Morphology, elemental content and isotopic composition of leaves of the seagrasses Posidonia oceanica and Cymodocea nodosa were highly variable across the Illes Balears, a Spanish archipelago in the western Mediterranean, and varied seasonally at one site in the study area. The data presented in this paper generally expand the reported ranges of nitrogen, phosphorus, iron and arsenic content and delta C-13 and delta N-15 for these species. Nitrogen and phosphorus content of P. oceanica leaves also showed significant seasonal variability; on an annual basis, P. oceanica leaves averaged 1.55% N and 0.14% P at this monitoring site. Both N and P were more concentrated in the leaves in winter than in summer, with winter maxima of 1.76% N and 0.17% P and summer minima of 1.34% N and 0.11% P. There was no significant annual pattern observed in the delta C-13 of P. oceanica leaves, but there was a repeated 0.6C degrees seasonal fluctuation in delta N-15. Mean annual delta N-15 was 4.0C degrees; delta N-15 was lowest in May and it increased through the summer and autumn to a maximum in November. Over the geographic range of our study area, there were interspecific differences in the carbon, nitrogen and phosphorus content of the two species. Posidonia oceanica N:P ratios were distributed around the critical value of 30:1 while the ratios for C. nodosa were lower than this value, suggesting P. oceanica we collected was not consistently limited by N or P while C. nodosa tended toward nitrogen limitation. Nutrient content was significantly correlated to morphological indicators of plant vigor. Fe content of P. oceanica leaves varied by a factor of 5x, with a minimum of 31.1 mu g g(-1) and a maximum of 167.7 mu g g(-1). Arsenic was present in much lower tissue concentrations than Fe, but the As concentrations were more variable; the maximum concentration of 1.60 mu g g(-1) was eight times as high as the minimum of 0.20 mu g g(-1). There were interspecific differences in delta C-13 of the two species; C. nodosa was consistently more enriched (delta C-13 = -7.8 +/- 1.7C degrees) than P. oceanica (-13.2 +/- 1.2C degrees). The delta C-13 of both species decreased significantly with increasing water depth. Depth related and regional variability in the delta C-13 and delta N-15 of both species were marked, suggesting that caution needs to be exercised when applying stable isotopes in food web analyses.