El Niño and marine heatwaves: Ecological impacts on Oregon rocky intertidal kelp communities at local to regional scales

Abstract

El Niños and marine heatwaves (MHWs) are predicted to increase in frequency under greenhouse warming. The impact of climate oscillations like El Niño‐Southern Oscillation on coastal environments in the short term likely mimics those of climate change in the long term; therefore, El Niños may serve as a short‐term proxy for possible long‐term ecological responses to an increasingly variable climate. Understanding and prediction of ecosystem responses requires elucidating the mechanisms underlying different organizational scales (organism, space, and time). We analyzed spatiotemporal variation in the effect of the 2015–2016 El Niño and the overlapping 2014–2016 East Pacific MHW on three intertidal kelps (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis) at seven sites across 300 km of the Oregon coast and over three years post El Niño. We measured percent cover, density, maximum length, growth, and carbon : nitrogen (C:N) ratios monthly in spring/summer at each site from 2016 through 2018. Results revealed a complex interplay between spatial, temporal, and biological factors that modified the effects of these thermal anomalies on Oregon intertidal kelp populations. Our findings generally agree with prior literature showing detrimental effects of El Niño on kelp. However, El Niño and possibly MHW effects can be mitigated or amplified by environmental processes and kelp life history strategies. In our study, coastal upwelling provided regional relief for the kelp individuals with respect to their growth needs and mitigated the adverse effects of warming. On the other hand, we also found that coastal upwelling amplified, or compounded, detrimental effects of El Niño by increasing phytoplankton‐induced shading and mollusk grazing on juvenile and adult kelps, thereby reducing their density. Given the greater uncertainty associated with warming events and climate change in the California Current Upwelling System and its biological implications, our findings reiterate the importance of acquiring better understanding of how context‐specific underlying conditions modify ecosystem processes. More specifically, understanding how demographic traits and life history stages of kelp change with biological interactions and environmental forcing over temporal and spatial scales is crucial to anticipating future climate change ramifications.

Heatwave restructures marine intertidal communities across a stress gradient

“…For example, while A. marginata collapsed in 2016 and partially recovered, H. sessile steadily increased in cover at low elevations (Figures 3 and 4), compensating for canopy loss. Although it may be surprising that Hedophyllum increased in abundance given our understanding of how temperature impacts the ranges and physiologies of kelps (Lüning & Freshwater, 1988; Spiecker & Menge, 2022), opposing fluctuations of these two kelp species are likely to reflect known physiological tolerances and competitive interactions. Alaria is generally competitively dominant unless otherwise excluded (e.g., when grazers are rare; [Dayton, 1975; Paine, 2002; Widdowson, 1965]), yet it is also sensitive to acute warming, particularly during reproduction and recruitment (Muth et al, 2019).…”

“…Because intertidal organisms often live at or near their physiological limits (Somero, 2002), warming is expected to alter vertical distributions by compounding existing stressors (e.g., desiccation, acute heat stress, photooxidation) that, together with species interactions, determine a stereotypical intertidal pattern of horizontal bands of dominant species and their associates (“zonation”). Patterns of zonation may shift with changes in individual populations or community interactions (Harley, 2011; Harley & Paine, 2009), but there are few researched examples of intertidal community responses to heatwaves that include more than a few dominant taxa (Spiecker & Menge, 2022; Suryan et al, 2021, but see Miner et al, 2021).…”

A decade of invertebrate recruitment at Santa Catalina Island, California

“…The present tiles were submerged annually from 2010 to 2021, and they sampled propagules originating from the ~503 invertebrates and ~174 macroalgae recorded within the MPA in which the study occurred ( Looby & Ginsburg, 2021 ), and probably also from more distant sources. Together, recruits varied among years, which included the strong El Niño of 2015–2016, the overlapping marine heat wave of 2014–2016, and the weaker El Niño of 2018 ( Spiecker & Menge, 2022 ; Walker et al, 2020 ), when seawater temperature was elevated, and nutrients and chlorophyll a concentrations were low ( Chavez et al, 2002 , see also Fig. 4 ).…”

Temperature and food chain length, but not latitude, explain region-specific kelp forest responses to an unprecedented heatwave

“…Various abiotic and biotic factors may make kelp more resilient in the face of warming and shifts in trophic dynamics (Hollarsmith et al, 2022). Spatial variation in temperature and other environmental variables can mediate the responses of kelp forests to large-scale warming (Wernberg et al, 2016; Cavanaugh et al, 2019; Starko et al, 2019; Filbee-Dexter et al, 2020), with factors like water motion, upwelling dynamics, and nutrient pollution leading to complexity in the distribution of environmental variation across the coastal fringe (Druehl, 1978; Hollarsmith et al, 2022; Spiecker & Menge, 2022; Starko et al, 2022). For example, while inland waters may warm up in the summer, areas on the outer coast with high wave exposure or currents can stay cool and nutrient rich through upwelling and mixing, potentially facilitating kelp persistence (Berry et al, 2021; Starko et al, 2022).…”