WATER CHEMISTRY AND THE DISTRIBUTION OF MEXICAN PODOSTEMACEAE - A PRELIMINARY EVALUATION

Tropical rivers represent the most heavily polluted of tropical aquati c habitats. Podostemaceae often represent the dominant submerged veget ation in tropical rivers, especially in river-rapid and waterfall habi tats. Anecdotal accounts implicate water pollution as detrimental to p opulations of Podostemaceae. This paper examines chemical variables an d their relationship to the occurrence of species of Podostemaceae. Tw enty-eight tropical rivers in which Podostemaceae occur were sampled i n the Mexican states of Nayarit, Jalisco, Guerrero and Oaxaca on the P acific slope, and Veracruz and Oaxaca on the Atlantic slope. Assays we re conducted for total nitrogen, total phosphorus, ammonia, nitrates, orthophosphates, major cations (Na, K, Mg, Ca), temperature and pH. Se venteen rivers had nutrient levels below the level of detection; Podos temaceae occurred primarily in low nutrient (oligotrophic) rivers. Six rivers had detectable levels of total nitrogen (160-2050 mu g l(-1)) and total phosphorus (100-720 mu g l(-1)). Five additional rivers had detectable levels of total nitrogen, but lacked phosphorus. Sodium was the dominant cation in all but one river. These results indicate that some species of Podostemaceae can tolerate high levels of nutrients, at least during the period of low water (the period of sampling for th is study). Studies of ambient water chemistry throughout the year are needed. The need for additional study is discussed. (C) 1997 Elsevier Science B.V.