Effect of eutrophication on the distribution of arsenic species in eutrophic and mesotrophic lakes

Abstract

Effects of eutrophication on arsenic speciation were studied in eutrophic Lake Kiba and mesotrophic Lake Biwa, Japan. By combining hydride generation atomic absorption spectrometry with ultraviolet irradiation, inorganic, methyl and ultraviolet-labile fractions of arsenic were determined. In both Lakes, inorganic species (As(V+III)) dominated over other forms of arsenic all the year round. Most of methylarsenic fraction was dimethylarsinic acid (DMAA), and the concentration of monomethylarsonic acid (MMAA) was below the detection limit. Measurements of size-fractioned arsenic concentrations in water column indicate that most of the DMAA was distributed in truly dissolved fraction (<10 kDa), while ultraviolet-labile fractions were distributed in particulate (>0.45 microm) and colloidal (10 kDa-0.45 microm) fractions. Arsenic speciation in eutrophic Lake Kiba fluctuated greatly with season. The ultraviolet-labile fractions were observed with the increase of DMAA from May to October, and they disappeared with the decrease of DMAA in January. In mesotrophic Lake Biwa, the ultraviolet-labile fractions of arsenic were not influenced as much as those in eutrophic Lake Kiba. On the other hand DMAA concentration was higher in Lake Biwa compared to that in Lake Kiba. The results suggest that the biosynthesis of complex organoarsenicals was enhanced by eutrophication, and the arsenic speciation would be influenced by the balance of biological processes in natural waters.

Seasonal changes of arsenic speciation in lake waters in relation to eutrophication

“…Both in eutrophic and mesotrophic lakes, the total As concentrations in the summer were higher than in the winter. These trends of As occurrence and distribution in lake waters were also reported previously (Crecelius, 1975;Hasegawa et al, 2009;Takamatsu and Kawashima, 1985). Inorganic As was released into the water from the sediments in the bottom layer under anaerobic conditions in summer, while As(V) was adsorbed onto Fe/Mn oxides and then settled down into the sediments in winter.…”

“…Results suggest that the occurrence of UV-degradable fractions of As is higher in high eutrophic lakes and the production of DMAA is related to the water temperature in low eutrophic lakes. The occurrence of DMAA has been reported in other geographical areas (Anderson and Bruland, 1991;Hasegawa et al, 2009), and in laboratory experiments (Hasegawa et al, 2001). …”

“…Therefore, it is necessary for the organoarsenicals to be converted to hydride reactive forms using UV irradiation (Hasegawa et al, 1999). Although the distribution and speciation of As in 5 | P a g e eutrophic and mesotrophic lakes were discussed in our previous article (Hasegawa et al, 2009), it was difficult to interpret the influence of eutrophication on As speciation because of limited data from a less number of lakes. In the present study, the correlation between As speciation and eutrophication are inferred based on samples collected from 18 lakes in Japan during July 2007 andFebruary 2008.…”

“…The UV-As was transformed to As(V) directly. The detail of the organoarsenic speciation is shown elsewhere (Hasegawa et al, 2009). The UV-As and UV-DMAA were estimated from the concentration changes of As(V+III) and DMAA during the ultraviolet irradiation by a non-linear least-squares computation, respectively (Hasegawa et al, 1999).…”

“…The DMAA is produced by bacterial degradation of complex organoarsenicals in the suspended solids, which is subsequently released into the water column. Thus bacterial activity is responsible for the increase of DMAA concentration in surface water (Hasegawa et al, 2009). Higher concentrations of DMAA might also be resulted from the long-term accumulation of DMAA excreted by phytoplankton.…”