Nitrogen budget of the seagrass Thalassia testudinum in the western Gulf of Mexico




Lee, Kun-seop, 1961-

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The nitrogen (N) budget of the seagrass Thalassia testudinum was examined with respect to inorganic-N acquisition and the effects of sediment NH₄⁺ enrichment on two distinct populations in south Texas. The two populations exhibit different biomass allocation patterns at Corpus Christi Bay (CCB) and lower Laguna Madre (LLM): plants at CCB have a higher above-ground biomass while plants at LLM have a higher below-ground biomass. Ambient sediment pore water NH₄⁺ concentrations at CCB (ca. 100 μM) were significantly higher than at LLM (ca. 30 μM). Therefore, it was hypothesized that 1) differences in biomass allocation are a result of the differential sediment N availability, 2) sediment NH₄⁺ enrichment will affect growth, leaf morphology and tissue nutritional content of T. testudinum to a greater degree at low sediment N conditions, and 3) the relative contributions by leaf and root tissues to total N acquisition will differ between the two study sites. To examine the effects of sediment NH₄⁺ enrichment, the seagrass bed sediments were fertilized with commercial N fertilizer, and changes in production, biomass, leaf morphology, tissue nutritional content and carbon (C) reserves were monitored. Additionally, N uptake by leaves and roots of T. testudinum from the two sites were measured seasonally. After fertilization, leaf production rates and shoot height at LLM increased to reach levels equivalent to CCB. However, sediment NH₄⁺ enrichment had little effect on production and leaf size of T. testudinum at CCB. These results suggest that sediment N availability at LLM limits seagrass production. Rhizome non-structural carbohydrates (NSC) decreased in response to sediment NH₄⁺ enrichment during the early periods of the experiment which suggests that C was reallocated from rhizome to leaf tissues to support the stimulated leaf growth. Thus, the NH₄⁺ enrichment affected concentration and allocation of C as well as N. Root NH₄⁺ uptake accounted for about 52 % of total N acquisition, while leaf NH₄⁺ uptake contributed about 38 % and leaf NO₃⁻ uptake accounted for the remaining 10 % at both sites. The high biomass, chlorophyll, and C content in leaf tissues at CCB and the high biomass, C and NSC content in rhizome tissues at LLM demonstrated that plants responded to high sediment N conditions by enhancing leaf function, and to low N conditions by enhancing function of below-ground tissues