北部湾北部滨海湿地水体和表层沉积物中营养元素分布与污染评价

2009年4～5月,对北部湾北部滨海湿地水体和表层沉积物进行了取样调查,研究水体中营养盐和表层沉积物中碳、氮和磷元素含量、分布及其影响因素,并在此基础上评价了滨海湿地水体富营养化水平和表层沉积物的污染状况.研究区水体中的溶解无机氮(dissolved inorganic nitrogen,DIN)、活性磷酸盐PO43-—P和化学需氧量(CODMn)平均含量分别为(211.84±37.44) μg/L、(11.01±12.11) μg/L和(0.92±0.32) mg/L;表层沉积物中总氮(TN)、总磷(TP)和总有机碳(TOC)的平均含量分别为(373±355) μg/g、(232.28±157.34) μg/g和(0.45±0.46)％.北部湾北部滨海湿地水体中的DIN含量和PO43-—P含量在茅尾海和廉州湾东部西场海区都很高,且水体富营养化比较严重,大规模的海水养殖可能是造成该现象的主要原因.研究区水体中CODMn含量空间分布呈近岸高、远岸低的特征.研究区铁山港顶部、南流江口、大风江口、钦江和茅岭江口较细的表层沉积物中的TN、TP和TOC含量较高.研究区表层沉积物的TOC/TN值分布表明,钦江口、钦州港区、防城港、企沙港区和珍珠湾东北部的沉积物有机质含量受到了强烈的陆源影响.根据加拿大安大略省环境质量评价标准,除了钦州湾顶部的龙门港区、大风江口中部和铁山港顶部西北角一小部分区域的表层沉积物中的TOC、TN和TP含量分别略高于1％、550μg/g和600μg/g,已被明显污染,而处于最低级别或严重危害级别,会对部分底栖生物产生影响外,其他绝大部分区域都属安全级别,为清洁或部分已被轻微污染.

Nutrients Distribution and Contamination Assessment in Seawater and Surface Sediment of the Coastal Wetlands,Northern Beibu Gulf

Coastal wetlands have important ecological service functions in reducing nutrients into the sea and diminishing eutrophication of the estuaries.But the input of nitrogen and phosphorous to the costal wetlands in China has increased significantly with the economic development of coastal areas in these years.The induced seawater eutrophication and sediment contamination have caused negative impacts on fishery resources,human health and seawater utilization.The content,distributions and contamination status of nutrients in seawater and surface sediment of the coastal wetlands,northern Beibu Gulf(with water depth less than 6 m) are analyzed and assessed based on the investigation data obtained in April and May,2009.The results indicated that the surface sediment in the eastern study area,bounded by Dafeng River estuary,is sandy deposit;while salty and clayey sediment dominated in the western part.The mean contents of dissolved inorganic nitrogen(DIN,including NO 3 —N,NO 2 —N,NH 4 + —N),PO 4 3—P and COD Mn in the seawater were(211.84 ± 37.44) mg/L,(11.01±12.11) mg/L and(0.92±0.32) mg/L.The mean contents of total nitrogen(TN),total phosphorus(TP) and total organic carbon(TOC) in the surface sediments were(373±355) mg/g,(232.28±157.34) mg/g and(0.45±0.46)%,respectively.Medium to strong positive correlations exsit among the nutrients in the seawater or surface sediment,suggesting these nutrient elements originate from similar sources.High contents of DIN and PO 4 3—P distributes in the sea area of Maowei sea and eastern part of Lianzhou bay.It is speculated to be caused by the large scale aquacuture developed in these areas.However,chemical oxygen demand(COD) content showed decreasing seaward.Seawater eutrophication in the area of Maowei sea and eastern part of Lianzhou bay was evaluated serious employing the method of eutrophication index.The contents of TN,TP and TOC in the surface sediments were controled clearly by particle size,which was evidenced by the strong positive correlations between the sediment median diameter and the contents of nutrients.The high contents of nutrients were found in fine-grained sediments because of their higher absorption ability and lower porosity to prevent degradation of organic matters.Sediment TOC/TN ratios demonstrated that the organic matter is predominantly terrigenous in the surface sediment of Qin River estuary,Qinzhou harbour,Fangcheng habour,Qisha harbour and the northeasten part of Pearl bay.According to the’guidelines for the protection and management of aquatic sediment quality in Ontario’,the contents of TOC,TN and TP in the surface sediments lied in their lowest effect levels of 1%,550 μg/g and 600 μg/g for most part of the study area,meaning the sediment was clean-marginally polluted and had potential to affect fish or sediment-dwelling organisms except the tops of Qingzhou bay and Tiehangang bay along with the middle estuary of Dafeng River,where the content was slightly greater than the lowest effect levels,implying the sediment is marginally-significantly polluted and could affect sediment use by some benthic organisms.