Minhan Dai

First data on trace metal level and behaviour in two major Arctic river-estuarine systems (Ob and Yenisey) and in the adjacent Kara Sea, Russia

Dissolved and colloidal trace element concentrations (Cd, Cu, Fe, Ni and Pb) were determined in the Ob and Yenisey river-estuary systems and in the adjacent Kara Sea in September 1993. The data show a natural low concentration level of ‘dissolved’ (< 0.4 μm) trace metals (Cd, Cu, Fe, Ni and Pb) in the two rivers and in the Kara Sea as compared to world unpolluted rivers and the central Arctic Ocean, suggesting that the region studied is pristine with respect to trace metals. The pathway of trace metals transported from rivers to the ocean seems to be complicated, and largely influenced by biogeochemical processes taking place in the estuarine mixing zone. Colloidal material (104 Daltons-0.4 μm), in addition to its significant contribution to the so-called ‘dissolved’ fraction, has been shown to play a fundamental role in determining the behaviour of both conservative and non-conservative trace metals during estuarine mixing. Hence, colloids may control to a large extent the fate of ‘dissolved’ trace metals as well as their net input from the rivers to the Kara Sea.

The significant role of colloids in the transport and transformation of organic carbon and associated trace metals (Cd, Cu and Ni) in the Rhône delta (France)

The distribution of organic C and trace elements (Cd, Cu, and Ni) between truly dissolved and colloidal phases was determined in May 1992, in order to better understand their behavior during estuarine mixing in the Rhone delta (France). Colloidal material (104 Daltons-0.4 μm) was separated with an ultra-clean cross-flow filtration system. A considerable part of the organic C and trace metals studied, used to be considered to occur in the so-called “dissolved” pool, is associated with colloidal material. The significant correlation between colloidal organic C and colloidal Cu suggests an important involvement of colloidal organic material in the transport of this element. The correlation between Ni, Cd and organic matter mainly occurs in the truly dissolved fraction, suggesting that the truly dissolved organic C is important for these two elements. Truly dissolved and colloidal organic C and trace-metal fluxes transported from the River Rhone to the northwestern Mediterranean Sea were estimated.

Observed three-dimensional structure of a cold eddy in the southwestern South China Sea

National Basic Research Program of China [2009CB421208]; Natural Science Foundation of China [40821063, 40521003]; Programme of Introducing Talents of Discipline to Universities [B07034]; Research Grants Council of Hong Kong [CERG 601009]

Biological response to intensified upwelling and to a river plume in the northeastern South China Sea: A modeling study

National Key Basic Research Development Program [2009CB421208]; National Natural Science foundation of China [N_HKUST623/07, NSFC 40731160624]; Hong Kongs Research Grants Council [CERG-601008]; SCOPE project

Evaluation of two cross-flow ultrafiltration membranes for isolating marine organic colloids

Abstract Laboratory and field studies were performed to evaluate two 1 kilo-Dalton (kD) cross-flow ultrafiltration (CFF) membranes (a Millipore Prep-scale CFF membrane constructed primarily from regenerated cellulose and an Amicon CFF polysulfone membrane) to isolate colloids (operationally defined as particles or macromolecules between 1 kD and 0.2–1 μ m) from sea water. We focused on three crucial aspects when applying the CFF technique: retention characteristics, sorptive potential and ultrafilter breakthrough. Lab results showed that both CFF systems retained ≥91% of a 3000 nominal molecular weight (NMW) dextran standard, consistent with the manufacturers rated cutoff. The Millipore membrane showed essentially no loss of a dextran standard, while 33% was lost for the same molecule onto the Amicon CFF membrane. Both membranes showed higher losses of a protein standard (Lactalbumin) added to sea water. For bulk organic carbon (OC), both membranes usually had reasonable recovery (100±10%) as long as the membranes were preconditioned. This was true for both lab experiments and field investigations in open ocean water off Bermuda. However, data from 234 Th and 230 Th analysis of samples from a station off Bermuda showed very large losses and hence low recovery from CFF. Results of these fractionated OC and 234 Th distributions are also discussed in the context of prior studies. Ultrafilter breakthrough of both high molecular weight (HMW) and low molecular weight (LMW) compounds may occur throughout the CFF process, especially when processing coastal sea water where COC is relatively enriched. A permeation coefficient model provides an overall reasonable fit to the data characterizing the permeation behaviour of CFF; the retentate prediction based on the model indicates that breakthrough becomes more significant after the concentration factor (cf) is higher than 5, which implies that fractionation of organic components increases at higher cf.

Diversity and Abundance of Ammonia-Oxidizing Archaea and Bacteria in Qinghai Lake, Northwestern China

The abundance and diversity of amoA gene, a gene putatively encoding ammonia monooxygenase subunit A, was evaluated in oxic lake water and anoxic sediments from two adjacent sites of Qinghai Lake, China. An integrated approach was employed including geochemistry, clone library construction, and reverse-transcription quantitative polymerase chain reaction. Whereas ammonia-oxidizing archaea (AOA) were more abundant than ammonia-oxidizing bacteria (AOB) in oxic lake water, the opposite was true in anoxic sediments. The AOB community was the same in the water and sediments and consisted of Nitrosomonas- and Nitrosospira-like sequences, suggesting that the sediment-associated AOB may be derived from the water column, but metabolically inactive. In contrast, the AOA community was distinct between the water column and the sediments, and clone sequences were related to those from aquatic environments and soils, respectively, suggesting that AOA in the sediments may be native and metabolically active. Our data have important implications for a better understanding of the potential role of ammonia oxidizers in lake ecosystems.

Drivers shaping the diversity and biogeography of total and active bacterial communities in the South China Sea

To test the hypothesis that different drivers shape the diversity and biogeography of the total and active bacterial community, we examined the bacterial community composition along two transects, one from the inner Pearl River estuary to the open waters of the South China Sea (SCS) and the other from the Luzon Strait to the SCS basin, using 454 pyrosequencing of the 16S rRNA and 16S rRNA gene (V1‐3 regions) and thereby characterizing the active and total bacterial community, respectively. The diversity and biogeographic patterns differed substantially between the active and total bacterial communities. Although the composition of both the total and active bacterial community was strongly correlated with environmental factors and weakly correlated with geographic distance, the active bacterial community displayed higher environmental sensitivity than the total community and particularly a greater distance effect largely caused by the active assemblage from deep waters. The 16S rRNA vs. rDNA relationships indicated that the active bacteria were low in relative abundance in the SCS. This might be due to a high competition between active bacterial taxa as indicated by our community network models. Based on these analyses, we speculate that high competition could cause some dispersal limitation of the active bacterial community resulting in a distinct distance‐decay relationship. Altogether, our results indicated that the biogeographic distribution of bacteria in the SCS is the result of both environmental control and distance decay.

Dynamics of the carbonate system in a large continental shelf system under the influence of both a river plume and coastal upwelling

National Basic Research Program of China (973 Program) [2009CB421201]; National Science Foundation of China (NSFC) [90711005, 40821063]; NSFC-RGC [40731160624, N_HKUST623/07]; SCOPE project

Dissolved insecticides and polychlorinated biphenyls in the Pearl River Estuary and South China Sea

Persistent organic pollutants (POPs) such as organochlorine (OCl) insecticides and polychlorinated biphenyls (PCB), together with the new generation of organophosphorus (OP) insecticides, are of global concern, due to their widespread occurrence, persistence, bioaccumulation and hormone disruption potential. This paper represents an attempt to study the source and transportation of such pollutants in estuarine and coastal environments as an integrated ecosystem, by determining the levels of 18 OCl insecticides, 21 PCB congeners, and 17 OP insecticides in the Pearl River Estuary and South China Sea. The total concentrations varied from 126-1198 ng l(-1) for OCl insecticides, 33.38-1064 ng l(-1) for PCB congeners, and 4.44-6356 ng l(-1) for OP insecticides in the Pearl River Estuary. In comparison, their levels in the South China Sea were significantly lower, varying from 57.09-202 ng l(-1) for OCl insecticides, 21.72-144 ng l(-1) for PCBs, and 1.27-122 ng l(-1) for OP insecticides, respectively. The predominance of beta-HCH in HCHs, and DDE in DDTs in all water samples was clearly observed, suggesting beta-HCH and DDEs resistance to further degradation. The PCBs were dominated by those with 3-6 chlorines. The distribution characteristic of OP insecticides shows that five compounds (methamidophos, dimethoate, malathion, dichlorvos and omethoate) accounted for 56% and 72% of the total OP insecticide concentration. The relationship between pollutant concentrations and salinity in the estuary showed that they were all removed during the mixing process, therefore behaving non-conservatively.

Colloidal organic carbon and 234Th in the Gulf of Maine

Abstract The role of colloids in the biogeochemical cycling of organic carbon and 234 Th was examined in the Gulf of Maine during the summer of 1996 and 1997. The colloidal fraction was separated using a 1 kiloDalton (kDa) cross-flow ultrafiltration (CFF) membrane. The mass balance of CFF was carefully checked in order to evaluate the performance of the CFF system, since this is one of the primary concerns when using CFF methods. For bulk organic carbon (OC), the membrane showed excellent recoveries (94–107%) as long as preconditioning techniques were employed. Although our mass balances for 234 Th were higher than previous efforts (>55% for most samples), significant losses still occurred. Furthermore, a large percentage of the “lost” 234 Th was irrecoverable even after the CFF membranes were cleaned with weak acid and base rinses. The colloidal organic C (COC) represented 16–25% of the organic carbon pool, and was correlated with the Chl a fluorescence maximum. The profile of colloidal 234 Th, 7–36% of the total 234 Th, followed a similar pattern. The difference between the mass balances for 234 Th and OC suggests that 234 Th may follow only a fraction of COC within the water column. This fraction appears to be the more reactive component of the COC pool. Given the higher and more consistent 234 Th recoveries found using our CFF techniques, we were able to constrain the turnover rate of COC to between 3 and 30 days. Turnover rates of dissolved and particulate OC varied from 14 to 41 and 19 to 68 days, respectively.