The dynamics and stoichiometry of dissolved organic carbon release by kelp: Carbon fixation, DOC release and nutrient uptake data

Canopy-forming kelps are foundational species in coastal ecosystems,
fixing tremendous amounts of carbon, yet we know little about the
ecological and physiological determinants of dissolved organic carbon
(DOC) release by kelps. We examined DOC release by the bull kelp,
Nereocystis luetkeana, in relation to carbon fixation, nutrient uptake,
tissue nitrogen content, and light availability. DOC release was
approximately 3.5 times greater during the day than at night. During the
day, N. luetkeana blades released an average of 16.2% of fixed carbon as
DOC. Carbon fixation increased with light availability but DOC release did
not, leading to a lower proportion of fixed carbon released as DOC at high
light levels. We found no relationship between carbon fixation and DOC
release rates measured concurrently. Rather, DOC release by N. luetkeana
blades declined with marginal significance as blade tissue nitrogen
content increased and with experimental nitrate addition, supporting the
role of stoichiometric relationships in DOC release. Using a stable
isotope (13C) tracer method, we demonstrated that inorganic carbon is
rapidly fixed and released by N. luetkeana blades as 13DOC, within hours.
However, recently fixed carbon (13DOC) comprised less than 20% of the
total DOC released, indicating that isotope studies that rely on tracer
production alone may underestimate total DOC release, as it is decoupled
from recent kelp productivity. Comparing carbon and nitrogen assimilation
dynamics of the annual kelp N. luetkeana with the perennial kelp
Macrocystis pyrifera revealed that N. luetkeana had significantly higher
carbon fixation, DOC production and nitrogen uptake rates per unit dry
mass. Both kelp species were able to perform light-independent
carbon fixation at night. Carbon fixation by the annual kelp N. luetkeana
is as high as 2.35 kg C m-2 yr-1, but an average of 16% of this carbon
(376 g C m-2 yr-1) is released as DOC. As kelp forests are increasingly
viewed as vehicles for carbon sequestration, it is important to consider
the fate of this substantial quantity of DOC released by canopy-forming
kelps.