2016年夏季南海海盆水体颗粒物粒径分布特征*

颗粒物粒径分布(Particle Size Distribution, PSD)代表了颗粒物浓度与颗粒物粒径之间的关系, 影响着海洋生态环境和水体光学特性等。文章基于2016年夏季航次调查的生物光学剖面数据, 研究了南海海盆海域PSD的分布特征。研究发现, 幂律函数可以较好地拟合南海海盆区域的PSD, 对数空间中的实测的PSD与模拟的PSD平均决定系数高达0.95。PSD斜率(ξ)的分布范围为[1.27, 7.65], 均值为3.93±0.56。南海海盆区域表层水体的ξ均值与全球大洋表层水体的ξ均值相近, 但高于海湾等表层水体的ξ均值。ξ能较好地表征颗粒物平均粒径D_A的大小, 两者存在明显负相关关系, 即ξ值越高, D_A越小; 反之, D_A越大。通过分析T1断面的生物光学剖面数据及总体平均的PSD剖面数据, 发现PSD剖面分布特征如下: 1)表层水体的ξ值相对较高, 且D_A值相对较低, 推测可能是由于微微型藻类为主导颗粒物所致; 2) ξ值极小值层出现在次表层叶绿素浓度极大值层(Subsurface Chlorophyll Maximum Layer, SCML)中, 并伴随D_A极大值层的出现, 其原因可能是SCML中的大粒径浮游植物占比显著增加; 3)弱光层中的ξ值较SCML中的高, 但略低于表层的ξ值, 而D_A则位于表层与SCML的D_A之间, 这可能与浮游植物及其碎屑的絮凝、分解、沉降等过程相关。PSD特征影响着海水的固有光学特性, 分析发现: 由于SCML中的叶绿素浓度增加, 颗粒物散射系数(b_p(532))和颗粒物后向散射系数(b_bp(532))也相应呈现显著增加的趋势。弱光层中的平均b_p(532)与平均b_bp(532)最小。ξ与颗粒物衰减光谱斜率之间呈高分散性, Boss 等(2001b)的模型适合用于粗略估算区域性的ξ分布范围及均值。

Characterization of particle size distribution in the South China Sea basin during summer 2016

Particle size distribution (PSD) describes the relationship between particle concentration and particle size, influencing marine ecosystem environment, the optical properties of sea water, and so on. Based on the in-situ profiles of biological and optical properties during summer 2016 in the South China Sea basin, characterization of PSD was studied. The power-law model was fit to describe the PSD, and the results indicated that the mean coefficient of determination between in-situ PSD and simulated PSD could reach 0.95 in the logarithmic space. The PSD slope (ξ) ranged in [1.27, 7.65] with a mean of 3.93±0.56. The mean of ξ in the surface water of the South China Sea basin was similar to the mean of ξ in global ocean surface water, but higher than that in the surface water of other areas such as the bay. There was a strong negative relationship between ξ and the mean diameter (D_A). Taking section T1 as an example, we analyzed the mean PSD profile of these stations. The features of PSD profiles are as follows. 1) At the surface, there were high values of ξ with low values of D_A because the dominate particle was pico-phytoplankton. 2) The minimum value of ξ appeared in the subsurface chlorophyll maximum layer (SCML) with higher D_A, which may result from the higher proportion of big phytoplankton. 3) In the twilight layer, the values of ξ as well as the values of D_A were between those at the surface and in the SCML. This phenomenon may be related to the process of flocculation, decomposition, and settlement of phytoplankton. The traits of PSD would influence the inherent optical properties (IOP) of seawater. We found that both particulate beam scattering coefficient at 532 nm (b_p(532)) and particulate beam backscattering coefficient at 532 nm (b_bp(532)) would be higher in the SCML because of the increasing chlorophyll-a concentration. However, the lowest mean b_p(532) and the lowest mean b_bp(532) were found in the twilight layer. Furthermore, although the relationship between ξ and the particulate beam attenuation sepctral slope was weak, the model of Boss et al. (2001b) is suitable for estimating the regional range and mean value of ξ roughly.