筏式养殖牡蛎的生物沉积对底质富集Cu、Zn的影响

China is the largest mariculture production country in the world, and bivalve production represents the largest contribution to mariculture production in China. The previous studies have demonstrated the enrichment of heavy metal in the sediment collected from bivalve mariculture zone, but it is difficult to explain this phenomenon on the perspective of exogenous source. Therefore, we speculate that this may be due to the biodepositon process of cultured bivalve. Filter-feeding bivalves, such as oysters and mussels, have strong water-filtration ability. They are able to filter a large number of fine particulate matters and phytoplanktons, and produce large organic particles as the form of feces and pseudofeces through the digestion process. Those suspended solids usually load with heavy metals due to its enrichment of metal-binding organic ligands, and transport to the sediment by biodeposition. This process finally leads to heavy metal enrichment in the sediment of the mariculture zone. This project intends to 1) estimate the fluxes of Cu and Zn biodeposition by cultured oysters; 2) reveal the accumulation of Cu and Zn in sediment due to the biodeposition by analysis of the spatial and temporal distribution of Cu and Zn in aquaculture water, suspended particles, and sediment combined with related environmental parameters; 3) evaluate the contribution of Cu and Zn biodeposition to the accumulation of Cu and Zn in sediment according to stable isotope tracer technology through setting up the synchronous field control culture experiment in the typical oyster raft mariculture zone of Xiangshan Bay. The results of this project will help to understand the biogeochemical cycling of heavy metals in the bivalve mariculture environment, and provide a scientific basis for comprehensive assessment of the impact of mariculture on the environment.

我国是海水养殖大国，贝类养殖在我国海水养殖中占有重要地位。已有研究显示，贝类养殖区沉积物中重金属富集程度较高，而从外源污染角度很难解释这个现象。我们推测这与养殖贝类的生物沉积有关。贝类的滤水能力很强，可大量摄食水体中的细小颗粒物、浮游植物等物质，将其消化处理后以粪粒和“假粪”的形式排出体外。这些可沉降的有机颗粒物会吸附水体中的重金属，并通过生物沉积将其输送至底层并固定下来，从而在底质出现重金属富集现象。本项目拟在象山港牡蛎筏式养殖海区，通过设立同步对照实验，现场测定牡蛎对Cu、Zn的生物沉积通量；分析Cu、Zn在养殖水体和沉积物中的时空分布特征，结合环境参数，揭示生物沉积对底质富集Cu、Zn的影响规律；并借助稳定同位素示踪技术，量化牡蛎的生物沉积对底质富集Cu、Zn的贡献。研究结果将有助于深入认识重金属在贝类养殖区环境中的生物地球化学过程，为全面评估海水养殖对海区环境的影响提供科学依据。

我国是海水养殖大国，贝类养殖在我国海水养殖中占有重要地位，其在为人类提供优质食物资源的同时，还会对养殖海区环境造成影响。贝类养殖区沉积物中重金属富集程度较高，而从外源污染角度很难解释这个现象，我们推测这与养殖贝类的生物沉积有关。为此，本项目在象山港牡蛎筏式养殖区设立同步对照实验，测定养殖牡蛎的生物沉积速率及其季节性变化，分析养殖水体和沉积物中14种常见重金属的时空分布特征，量化生物沉积对底质富集重金属的贡献，以揭示贝类的生物沉积对底质富集重金属的影响规律。研究结果显示，象山港筏式养殖牡蛎的生物沉积速率在夏季和秋季较高，春季最低，这主要受养殖牡蛎个体大小、水温和盐度的共同影响；牡蛎的生物沉积降低了水体中悬浮颗粒物的含量，综合水体、生物沉积物、可沉降颗粒物和表层沉积物中重金属的研究结果可以发现，牡蛎大量滤食水体中的细小颗粒物，自身同化一部分后，剩余部分成为更大颗粒的生物沉积物向下沉降，并在沉降过程中不断吸附水体中的重金属，这个过程将上层水体中重金属向底层输送，从而导致养殖区底层沉积物中重金属富集。