Algal blooms modulate organic matter remineralization in freshwater sediments: A new insight on priming effect.

This study provides a novel insight into the degradation of sediment organic matter (SOM) regulated by algae-derived organic matter (AOM) based on priming effect. We tracked the dynamics of SOM mineralization products and pathways, together with priming effects (PE) using the compound-specific stable isotope (δ13C) technique following addition of low- and high-density algal debris in sediments. We found that algal debris increased the total carbon oxidation rate, and resulted in denitrification and methanogenesis-dominated SOM mineralization. While iron reduction and sulphate reduction played important roles in the early period of algal accumulation. Total carbon oxidation rate and anaerobic rates (Ranaerobic) were higher in the amended treatments compared with that in the control. Analysis indicated that algal debris had a positive PE on SOM mineralization, which caused an intensified mineralization in the initial phase with over 80% of dissolved inorganic carbon deriving from SOM degradation. Total carbon oxidation rate of SOM deduced from priming effect (RTCOR-PE) was similar to Ranaerobic, further indicating SOM mineralization was a critical source of the end products. These findings deviate the causal focus from the decomposition of AOM, and confirm the accumulation of AOM as the facilitator of SOM mineralization. Our study offers empirical evidences to advance the traditional view on the effect of AOM on SOM mineralization.

Abundant sediment organic matter potentially facilitates chemical iron reduction and surface water blackness in a Chinese deep lake.

Black bloom has become an increasingly severe environmental and ecological problem in lots of lakes. Ferrous monosulfide (FeS), which is closely related to chemical iron reduction (CIR), is considered the major cause for black water in shallow lakes, but few studies focus on the effect of organic matters (OM) content on iron and sulfate reduction and its contribution to the black bloom in deep lakes. Here, in Lake Fuxian, a Chinese deep lake which has also suffered from black bloom, FeS was identified responsible for the surface water blackness by using multiple microscopy and element analyses. Dissolved oxygen (DO) penetrated 1.6-4.2 mm in all sediment sites, further indicating FeS formed in the sediments instead of the permanently oxic water column. Geochemical characteristics revealed by diffusive gradients in thin films (DGT) showed that DGT-Fe2+ concentration was 57.6-1919.4 times higher than the DGT-S2- concentration and both were positively correlated with DGT-PO43-. Combining DGT profiles and anaerobic OM remineralization rate according to bag incubation, iron reduction is more effective than sulfate reduction although the two processes coexisted. Moreover, correlation of DGT-Fe2+ and DGT-PO43- was better than that of DGT-PO43- and DGT-S2- at OM-depleted sites but opposite at OM-rich sites. In addition, total organic carbon (TOC) was significantly positively related to acid volatile sulfide (AVS). We therefore conclude that abundant OM potentially exacerbate chemical iron reduction and further lead to surface water blackness. Our study revealed the mechanisms behind the black bloom and gives credence to the management strategy of reducing OM loading to protect water quality in deep lakes.

The establishment of preliminary safety threshold values for cyanobacteria based on periodic variations in different microcystin congeners in Lake Chaohu, China.

As harmful cyanobacterial proliferation threatens the safety of drinking water supplies worldwide, it is essential to establish a safety threshold (ST) for cyanobacteria to control cyanobacterial density effectively in water sources. For this purpose, cyanobacterial abundance, microcystin (MC) production, and environmental parameters were monitored monthly from September 2011 to August 2012 in one drinking water source of Lake Chaohu. The cyanobacterial density ranged from 1400 to 220 000 cells per mL with the succession of two dominant species Microcystis and Dolichospermum, which was determined by water temperature and nutrient loading. The MC concentrations were correlated significantly with the cyanobacterial density and they varied between 0.28 and 8.86 µg L(-1). Therefore, the characteristics of MC cell quotas were classified according to four stages of the development of cyanobacteria, namely: recruitment, multiplication, decline and dormancy. The ST for cyanobacteria was established for different periods based on the MC cell quota and its guideline wherein three commonly monitored MC congeners (MC-LR, -RR and -YR) were considered in the present study. Its reliability was verified in the water source using the data collected between June 2013 and May 2014. The results highlighted the necessity to classify the ST-values in different periods referring to the main MC congeners rather than MC-LR, which will facilitate the management and control of toxic cyanobacterial proliferation in drinking water sources.