SELENIUM DETOXIFICATION IN WETLANDS BY PERMANENT FLOODING .1. EFFECTS ON A MACROALGA, AN EPIPHYTIC HERBIVORE, AND AN INVERTEBRATE PREDATOR IN THE LONG-TERM MESOCOSM EXPERIMENT AT KESTERSON RESERVOIR, CALIFORNIA

Storage of Se-rich agricultural drainwater in an inland saline marsh (Kesterson Reservoir) resulted in heavily contaminated biota (100 to 300 ppm Se as dw). A hypothesis was proposed that permanent flooding of the marsh with low-Se water (to create anoxic sediments) would immobilize Se in an insoluble fraction unavailable to aquatic biota. This was tested in a 0.4 ha mesocosm at Kesterson Reservoir over 2.3 yr by measuring the decline in Se in plants and animals. The study was part of a larger project, only data for soluble and particulate Se, in the submerged macroalga Chara, and in the aquatic and aerial forms of typical epifloral invertebrates - a herbivore (chironomid), and one of its predators (damselfly) are reported here. Permanent flooding resulted in biologically immobile Se. A rapid initial decline in Se lasting a few months was followed by a slower, irregular but persistent decrease. Soluble Se declined most quickly, followed by that in damselfly and chironomid populations. Declines in the vegetation were slowest but Se fell to a lower level (3 to 4 ppm) than in the animals (14 to 15 ppm), although Se levels were still dropping in all biota at the end of the experiment. Between 85 and 93% of initial Se was lost by the 3 types of organisms over 2.3 yr. There was little difference in loss rates between predator and herbivore. Because Chara was the most common submerged vegetation, its loss of 94% of initial Se to concentrations considered safe for wildlife, is most important to detoxification of the entire wetland.