Summary

We investigated the effect of various temporal, spatial, and enviromental parameters on ‘Fo’, ‘Fv/Fm’ and ‘ETR’. For this two different data sets were utilized: underway samples (n=44) and CTD samples (n=115).

For the CTD samples we considered temperature, salinity, time, fluorescence, depth, mixed layer depth (MLD), PAR and optical depth. All data was acquired from the CTD directly, except optical depth, which was calculated based on 1% irradiance depth (here assumed be the DCM) and assumed to increase logarithmically towards the surface. As this represents a simplication, care should be taken when interpreting this metric. For instance, an obvious problem in this data set is the optical depth values >100%.

For the underway samples we considered temperature, salinity, time, fluorescence and ‘bbprime’. The latter of which is a measurement of PIC scatter. We noticed unrealistically low ETR measurements (<10), which skewed some of the results. Therefore beside the full underway data set, we additionally investigated correlations between the ETR measurements and environmental parameters for samples with ETR values >10. Another issue with the underway samples is that the relationship between Fo and Fluorescence is non-linear when theoretically the two should be perfectly linearily correlated. It is not clear if this due to an issue with the underway fluorescence sensor, or a mismatch in time/space between our measurements and the underway measurements. As such the underway measurements presented here should be considered strictly preliminary and interpreted with caution.

Overall the CTD and underway samples generally agreed, except the relationship between Fluorescence and Fv/Fm and Time and Fv/Fm. The former of which showed a binomial trend for the CTD samples but was non-significant for the underway samples. This might be due to the issue with the fluorescence data mentioned above, or may be depth related. The difference in correlation between time and Fv/Fm for the CTD and underway samples is not clear (but may be due to the time/space mismatch). At first glance the relationship between Fo and temperature may appear different, as it is binomial for the CTD data but linear for the underway data. This is however due to the different sampling range of the two measurements (e.g. the underway samples did not sample water <15.5C).

CTD samples

CTD Fo

CTD Fv/Fm

CTD ETR

Underway samples

Underway Fo

Underway Fv/Fm

Underway ETR

Underway ETR (ETR >10)