Interspecific variation in the elemental and stable isotope content of seagrasses in South Florida

The elemental (C, N, and P) and isotope (delta C-13, delta N-15) content of leaves of the seagrasses Thalassia testudinum, Halodule wrightii, and Syringodium filiforme were measured across a 10 000 km(2) survey of the seagrass communities of South Florida, USA, in 1999 and 2000. Trends at local and broad spatial scales were compared to examine interspecific variation in the seagrass characteristics often used as ecological indicators. The elemental and stable isotope contents of all species were variable and demonstrated marked interspecific variation. At broad spatial scales, mean N:P ratios were lowest for T testudinum (36.5 +/- 1.1) and S. filiforme (38.9 +/- 1.3), and highest for H. wrightii (44.1 +/- 1.8). Stable carbon isotope ratios (delta C-13) were highest for S. filiforme (-6.2 +/- 0.2 parts per thousand), intermediate for T testudinum (-8.6 +/- 0.2 parts per thousand), and lowest for H. wrightii (-10.6 +/- 0.3 parts per thousand). Stable nitrogen isotopes (delta N-15) were heaviest for T testudinum (2.0 +/- 0.1 parts per thousand), and lightest for H. wrightii (1.0 +/- 0.3 parts per thousand) and S. filiforme (1.6 +/- 0.2 parts per thousand). Site depth was negatively correlated to delta C-13 for all species, while delta N-15 was positively correlated to depth for H, wrightii and S. filiforme. Similar trends were observed in local comparisons, suggesting that taxon-specific physiological/ecological properties strongly control interspecific variation in elemental and stable isotope content. Temporal trends in delta C-13 were measured, and revealed that interspecific variation was displayed throughout the year. This work documents interspecific variation in the nutrient dynamics of 3 common seagrasses in South Florida, indicating that interpretation of elemental and stable isotope values needs to be species specific.