ from patterns within other populations based on spatial, temporal, and characteristic differences in the surrounding ecosystems.

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