Limnol. Oceanogr., 44(5), 1999, 1204–1215

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 NH and NO 1 NO concentrations were not significantly different 1 2 2 4 3 2 between study sites (ca. 1.2 mM [NH ] and 0.7 mM [NO 1 NO ] at both sites), but sediment NH concentrations 1 2 2 1 4 3 2 4 in CCB (87 mM) were significantly higher than in LLM (26 mM). The higher sediment NH levels at CCB correlated 4 with significantly higher leaf biomass at CCB, but there was no difference in root biomass between study sites. Leaf NH uptake showed clear seasonal variation: Vmax was highest in summer and fall, but Km was highest in 4 winter. Vmax of leaf NO uptake did not change with season, but Km decreased with increasing incubation temperature. 3 There were no clear differences in leaf NH and NO uptake rates between study sites, although leaf NH uptake 1 2 1 4 3 4 affinity was higher than that of NO . Root NH uptake was variable with season and did not saturate at the 2 1 3 4 experimental NH concentrations at either site (0–300 mM). Based on these measurements, N acquisition was 4 highest during summer and fall and lowest during winter and spring. Roots and leaves contributed nearly equally to total plant N acquisition (root NH 5 52%; leaf NH 5 38%; and leaf NO 5 10%) at both sites. Annual N 1 1 2 4 4 3 acquisition in CCB was double that of LLM (97.03 and 53.49 g N m22 yr21, 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 NH pool sizes, results from the higher fraction of biomass allocated to below4 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. Throughout coastal ecosystems of the world, seagrasses are known to achieve high levels of production (McRoy and McMillan 1977; Zieman and Wetzel 1980). Because of their rapid seasonal growth, seagrasses require high N incorporation and play an important role in the cycling of N in shallow estuarine ecosystems, which likely contributes to the high productivity of these regions (Kenworthy et al. 1982; Hemminga et al. 1991; Blackburn et al. 1994). Dissolved organic matter (DOM) released through exudation, leaching, and decomposition of seagrass tissues is an important source of nutrients in the water column and sediments (Wood and Hayasaka 1981; Moriarty and Pollard 1982; Smith et al. 1984; Boon et al. 1986; Moriarty et al. 1986; Chin-Leo and Benner 1991). This organic matter also can control abun1 Corresponding author.