|Place of Conferral||中国科学院海洋研究所|
|Keyword||獐子岛海域 虾夷扇贝 底播养殖 营养盐限制 浮游植物群落|
贝类养殖的下行控制作用会增加无机氮和磷酸盐的消耗，并在外源输入较少的春季，加剧营养盐尤其是硅酸盐限制。根据2009年至2010年的周年调查结果，研究海域在6–10月份水体出现强烈的垂直分层现象，而在冬季温跃层消失，水体混合均匀；盐度的明显降低发生在6–8月份的上层水体中。得益于淡水输入以及温跃层消失过程中的补充作用，水柱平均营养盐浓度在7–12月呈现增加趋势。2010年2月至3月期间，伴随着叶绿素浓度的增加，营养盐浓度骤降，并在整个春季出现营养盐的净消耗。3月养殖海区的硅酸盐浓度在所有站位均降至浮游植物吸收最小阈值2 µM以下，而在非养殖海区硅酸盐限制最早出现于4月份。养殖海区的无机氮和磷酸盐浓度在全年均低于非养殖海区（P < 0.01）。上述研究结果表明，贝类养殖可以通过下行控制作用降低营养盐的浓度，这种作用在没有外源输入补充时尤为明显，具体表现为导致了硅酸盐限制，进而引起浮游植物群落结构的改变。
伴随着春季硅酸盐限制，浮游植物群落优势种由硅藻向甲藻转变。研究结果表明，獐子岛春季无机氮浓度一直处于较高水平，3月平均值高达5.74 µM，且随时间变化较小。而磷酸盐和硅酸盐浓度低于浮游植物最低吸收阈值存在于整个春季以及各层水体样品中（磷酸盐浓度除了五月底层水体外）。3月，硅酸盐限制发生频率高达71.2%，水柱平均硅酸盐浓度低至1.7 µM。3月至5月，上层水体磷酸盐浓度由0.12 µM降至0.05 µM，4月和5月77.3-90%的站位出现了磷酸盐限制。相应地，浮游植物丰度在春季急剧下降，从3月的7.16×104降至5月的1.70×104 cells L-1；浮游植物粒级组成的占比优势由小型浮游植物逐渐转变为微型浮游植物，群落优势种由硅藻向甲藻转变。从浮游植物种类鉴定结果可知，甲藻优势度和优势种群增加，而硅藻优势种群由3月的具槽帕拉藻变为5月的格氏圆筛藻。结果表明，硅甲藻优势地位的转变是由硅酸盐限制引起的，而磷酸盐缺乏进一步促进了优势种群由硅藻向甲藻转变。浮游植物群落结构的改变是由营养盐的上行控制决定的，而不是贝类选择性滤食（下行控制）的结果。
As mariculture expands offshore in response to the increasing demand for seafood, a new set of ecological concerns arises. While bivalve farming is well recognized modifying biogeochemical cycle in water column through filter-feeding and biodeposition, its impacts on nutrient concentrations in various ecosystems may vary from depletion to addition. Located in northern Yellow Sea, waters around the Zhangzi Island (50 km offshore) is a typical offshore shellfish farming area in China, where bottom-seeding aquaculture of Japanese scallops Patinopecten yessoensis has been performed since 1998. As natural variability in this area has been well documented, it was selected as an ideal place to investigate the ecological consequences of shellfish farming.
Annual variations of nutrients, Chlorophyll-a (Chl-a) and size-fractionated Chl-a concentrations were investigated from July 2009 to June 2010, and compared between mariculture area and open waters, in order to distinguish the effects of scallop farming from influences of natural variabilities. Furthermore, in order to figure out the causes of silicate limitation in spring, the temporal and spatial distribution of nutrients, biogenic silica, Chl-a concentrations as well as physical conditions were investigated in this area from March to May in 2014. We analyzed the occurrence frequency of nutrients limitation and the way in which it regulates the phytoplankton community structure. Additionally, a new method of Danish Unisense microelectrode was applied in our study to determine the physicochemical properties of sediments in the Laizhou Bay, the Muping area, the Jiaozhou Bay and the Zhangzi Island area, so as to explore the correlation between biodeposition and shellfish farming in water columns.
According to the annual survey in the Zhangzi Island area, strong vertical stratification was observed from June to October and disappeared in winter with vertical homogeneity. Significant decrease of salinity was observed in the upper layers from June to August. Nutrient concentrations in monthly average showed similar trends in mariculture area and open waters, increasing continuously from July to December. This can be attributed to the coefficient supplement by freshwater discharge of the Yalu River and the collapse of the YSCWM. From February to March, nutrient concentrations decreased dramatically and net consumption occurred overwhelmingly in spring. Correspondingly, increase in Chl-a concentration was recorded in March. Silicate concentration lower than the minimum threshold for phytoplankton growth occurred in March in all stations in mariculture area, while in open waters silicate limitation was recorded firstly in April in upper layers. Dissolved inorganic nitrogen (DIN) and phosphate concentrations were significantly lower in mariculture area compared to those in open waters all through the year (P < 0.01). Silicate concentration, however, was higher in mariculture area in summertime (July to September) and lower during November-June (P < 0.05). According to our results, shellfish farming can work as nutrient sink through top-down control on nutrient concentration and structure. Nutrients removal was extremely significant in spring when exogenous supplement is scarce, leading to silicate limitation and shift in size-fractionized phytoplankton community structure.
Along with silicate limitation, dominance shift from diatoms to dinoflagellates was recorded in phytoplankton community. The monthly averaged DIN concentration was comparatively high, with the maximum of 5.74 µM in March and slight change during the sampling period. Phosphate and silicate deficiencies were recorded in spring in all layers (except the bottom layer in May for phosphate). Silicate limitation presented at up to 71.2% of stations, with the average concentration as low as 1.7 µM in March. Meanwhile, phosphate concentration decreased from 0.12 to 0.05 µM in the upper layers from March to May. Stoichiometric ratios and absolute concentrations indicate that 77.3-90% of stations showed phosphate limitation in April and May. Accordingly, phytoplankton abundance decreased sharply in spring, from 7.16×104 cells L-1 in March to 1.70×104 in May. The dominant species in phytoplankton community changed from diatoms to dinoflagellates. On species level, both increased dominance of dinoflagellate and shift in dominant diatom species were observed. The dominant diatom species changed from Paralia sulcate in March to Coscinodiscus granii in May. It suggested that diatom/dinoflagellate shift in dominance was triggered by silicate limitation and further promoted by phosphate deficiency. The dominance shift was proposed to be determined by bottom-up control of nutrient concentrations rather than selective feeding of scallop (top-down).
Specifically, the silicate limitation in spring was attributed to net consumption of photosynthesis and defficiency in local re-mineralization. During continuous observation in spring, silicate concentration in water column was significantly higher in nighttime than daytime, whereas that of DIN and phosphate showed no diel difference. Comparing to dissolved silicate, biogenic silica concentration was lowest in April (one month later), and bottom enrichment was not evident. The sediment type in the Zhangzi Island area is silty sand, and the organic matter content was slightly higher than that in the Laizhou Bay, where the sediment type is a mix of silty sand and sandy silt. Compared with the Jiaozhou Bay, another bottom seeding shellfish farming area, the probability of hypoxic conditions is lower in the Zhangzi Island area, but that of nutrient limitation is higher.
It is thus concluded that, even in the same shellfish farming area, the observed ecological consequences may vary with seasons. In summer and autumn, the farmed shellfish population helped in removal of extra DIN and phosphate, without any significant negative impacts on nutrient structure and phytoplankton community, whereas in spring it resulted in silicate limitation and in turn diatom/dinoflagellate shifted. On future perspective, presence of silicate limitation can be used as an index of over cultivation in shellfish farming for both seafood production and eutrophication mitigation. On the other hand, the “bottle-neck” effect of nutrient limitation on food availability in spring suggests that carrying capacity might be originally overestimated, when calculated from averaged annual primary production.
|MOST Discipline Catalogue||理学::海洋科学|
|Table of Contents|
|梁艺. 獐子岛海域贝类养殖与营养盐限制的关系研究[D]. 中国科学院海洋研究所. 中国科学院大学,2019.|
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