近25年广西北部湾海域营养盐时空分布特征

Temporal and Spatial Distribution Characteristics of Nutrients in the Coastal Seawater of Guangxi Beibu Gulf during the Past 25 Years

营养盐含量和结构的变化可反映海域潜在的生态安全.为广西北部湾经济区实施差别化的分区可持续发展战略提供数据基础,找出广西不同海湾营养盐的时空变化差异特征及影响因素,在全面梳理1990─2014 年广西北部湾近岸海域水质监测数据的基础上,采用富营养化指数法对该水域主要海湾的湾内、湾口、湾外的富营养化状态、营养盐及无机氮组成的时空变化特征进行研究.结果表明,广西北部湾近岸海域富营养化状态总体良好,时空分布受入海径流及海域水团的影响,年际随入海径流量波动,营养盐质量浓度呈湾内-湾口-湾外递减趋势,位于沿岸水团的湾内采样点,出现中度/重/严重富营养,富营养化指数范围4.5-17.2,位于混合水团的湾口、湾外采样点,处于贫营养,富营养化指数低于0.5.溶解态无机氮与溶解态无机磷高值区的空间分布与富营养化指数基本一致,以硝态氮为主的无机氮污染比无机磷污染严重.多数点位氮磷比常年大于Redfield 比值,处于磷限制状态.钦州湾内、廉州湾内、廉州湾口和大风江口是富营养化最为严重的水域,无机氮磷质量浓度长期劣于四类海水标准限值,而呈磷限制状态,氮磷比分别高达202、132、142、224,容易在磷污染增高时,爆发富营养化,应特别加以监测与控制.

The concentrations of nutrients and their changes;can indicate potential ecological risks in seas. In order to support the implementation of a differentiated and sustainable development strategy in the Beibu Gulf Economic Zone, we studied the distributions of nutrients in Beibu Gulf, Guangxi, China, using seawater monitoring data from 1990─2014, where we focused on the temporal and spatial variations in nutrients and related factors. The eutrophication index was employed to study the eutrophication status, nutrient levels, and inorganic nitrogen composition from the inner gulf to the outer gulf. The results showed that the overall eutrophication status was good. The concentration of nutrients varied with the annual runoff and the distribution of nutrients controlled by water masses decreased from the inner gulf to the outer gulf. Thus, mesotrophic, eutrophic, and hypertrophic conditions were detected in the inner bay sites located in the coastal water mass, where the eutrophication index ranged from 4.5 to 17.2. Oligotrophic conditions were found in the outer bay sites in the mixed water mass, where the eutrophication index was<0.5. The spatial variations in the area with high dissolved inorganic nitrogen and dissolved inorganic phosphorus agreed with the area with a high eutrophication index. Nitrate dominated the dissolved inorganic nitrogen pollution, which was much higher than the dissolved inorganic phosphorus. The nitrogen to phosphorus ratios were higher than the Redfield value (16) in most years when monitoring was conducted, thereby indicating that the water was in a phosphorus-limited state. The Qinzhou inner bay, Lianzhou inner bay, the mouth of Lianzhou bay, and Dafeng River were the most eutrophic sites, where the water qualities were worse than the IV level in terms of the dissolved inorganic nitrogen, and the nitrogen to phosphorus ratios were as high as 202, 132, 142, and 224, respectively. Eutrophication control should be conducted in these sites, especially to reduce the phosphorus levels, due to the high risk of eutrophication with large amounts of nitrogen pollutants and in phosphorus-limiting conditions.