Nitrogen limitation affects the sinking property of Microcystis by promoting carbohydrate accumulation.

Nitrogen limitation has been proven to inhibit Microcystis proliferation, and the significant decline in Microcystis blooms in late summer or autumn has been considered to be related to the nitrogen depletion in water. Sinking loss is another factor that influences the dynamics of cyanobacteria in lakes. However, to date, it is still unclear how the sinking property of Microcystis responds to nitrogen availability. Our results suggest that nitrogen limitation would directly influence sinking property of Microcystis, through a significant increase in the specific density of cells. In the short term, carbohydrate accumulation was mainly responsible for the high specific density, showing a high correlation among the NO3--N concentration, specific density and carbohydrate content. Furthermore, carbohydrates could rapidly accumulate after one light/dark cycle, which was mainly due to the reduction in carbohydrate consumption in the darkness under nitrogen limitation. Under nitrogen-light coupling conditions, the specific density ranged from 1.060 to 1.068, except for the treatment with high-nitrogen plus low-light, which showed the value of 1.032. More importantly, when coupled with low nitrogen, the low light did not decrease the carbohydrate content and the specific density, which implied that the sinking cells could not migrate back to the surface. Accordingly, a hypothesis was proposed that the carbohydrate accumulation induced by low nitrogen availability caused an increase in specific density, which invalidates the buoyancy regulation, and cells sink continually out of the water column. This study explores a new understanding on the disappearance mechanisms of Microcystis blooms in the late summer and fall.

[Distribution of organochlorine pesticides in surface sediments from Huangpu River and its risk evaluation].

This paper reports 20 organochlorine pesticides (OCPs) in surface sediments of Huangpu River in Shanghai, China. Pesticides were analyzed by GC/ECD with dual column. The concentration range of sigmaOCPs in sediments was 2.65-19.54 ng/g. DDTs, BHCs, methoxychlor and dieldrin had higher concentration than others. The concentration range of DDTs and BHCs was 0.68-4.43 ng/g and 0.14-0.77 ng/g, respectively, which shows concentration of DDTs was higher than BHCs. OCPs content increasing from upper to downstream Huangpu River implicats that discharging of industrial effluents and inflow of Suzhou River offered lots of OCPs to middle-lower Huangpu River. Distribution of OCPs components indicate that OCPs in sediments may be derived from the aged and weathered agricultural soils. According to correlation analysis between OCPs and TOC of sediments, TOC content influences the distribution of OCPs in sediment strongly. Contamination level of OCPs in sediment of Huangpu River is relatively lower than other areas. Comparing with ERL and ERM value of risk evaluation, sediments in middle-lower Huangpu River pose a bit high risk to consumers of bottom feeders.