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.

Ship-of-Opportunity Monitoring of the Chilean Fjords Using the Pocket FerryBox

“…The presence of high concentrations of CDOM following the period of heavy rain is likely to derive from the leaching of organic matter from the mature forests and grasslands that cover the adjacent land area, much of which is protected within nature reserves and/or is pristine. Significant CDOM levels have also been observed in New Zealand fjords (Gonsior et al 2008) and Arctic rivers (Retamal et al 2007), with similar characteristics to this region of Chile, but have not been previously reported within the Laguna San Rafael area. Because the estuarine water in this region tends to have high turbidity, either due to high concentrations of suspended fine particulate matter of glacial origin (rock flour) close to Laguna San Rafael or sediment from the Aysé n River, salinity is strongly anticorrelated with turbidity and CDOM in this section.…”

An ultrahigh‐resolution mass spectrometry index to estimate natural organic matter lability

“…The Doubtful Sound contains very high, nearly exclusive terrestrial chromophoric OM character confirmed first by fluorescence spectroscopy . The southwestern slope of the South Island of New Zealand and the northeastern slope of Stewart Island accrue an estimated 400–500 tons of carbon vegetation per hectare with considerable carbon storage values .…”

“…The Doubtful Sound contains very high, nearly exclusive terrestrial chromophoric OM character confirmed first by fluorescence spectroscopy. [56] The southwestern slope of the South Island of New Zealand and the northeastern slope of Stewart Island accrue an estimated 400-500 tons of carbon vegetation per hectare with considerable carbon storage values. [57] The lignin-like OM character and recalcitrant nature in both ecosystems could be a result of the higher annual rainfall percentages (>7 m year -1 at Doubtful Sound) and the fjord slope mass-wasting soil and vegetation events that deliver terrestrial OM to the soils and surrounding waters.…”

Correlation between molecular absorption spectral slope ratios and fluorescence humification indices in characterizing CDOM

“…The chromophores-containing DOM is termed chromophoric dissolved organic matter (CDOM) in the literature (Laurion et al 2000;Gonsior et al 2008). CDOM is sometimes referred to as colored dissolved organic matter (Del Castillo et al 1999;Sulzberger and Durisch-Kaiser 2009).…”