Inorganic nitrogen acquisition in the seagrass Thalassia testudinum: Development of a whole‐plant nitrogen budget

Abstract

Whole‐plant nitrogen (N) uptake experiments were used to quantify the N budget of Thalassia testudinum growing under different sediment nutrient regimes at two locations in the western Gulf of Mexico. At both sites, Corpus Christi Bay (CCB) and lower Laguna Madre (LLM), Texas, concurrent measurements of plant biomass and levels of dissolved inorganic nitrogen (DIN) in the water column and sediments were made over a 12–month period (October 1996–October 1997). Water‐column NH4+ and NO3− + NO2‐ concentrations were not significantly different between study sites (ca. 1.2 µM [NH4+] and 0.7 µM [NO3− + NO ] at both sites), but sediment NH4+ concentrations in CCB (87 µM) were significantly higher than in LLM (26 µM). The higher sediment NH4+ levels at CCB correlated with significantly higher leaf biomass at CCB, but there was no difference in root biomass between study sites. Leaf NH4+ uptake showed clear seasonal variation: Vmax was highest in summer and fall, but Km was highest in winter. Vmax of leaf NO3‐ uptake did not change with season, but Km decreased with increasing incubation temperature. There were no clear differences in leaf NH4+ and NO3− uptake rates between study sites, although leaf NH4+ uptake affinity was higher than that of NO3−. Root NH4+ uptake was variable with season and did not saturate at the experimental NH4+ concentrations at either site (0–300 µM). Based on these measurements, N acquisition was highest during summer and fall and lowest during winter and spring. Roots and leaves contributed nearly equally to total plant N acquisition (root NH4+ = 52%; leaf NH4+ = 38%; and leaf NO3− = 10%) at both sites. Annual N acquisition in CCB was double that of LLM (97.03 and 53.49 g N m−2 yr−1, respectively), but >50% of N uptake was not incorporated into biomass at either site. DIN turnover time ranged from 0.21 to 0.91 d in the water column and from 0.95 to 1.75 d in sediment pore water, indicating the importance of DIN regeneration processes for supporting seagrass production. The similarity in the relative tissue contributions between plants at both sites, despite a significant difference in sediment NH4+ pool sizes, results from the higher fraction of biomass allocated to below‐ground tissues in plants living under low‐sediment N conditions (LLM). In N‐sufficient sediments, overall plant productivity is greater as T. testudinum is able to allocate a greater proportion of its biomass into photosynthetic aboveground tissues.