In situ degradation of kelp detritus: Implications for coastal carbon cycling

Abstract

Marine macroalgae (i.e., seaweeds) are crucial components of coastal carbon cycles and may be important contributors to marine carbon sequestration. Understanding the fate of macroalgal-derived carbon in marine systems requires an understanding of the degradation dynamics of macroalgae across environmental gradients. Therefore, we conducted an in situ kelp tissue degradation study in Tīkapa Moana/the Hauraki Gulf, Aotearoa New Zealand to assess the longevity of carbon contained within Ecklonia radiata kelp tissues. We utilized a set of six study sites which represent a natural gradient in turbidity to assess the influence of light availability on kelp decay dynamics, kelp tissue composition, and kelp surface/biofilm bacterial community. We found that half of kelp detritus disappeared within, on average, 58.37 ± 7.9 d. Light availability did not significantly influence kelp tissue longevity, although the shape of the kelp degradation curves differed across the light availability gradient. Kelp tissues maintained baseline concentrations of photosynthetic pigments throughout decomposition, while C : N (carbon : nitrogen) ratio decreased through degradation time at all sites. Degradation was associated with increased alpha diversity of kelp epiphytic bacterial communities and significant changes in kelp epiphytic bacterial community structure. This research shows that kelp tissues persist on subtidal rocky reefs in Tīkapa Moana/the Hauraki Gulf for no longer than 4 months, and that kelp degradation involves complex biogeochemical transformations that are not captured by biomass loss alone. These findings have critical implications for assessing the carbon sequestration potential of kelp forests in Aotearoa New Zealand and beyond.