Sources and properties of natural organic matter (NOM) in water along the Dongjiang River (the Source of Hong Kong's drinking water) and toxicological assay of its chlorination by-products.

The Dongjiang River is the major source of the drinking water supply for Hong Kong and also other parts of the Pearl River Delta in China, and the deterioration in the water quality of this river and the excessive levels of trihalomethanes (THMs) in the tap water of some districts in Hong Kong have become a matter of public concern. The main objective of the present study is to investigate the distribution patterns of natural organic matter (NOM) and their association with THM production in the Dongjiang River. We examined the physicochemical and biological properties of the river water and the corresponding sediment elutriate collected from four sampling sites along the Dongjiang River from upstream to downstream and chlorination experiments were conducted. Algal bioassays were performed in order to test the chlorination effects. The results showed that: (1) upstream NOM was derived from terrestrial input, while that at mid- and downstream was most likely derived from phytoplankton; (2) phytoplankton is a major contributor to NOM in the sediments, whereas sediments seem to be the site for major microbial degradation of NOM, biogeochemical recycling of nutrients and a potential NOM pool for the overlaying water during sediment resuspension; (3) dissolved organic carbon (DOC) in surface water is a good indicator for THM production, whereas ultraviolet (UV) absorbance at 254 nm (UV(254)) is a better predictor for THM formation in the elutriates; (4) the bioassay results showed that toxic compounds other than THMs in the chlorinated water are the major factors causing algal growth inhibition.

Modeling of trihalomethane (THM) formation via chlorination of the water from Dongjiang River (source water for Hong Kong's drinking water).

The Dongjiang River is the major source of drinking water supply for Hong Kong. The deterioration of the water quality of the Dongjiang River and excessive trihalomethanes (THMs) in the tap water of some districts in Hong Kong have become causes for public concern. The main objective of the present study is to investigate and model THM formation due to the chlorination of the Dongjiang River water under different chlorination conditions. The results showed that the total THM formation ranged between 11.7 and 91.8 mg L(-1) and that control of the levels was primarily due to the reaction time and the Br(-) level in the water. Bromide concentration was a key factor in determining bromine-containing THM formation and consequently the speciation of THMs. Higher concentrations of bromide shifted THM species to more-bromine-containing ones, while the kinetics reflected the competing halogenation reactions. As the two mixed-halogen THMs had high cancer potency, the cancer risk of total THMs appeared to reach a peak at a bromide concentration ranging between 218 and 262 mg L(-1) (with a bromide to dissolved organic carbon molar ratio (Br(-)/DOC) ranging between 15 and 18 mM/mM).

Control mechanisms of diel vertical migration: theoretical assumptions.

We explore control mechanisms underlying the vertical migration of zooplankton in the water column under the predator-avoidance hypothesis. Two groups of assumptions in which the organisms are assumed to migrate vertically in order to minimize realized or effective predation pressure (type-I) and to minimize changes in realized or effective predation pressure (type-II), respectively, are investigated. Realized predation pressure is defined as the product of light intensity and relative predation abundance and the part of realized predation pressure that really affects organisms is termed as effective predation pressure. Although both types of assumptions can lead to the migration of zooplankton to avoid the mortality from predators, only the mechanisms based on type-II assumptions permit zooplankton to undergo a normal diel vertical migration (morning descent and evening ascent). The assumption of minimizing changes in realized predation pressure is based on consideration of DVM induction only by light intensity and predators. The assumption of minimizing changes in effective predation pressure takes into account, apart from light and predators also the effects of food and temperature. The latter assumption results in the same expression of migration velocity as the former one when both food and temperature are constant over water depth. A significant characteristic of the two type-II assumptions is that the relative change in light intensity plays a primary role in determining the migration velocity. The photoresponse is modified by other environmental variables: predation pressure, food and temperature. Both light and predation pressure are necessary for organisms to undertake DVM. We analyse the effect of each single variable. The modification of the phototaxis of migratory organisms depends on the vertical distribution of these variables.

Photosynthesis-irradiance response at physiological level: a mechanistic model.

A mechanistic model is developed to present the photosynthetic response of phytoplankton to irradiance at the physiological level. The model is operated on photosynthetic units (PSU), and each PSU is assumed to have two states: reactive and activated. Light absorption that drives a reactive PSU into the activated state results from the effective absorption of the PSU. Transitions between the two states are asymmetrical in rate. A PSU in the reactive state becomes activated much faster than it recovers from the activated state to the reactive one. The turnover time for an activated PSU to transit into the reactive one is defined by the turnover time of the electron transport chain. The present model yields a photosynthesis-irradiance curve (PE-curve) in a hyperbola, which is described by three physiological parameters: effective cross-section (sigmaPSII), turnover time of electron transport chain (tau) and number of PSUs (N). The PE-curve has an initial slope of sigmaPSII x N, a half-saturated irradiance of 1/(tau sigmaPSII), and a maximal photosynthetic rate of N/tau at the saturated irradiance. The PE-curve from the present model is comparable to the empirical function based on the target theory described by the Poisson distribution.