Water samples (75-250 mL) were gently (<150 mbar) filtered through 25 mm, 0.40 μm polycarbonate filters (Millipore, Isopore, HTTP02500). Post-filtration, filters were stained with 500 μL of a solution of 0.04% Alcian Blue (AB) and 0.06% acetic acid (pH 2.5) and were frozen at -20°C. For extraction, filters were immersed in sulfuric acid (80%) for 2 h and their absorbance was then measured spectrophotometrically (Agilent 8453 UV-visible spectrophotometer) at fixed wavelength of 787 nm. The AB-dye was calibrated using methods found in Bittar et al. (2018), where AB was used to stain known concentrations of xanthan gum (XG, Sigma-Aldrich). Two separate AB solutions (f factors 169 and 129) were used during the Bergen experiment. TEP concentrations (μg xanthan gum (Xg) equivalents L-1; μg XG eq. L-1) were calculated according to Passow and Alldredge (1995).
Mesocosm treatments are as follows:
P-limited: N:P added in a 60:1 ratio during the first 3 days of the experiment, no shading
Redfield: N:P added in a 16:1 ratio during the first 3 days of the experiment, no shading
Shaded: N:P added in a 16:1 ratio during the first 3 days of the experiment, top shaded of the mesocosm added on May 20, 2017
Ambient: no nutrients added, no shading
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