Values given are the average of 3e5 filters. For chl a determination, pigments
were extracted from the filters in 10 ml 90% acetone at 4 C in the dark for approximately 24 h, and vials were occasionally shaken.
Chl a in the supernatant of samples collected in 2008 was deter-
mined with a Lovibond PC Spectro 1.0 photometer and calculated
after Lorenzen (1967). Chl a in samples collected in 2009 was
determined with a TURNER 10-AU field fluorometer after Arar and
Collins (1997). Significant differences between standards measured
with both devices could not be detected.