St Helena Marine Water Quality: Background Conditions and Development of Assessment Levels for Coastal Pollutants

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 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 (