Changes of dissolved organic matter following salinity invasion in different seasons in a nitrogen rich tidal reach

Dissolved organic matter (DOM) is a heterogeneous mixture of dissolved material found ubiquitously in aquatic systems and dissolved organic nitrogen is one of its most important components. We hypothesised nitrogen species and salinity intrusions affect the DOM changes. Here, using the nitrogen rich Minjiang River as an easily accessible natural laboratory 3 field surveys with 9 sampling sites (S1-S9) were conducted in November 2018, April and August 2019. The excitation emission matrices (EEMs) of DOM were explored with parallel factor (PARAFAC) and cosine-histogram similarity analysis. Four indices including fluorescence index (FI), biological index (BIX), humification index (HIX) and the fluorescent DOM (FDOM) were calculated and the impact of physicochemical properties was assessed. The results suggested that the highest salinities of 6.15, 2.98 and 10.10, during each campaign corresponded to DTN concentrations of 119.29-240.71, 149.12-262.42 and 88.27-155.29 μmol·L^-1, respectively. PARAFAC analysis revealed the presence of tyrosine-like proteins (C1), tryptophan-like proteins or a combination of the peak N and tryptophan-like fluorophore (C2) and the humic-like material (C3). The EEMs in the upstream reach (i.e. S1-S3) were complex with larger spectra ranges, higher intensities and similar similarity. Subsequently, the fluorescence intensity of three components decreased significantly with low similarity of EEMs (i.e. S4-S7). At the downstream, the fluorescence levels dispersed significantly and no obvious peaks were seen except in August. In addition, FI and HIX increased, while BIX and FDOM decreased from upstream to downstream. The salinity positively correlated with FI and HIX, and negatively related to BIX and FDOM. Besides, the elevated DTN had a significant effect on the DOM fluorescence indices. Altogether, salinity intrusion and elevated nitrogen are relevant for the distribution of the DOM, which is helpful for the water management tracing the DOM source according to the on-line monitoring of salinity and nitrogen in estuaries.

Isotopic compositions (δD, δ18O) and end-member mixing for the control interface in a complex tidal region

Identifying the mixing processes of waters and currents in tidal reach is an important aspect of environmental management to protect freshwater resources and prevent water pollution. In this study, three field investigations conducted in a typical tidal reach in August, November and the following April focused on two isotopes (δD and δ¹⁸O) and salinity. A salinity-isotope conservative mixing model was established to differentiate water flows of the important control interface (CI) from freshwater, transition zone and saltwater end-members. Results suggested that the average δD and δ¹⁸O values during the ebb and flood tides depleted from August to November, then enriched significantly in the following April and were even higher than those in August. The δD and δ¹⁸O values in the saltwater zone enriched markedly compared with those in freshwater zone and transition zone due to the stronger evaporation occurring in the saltwater zone. Based on the revised model, the average contributions of freshwater end-member, transition zone end-member and saltwater end-member in three months were, respectively, 51.50 %, 36.93 % and 11.57 %. However, the contributions of freshwater and transition zones in April end-member were equivalent (47.45 % vs 44.31 %). Meanwhile the largest contribution of saltwater end-member was 20.56 % and occurred in August. The proportions of three end-members that contributed to CI changed with different evaporation scenarios and moisture sources of precipitation. Our research provides important information that furthers our understanding of the isotopes and their applications to environmental management in estuarine regions.

Seasonality in a tidal reach: Existence, impact and a possible approach for design flood level estimation

Heterogeneity caused by seasonality could lead to the estimation error of the design flood level (DFL). This research intended to examine the existence of seasonality in the extreme water levels in a tidal reach and to quantify its impact on the DFL estimation. The mixed-distribution, a commonly used method for design value estimation with heterogeneous samples, was tested. A case study was carried out in the Pearl River Delta, South China. Results showed that a significant seasonality existed in the extreme water levels that were generated from the flood-tide interactions in the delta. If the DFL was estimated with a single distribution, the DFL with a return period smaller than 1.1 years would be underestimated and more information of the non-flood season would be lost. The mixed-distribution was superior in its consideration of seasonality, however, when the return period was over 10 years or smaller than 5 years, the DFL estimation results of this approach were only shifted by <1% from that of a single distribution.