Utilization of terrestrial organic matter by the bivalve <Emphasis Type="Italic">Corbicula japonica</Emphasis> estimated from stable isotope analysis

ABSTRACT:   Carbon and nitrogen isotopic ratios in tissue of the bivalve corbicula ( Corbicula japonica ) and particulate organic matter (POM) were measured along a salinity gradient in the Kushida Estuary, Japan. The bivalve exhibited a gradual isotopic enrichment from the uppermost estuarine site (δ¹³C = −24.8‰ and δ¹⁵N = 8.6‰) to the marine site (δ¹³C = −16.1‰ and δ¹⁵N = 11.8‰). Using the concentration-weighted mixing model, the bivalves' food source is estimated from the isotope values for the bivalves and POM from terrestrial plants, marine phytoplankton and benthic microalgae. The results indicated that the contributions of benthic micro algae and phytoplankton were small, while terrestrial particulate matter is significantly important for the corbicula diet, although the contribution varies among sampling sites.     

Suspended culture of the sea cucumber Apostichopus japonicus below a Pacific oyster raft – potential for integrated multi‐trophic aquaculture

To discuss the possibility of co‐culturing Pacific oyster with the sea cucumber Apostichopus japonicus, a field experiment was conducted in an oyster farm. Apostichopus japonicus juveniles (mean wet weight, 0.08 g) were cultured below an oyster raft and at a control station for 216 days, and the wet weight and stable isotope ratios (δ¹³C, δ¹⁵N) were analysed together with settling organic matter (OM) collected using sediment traps. All sea cucumbers cultured below the raft survived (survival rate, 100%), while at the control station one individual disappeared (96%). During 216 days, the juveniles at the oyster and control stations grew to a mean weight of 5.5 and 2.6 g, attaining respective specific growth rates of 2.0% and 1.6% (paired t‐test, P < 0.001). Settlement rates of carbon and nitrogen at the oyster station were ~5 times larger than those at the control station. The stable isotope analysis showed that settling OM at both stations originated from coastal phytoplankton and that phytoplankton represented the primary food source for A. japonicus. The rapid growth of A. japonicus at the oyster station was concluded to be due to the abundant supply of oyster biodeposits, which could be ingested by this species.     

Seasonal and spatial variations of the food web structure in a shallow eutrophic lake assessed by stable isotope analysis

Stable isotope analysis was used to investigate seasonal and spatial variations of the food web structure in a large eutrophic lake ecosystem (Lake Taihu, China). Basal food sources, invertebrates and fish were sampled in two lake regions with different environmental conditions and spatial variations in the isotopic composition of lake food webs were found. Overall, more depleted δ¹³C and enriched δ¹⁵N isotope values of organic matter sources and consumers were found in the phytoplankton-dominated lake region than in the macrophyte-dominated region. Wide seasonal variations in the isotopic ratios were also observed in the lake biota, with a general trend towards enriched δ¹³C and δ¹⁵N values in summer and depleted values in winter. This pattern could be explained by a combination of environmental (e.g., irradiance and nutrient inputs) and biotic (e.g., availability of food sources and plasticity in prey item choice) features. Results of isotope mass balance suggest that macrophytes provide some trophic support in the macrophyte-dominated area, but in both lake regions it is more likely that aquatic food webs are phytoplankton based rather than macrophyte based under eutrophic conditions.     

A non‐classical biomanipulation experiment in Gonghu Bay of Lake Taihu: control of Microcystis blooms using silver and bighead carp

A non‐classical biomanipulation experiment was carried out in Gonghu Bay of Lake Taihu in 2009. Silver and bighead carp were stocked in a large fish enclosure to control cyanobacterial blooms. Water quality, plankton abundance, and the intracellular and extracellular microcystins (MCs) in lake water were investigated monthly in 2009. The concentrations of nitrogen nutrients were significantly lower in the fish enclosure than in the surrounding lake, while phosphorus (especially total phosphorus) concentration was higher in fish enclosure. During the blooming period, Cyanophyta contributed to more than 90% of the phytoplankton in the surrounding lake, whereas it represented only 40–80% in the fish enclosure. The phytoplankton and crustacean zooplankton biomasses and the zooplankton/phytoplankton ratios were all significantly lower in the fish enclosure than in the lake. This result suggested that silver and bighead carp can effectively suppress the phytoplankton biomass with the initial stocking density of 7.5 g m^?3 for silver carp and 1.1 g m^?3 for bighead carp, despite a simultaneous decrease in the grazing pressure of the zooplankton on the phytoplankton. During the blooming period, the intracellular and extracellular MCs in the fish enclosure were reduced by 93.8% and 69.8% compared with the surrounding lake. MCs content varied from 0.34 to 18.8 ng (mean 4.8 ng) MC‐LReqg^?1 wet weight in the muscle sample of silver and bighead carp in the experimental enclosure, which suggested that these fish were safe to consume for human. However, the long‐term effects of MCs on aquatic ecosystem and on public health cannot be overlooked.     

Population structure and reproduction of the African bonytongue Heterotis niloticus in the Sô River-floodplain system (West Africa): implications for management

Abstract –  Population structure of the African bonytongue, Heterotis niloticus , was examined in southern Benin within Lake Hlan and a region of the Sô River floodplain located approximately 60 km downstream from the lake. Both locations support important fisheries in which Heterotis is the principal target species during the flood period. Ripe adults comprised over 40% of the population in Lake Hlan, whereas only 3.5% of individuals captured from river sites were adults. Monthly averages for the gonadosomatic index and percentages of individuals with mature gonads peaked as water levels increased during the wet season then declined during the peak flood period. Oocyte size frequency distributions from ovaries suggested a potential to produce an additional cohort in the event of nesting disruption. During the peak spawning period (May to August), between 37 and 51 active nests per hectare per month were observed in Lake Hlan. The number of larvae per nest ranged from 3953 to 6125. Lake Hlan bonytongues appear to constitute an important source subpopulation that exports new recruits to river/floodplain areas downstream where intense fisheries harvest mostly juveniles and subadults. Consequently, restriction of harvest of adult bonytongues in Lake Hlan may be essential for sustenance of commercial fishing in downstream reaches of the Sô River.     

滆湖北部底泥疏浚的生态效应研究

为研究滆湖北部湖区生态系统对底泥生态疏浚的动态响应,2012年10月~2013年9月在滆湖北部湖区采集沉积物和生物样品,分析底泥疏浚对底泥营养盐含量、浮游植物、底栖动物和大型水生植物群落结构的影响。结果表明,滆湖北部疏浚区的总氮、总磷和有机质营养盐含量低于未疏浚区,说明底泥疏浚是削减沉积物内源负荷的有效手段。与未疏浚区相比,疏浚区浮游植物的密度、生物量有所减少,且群落中蓝藻所占的比例下降,浮游植物多样性指数上升,一定程度上减少了滆湖北部发生蓝藻水华的风险。疏浚区底栖动物中的寡毛类的数量和生物量与未疏浚区基本相等,而摇蚊类和软体动物的密度和生物量则下降了80%以上。底泥疏浚区大型水生植物的种类、覆盖度和生物量与未疏浚区相当。     

澜沧江源区浮游植物群落特征及其对水质的指示作用

为了探究澜沧江源区水生态现状与变化趋势,在干流囊谦段布置多昌、扎曲大桥及香达3个监测点,支流选择子曲的下拉秀、尕麻和野吉尼玛3个监测点,共计6个监测点位,于2016年7月(夏季)和10月(秋季)对澜沧江源区干流及其支流浮游植物进行了调查,以期为三江源区水生态保护与管理提供科学依据。结果表明:(1)记录浮游植物7门、47种,其中硅藻(21种)和绿藻(14种)为主要成分,分别占总种类数的45%和30%;时空差异明显,夏季较秋季种类丰富,支流较干流种类丰富;优势种以硅藻为主,其中针杆藻(Synedra sp.)为两季度绝对优势种;(2)澜沧江源区浮游植物密度为5.4×10~4～230×10~4个/L,平均密度为79×10~4个/L,硅藻贡献最大;时空差异明显,秋季高于夏季,支流高于干流;(3)浮游植物Shannon-Wiener多样性指数(H)夏季为2.32,秋季为2.72,Pielou均匀度指数(J)夏季为0.73,秋季为0.70。研究显示,澜沧江源区水质介于极贫营养至中营养之间,处于轻度污染至无污染状态,水生态状况整体优良。     

嘉兴南湖水系浮游植物群落结构及其环境影响因子

为进一步了解嘉兴南湖的水生态特征,探究环境因子对浮游植物分布的影响规律,于2018年5月在南湖主干河流、主要出入湖口及南湖分别设置7个、6个和3个采样点,对浮游植物群落结构及分布进行调查。结果显示,南湖及其水系共发现浮游植物64种,隶属6门、47属,优势种有6种,分别是绿藻门游丝藻(Planctonema lauterbornii)、小球藻(Chlorella)和衣藻(Chlamydomonas)、硅藻门小环藻(Cyclotella)、隐藻门啮蚀隐藻(Cryptophyceae)和具尾蓝隐藻(Chrcomonas caudata),优势度分别为0.290、0.039、0.020、0.199、0.065和0.049。从空间分布来看,浮游植物平均密度为1021×10⁴个/L,平均生物量为7.43 mg/L,其中南湖及主要出入湖口浮游植物种类(20种、25种)、密度(1958×10⁴个/L、1303×10⁴个/L)及生物量(14.95 mg/L、7.66 mg/L)均高于南湖7条主干河流(16种、378×10⁴个/L、4.01 mg/L),区域分布差异明显。各采样点浮游植物Shannon-Weiner多样性指数(H′)为1.59~2.70,平均值为2.18;Margalef丰富度指数(D)为0.77~1.53,平均值为1.20;Pielou均匀度指数(J′)为0.57~0.92,平均值为0.74;南湖各出入湖口多样性指数和丰富度指数较高,7条主干河流显示出较高的均匀度,而南湖水体多样性指数和均匀度指数均较低。营养状态评价发现,南湖水系大部分水体处于中-富营养水平,无贫营养水体。物种与环境典型对应分析表明,南湖水系浮游植物分布受电导率(EC)、pH、溶解氧(DO)、总溶解性固体(TDS)、浊度(TUB)、水温(WT)等水体基本理化指标以及总氮(TN)、总磷(TP)、亚硝态氮(NO 2-N)等水质指标双重作用影响,这些指标是决定南湖水系浮游植物分布的主要影响因子。     

Stable isotopes and fatty acids as dietary tracers of intertidal bivalves

We evaluated the potential food sources of six commercially important bivalve species (Mactra veneriformis, Mactra chinensis, Ruditapes philippinarum, Cyclina sinensis, Dosinia laminate, and Sinonovacula constricta) that coexist in intertidal areas of Zhuanghe Bay, northern China. The δ¹³C values of bivalves (?19.9 to ?19.0 ‰) were between those of particulate organic matter (POM) (?22.5 ‰) and sedimentary organic matter (SOM) (?14.9 ‰). Based on the isotope two-source mixing model, the relative contributions of POM and SOM to the dietary regime of intertidal bivalves were 71.6 and 29.4 %, respectively. This result is consistent with the traditional view that POM is the principal food source of bivalves. The lack of significant differences in δ¹³C values among species suggests that they had the same primary food source. High proportions of dinoflagellate fatty acid markers in all species supported the premise that POM was their primary food source. Levels of bacterial and terrestrial organic matter (TOM) fatty acid markers were also high in all species, indicating that bacteria and TOM can be important supplemental food sources for intertidal bivalves.     

Shell growth and age determination of the brackish water bivalve Corbicula japonica in Lake Shinji, Japan

ABSTRACT:   Shell growth and age determination of Corbicula japonica were investigated from samples collected monthly at two sites and field growth experiments from November 1999 to November 2000 in Lake Shinji. Both specimens did not grow in winter when the water temperature was below 11°C, but grew rapidly from spring to early summer when the water temperature rose from 15°C to 30°C, before further growing slowly from summer to autumn after the main breeding period. The monthly marginal growth distribution of the samples indicated that the concentric groove on the shell surface was an annual growth ring. It is concluded that the age of C .  japonica in the i -ring group when the shell growth resumed in spring was i  + 0.5 ( i  = 1, 2, · · ·), because the first true growth ring was formed at 0–2 mm in shell length, which then faded with growth. Lee's phenomenon was observed in the age determination results, and thus the growth coefficient and the asymptotic shell length of the von Bertalanffy growth equation were not estimated from the samples. However, these parameters were estimated from the annual shell length increments of the experimental individuals, which were 0.331/year and 28.2 mm, respectively.