Arsenic and phosphorus in seagrass leaves from the Gulf of Mexico

Arsenic is a common contaminant in the marine environment, but little is known about arsenic in seagrasses. Arsenate is taken up by the phosphate uptake systems of plants, and there is often a relationship between P availability and As uptake. We sampled green leaves of the seagrass Thalassia testudinum from six estuaries in the Gulf of Mexico to document the As content of seagrasses and to determine the relationships between P availability and As. Arsenic content of seagrasses was generally lower than literature values of other marine primary producers. Arsenic content varied from 0.90 to 3.36ppm and phosphorus content varied from 544 to 6294ppm, and there were significant differences among the estuaries studied. Nutrient stoichiometry (N:P) indicated that there were differences in P availability among estuaries: Charlotte Harbor had high P availability, while St. Joseph Bay and Florida Bay were P-limited. The Homosassa River, the Anclotte Estuary and Tampa Bay had intermediate P availability. When data from all estuaries were pooled, there was a significant, negative relationship between P and As content, as predicted by the model of competitive uptake of As and P. However, more extensive sampling within one estuary (Florida Bay) showed a significant positive relationship between P and As, suggesting that factors other than strict competitive uptake kinetics influence the relative content of As and P. We suggest that the P and As availability ratio in estuaries, and therefore in the seagrasses that inhabit them, is largely controlled by the relative importance of freshwater and marine inputs of the two elements.