海洋生态与环境科学重点实验室知识库

营养盐是生态系统食物网的基石，对海洋生态系统，尤其是海洋中养殖生态系统的运转起到重要的调控作用。大型藻类和贝类是海水养殖业的两大支柱型类群，我国贝藻养殖产量占海水养殖的八成以上。自藻类养殖和贝类养殖先后获得成功并在全国掀起第一和第三次海水养殖浪潮以来，我国贝藻养殖业的蓬勃发展带来了巨大的经济利益和生态效益，同时也面临着诸多问题。首先，由于贝藻养殖几乎完全依赖自然生态系统提供物质和能量，其发展逐渐受到物质和能量供给的限制，产量提升困难，养殖品质下降。其次，养殖海域环境发生异常与长期养殖带来的生态问题，使得大规模减产和死亡事件时有发生。第三，在贝藻混养模式中，贝藻互利作用的实现往往受到养殖周期和空间分布差异的限制。目前，我国的海水养殖大多分布在近海，而近海生态系统在全球变化和人类活动的多重压力下复杂且多变，处于动荡的状态，我们对近海养殖生态系统的承载力还缺乏了解。随着贝藻产量的不断增长，养殖生态系统承载力的问题逐渐凸显。所以，我们有必要从生态系统物质能量流动、生态系统服务与产出、养殖种类之间的相互作用等方面对这个系统进行系统研究与分析，为海洋生态系统健康、海洋生物资源可持续利用做出贡献。

本研究以桑沟湾养殖生态系统为研究对象，通过构建基于贝藻养殖的桑沟湾生态系统营养盐收支模型，解析了营养盐供给和养殖生物的营养盐需求状况的季节变化和长期变化；通过室内实验和野外实验相结合的营养盐加富实验，探索和分析了不同实验条件下浮游植物对营养盐添加的响应及其影响因素，以及野外施加营养盐对海带生长的促进作用、是否会产生级联效应等；另外，对研究期间发生的一次海带大规模减产事件的成因进行了系统剖析。主要结果如下：

1981~2020年桑沟湾营养盐收支年代际变化的分析结果显示，桑沟湾的营养盐水平较低，20世纪80年代以来，桑沟湾的溶解无机氮浓度先增高后下降，磷酸盐浓度呈现波动状态。随着贝藻养殖产量的增加，贝藻收获移出的氮磷通量持续增加。过去40年，桑沟湾养殖生态系统的氮磷输入与输出的差持续增加，即系统对氮磷的支持作用在增强。桑沟湾营养盐收支的季节变化的分析结果显示，桑沟湾海水的氮磷浓度季节变化明显，秋季丰富，春季较低，氮磷比夏秋季偏高。冬春季藻类收获移出氮磷通量较多，夏秋季贝类收获移出氮磷通量较多。氮磷收支不平衡在春季最为明显。从营养盐的入海来源减少和海湾水动力减弱分析，近几年，桑沟湾的营养盐供给在持续减少。所以，桑沟湾贝藻养殖需求的增加对养殖生态系统的营养盐供给和承载力的压力在增大。

贝藻养殖是桑沟湾浮游植物变动的重要驱动因素之一，自上世纪80年代以来，桑沟湾浮游植物的丰度、物种类总数和多样性指数均呈明显的下降趋势。桑沟湾的浮游植物总体上是以硅藻为主的群落，近几年在8月出现了以甲藻为主的浮游植物群落结构。通过营养盐加富实验，发现在各次实验中，添加营养盐后浮游植物颗粒物浓度均显著增加，且以硅藻为主。实验结果表明，桑沟湾营养盐水平对浮游植物有明显的限制作用，添加营养盐可提高桑沟湾的浮游植物生产力。实验中浮游植物对营养盐加富的响应的影响因素主要体现在三个方面，一是不同区域，3月湾内区浮游植物的增长显著低于湾口区和湾外区，5月的实验中湾口区的浮游植物增长显著低于其他两区。二是添加营养盐的浓度，在设置的低、中、高浓度下，各组浮游植物均出现快速生长，但高浓度组浮游植物丰度维持的时间更长。三是添加营养盐的组合和比例，10月，硅氮对浮游植物的限制最明显。

2021年12月~2022年1月和2023年1月~2月在桑沟湾海带养殖区开展了两次野外营养盐加富实验，结果表明，海带生长早期存在营养盐限制，施加营养盐可以促进海带生长，具体表现在海带的长度、宽度、叶片的垂直投影面积及生物量等方面。同时，养殖区浮游植物生物量和海水营养盐的监测结果显示，尚未发现负面的生态影响。从系统物质平衡的角度来看，海带养殖业的长期可持续发展需要增加系统的物质输入，且适当施加营养盐的生态风险较小。在2021年冬季，荣成市发生了历史上罕见的养殖海带大规模减产事件，经综合分析，造成海带白烂的主要原因是海水透明度较高，使得海带接受到的光照过强而引起的光损害。浮游植物大量繁殖消耗了大量营养盐，也对海带的生长产生了负面影响。在这种情况下，海带生长滞后，光耐受性减弱，不足以抵抗强光伤害而发生白烂，最终造成海带大面积减产甚至绝产的现象。从海带养殖业兴起至今，海带产量的不断增加和高产量的维持，对养殖生态系统的健康产生了巨大压力，探索新认知、新模式和新技术将成为实现海带养殖业可持续发展的必经之路。

The provision of nutrients serves as the fundamental basis for sustaining the food web in ecosystems and plays a crucial role in regulating marine ecosystems, especially those associated with aquaculture. Seaweed and shellfish are the two main pillars of the mariculture industry. The production of seaweed and shellfish aquaculture in China accounts for over 80% of mariculture. Since seaweed and shellfish aquaculture has been successful and triggered the first and third waves of mariculture in China, the vigorous development of seaweed and shellfish aquaculture in our country has brought huge economic and ecological benefits but also faced numerous problems. First of all, since seaweed and shellfish aquaculture almost completely rely on natural ecosystems, the development of seaweed and shellfish aquaculture is gradually limited by the supply of materials and energy, making it difficult to increase production and compromising cultivation quality. Secondly, the abnormal environment of the aquaculture sea area, along with the ecological problems caused by long-term aquaculture, leads to periodic reductions in production and occurrences of mass mortality. Thirdly, in the polyculture mode of seaweed and shellfish, the realization of the mutually beneficial interaction between seaweed and shellfish is often hindered by disparities in their cultivation cycles and spatial distributions. Currently, the majority of mariculture in China is primarily located along the coastal regions. However, coastal ecosystems are complex, volatile and in a state of turbulence due to the multiple pressures of global change and human activities. The carrying capacity of aquaculture ecosystems in coastal regions is still poorly understood. And the issue of carrying capacity in aquaculture ecosystems has become increasingly prominent due to the continuous growth in seaweed and shellfish production. Therefore, conducting systematic research and analysis on this system is imperative for us. This should encompass various aspects including the flow of material energy, ecosystem services and outputs, as well as interactions among cultured species. All these efforts are aimed at contributing to the health of marine ecosystems and promoting sustainable utilization of marine biological resources.

This study focuses on the aquaculture ecosystem in Sanggou Bay. By constructing a nutrient budget model for the system based on a seaweed and shellfish polyculture ecosystem, we analyzed the seasonal and long-term changes in nutrient supply and demand. Through a combination of laboratory experiments and field enrichment trials, we explored and analyzed the response of phytoplankton to nutrient addition and its influencing factors. Additionally, we investigated the promotion effect of nutrient-enriched on kelp growth and whether it would produce a cascade effect. The main results are as follows:

The analysis of interdecadal variations in the nutrient budget spanning from 1981 to 2020 reveals that the nutrient concentration in Sanggou Bay consistently remains at low levels. Since the 1980s, there has been an initial increase followed by a subsequent decrease in the concentration of dissolved inorganic nitrogen, as well as fluctuations in the concentration of phosphate. With the expansion of aquaculture scale and increased production, the removal of nitrogen and phosphorus fluxes through harvest has been continuously rising. In the past 40 years, there has been an increasing gap between influxes and outfluxes in Sanggou Bay, indicating an enhanced supportive effect of the system on nitrogen and phosphorus. The analysis of seasonal variations in the nutrient budget reveals that the nitrogen and phosphorus concentrations in Sanggou Bay exhibit significant seasonal fluctuations, with higher levels observed during autumn and lower levels observed during spring. The N/P ratio is higher in summer and autumn. Seaweeds contribute more to nitrogen and phosphorus removal during winter and spring, while shellfish play a larger role in nutrient removal during summer and autumn. And the nutrient balance is most unbalanced during spring. The analysis reveals a recent decline in nutrient supply to Sanggou Bay, which is attributed to the diminishing sources of nutrients and the weakening hydrodynamic power. Therefore, the increasing demand for seaweed and shellfish aquaculture in Sanggou Bay has placed growing pressure on nutrient availability and the carrying capacity of the ecosystem.

The cultivation of seaweed and shellfish is a significant driving force behind the observed changes in phytoplankton dynamics within Sanggou Bay. Since the 1980s, there has been an overall decline in phytoplankton abundance, species count, and the diversity index of phytoplankton in Sanggou Bay. The phytoplankton community in Sangou Bay is dominated by diatoms, but in recent years, a dominance shift has occurred with dinoflagellates appearing in August. Nutrient enrichment experiments revealed that nutrient addition substantially increased the concentration of phytoplankton particles in all experiments, with diatoms being predominant. It can be seen that the nutrient level in Sanggou Bay has an obvious limiting effect on phytoplankton, and the addition of nutrient can enhance the potential for phytoplankton production. The main factors that influence the response of phytoplankton to nutrient enrichment were as follows. Firstly, regional variations played a significant role. The growth of phytoplankton in the inner bay station was significantly lower than that in the bay mouth station and the outer bay station in March, while the growth of phytoplankton in the bay mouth station was significantly lower than that in the other two stations in May. The second factor was the concentration of added nutrients. Under low, medium, and high concentrations established, all groups of phytoplankton demonstrated rapid growth, but the phytoplankton abundance in the high concentration group was maintained for a longer period of time. The third factor was the combination and ratio of added nutrients. In October, the limitation of silicon-nitrogen on phytoplankton was most evident.

Two in-field nutrients enrichment experiments were carried out in the bay mouth of a kelp farm from December 2021 to January 2022 and January 2023 to February 2023, and the results showed that nutrient addition had a positive effect on early-stage kelp growth. The length, vertical projected area, and biomass of kelp in the nutrient-enriched area were significantly greater than those in the control area. There was no significant difference in phytoplankton biomass between the nutrient-enriched area and the control area after nutrient addition, and no negative effects were found. From the perspective of system material balance, it is imperative to enhance the input of materials in order to ensure the sustainability of kelp aquaculture while minimizing ecological risks through moderate nutrient application. In the winter of 2021, there was a rare large-scale decrease in kelp production in Rongcheng City. The comprehensive analysis showed that the primary cause of kelp bleaching was the high transparency of seawater, which resulted in excessive light exposure for the kelp. And the proliferation of phytoplankton consumed a lot of nutrients, which also negatively impacted the growth of kelp. In this case, kelp growth lagged behind, with weak light tolerance that was not enough to resist strong light damage, eventually resulting in a reduction in production or even extinction phenomenon. Since the rise of kelp aquaculture, the continuous increase in kelp production and the maintenance of high production have exerted great pressure on the health of the aquaculture ecosystem. Exploring new perceptions, models, and technologies will be the only way to achieve sustainable development in kelp aquaculture.