Phycoerythrin Corrections for Turbidity Artifact

by | May 21, 2013


The Turner Designs Cyclops Phycoerythrin  (PE) sensor has a significant turbidity artifact.  This artifact is so extreme that it often exceeds the signal due to PE containing cells.  Without correction, the data from this sensor are difficult to interpret.   In general, the CyanoWatch is less affected by turbidity at low turbidity levels, but does have an artifact at higher turbidity levels.  In the two figures below, the PE data are plotted against the turbidity data and are colored by chlorophyll (all scales the same in both figures).

In November there was low chlorophyll and the linear relationship between the PE and turbidity represents the turbidity artifact.  In October the same signal due to turbidity is present but a second signal associated with chlorophyll & PE containing cells can be seen.  This second population is due to the presence of M. rubrum.  The turbidity corrections aim to reduce/remove the signal due solely to the light scattering in turbid waters.   The turbidity artifact is sensor specific and details of the individual corrections can be found in the table above.

The correction  for turbidity is calculated as the linear fit of phycoerythrin vs. turbidity in that population of points not associated with chlorophyll:

  • Corrected PE =  raw PE  –  turbidity artifact
  • Corrected PE = raw PE – (turbidity * slope of fit + intercept of fit)

Another result of this correction is the zeroing of the data so that when there is no PE  the signal is close to zero.  On occasion there is a shift in the relationship between PE and turbidity.  It is possible that the changing relationship is partly a function of temperature or other seasonal parameters and this is still being investigated.  The corrections are evaluated on an ongoing basis and are corrected/updated when needed.

If there is a minimum and maximum turbidity listed in the table below, the correction is only applied to the phycoerythrin data when turbidity values are within that range.  It is also worth noting that because the turbidity correction for phycoerythrin is a function of both PE and  turbidity data, the availability and quality of verified phycoerythrin data is dependent upon the availability of turbidity data and is affected by the quality of those data.


Phycoerythrin Corrections for Turbidity Artifact

Station Sensor Type Dates Slope Intercept Min. Turb. PDF with Details
Saturn03 Cyclops 7/1/10 – 11/15/10 4035 6807 s3pe-turbcor01
Saturn03 Cyclops 11/15/10 – 5/2/11 4820 4406
Saturn03 Cyclops 5/3/11 – 8/30/12 multiple:  see PDF s3pe-turbcor02
Saturn03 Cyanowatch 8/17/12 – 9/29/13 0.0932 -0.3005 3.224 s3pe-turbcor03