McPherson ML et al. (2021), Large-scale shift in the structure of a kelp fo...

ECB-ART-49289

Commun Biol 2021 Mar 05;41:298. doi: 10.1038/s42003-021-01827-6.

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Large-scale shift in the structure of a kelp forest ecosystem co-occurs with an epizootic and marine heatwave.

McPherson ML , Finger DJI , Houskeeper HF , Bell TW , Carr MH , Rogers-Bennett L , Kudela RM .

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Climate change is responsible for increased frequency, intensity, and duration of extreme events, such as marine heatwaves (MHWs). Within eastern boundary current systems, MHWs have profound impacts on temperature-nutrient dynamics that drive primary productivity. Bull kelp (Nereocystis luetkeana) forests, a vital nearshore habitat, experienced unprecedented losses along 350 km of coastline in northern California beginning in 2014 and continuing through 2019. These losses have had devastating consequences to northern California communities, economies, and fisheries. Using a suite of in situ and satellite-derived data, we demonstrate that the abrupt ecosystem shift initiated by a multi-year MHW was preceded by declines in keystone predator population densities. We show strong evidence that northern California kelp forests, while temporally dynamic, were historically resilient to fluctuating environmental conditions, even in the absence of key top predators, but that a series of coupled environmental and biological shifts between 2014 and 2016 resulted in the formation of a persistent, altered ecosystem state with low primary productivity. Based on our findings, we recommend the implementation of ecosystem-based and adaptive management strategies, such as (1) monitoring the status of key ecosystem attributes: kelp distribution and abundance, and densities of sea urchins and their predators, (2) developing management responses to threshold levels of these attributes, and (3) creating quantitative restoration suitability indices for informing kelp restoration efforts.

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	Fig. 1. Spatial and temporal variability of bull kelp canopy area in northern California from 1985 to 2019.a Sonoma and Mendocino county region of study and SST domain (Esri World Imagery – Esri, CGIAR, USGS HERE, Garmin, FAO, METI/NASA, EPA, Earthstar Geographics) and inset with a global map indicating the northern California region with a red star, b annual timeseries heatmap of kelp canopy summed within 90 m latitudinal bins. Esri. “World Imagery” [basemap]. Scale Not Given. “World Imagery Map”. December 12, 2009. https://www.arcgis.com/home/item.html?id=10df2279f9684e4a9f6a7f08febac2a9. (Jan 26, 2021).
	Fig. 2. SST distribution and kelp canopy area in northern California during prominent El Niño and MHW events from 1985 to 2019.Kernel density functions for SST anomalies during a the 1997/1998 El Niño, b the 2014–2015 NE Pacific MHW event (i.e., ‘blob’ and El Niño), and c relatively normal conditions before and after the MHW event (2012/2013 and 2018/2019, respectively). Shaded gray areas (a–c) represent ±1 SD from the long-term mean SST index. Solid black lines (a–c) represent the physiological threshold for bull kelp at 17 °C65 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma _{1985 - 2019}$$\end{document}σ1985−2019 = 3.5) and the NO3 deplete (NO3 = 0) threshold (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sigma _{1985 - 2019}$$\end{document}σ1985−2019 = 0.48). d Kelp canopy coverage through time with relevant oceanographic and biological events overlaid onto the timeseries as follows: a shaded yellow bar during the 1997/1998 El Niño; a shaded red bar during the 2014/2015 ‘blob’; a shaded orange bar during the overlapping ‘blob’ and 2015/2016 El Niño; a shaded yellow bar during the 2015/2016 El Niño; and a dashed gray line in which SSWD in sunflower stars was first observed in 2013. Annual error estimates (black error bars) for kelp canopy area were determined using the normalized root mean square error (NRMSE) between CDFW arial flyover surveys and USGS Landsat imagery66.
	Fig. 3. Results of Partial Least Squares Regression analysis for environmental and biological drivers of kelp canopy area from 1985 to 2019.Component 1 partial least squares regression (PLSR) x weights (top row) from environmental indices across 1985 to 2016 (a) and both environmental and biological indices from 2003 to 2016 (b). PLSR models and forecasts using all components overlaid on satellite-derived kelp canopy (c, d). See supplementary data for detailed PLSR results (Supplementary Table 1). Predictor variable acronyms are as follows: purple urchin density—‘Purple Urchin’; seasonal nitrate concentrations—‘Summer NO3’ and ‘Spring NO3’; marine heatwave days—‘MHW Days’; seasonal sea surface temperature— ‘Summer SST’ and ‘Spring SST’; mean significant wave height—‘Mean Hs’; Pacific Decadal Oscillation—‘PDO’; North Pacific Gyre Oscillation—‘NPGO’; Multivariate El Niño/Southern Oscillation Index—‘MEI’. See Methods for a detailed description of how each environmental variable influence kelp canopy dynamics.
	Fig. 4. Temporal trends of important environmental and biological drivers of ecosystem change in northern California kelp forests.Standardized indices of (a) bull kelp canopy coverage, MHW days, purple urchin density, and sunflower star density where data are available. Standardized indices overlaid with Ordinary Least Square Regression (OLSR) fits (except in 4c 2003–2013 where a second degree polynomial LSR is applied) prior to and after the NE Pacific MHW for (b) bull kelp canopy coverage and nitrate concentration, (c) sunflower star density, (d) MHW days, and (e) purple urchin density. See supplementary data for detailed LSR results and error statistics (Supplementary Fig. 2 and Supplementary Table 2).

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