Short-term effects of hypoxia are more important than effects of ocean acidification on grazing interactions with juvenile giant kelp (Macrocystis pyrifera)

Abstract

Species interactions are crucial for the persistence of ecosystems. Within vegetated habitats, early life stages of plants and algae must survive factors such as grazing to recover from disturbances. However, grazing impacts on early stages, especially under the context of a rapidly changing climate, are largely unknown. Here we examine interaction strengths between juvenile giant kelp (Macrocystis pyrifera) and four common grazers under hypoxia and ocean acidification using short-term laboratory experiments and field data of grazer abundances to estimate population-level grazing impacts. We found that grazing is a significant source of mortality for juvenile kelp and, using field abundances, estimate grazers can remove on average 15.4% and a maximum of 73.9% of juveniles per m 2 per day. Short-term exposure to low oxygen, not acidification, weakened interaction strengths across the four species and decreased estimated population-level impacts of grazing threefold, from 15.4% to 4.0% of juvenile kelp removed, on average, per m 2 per day. This study highlights potentially high juvenile kelp mortality from grazing. We also show that the effects of hypoxia are stronger than the effects of acidification in weakening these grazing interactions over short timescales, with possible future consequences for the persistence of giant kelp and energy flow through these highly productive food webs. Species interactions play an important role in the organization and persistence of communities 1-3. Competition between species can drive distributional ranges 4 , predation can promote the coexistence of competing species 5 , and positive interactions can increase diversity by ameliorating the effects of stressors 6. Following disturbance, species interactions can determine whether a system persists in an alternate state or reverts to its original state. These phase shifts have been well documented in ecosystems such as coral reefs 7,8 and kelp forests 9,10 where herbivores play important roles in community dynamics. One of the best documented examples of a phase shift is the transition from kelp forests to barrens where sea urchins destructively graze macroalgae, destroying habitat and structural complexity 9. These changes can be dramatic in giant kelp forests, one of the most biodiverse habitats on earth due to the habitat-forming algae, giant kelp (Macrocystis pyrifera), the largest macroalga described to date 11. In addition to over-grazing by urchins, giant kelp forests are also subject to periodic storms and the threat of extreme heatwaves associated with climate change 12. This leads to loss of adult giant kelp biomass, opening of the canopy, changes in benthic community structure 13 , and impacts on whole food webs 14. M. pyrifera can quickly recover from disturbance events in part due to its fast growth rate 15. However, for successful recovery, juvenile M. pyrifera must survive many factors. A lot is known about abiotic influences such as light, sand scour, and nutrients 16 , but much less is known about biotic factors ...

Coupled changes in pH, temperature, and dissolved oxygen impact the physiology and ecology of herbivorous kelp forest grazers

“…franciscanus . These results are consistent with past studies showing reduced grazing in sea urchins due to reduced oxygen (Low & Micheli, 2018, 2020; Ng & Micheli, 2020) as well as acidification (Brown et al, 2014; Donham et al, 2021); however, the impacts of pH, temperature, and DO on gastropods are more variable.…”

Resilient consumers accelerate the plant decomposition in a naturally acidified seagrass ecosystem

“…Interestingly, similar ecological patterns have been reported by other studies conducted in different temperate CO 2 vent systems, where amphipods attain higher population densities and/or richness under acidification (Auriemma et al, 2019; Baggini, 2015; Garrard et al, 2014; Kroeker et al, 2011; Ravaglioli et al, 2019; Scipione et al, 2017; Vizzini et al, 2017). Because gammarids function as important consumers of plant detritus in diverse coastal marine habitats (e.g., Gilson et al, 2021; Haram et al, 2020; Middleton & McKee, 2001; Remy et al, 2018), a broader examination of how this group responds to OA (e.g., metabolic and feeding rates, population dynamics, and community structure; Borges et al, 2018; Ng & Micheli, 2020; Schram et al, 2016; Timmers et al, 2021) may shed further light on detrital dynamics in the future ocean.…”

Multi-stressor Extremes Found on a Tropical Coral Reef Impair Performance

“…The juvenile oyster, Crassostrea virginica, raised under diel oxygen cycles where concentrations dipped to 0.5 mg/L exhibited reduced growth rates (Keppel et al, 2016). Herbivory rates of a temperate sea urchin, an amphipod and a snail, all decreased after 48-h reductions of DO from 7.2 to 2.0 mg/L but not after reductions of pH from 7.9 to 7.4, and the combination of the two stressors had the same effect as the hypoxia alone (Ng and Micheli, 2020). Abalone exposed to a 24-h combination of reduced pH (7.5) and DO (5 mg/L) once every 15 days, showed decreased growth and survival compared to controls, as did those exposed only to reduced DO (Kim et al, 2013).…”