In this study, we conducted a 14-month investigation in Daya Bay, southern China to understand the effects of oyster farming on phytoplankton community and biomass by using size-fractionated phytopigments. Results proved the filtering effects of oysters on phytoplankton biomass. During the oyster culture period, the average concentration of total chlorophyll a (sum of size-fractionated Chl a) within the farming area was approximately 60% lower than that at the reference site. Phytoplankton depletion in the aquaculture zone mainly occurred in micro-sized fractions (>20 μm) of Chl a, fucoxanthin, and peridinin. The influence of oyster filtration on nano-sized (2.7–20 μm) pigments seemed less than that on micro-sized ones. The depletion of peridinin and 19′-hex-fucoxanthin in aquaculture zone was higher than those of the other pigments, which indicated that flagellated cells might be selectively filtered by oysters and could be more easily influenced by oyster aquaculture. The pico-sized Chl a (<2.7 μm) comprised 24% of total Chl a on the average in the aquaculture zone during the cultural period compared to 6% in the reference site. Picoeukaryote abundance, which was determined via flow cytometry, was significantly higher in the aquaculture zone than in the non-aquaculture areas. The abundance of picoeukaryote is significantly and positively correlated with the concentrations of pico-sized prasinoxanthin, violaxanthin, and neoxanthin, indicating that picoeukaryote is dominated by those in prasinophyte. The results suggest that oyster aquaculture might stimulate the growth of prasinophyte, although the seasonal variations are mainly controlled by the water temperature in the study area. This research highlights the successful use of size-fractionated phytopigments to estimate size-specific phytoplankton biomass and community, which can be applied as a routine method to monitor the environmental effect and food resources of bivalve aquaculture.

Vibrio parahaemolyticus is the most prevalent gastroenteritis-causing pathogen in Korea and in some other Asian countries. It is frequently found in oysters and other seafood. This study monitored changes in the prevalence of V. parahaemolyticus and environmental parameters in oyster aquaculture environments in Korea. From June to October 2014, we tested oysters (Crassostrea gigas) from shellfish-harvesting areas off the west coast of Korea. These 71 isolates were the sum of 16 (22.5 %), 19 (26.8 %), 23 (32.4 %), and 13 (18.3 %) isolates collected in July, August, September, and October, respectively. These 71 isolates had the following profiles of resistance against 16 antibiotics: all isolates were resistant to ampicillin and vancomycin, and 52.2, 50.7, and 50.7 % of isolates exhibited resistance to cephalothin, rifampin, and streptomycin, respectively. PCR analysis for the presence of the species-specific toxR gene confirmed that 38 (53.5 %) of the total 71 isolated strains were positive for V. parahaemolyticus. In PCR analysis for virulence of V. parahaemolyticus, of the 71 isolates tested in the present study, only 38 (53.5 %) were positive for the trh virulence gene and 71 (100 %) was negative for the tdh virulence gene.