Erlania1, Adam Miller1, Mary A. Young1, Owen J. Holland1, Rafael C. Carvalho2, David Kennedy3, Daniel Ierodiaconou1, Peter Macreadie4, Alecia Bellgrove1
1Deakin University School of Life and Environmental Sciences, Warrnambool, VIC 3280, Australia
2School of Earth, Atmosphere and Environment, Monash University, Australia
3School of Geography, Earth and Atmospheric Sciences, The University of Melbourne, Australia
4Deakin University School of Life and Environmental Sciences, Burwood, VIC 3125, Australia
Demonstrating evidence of seaweed carbon sequestration has been the key challenge in incorporating seaweed into blue carbon (BC) inventories since the fate of seaweed carbon is largely unknown. We investigated the seaweed carbon fingerprint in marine sediments collected from 112 near-shore sites across seven biogeographical units (biounits) in southwestern Victoria, Australia using an environmental DNA (eDNA) approach. To understand the environmental drivers of seaweed transport and deposition, we assessed the effects of oceanographic parameters on the richness and relative abundance of seaweed taxa identified, and organic carbon preserved, in the sediments using generalized additive models (GAMs). A total of 67 macroalgal taxa (15 Phaeophyceae, 38 Rhodophyta, and 14 Chlorophyta) were detected in sediments from >91% of sampling sites. Modelling revealed the interaction of oceanographic parameters drove the diversity of macroalgae deposited in the sediments, and the strong effects of environmental parameters on potential exportation of coastal organic carbon sources, such as seaweed, towards the ocean. There were significant differences in the occurrence of taxa among biounits, in which 3 Chlorophyta, 7 Phaeophyceae, and 6 Rhodophyta taxa contributed to 70% of the variability in taxa among biounits. Caulerpa longifolia and a group of Phaeophyceaen taxa (assigned as Phaeophyceae due to database limitations) appear to be the most common taxa detected across biounits and contributed most to the similarity within each biounit. Overall, our findings have confirmed that macroalgal biomass is exported to and deposited in near-shore sediments. The high resolution in taxonomic identification obtained allows understanding of the patterns of macroalgal diversity and the key taxa preserved within the sediments, from which we can begin to understand the important macroalgal species with high recalcitrance that may contribute significantly to carbon sequestration.