Short-term temporal and spatial variations in physical, optical and biogeochemical parameters in June of 2013 in Alfacs Bay (NW Mediterranean)@en

The detection and prediction of changes in coastal ecosystems require a better understanding of the complex physical, chemical and biological interactions, which involves that observations should be performed continuously. For this reason, there is an increasing demand for small, simple and cost-effective in situ sensors to analyze complex coastal waters at a broad range of scales. In this context, this study seeks to explore the potential of beam attenuation spectra, c(lambda), measured in situ with an advanced-technology optical transmissometer, for assessing temporal and spatial patterns in the complex estuarine waters of Alfacs Bay (NW Mediterranean) as a test site. In particular, the information contained in the spectral beam attenuation coefficient was assessed and linked with different biogeochemical variables. The attenuation at lambda = 710 nm was used as a proxy for particle concentration, TSM, whereas a novel parameter was adopted as an optical indicator for chlorophyll a (Chl-a) concentration, based on the local maximum of c(lambda) observed at the long-wavelength side of the red band Chl-a absorption peak. In addition, since coloured dissolved organic matter (CDOM) has an important influence on the beam attenuation spectral shape and complementary measurements of particle size distribution were available, the beam attenuation spectral slope was used to analyze the CDOM content. Results were successfully compared with optical and biogeochemical variables from laboratory analysis of collocated water samples, and statistically significant correlations were found between the attenuation proxies and the biogeochemical variables TSM, Chl-a and CDOM. This outcome depicted the potential of high-frequency beam attenuation measurements as a simple, continuous and cost-effective approach for rapid detection of changes and patterns in biogeochemical properties in complex coastal environments.海岸生态系统变化的监测与预测，需更深入理解其复杂的物理、化学与生物相互作用，这要求开展连续观测。为此，学界对小型、简易且经济高效的原位（in situ）传感器的需求日益增长，以期在广泛尺度下分析复杂近岸水域。在此背景下，本研究旨在探索利用先进技术光学透射仪（optical transmissometer）原位测得的光束衰减光谱c(λ)的潜力，以地中海西北部阿尔法克斯湾这一试验场地的复杂河口水域为研究对象，分析其时空分布模式。研究重点评估了光谱光束衰减系数所蕴含的信息，并将其与不同生物地球化学变量建立关联：其中，波长710 nm处的衰减值被用作颗粒物浓度——总悬浮颗粒物（Total Suspended Matter，TSM）的替代指标；同时基于红光波段叶绿素a（Chl-a）吸收峰长波侧观测到的c(λ)局部最大值，提出了一种新的光学指标用于表征叶绿素a浓度。此外，鉴于有色溶解有机物（CDOM）对光束衰减光谱形态具有重要影响，且研究中已获取粒径分布的补充测量数据，本研究采用光束衰减光谱斜率来分析CDOM含量。研究结果与同步采集水样的实验室分析所得光学及生物地球化学变量进行了对比，结果吻合良好；且各衰减替代指标与TSM、Chl-a及CDOM等生物地球化学变量之间均存在统计学显著的相关性。该结果表明，高频光束衰减测量作为一种简易、连续且经济高效的手段，有望快速识别复杂近岸环境中生物地球化学特性的变化与分布模式。