Extreme Marine Heatwaves Alter Kelp Forest Community Near Its Equatorward Distribution Limit

Abstract

Climate change is increasing the frequency and severity of marine heatwaves. A recent extreme warming event (2014-2016) of unprecedented magnitude and duration in the California Current System allowed us to evaluate the response of the kelp forest community near its southern (warm) distribution limit. We obtained sea surface temperatures for the northern Pacific of Baja California, Mexico, and collected kelp forest community data at three islands, before and after the warming event. The warming was the most intense and persistent event observed to date, with low-pass anomalies 1 • C warmer than the previous extremes during the 1982-1984 and 1997-1998 El Niños. The period between 2014 and 2017 accounted for ∼50% of marine heatwaves days in the past 37 years, with the highest maximum temperature intensities peaking at 5.9 • C above average temperatures for the period. We found significant declines in the number of Macrocystis pyrifera individuals, except at the northernmost island, and corresponding declines in the number of fronds per kelp individual. We also found significant changes in the community structure associated with the kelp beds: half of the fish and invertebrate species disappeared after the marine heatwaves, species with warmer affinities appeared or increased their abundance, and introduced algae, previously absent, appeared at all islands. Changes in subcanopy and understory algal assemblages were also evident; however, the response varied among islands. These results suggest that the effect of global warming can be more apparent in sensitive species, such as sessile invertebrates, and that warming-related impacts have the potential to facilitate the establishment of tropical and invasive species.

Geographic variation in responses of kelp forest communities of the California Current to recent climatic changes

“…Clear signs of tropicalization have been reported for temperate marine ecosystems (Arafeh-Dalmau et al, 2019;Horta e Costa et al, 2014;Vergés et al, 2016). In the present study, we could not focus on species thermal affinities because we grouped species into Islands (Caselle et al, 2015;, as well as the establishment of voluntary, community-based no-take marine reserves in Baja California (Fulton et al, 2018;Micheli et al, 2012) may have contributed to positive rates of change in species targeted by fisheries.…”

“…These marked declines in kelp abundance are likely related to reduced nutrient concentrations associated with increased water temperatures (e.g., Reed, Washburn, et al, 2016) California, Mexico, have been noted in previous warming events (Edwards, 2004) and by others in response to the recent marine heatwave (Arafeh-Dalmau et al, 2019;Cavanaugh, Reed, Bell, Castorani, & Beas-Luna, 2019). In contrast, and similar to our results, Reed, Washburn, et al (2016) could not attribute any change in the long-term declining trajectory of giant kelp abundance and several associated species in southern California to the same marine heatwave we evaluated.…”

Remnant kelp bed refugia and future phase-shifts under ocean acidification

“…Sea urchin pH thresholds were determined from experimental data on fertilization and early development rates across pH for Centrostephanus rodgersii, plus data from other sea urchin species defining performance of juvenile and adult urchins across pH (S4 Fig in S1 File). Reduction in urchin performance was assumed to translate directly to reduced grazing on barren grounds to below the threshold required for maintenance of barrens (7). Furthermore, rapid local recovery of kelp beds (within 18 months in Tasmania (31)) occurs on barrens once grazing pressure falls below this critical threshold (7).…”

Ecological performance differs between range centre and trailing edge populations of a cold-water kelp: implications for estimating net primary productivity

“…Müller et al 2009;Raybaud et al 2013;Franco et al 2018;Assis et al 2018;Khan et al 2018) or documenting species' range shifts (Wernberg et al 2011;Smale et al 2015), including population losses at trailing edges (e.g. Wernberg et al 2016;Arafeh-Dalmau et al 2019). In contrast, far less is known about how ocean warming may alter ecological performance and carbon dynamics within any given species range (but see Pessarrodona et al 2018).…”

“…The latitudinal distributions of kelp species are strongly constrained by temperature (Eggert 2012) and, as such, they are influenced by contemporary and near-future ocean warming trends (reviewed by Smale 2020). Assuming no local adaptation or plasticity, climate-driven poleward range shifts may result in considerable losses of standing biomass and reduced rates of primary productivity from any given species' trailing edge (Wernberg et al 2016;Arafeh-Dalmau et al 2019). Moreover, rising temperatures will likely impact performance throughout the entire species range, with current 'cold' populations likely to function similarly to current 'warm' populations over the coming decades (Pessarrodona et al 2018).…”