The spatial and temporal dynamics of chromophoric dissolved organic matter (CDOM) were studied using excitation emission matrix fluorescence spectroscopy (EEMs) and parallel factor analysis (PARAFAC) during five cruises in the subtropical Jiulong Estuary from August 2008 to June 2009. Two humic-like (C1 and C3), one tryptophan-like (C4) and one possible protein-like (C2) component was identified by PARAFAC and their behavior in the river–estuary–coastal interface was evaluated. The spatial distributions of the maximum fluorescence (Fmax) for the fluorescent components showed a remarkable loss in the upper estuary. The following significant addition of all components in the low salinity turbidity maximum zone suggested the inputs from riverine source, sediment resuspension and the surrounding mangrove ecosystem. C1, C2 and C3 showed conservative behavior in the middle and lower estuary indicated by the linear relationship between their fluorescence intensities and salinity in the five cruises. However, the tryptophan-like C4 received widespread additions (likely from autochthonous production) in the estuary. Although the humic-like C1 and C3 showed no significant variation in the estuary–coastal interface, C2 and C4 decreased more rapidly beyond this interface, indicating the significant influence of coastal current to this estuarine environment. The seasonal variation of tryptophan-like C4 was characterized by higher Fmax values in the upper to middle estuary in the dry season, which is in contrast to that of the humic-like C3. Correlation analysis with DOC showed that the fluorescence intensity of C1 (or C2 and C3) was suitable for tracing DOC dynamics in this estuary. These results indicated different mixing behavior and temporal variability for different fluorescent components in this dynamic estuarine environment.Research highlights►Four fluorescent components were identified using EEMs-PARAFAC. ►They were non-conservative at the river–estuary interface. ►The humic-like and protein-like fluorophores had different seasonal variation. ►The fluorescence intensities of C(1–3) were suitable for tracing DOC.

Land reclamation has become one of the great concerns of many coastal states for its serious environmental and ecological effects. User charge is an effective market-based approach for regulating land reclamation. The user change should be designed to reflect the full effect of converting sea to land to ensure sustainable use of marine resources. In this paper, we present an ecological-economic framework to estimate the user charge that covers both the ecological damages and the rent associated with the reclaimed land. We apply the framework to Xiamen’s West Sea. Results of the case study suggest that the rents of reclaimed land for industrial and commercial uses are 49 yuan/m2 and 1,066–2,704 yuan/m2, respectively. The unit value of ecological damages of land reclamation is 605 yuan/m2. The present user charge is too low to regulate excessive land reclamation. The user charge for land reclamation in the study area should be modified to 653 yuan/m2 for industrial uses and 35–40% of adjacent land prices for commercial uses.

Seasonal variation, horizontal and vertical distribution, and cell size of nanoflagellates, together with physico-chemical and biological factors, were studied in the northern South China Sea (SCS). It was found that nanoflagellate abundance ranged from 0.157 × 103 to 9.193 × 103 cells/ml (with a mean of 0.891 × 103) in winter (February, 2004), while it ranged from 0.107 × 103 to 5.417 × 103  cells/ml (with a mean of 0.599 × 103) in summer (July, 2004). Nanoflagellates were more abundant in winter than summer in offshore regions, showing an unique seasonal pattern in this subtropical marginal sea. The abundance of nanoflagellates decreased from the estuary to the offshore region. Vertical distribution of nanoflagellates coupled well with that of bacteria and Chl a. The small size fraction of less than 5 μm dominated the nanoflagellate populations. Wind-driven mixing, eddies, availability of nutrients as well as Chl a and abundance of picoplankton seemed to be the major controlling factors for the spatial distribution and seasonal variation of nanoflagellates in the study area.

This paper presents the study carried out for controlling agricultural non-point source pollution (NSP) in a medium-sized watershed covering 1.47 × 104 km2 in Southeast China using quantitative analysis coupled with geographic information system (GIS), universal soil loss equation (USLE), soil conservation service-curve number (SCS-CN), nutrient loss equations, and annualized agricultural nonpoint source model (AnnAGNPS). Based on the quantitative results derived from GIS and environmental models, five control division units were generated for NSP control in Jiulong River watershed, namely, controlling unit for soil losses, controlling unit for livestock breeding and soil losses, controlling unit for excessive fertilizer use and livestock breeding, controlling unit for soil losses and fertilizer use, and controlling unit for excessive fertilizer use and soil losses. This study proved that integrating GIS with environmental models can be adopted to efficiently evaluate major sources and contributors of NSP, and identify the critical source areas of NSP, which enables adjusting measures to local conditions by further control division units developed through such study for control and management of water quality degradation induced by NSP in the Jiulong River watershed.

Eutrophication and harmful algal blooms (HAB) have become the primary water quality concerns in China. A comprehensive analysis was conducted using a dataset obtained from long-term monitoring of a coastal bay–river system in southeast China (Xiamen Bay–Jiulong River Basin), to examine the nutrient [nitrogen (N) and phosphorus (P)] dynamics and the management options. The analysis showed that increasing human stresses and external nutrient loads over the past 30 years were the main causes of water degradation and eutrophication. A significant decline of N:P ratio was observed in both river and estuarine waters since the 1990s due to relatively high P loadings as a consequence of waste discharges from the proliferating husbandry of livestocks and the application of excessive phosphate fertilizers to cash crops. Continued nutrient enrichment and the decline of N:P ratio have changed the nutrient stoichiometry and supply ratio in waters, which have increased the risk of nutrient-enhanced algal bloom. According to the findings and current knowledge concerning eutrophication and HAB processes, a dual nutrient (N and P) management strategy is necessary for management of water quality in the Xiamen Bay–Jiulong River Basin with a focus on animal wastes in the north Jiulong tributary and over-fertilization in the west Jiulong tributary.Highlights► Long-term nutrient dynamics in the Xiamen Bay–Jiulong River Basin were examined. ► A trend of nutrient enrichment and N:P ratio decline in water were observed. ► Proliferating livestocks and fertilization explain the long-term nutrient dynamics. ► A dual nutrient (N & P) management strategy is necessary to restore water quality.

Measurements were conducted during 2004–2005 and 2009–2010 to characterize atmospheric nitrogen (N) deposition to the Jiulong River Estuary – Xiamen Bay area in southeast China. Isotopic analysis and long-term data (1990–2009) for inorganic N extracted from the national acid deposition dataset were used to determine the dominant source of atmospheric nitrate and N component dynamics. The results showed that the mean dissolved total N concentration in rain water for the three coastal area sites was 2.71 ± 1.58 mg N L−1 (n = 141) in 2004. The mean dissolved inorganic N at the Xiamen site was 1.62 ± 1.19 mg N L−1 (n = 46) in 2004–2005 and 1.56 ± 1.39 mg N L−1 (n = 36) in 2009–2010, although the difference is not significant, nitrate turnover dominates the N component in the latter period. Total deposition flux over Xiamen was 30 kg N ha−1 yr−1, of which dry and wet deposition contributed 16% and 84%, respectively. Nitrate in wet deposition with low isotopic value (between −3.05 and −7.48‰) was likely to have mostly originated from combustion NOx from vehicle exhausts. The inorganic N in acid deposition exhibited a significant increase (mainly for nitrate) since the mid-1990s, which is consistent with the increased gaseous concentrations of NOx and expanding number of automobiles in the coastal city (Xiamen). The time series of nitrate anions and ammonium cations as well as pH values during the period 1990–2009 reflected an increasing trend of N emission with potential implication for N-induced acidification.Highlights► Dissolved total nitrogen in rain waters was 2.71 ± 1.58 mg L−1 in the southeast China coastal area. ► Dry and wet deposition contributed 16% and 84% respectively to the total flux (30 kg ha−1 yr−1). ► Nitrate in rain waters with low isotopic value seems originated from combustion NOx from vehicle exhausts. ► There is an increasing trend of N emission with potential implication for N-induced acidification.

Dissolved organic matter (DOM) has not been studied fully for tropical and subtropical rivers, in particular during storm events. DOM dynamics during a summer storm event in June, 2011 were examined in a subtropical river (the North Jiulong River, China). Stormwater runoff was sampled for measurement of dissolved organic C (DOC), absorption spectroscopy and fluorescence excitation emission matrix spectroscopy (EEMs). Three different fluorescent components were identified using parallel factor analysis (PARAFAC), including humic-like C1 and C3, and C2 as a combination of humic-like and protein-like fluorophores. The flux of DOC increased fivefold from 0.4 kg s−1 at baseflow to 2.0 kg s−1 at peak flow. Chromophoric DOM (CDOM) and fluorescent components also showed large increases with stormflow. The flux of DOC was similar during the rising and falling hydrographs at equal water discharge, while those of CDOM and fluorescent components were much higher during the falling-hydrograph. Carbon specific CDOM absorption coefficient at UV (SUVA254) and the fraction of C3 which fluoresced at long emission wavelength correlated positively to each other but negatively to absorption spectral slope ratio (SR) and C2%. They showed notable temporal variations indicative of increases in aromaticity, C3% and average molecular weight of DOM during the storm event. Changes in DOM composition lagged behind those in DOM flux. Changes in the flux and quality of DOM during the storm event, which were monitored effectively by absorption spectroscopy and EEMs–PARAFAC, could affect biogeochemical processes in the river and receiving coastal waters.Highlights► Dynamics of DOM in a storm event were studied in a subtropical river. ► The flux of DOM increased greatly in the storm flow. ► The aromaticity and molecular weight of DOM increased during the storm event. ► The fraction of a humic-like fluorescent component (C3) increased in the event. ► Changes in DOM composition lagged behind those in DOM flux.

Probabilistic risk assessment (PRA) uses probability theory to quantify the probability of occurrence of an event. In this study, PRA was conducted for the discharge of excess nitrogen (N) from an agricultural (Jiulong River) watershed in Southeast China. Using fault tree analysis, a type of PRA, the probability of occurrence of excess N discharge into the river during a runoff event was evaluated both qualitatively and quantitatively. Land use maps, soil maps, fertilizer use records, and expert opinions were used to determine probabilities of individual events within the fault tree and to calculate the overall probability of excess N discharge during a runoff event. Qualitative analysis showed that the risk of excess N discharge was mainly related to crop and livestock practices in the watershed. Proper management of tillage, fertilizer, and manure was necessary to control N releases. Quantitative assessment results indicated that alternative practices including reduction of fertilization, installation of vegetative strip buffer around the pig farms, and installation of more riparian buffers along the Jiulong River could reduce the likelihood of N discharge through runoff.

Chromophoric dissolved organic matter (CDOM) may show notable non-conservative behaviors in many estuaries due to a variety of biogeochemical processes. The partition between CDOM and chromophoric particulate organic matter (CPOM) was examined in the Jiulong Estuary (China) using absorption and fluorescence spectroscopy, which was also compared with microbial and photochemical degradations. The absorption coefficient of water-soluble CPOM (aCPOM(280)) at ambient Milli-Q water pH (6.1) ranged from 0.11 to 7.94 m−1 in the estuary and was equivalent to 5–101% of CDOM absorption coefficient. The aCPOM(280) correlated significantly with the concentration of suspended particulate matter and was highest in the bottom water of turbidity maximum zone. Absorption spectral slope (S275–295) and slope ratio (SR) correlated positively with salinity for both CPOM and CDOM, suggesting decreases in the average molecular weight with increasing salinity. The adsorption of CDOM to re-suspended sediments (at 500 mg L−1) within 2 h was equivalent to 4–26% of the initial aCDOM(280). The adsorption of CDOM to particles was less selective with respect to various CDOM constituents, while the microbial degradation resulted decreases in S275–295 and SR of CDOM and preferential removal of protein-like components. The partition between CPOM and CDOM represented a rapid and important process for the non-conservative behavior of CDOM in turbid estuaries.