Grazing and Recovery of Kelp Gametophytes Under Ocean Warming

Abstract

Kelp forests are economically important ecosystems that support a wealth of biodiversity but are declining globally. They are often replaced by biologically depauperate alternate stable states dominated by turfing algae. Hysteresis maintains algal turfs by inhibiting kelp recruitment, preventing the reestablishment of kelp forests. The mechanisms inhibiting kelp recruitment remain poorly understood as microscopic stages of kelp are difficult to study in situ. A potential mechanism contributing to the suppression of recruitment may be turf-facilitated grazing of kelp gametophytes, the haploid free-living reproductive life stage. Here we assess the resilience of kelp gametophytes to grazing pressure from a gastropod micrograzer commonly present in turf under current and future ocean warming scenarios. Gametophyte coverage and abundance were significantly reduced following grazing under all temperatures, however there was no significant effect of temperature on grazing rates. Once grazing pressure was removed, gametophyte abundance recovered to control levels, but the total coverage and length of gametophytes continued to decline in all treatments. Gametophytes were found to survive micrograzer ingestion and continued to grow in aggregations in the gastropod’s mucus trail and faeces, even producing sporophytes. Gametophyte survival post-ingestion may positively contribute to dispersal and sporophyte recruitment, however the lack of gametophyte recovery at elevated temperatures may counteract this effect under future ocean warming. Taken together, this study demonstrates complex interactions that take place in the turf micro-habitat of kelp gametophytes and highlights biotic factors influencing transitions between kelp forests and algal turfs.

Urchin grazing of kelp gametophytes in warming oceans

“…The shredding and mixing that might occur during T. gratilla grazing and ingestion could create the right density for gametophyte recruitment. A similar effect of increased sporophyte recruitment was seen following grazing of E. radiata by a micro snail Anachis atkinsoni (Veenhof, Dworjanyn, et al, 2022). It is therefore interesting that there was no recruitment after ingestion by Centrostephanus rodgersii , despite a high proportion of healthy gametophytes (>50%) surviving in fecal pellets, and this could partly explain the lack of natural recovery of kelp forests from barrens created by this particular species (Ling et al, 2015; Ling & Johnson, 2009).…”

“…The effects of ocean warming on urchin grazing activity may thus be dependent on size, as larger individuals increase grazing rates in response to warming (Carey et al, 2016; Dworjanyn & Byrne, 2018; Provost et al, 2017). This size‐dependent response to warming should be further explored, especially in the context of the separate ecological roles of smaller grazers and large grazers (this study; Veenhof, Dworjanyn, et al, 2022). Due to their microscopic size, gametophytes are hard to study in situ.…”

“…These temperatures are representative of the warm range edge of the distribution of E. radiata , which is one of the few places where it co‐occurs with both Centrostephanus rodgersii and Tripneustes gratilla . We tested the predictions that (1) juvenile urchins would graze kelp gametophytes, reducing their length and abundance, but would not show a preference for male or female gametophytes; (2) grazing would be impacted differently by temperature concurrent with the urchins' thermal ranges, where the lower temperatures would increase C. rodgersii grazing, and higher temperatures would increase T. gratilla grazing; (3) gametophytes would be able to recover post‐grazing, but recovery would be hampered under higher temperatures; and (4) gametophytes would survive ingestion by sea urchins, and recruitment would be facilitated by ingestion (Veenhof, Dworjanyn, et al, 2022). Through this, we aimed to provide novel insights into ecological interactions between juvenile urchins and gametophyte populations under current and future climate scenarios and the mechanisms behind the persistence of urchin barrens over kelp forests.…”

What does the future look like for kelp when facing multiple stressors?

“…Although some kelp populations may survive and even increase productivity and resilience with anthropogenic stressors, many populations will decline in diversity and biomass, causing shifts to irreversible alternative states (Christie et al, 2019 ; Filbee‐Dexter & Wernberg, 2018 ; Kumagai et al, 2018 ; Miranda et al, 2019 ; Veenhof et al, 2022 ). In nutrient‐enriched and polluted waters, the green algae Ulva spp.…”

Kelp gametophytes can survive and reproduce after being eaten in a warming ocean

“…Most studies of urchin grazing have focused on their impacts on large kelp sporophytes, and few studies have examined the effects of grazing on kelp microscopic life stages. Those that have, have largely focused on grazing by adult sea stars (Leonard, 1994), urchins (Dean et al, 1988), and/or gastropods (Henríquez et al, 2011), and even fewer have examined how this varies under ocean warming (but see Zacher et al, 2016; Veenhof et al, 2022) or according to different life stages of the grazers. Additionally, while it has generally been assumed that grazing on kelp microscopic stages results in their complete mortality, the survival of kelp in these stages after ingestion is an area of research that has been overlooked.…”