Hydrogen peroxide production in marine bathing waters: Implications for fecal indicator bacteria mortality

Hydrogen peroxide concentrations [H2O2] have been measured over the last two decades in multiple studies in surface waters in coastal, estuarine and oceanic systems. Diurnal cycles consistent with a photochemical production process have frequently being observed, with [H2O2] increasing by two orders of magnitude over the course of the day, from low nM levels in the early morning to 102 nM in late afternoon. Production rates range from <10 for off-shore ocean waters to 20–60 nM h−1 for near-shore coastal and estuarine environments. Slow night-time loss rates (<10 nM h−1) have been attributed to biological and particle mediated processes. Diurnal cycles have also frequently been observed in fecal indicator bacteria (FIB) levels in surf zone waters monitored for microbial water quality. Measured peak peroxide concentrations in surface coastal seawaters are too low to directly cause FIB mortality based on laboratory studies, but likely contribute to oxidative stress and diurnal cycling. Peroxide levels in the surf zone may be increased by additional peroxide production mechanisms such as deposition, sediments and stressed marine biota, further enhancing impacts on FIB in marine bathing waters.