Temporal variations in dissolved selenium in Lake Kinneret (Israel)

Selenium is an essential micronutrient for the growth of the dinoflagellate Peridinium gatunense that dominates the spring algal bloom in Lake Kinnere t (LK). The relationship between the levels of dissolved selenium species a nd the occurance of algal blooms in this lake was studied. During algal blo oms of P. gatunense in spring and of the blue-green Aphanizomenon ovalispor um in fall (in 1994) the concentration of epilimnetic dissolved organic Se (Se-org) increased whereas that of selenite (SeIV) decreased, to levels bel ow the limit of detection: 5 ng/l. The disappearance of SeIV during these b looms is attributed to algal uptake and it is suggested that the growth of both algae may have depended on Se-org regeneration. A budget performed for selenate (SeVI) suggests that this species is also consumed by algae but t o a lesser extent than SeIV (in 1994 similar to 40% of the epilimnetic load ). During the stratification period the hypolimnion of Lake Kinneret become s anoxic, with high levels of dissolved sulfide. The affects of this enviro nment on the distribution of Se oxy-anions, selenite (SeIV) and selenate (S eVI), were also studied. At the onset of thermal stratification (March) abo ut 35% of the lake inventory of both Se oxidized species are entrapped in t he hypolimnion. During stages of oxygen depletion and H2S accumulation, SeI V is completely and SeVI partially removed from this layer. The removal is attributed to reduction followed by formation of particulate reduced produc ts, such as elemental selenium Se(o). The ratio between SeVI to total disso lved selenium (Se-T) in water sources to the lake is similar to 0.84, about twice the corresponding ratio in the lake (similar to 0.44, during holomix is). In the lake about 75% of annual SeVI inflow from external sources unde rgoes reduction to selenide (Se-II)and Se(o) through epilimnetic algal assi milation and hypolimnetic anoxic reduction, respectively. It is suggested t hat the latter oxidation of the dissolved organic selenide released from bi ogenic particles and of Se(o) only to the tetravalent species is the cause for the lower ratio of SeVI/Se-T in the lake.