Page 6 - NutrientsReadMe

samples (two subsamples of the same water sample) in the CitSci dataset, which were
obtained for this purpose, we estimate that these analytical errors are around ±0.3 µM for
nitrate, ±1 µM for silicic acid, and ±0.05 µM for phosphate (see Table below). However, note
that the scatter pattern in the 2016 and especially the 2017 CitSci dataset looks qualitatively
different than the pattern in other years, with a lower and rounder shape. Deep
concentrations are also lower. The reason for this is not clear, however the analytical
procedures used in those years were different than in the other years. A difference in data
quality for those years is also suggested by comparison with the more limited high-quality
data collected by government scientists, which does not show a qualitative difference in the
appearance of the scatter over all years. Thus, we suspect that the 2017 measurements
especially are also affected by procedural biases. These biases will not affect comparisons
within that year, but make year-to-year comparisons more uncertain.

Table 1: At many stations (typically one station per patrol per survey) duplicate water samples are taken. Shown
here are the pooled standard deviations of duplicate samples, after outliers were removed.

Duplicate Water Sample Analysis Results
Pooled Sample Standard deviation (Number of pairs)

Year

2015 2016 2017 2018 2019 2020 2021

Nitrate 0.2737 (336) N/A N/A 0.7594 (304) 0.1286 (155) 0.2036 (172) 0.0950 (195)

Silicate 0.7029 (290) N/A N/A 2.0675 (307) 0.6404 (176) 0.6426 (187) 0.9337 (187)

Phosphate 0.0553 (349) N/A N/A 0.0775 (309) 0.0378 (159) 0.0471 (209) 0.1274 (58)

Chlorophyll 0.1758 (38) 0.5032 (117) 0.3018 (147) 0.3305 (127) 0.3760 (63) 0.2540 (97) 0.3827 (87)