When a predetermined quantity or loading of the pollutants, nutrients or other undesirable species has precipitated onto the algal cell walls, {HEAD ~ a portion} of {TAIL ~ the algal turf} is harvested 
of
When a predetermined quantity or loading of the pollutants , nutrients or other undesirable species has precipitated onto the algal cell walls , { HEAD ~ a portion } of { TAIL ~ the algal turf } is harvested
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH