Spectral characterization of chromophoric dissolved organic matter (CDOM) in a fjord (Doubtful Sound, New Zealand)

Abstract.

Doubtful Sound, Fiordland National Park, New Zealand has a stable low salinity layer (LSL) at the surface due to the high annual rainfall and an additional freshwater input from the discharge of a hydroelectric power plant. Chromophoric dissolved organic matter (CDOM) distribution dynamics in this LSL were analyzed using Excitation Emission Matrix (EEM) fluorescence and UV/Vis absorption measurements. The levels of CDOM in surface water in Doubtful Sound were much higher than previously reported for coastal zones. CDOM analyzed by UV/Vis absorbance and EEM fluorescence decreased by about 90% within the first 5 m depth. In the first 5 m, the salinity gradient was extreme (5 – 34 salinity), creating a vertical mixing gradient, which appeared to be stable over time. The spectral slope S and the fluorescence index increased during vertical mixing within the first 5 m depth indicating changes in the CDOM chromophores. Observations during a strong rain event revealed that CDOM in a freshwater stream initially increased, but decreased with additional rain (same flow rate), which is an indication that most CDOM was quickly exported within these catchments. Furthermore, the lower CDOM input after substantial rain diluted the surface CDOM level and created a subsurface CDOM maximum. However, shortly after the rain event (<24 h) the surface CDOM maximum with a steady decrease with depth was restored. The combination of extreme rainfall, pristine temperate rain forest and steep mountains with distinct watersheds creates a highly dynamic environment with very high terrestrial CDOM input. These CDOM dynamics and the high degree of scatter in the linear correlations between CDOM levels and salinity cannot be explained solely in terms of conservative mixing, but suggest that biophysical controls in the catchments and mixing with marine derived CDOM are involved in the dynamics of CDOM in this system.