C:N ratio, C and N content
For analyzing the carbon and nitrogen content in the blades of adult
kelps, a sub-sample of meristematic discs (ø 24mm) was taken for each
kelp species across all age classes (≥ 2years – 8years; A.
esculenta: n = 11 (2.5m); n = 15 (5m); ‘Digitate Kelps’: n = 15 (2.5m);
n = 10 (5m); S. latissima : n = 15 (2.5m); n = 7 (5m)) from 2.5
and 5 m depths individuals. The blade samples were blotted dry with
Kimtech Science precision wipers (Kimberly-Clark Professional, USA),
immediately frozen at -80°C and later freeze-dried (freeze dryer:
CHRIST, ALPHA 2-4 LD plus with LyoCube 4-8, Germany; pump: vacubrand,
chemistry hybrid pump RC6, Germany). The freeze-dried algal material was
ground to powder using a ball mill (Retsch MM40, Germany) and five 3 mm
steel beads (frequency 30/s, 1 min). Subsamples of 5-6 mg were weighed
into tin cartridges (6 x 6 x 12mm) and the samples were combusted at
950°C following the protocol of Graiff et al. (2015). The total content
of C and N was automatically quantified in an elemental analyzer
(UNICUBE® Elementar Analysensysteme GmbH, Langenselbold, Germany) and
acetanilide was used as the standard (Verardo et al., 1990). Individual
%carbon, %nitrogen and C:N ratio was analyzed and these data were used
to extrapolate the absolute C and N content stored in the DW of kelp
blades per m-² in 2021.