Despite the importance of coastal ecosystems for the global carbon budgets, knowledge of their carbon storage capacity and the factors driving variability in storage capacity is still limited. Here we provide an estimate on the magnitude and variability of carbon stocks within a widely distributed marine foundation species throughout its distribution area in temperate Northern Hemisphere. We sampled 54 eelgrass (Zostera marina) meadows, spread across eight ocean margins and 36° of latitude, to determine abiotic and biotic factors influencing organic carbon (Corg) stocks in Zostera marina sediments. The Corg stocks (integrated over 25‐cm depth) showed a large variability and ranged from 318 to 26,523 g C/m2 with an average of 2,721 g C/m2. The projected Corg stocks obtained by extrapolating over the top 1 m of sediment ranged between 23.1 and 351.7 Mg C/ha, which is in line with estimates for other seagrasses and other blue carbon ecosystems. Most of the variation in Corg stocks was explained by five environmental variables (sediment mud content, dry density and degree of sorting, and salinity and water depth), while plant attributes such as biomass and shoot density were less important to Corg stocks. Carbon isotopic signatures indicated that at most sites <50% of the sediment carbon is derived from seagrass, which is lower than reported previously for seagrass meadows. The high spatial carbon storage variability urges caution in extrapolating carbon storage capacity between geographical areas as well as within and between seagrass species.

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 in CCB (87 M) were significantly higher than in LLM (26 M). The higher sediment NH levels at CCB correlatedwith significantly higher leaf biomass at CCB, but there was no difference in root biomass between study sites. Leaf NH uptake showed clear seasonal variation: V max was highest in summer and fall, but K m was highest inwinter. V max of leaf NO uptake did not change with season, but K m decreased with increasing incubation temperature.Ϫ 3There were no clear differences in leaf NH and NO uptake rates between study sites, although leaf NH uptakeaffinity was higher than that of NO . Root NH uptake was variable with season and did not saturate at theexperimental NH concentrations at either site (0-300 M). Based on these measurements, N acquisition washighest during summer and fall and lowest during winter and spring. Roots and leaves contributed nearly equally to total plant N acquisition (root NH ϭ 52%; leaf NH ϭ 38%; and leaf NO ϭ 10%) at both sites. Annual N 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.

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The effects of sediment ammonium (NH,') enrichment on biomass allocation, growth, and leaf morphology of the seagrass Thalassia testudrnum in Corpus Chnsti Bay (CCB) and lower Laguna Madre (LLM), Texas, were examined from May to October 1997. Prior stuhes had shown that shoot height and leaf biomass at CCB were significantly higher than those at LLM, and ambient sediment NH4+ concentrations in CCB (ca 100 PM) were significantly higher than those in LLM (ca 30 PM). It was hypothesized that the differences in plant morphology and biomass between the 2 areas could be related to differences in sediment nitrogen levels between 2 sites. To test this hypothesis, we conducted an in sihr fertilization experiment at both sites over a 6 mo period. Results of t h~s experiment revealed that seagrass growth, biomass and leaf size significantly increased as a result of sedlment NHAt enrichment at LLM, but had Little effect on plant denslty, biomass and leaf morphology at CCB. In unfertilized plots, average leaf production rate (7.4 g dry wt m-' d-') and shoot height (43.3 cm) at CCB were significantly higher than those at LLM (2.5 g dry wt m-2 d-' and 18.8 cm, respectively). After fertilization, leaf production rates and leaf size at LLIM increased to reach equivalent levels of the CCB site.Leaf biomass at LLM increased significantly as a result of sediment NH4+ enrichment, but there was Little change in below-ground biomass. The below-to above-ground biomass ratio at LLM (4.7) was about 3-fold higher than that at CCB (1.6) in unfertilized plots, hut decreased significantly at LLM with sediment NH,' enrichment, while the ratio at CCB remained unchanged. We conclude, based on seagrass growth responses to increases in sediment NH,,', that sediment nitrogen availabhty at LLM limits seagrass productivity. T. testudinum responded to lirmted nitrogen conditions by increasing below-to above-ground biomass ratios. An ambient sediment NH4+ level of about 100 PM was considered to be the threshold concentration for nitrogen limitation of seagrass growth.

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