St Helena is an isolated oceanic island located in the tropical South Atlantic, and knowledge of broadscale oceanography and productivity in its surrounding waters is limited. This study used data from model outputs (2007-2017), remote sensing data (1998-2017) and survey measurements (April 2018 and 2019) to determine background conditions for nutrients, chlorophyll and suspended particulate matter (SPM) in offshore waters and propose standards (thresholds) for assessing inshore water quality based on 50 % deviation from seasonal (usually June to November) or annual averages. Seasonal thresholds were proposed for surface nitrate (average 0.18 µM), phosphate (average 0.26 µM), silicate (average 2.60 µM), chlorophyll (average 0.45 µg chl l-1) and SPM (average 0.96 mg l-1). Associated background values for most surface parameters (phosphate 0.17 µM, silicate 1.57 µM, chlorophyll 0.30 µg chl l-1; from model outputs and remote sensing) were slightly higher than offshore observations (April 2019). For nitrate, the average background value (0.12 µM) was lower than the observed average (0.24 µM). At depth (150-500 m), annual background values from model outputs were high (nitrate 26.8 µM, phosphate 1.8 µM, silicate 17.3 µM). Observed water masses at depths >150 m, identified to be of Antarctic and Atlantic origin, were nutrient-rich (e.g. 16 µM for nitrate, April 2019) and oxygen deficient (<4-6 mg l-1). A thermocline layer (between ca. 10 m and 230 m), characterised by a sub-surface chlorophyll maximum (average 0.3-0.5 µg chl l-1) near the bottom of the euphotic zone (ca. 100 m), is likely to sustain primary and secondary production at St Helena. For assessing inshore levels of chemical contaminants and faecal bacteria estimated from survey measurements, standards were derived from the literature. A preliminary assessment of inshore observations using proposed thresholds from surface offshore waters and relevant literature standards indicated concerns regarding levels of nutrients and faecal bacteria at some locations. More detailed modelling and/or field-based studies are required to investigate seasonal trends and nutrient availability to inshore primary producers and to establish accurate levels of any contaminants of interest or risk to the marine environment.

中文翻译：

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圣赫勒拿岛海洋水质：沿海污染物的背景条件和评估水平的发展

圣海伦娜（St Helena）是位于热带南部大西洋的一个孤立的海洋岛，其周围水域的大规模海洋学和生产力知识有限。这项研究使用模型输出（2007-2017），遥感数据（1998-2017）和调查测量值（2018年4月和2019年）的数据来确定离岸水域中养分，叶绿素和悬浮颗粒物（SPM）的背景条件并提出建议基于季节性（通常是6月至11月）或年度平均值的50％偏差来评估近岸水质的标准（阈值）。提出了表面硝酸盐（平均0.18 µM），磷酸盐（平均0.26 µM），硅酸盐（平均2.60 µM），叶绿素（平均0.45 µg chl l-1）和SPM（平均0.96 mg l-1）的季节阈值。大多数表面参数的相关背景值（磷酸盐0.17 µM，硅酸盐1.57 µM，叶绿素0.30 µg chl l-1；来自模型输出和遥感的数据）略高于海上观测（2019年4月）。对于硝酸盐，平均背景值（0.12 µM）低于观察到的平均值（0.24 µM）。在深度（150-500 m）处，模型输出的年背景值很高（硝酸盐26.8 µM，磷酸盐1.8 µM，硅酸盐17.3 µM）。确认为南极和大西洋起源的深度大于150 m的水团富含营养（例如，硝酸盐为16 µM，2019年4月）和缺氧（<4-6 mg l-1）。一个热跃层（约10 m和230 m之间）的特征是在常温区底部（约100 m）附近的次表层叶绿素最大值（平均0.3-0.5 µg chl l-1）。维持圣赫勒拿岛的初级和二级生产。为了评估根据调查测量值估算的近岸化学污染物和粪便细菌的水平，从文献中得出了标准。利用近海表层水域的建议阈值和相关文献标准对近海观测资料进行了初步评估，这表明人们对某些地方的营养物质和粪便细菌的含量感到担忧。需要进行更详细的建模和/或基于野外的研究，以调查季节性趋势和近岸初级生产者的养分供应情况，并确定对海洋环境有害或有风险的任何污染物的准确水平。利用近海表层水域的建议阈值和相关文献标准对近海观测资料进行了初步评估，这表明人们对某些地方的营养物质和粪便细菌的含量感到担忧。需要进行更详细的建模和/或基于野外的研究，以调查季节性趋势和近岸初级生产者的养分供应情况，并确定对海洋环境有害或有风险的任何污染物的准确水平。利用近海表层水域的建议阈值和相关文献标准对近海观测资料进行了初步评估，这表明人们对某些地方的营养物质和粪便细菌的含量感到担忧。需要进行更详细的建模和/或基于野外的研究，以调查季节性趋势和近岸初级生产者的养分供应情况，并确定对海洋环境有害或有风险的任何污染物的准确水平。