Abstract
Dissolved organic matter (DOM) is the most dynamic and least understood part of the global oceanic carbon cycle. Furthermore the molecular composition of DOM is largely unknown. This study focused on the distribution pattern, removal processes and molecular characterisation of DOM in a range of estuaries and coastal zones in New Zealand. Doubtful Sound, the longest fjord in Fiordland National Park, South Island, New Zealand was of particular interest, because of the combination of extreme rainfall, enhanced production of DOM within the temperate rainforest which largely appears in the relatively deep (≥ 5 m) low salinity layer (LSL) at the fjord surface. A typical river estuary (Freshwater River) located in Stewart Island, New Zealand was also investigated. Optical water properties such as the UV/Vis absorption coefficient at 355 nm (α[CDOM](355)) and excitation-emission matrix fluorescence (EEM) were determined for samples from freshwater, across the LSL into open ocean water. These optical properties showed a marked decrease with salinity and highest levels of EEM fluorescence and α[CDOM](355) in the brackish surface water. In addition to the observed changes in the optical properties, ultrahigh resolution Electrospray Ionisation Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FT-ICR-MS) determination of molecular formulae revealed that in the fjord about 20 % of these formulae changed along a vertical salinity gradient across the LSL between the brackish surface water and the saline water at 5 m depth. This trend was even more pronounced along the salinity gradient of the Fresh Water River Estuary in Stewart Island, where 60 % of all assigned molecular masses changed from freshwater over the mixing zone to ocean water. Associated with these changes was a marked increase in aromaticity with increasing salinity. Comparable behaviour with increasing salinity was also observed in estuarine samples from the Cape Fear River system, North Carolina, USA.
In contrast, only minor changes were determined in molecular formulae for surface water samples collected along a transect off the Otago Coast and across the Subtropical Convergence (STC) into Subantarctic Water (SAW). However, a comparison of the molecular formulae assigned to the DOM pool for the STC water and a freshwater stream in Doubtful Sound, revealed that 75 % of all the assigned formulae for the open ocean sample were common to these two markedly different types of natural waters. This seemingly refractory DOM contained nearly 600 assigned molecular formulae, which were all very similar (only spaced by two hydrogen and CH₂ groups) and could be explained with only 9 general molecular formulae. However, the comparison of all assigned formulae for the freshwater sample suggested that about 90 % of the assigned molecular formulae for the terrestrially-derived DOM changed as it moved from rivers to the open ocean and that only 10 % remained the same.
Singlet oxygen showed a very close relationship with the optical properties such as the absorption coefficients (a[CDOM](355)) and the EEM fluorescence intensities and these results suggested that singlet oxygen steady state concentrations are linked to CDOM.
Photodegradation processes were confirmed to be responsible for a significant destruction of CDOM. Samples collected from different salinity waters showed major differences in wavelength-dependent photo-decay of CDOM suggesting that the rate of photodegradation in the UV range decreased with increase in salinity whereas it was enhanced for longer wavelength radiation (≥400 nm). Additionally, the predominantly unsaturated compounds produced during estuarine mixing were found to be highly photolabile and were either destroyed or new unsaturated compounds were produced within 21 h of solar irradiation.