Map of Chlorophyll a concentrations trend between 1998 and 2019 in the Gulf of Biscay

For the 21 years of the study, an examination of trends in chlorophyll concentration revealed a general decline throughout the Gulf over the production period. These trends, extracted from dynamic linear model, also allowed this decline to be quantified. Expressed as a percentage, a large part of the area below the 50 m bathymetric line showed a decrease of at least 10% over the period, corresponding to a value of at least 0.1 µg.l-1. However, the spatial distribution reveals some more local phenomena. In southern Brittany, from Quimper to Vannes, a particular feature appears, with an upward trend over several kilometres along the coast, followed by a pronounced gradient along the coast. This gradient includes a zone where a continuous monotonic increasing trend is observed, then a zone where the trend becomes not significant and finally, about 15 km from the coast, a new zone where a significant continuous monotonic decreasing trend is observed. The increase in chlorophyll a concentration in the very coastal part is greater than 0.1 µg.l-1 over the period. Another peculiarity concerns the central part, located at the edge of the plateau at Cap Ferrat and Pente Aquitaine, where an increase in chlorophyll a was observed, but the variations remained small, being less than 0.1 µg.l-1. About a hundred kilometres south-west of Saint Nazaire, an area of about 40 by 50 km shows a decrease in chlorophyll a of more than 20%, quantified as more than 0.1 µg.l-1 over the period.

本研究为期21年，对叶绿素浓度变化趋势的分析显示，在整个研究周期内，该海湾全域叶绿素浓度整体呈下降态势。从动态线性模型（dynamic linear model）中提取的这些趋势数据，同时实现了该下降趋势的量化分析。以百分比计，水深低于50米的区域中，大部分在该周期内叶绿素浓度降幅至少达10%，对应浓度变化量至少为0.1 µg·l⁻¹。但空间分布特征显示存在若干局地异常现象：在布列塔尼南部，从坎佩尔至瓦讷沿岸区域出现一处特殊特征——沿岸数公里范围内呈上升趋势，随后沿岸形成显著梯度带。该梯度带包含三个区域：首先是呈现持续单调递增趋势的区域，随后是趋势不再显著的区域，最终在距海岸约15公里处，出现另一处呈现显著持续单调递减趋势的区域。该时段内，近岸区域的叶绿素a（chlorophyll a）浓度增幅超过0.1 µg·l⁻¹。另一处异常出现在费拉角（Cap Ferrat）与阿基坦斜坡（Pente Aquitaine）所在的海台边缘中部区域：该区域观测到叶绿素a浓度上升，但变化幅度较小，不足0.1 µg·l⁻¹。在圣纳泽尔西南约100公里处，一块面积约40×50公里的区域内，叶绿素a浓度降幅超过20%，经量化该时段内浓度变化量超过0.1 µg·l⁻¹。