The role of kelp in sediment transport: Observations from southeast New Zealand

Large marine algae such as kelp can, when dislodged, remove part of the substrate to which they are attached and subsequently transport it. Quantification of this process and the causes behind it have rarely been investigated. This study examines sediment transport by the kelps Durvillaea antarctica and Durvillaea willana in southeast New Zealand. Five beaches near Dunedin were surveyed weekly for 30 weeks and all beach-cast kelp was counted and identified; the weight and nature of any attached sediment was also recorded. Additionally, 60 sections of the southeast New Zealand coastline (totaling 130 km) were surveyed once and all beach-cast kelp examined. Rocks were found to be attached to 27% of examined holdfasts, usually occurring as a thin veneer. The weight per holdfast ranged from 0.1 g to 83 kg (with an unusual outlier of 365 kg) with a mean weight of 77.1 g (SE = 15.3, n = 1560). Most holdfasts transported less than 100 g. Carbonate material occurred on 84% of holdfasts with weights per holdfast ranging from 0.02 to 116.56 g with a mean of 7.8 g (SE = 0.3, n = 1695). The character of local rock strongly influences the kelp-mediated flux: South Coast beaches with schist rocks nearby received a significantly greater mean flux (7.49 g m2 year−1) than basalt-surrounded Peninsula beaches (mean flux 0.50 g m2 year− 1). Kelp-transported material provides a small but significant source of sediment to beaches in southeast New Zealand and therefore should be considered when constructing sediment budgets in coastal areas. This study has shown that it is often necessary for geologists to consider biological processes.

Research Highlights
► We measured rock and carbonate material on beach-cast kelp in SE New Zealand.
► Rocks and carbonate weighing up to 83 kg were transported onshore by kelp.
► One particularly large rock weighed 365 kg, but probably was only transported 50 m.
► Local rock type strongly influences the kelp-mediated flux.
► Kelp-transported material should be included inbeach sediment budgets.