The literature, and previously unpublished data from the authors’ laboratories, shows that the δ13C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from –3 to –35‰. While some marine macroalgae have δ13C values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between –30 and –35‰). The organisms with very negative δ13C (lower than –30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ13C values (higher than –10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ13C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ13C values lower than –30‰ have pyrenoids. Previous work showed a good correlation between δ13C values lower than –30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ13C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ13C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ13C values more positive than –10‰ (i.e. more positive than the δ13C of CO2 in seawater) must involve HCO3- use.

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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