Contrasting mechanisms of dislodgement and erosion contribute to production of kelp detritus

Abstract

We quantified simultaneously dislodgement and erosion for a dominant kelp species (Ecklonia radiata) over 1 yr, and related both to potential explanatory factors (wave exposure, temperature, and kelp fecundity). Erosion was the largest contributor of detritus, accounting for 80% of annual production. Most erosion occurred as a major pulse in autumn, whereas dislodgement was a minor and constant process throughout the year. Neither erosion nor dislodgement was correlated with water velocity (as often proposed), and this finding contradicts the common assumption that high dislodgement rates during peak wave action account for the bulk of detrital production. Together with low growth, the high erosion rate led to a severe reduction of individual kelp biomass in autumn (from 600 g to 300 g fresh weight kelp 21 ), reducing drag forces on kelp thalli by , 50%, likely reducing their susceptibility to dislodgement during peak wave action. Instead, a pulse of detrital production coincided with periods of peak kelp fecundity. We propose that sporogenesis weakens the tissue, making E. radiata more susceptible to erosion, and that the ensuing changes in kelp morphology decouple detrital production from the wave-action forces.

Can ecosystem functioning be maintained despite climate‐driven shifts in species composition? Insights from novel marine forests

“…ochroleuca might differ functionally from those formerly dominated by the cold‐temperate L. hyperborea , we compared processes related to the cycling of organic matter between the two species in two mixed kelp forests. We estimated kelp biomass accumulation—a widely used proxy for kelp primary production (de Bettignies et al., ; Krumhansl & Scheibling, )—and biomass loss (detrital production) over the course of an annual cycle (March 2016–February 2017). Produced biomass can either (a) be directly consumed by grazers in situ or (b) be exported as detritus and later consumed by a vast array of microbes, detritivores, and suspension‐feeders.…”

The rise of Laminaria ochroleuca in the Western English Channel (UK) and comparisons with its competitor and assemblage dominant Laminaria hyperborea

“…Productivity was calculated as biomass accumulation [g fresh weight (FW)·day −1 ] by multiplying blade extension (cm·day −1 ) with the biomass of the heaviest of the four first thallus sections (g FW·cm −1 ) (see de Bettignies et al . for further details and application of the method).…”

The 'Great Southern Reef': social, ecological and economic value of Australia's neglected kelp forests

“…Kelp forests are among the most productive ecosystems on the planet, with rates of productivity often exceeding that of the most intensively managed agricultural systems (Mann 1973). For example, the kelp forests of the GSR produce as much as 65 tonnes of biomass per hectare per year (de Bettignies et al 2013), which is over 16 times more than Australia's most fertile wheat fields (http://www.ausgrain.com.au, accessed 3 December 2014). This biological powerhouse then feeds directly into coastal ecosystems as food and detritus (Bustamante and Branch 1996;Wernberg et al 2006;Vanderklift and Wernberg 2010;Krumhansl and Scheibling 2012), critical for the energy and nutrient cycles supporting the rich marine life throughout the GSR and out into the wider ocean beyond shelf waters (Thompson et al 2011).…”