Green gravel as a vector of dispersal for kelp restoration

Abstract

Kelp forests are experiencing substantial declines due to climate change, particularly ocean warming and marine heatwaves, and active interventions are necessary to halt this decline. A new restoration approach termed “green gravel” has shown promise as a tool to combat kelp forest loss. In this approach, substrata (i.e. small gravel) are seeded with kelp propagules, reared in controlled conditions in the laboratory before out-planting to degraded reefs. Here, we tested the feasibility of cultivating Australia’s dominant kelp, Ecklonia radiata on green gravel with the aim of optimising the seeding conditions for E.radiata. We seeded substrata (i.e. gravel), that had different surface texture and size, with E. radiata gametophytes at two average seeding densities: high density of ~230 fragments mL-1 and low density of ~115 fragments mL-1. The tested substrata were small basalt, large basalt, crushed laterite and limestone. Gametophytes successfully adhered to all four tested substrata, however, gametophytes that adhered to the limestone gravel (the natural reef type off Western Australia) suffered extreme tissue bleaching likely due to dissolution and decrease in seawater pH. Gametophytes that adhered to the three other test substrata were healthy, fertilised following seeding and microscopic sporophytes were observed attaching to the gravel. Substrata and seeding density did not affect sporophyte growth (i.e. length) at the time of transferring into aquarium tanks (after three months of rearing in incubators) but over time substrata showed a significant effect on maximum lengths. After 12 months in aquarium tanks, sporophytes on both small and large basalt gravel were significantly larger than those on the crushed laterite. Gametophytes were also found to not only survive on the gravel itself but also detach from the gravel, settle successfully, fertilise and develop into healthy sporophytes ex situ on the surrounding substratum through lateral transfer. Substrata had a significant effect on density of detached gametophytes with rougher and larger gravel showing higher densities of detachment. Our results show the potential for green gravel to be a vector of dispersal for restoration in Western Australia where natural recovery of kelp forests has failed.

Cool-edge populations of the kelp Ecklonia radiata under global ocean change scenarios: strong sensitivity to ocean warming but little effect of ocean acidification

“…It is important to note that the study of Wernberg et al [15] measured short-term (45 min) incubations, whereas our study was conducted over three weeks, making direct comparisons difficult. Nevertheless, this observed difference in thermal optima may indicate substantial acclimatization or adaptation of E. radiata populations to local temperature conditions [67,68], supporting previous findings of similar adaptation or acclimatization for the gametophyte life stage [41,69]. Moreover, the difference in optima is similar to the difference in annual mean SST between our site and those in Wernberg et al [15] of approximately 6-8°C, further suggesting acclimatization or adaptation to local temperatures.…”

An assessment of the utility of green gravel as a kelp restoration tool in wave-exposed intertidal habitats

“…However, restoration of kelp forests is challenged by the dynamic nature of temperate reefs, alongside the need to scale-up restoration interventions to match the scale of loss (Filbee-Dexter et al ., 2022), as well as propagule limitation and a lack of hard substrate in some areas (Burek et al ., 2018; O'Brien and Scheibling, 2018; Eger et al ., 2022b). Green gravel has been advocated as a simple, cost-effective and scalable approach to kelp restoration, however its effectiveness has only been tested in limited environmental contexts (Fredriksen et al ., 2020; Alsuwaiyan et al ., 2022), although additional investigations are underway as part of the Green Gravel Action Group (see greengravel.org). Here we build on this growing body of literature by investigating the utility of green gravel as a kelp restoration technique along wave-exposed intertidal rocky shores.…”

“…For example, assessing the impact of seeding at different spore densities, as well as onto different rock types, textures, and shapes. Seeding density, which although has been found not to influence S. latissima and Ecklonia radiata growth on green gravel (Fredriksen et al ., 2020; Alsuwaiyan et al ., 2022), can result in reduced survival and growth in high density cultures due to competition for nutrients and space (Steen, 2003). While the influence of seeding rocks of different shapes (i.e., round vs thin and flat) has yet to be investigated, seeding different rock types and textures has been found to influence success, with greater detachment occurring on rocks with rougher textures (due to greater initial settlement), and severe tissue bleaching observed in individuals on limestone rocks (Alsuwaiyan et al ., 2022).…”

Seaweed restocking along the Chilean coast: History, present, and inspiring recommendations for sustainability

“…Although this technique can be effective, it is labor-intensive and requires upstream cultivation steps for the maintenance and growth of the seedlings. This methodology also has been evaluated in Ecklonia radiata (Alsuwaiyan et al, 2022) under indoor conditions.…”