Production of Inorganic Carbon From Aquatic Macrophytes by Solar Radiation

Solar radiation causes considerable photochemical effects on dissolved organic matter (DOM) from both fresh and marine waters. Photooxidation of DOM to dissolved inorganic carbon (DIC) has been shown to be a quantitatively significant process in the turnover of DOM. Less is known about photodegradation of particulate organic matter, e.g., plant detritus. We have evaluated direct and indirect (via photooxidation of DOM released from plant detritus) photooxidative inorganic-carbon production from detritus of the emergent macrophytes Phragmites australis, Typha angustifolia, and Juncus sp. Macrophyte leaves were sterilized and incubated in quartz tubes under three radiation regimes: darkness, ultraviolet (UV)-A, and UV-A + UV-B radiation, using fluorescent lamps with radiation intensities and spectral composition of UV-B and UV-A radiation similar to those of natural solar radiation. Inorganic carbon production was investigated with leaves both immersed in water and in the air. Photoproduction of DIC in both water and air was linear over time (72 h). Production of DIC ranged from 0.22 to 0.44 μg C·(mg dry mass)−1·(24 h)−1. Experiments with macrophyte leachate, excluding the particulate matter (POM), indicated that ∼56% of the DIC accumulating in the water originated from DOM leached from the plant detritus, while 44% of DIC production originated directly from POM. Carbon dioxide released from leaves incubated in air and exposed to UV radiation was lower than for leaves immersed in water (values including DIC produced from leachate). In most cases, DIC production with only UV-A radiation ranged between 60% and 75% of the values found in the UV-A + UV-B treatments, even though the artificial radiation was deficient in UV-A compared to solar radiation. Similar experiments, using natural solar radiation, indicated that PAR (photosynthetically active radiation) has a major effect on DIC production. This shows that wavelength bands other than UV-B can play a significant role in photodegradation of organic matter.