DOI: 10.25349/d9360j
title: El Niño and marine heatwaves: Ecological impacts on Oregon rocky intertidal kelp communities at local to regional scales

description: El Niños and marine heatwaves 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 2015-16 El Niño and the
 overlapping 2014-2016 East Pacific marine heatwave on three intertidal
 kelps (Hedophyllum sessile, Egregia menziesii, and Postelsia palmaeformis)
 at 7 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 effects 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 marine heatwave 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 populations 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.
description: <b><i>Study
 System</i></b> We studied
 three common intertidal kelp species (<i>Hedophyllum sessile,
 Postelsia palmaeformis, </i>and <i>Egregia
 menziesii</i>) along 300 km of the Oregon coast. Survey sites were
 nested within each of three capes or regions (from North to South): Cape
 Foulweather [Fogarty Creek, Boiler Bay, Depoe Bay], Cape Perpetua [Yachats
 Beach and Strawberry Hill], and Cape Blanco [Cape Blanco North and Rocky
 Point] (Appendix S1: Table S1; Fig. 1). The sea palm <i>P.
 palmaeformis </i>was mostly absent at Boiler Bay, so we added sea
 palm studies at Depoe Bay (South Point) as our second replicate site for
 this species. All aspects of the study: surveys, growth, density, and
 Carbon:Nitrogen (C:N) ratios were conducted monthly in spring/summer, when
 growth and reproduction occur, at each site from 2016 through
 2018. <b><i>Macroalgal
 Transect Surveys</i></b> We used transect
 surveys to examine changes in algal abundance and size across capes. At
 each site, we established five permanent (5 x 1 m) plots for each species.
 Since our goal was to document kelp performance and not characterize
 species populations at the site scale, plots were placed where the target
 species were most abundant. Further, sampling the same marked plots is in
 our view the best way to document temporal change. Plots were sampled
 using 0.5 x 0.5 m<sup>2</sup> quadrats placed contiguously on
 both sides of a transect line run through the middle of the plot along the
 5 m axis. Data collected monthly for each species<b><i>
 </i></b>were kelp percent cover, density, and maximum length
 of the longest individual in each quadrat, or for <i>P.
 palmaeformis</i>, maximum stipe and frond length. <b><i>In situ
 Macroalgal Growth and Breakage</i></b>
 We
 quantified growth of <i>H. sessile </i>and<i> E.
 menziesii</i> only through elongation of their blades because their
 growth with respect to the thickening of stipe, blade, and holdfast
 tissues are trivial compared to blade elongation, thus resulting in
 negligible short-term changes. We did not quantify <i>P.
 palmaeformis </i>growth because of their complex growth patterns and
 high breakage rate. The sea palm grows in two directions at a similar
 rate: (1) elongation of blades from the meristematic region, and (2)
 elongation and thickening of the stipe from the meristoderm beneath the
 cortex. Because of this complexity, there was no straightforward and
 non-intrusive way to measure growth in the field. Additionally, their high
 breakage rate (as a result of wave exposure) made it difficult to track
 individuals for growth rate measurements. For <i>H.
 sessile </i>and<i> E. menziesii</i>, growth rates were
 quantified using the hole-punch method. Monthly <i>H. sessile
 </i>growth<i> </i>rate was quantified by punching a hole
 in the longest vegetative blade of each individual 5 cm above the
 meristematic region. Growth was measured as the distance between the base
 of the blade and the hole which moves away from the holdfast as the blade
 grows. Monthly <i>E. menziesii </i>growth rate was determined
 by punching a hole in the longest vegetative blade 5 cm below the
 intercalary meristematic region.<b> </b>Growth was measured as
 the distance between the meristematic region and the previous hole. Twenty
 individuals of each species per site were identified using coded plastic
 tags attached to the substrate adjacent to each alga with a
 stainless-steel lag screw placed in pre-drilled holes in the
 rock. Using the same
 individuals tagged for growth measurements, we also quantified percent
 rachis breakage of <i>E. menziesii. </i>Individuals lacking a
 rachis beyond the site of the hole punch (for the growth measurement) was
 recorded as “broken.” Percent rachis breakage was calculated by dividing
 the number of “broken” individuals by the total number of the tagged
 individuals. <i>H. sessile </i>experienced no blade breakage
 throughout the survey obviating the need for its estimation.
 <b><i>Elemental
 Composition</i></b> Elemental
 composition provides a measure of kelp performance with regard to nutrient
 uptake. To identify biogeographic patterns of elemental composition (%C,
 %N, and C:N), we quantified the Carbon:Nitrogen ratio (C:N) for each
 species. We randomly collected samples from twenty separate individuals of
 each species: one-inch square sections of <i>H. sessile
 </i>blades, five fronds of <i>P. palmaeformis </i>and
 5-cm sections of <i>E. menziesii </i>terminal blades. All
 samples were placed in plastic zip-top bags in the field, kept cool, and
 subsequently stored in a –20°C freezer. Samples for the
 C:N analysis were prepared by thawing at room temperature and removal of
 epiphytes and fouling organisms. Samples were rinsed with deionized water
 and dried in ashed foil packets at 60°C for 48 hours. They were then
 ground to a powder using a Spex Sigma Prep 8000D Mixer/Mill, and stored in
 2 mL microcentrifuge tubes. Carbon-13 and Nitrogen-15 contents were
 analyzed by Oregon State University Stable Isotope Lab with a Carlo Erba
 NA1500 elemental analyzer and a DeltaPlus isotope ratio mass spectrometer.
 Due to financial constraints, only <i>H. sessile </i>samples
 taken each July from 2016 to 2018 were analyzed. <b><i>Environmental
 Parameters</i></b> Environmental data
 (chlorophyll-a [Chl-a], dissolved inorganic nitrogen [DIN], sea surface
 temperature [SST], surface air temperature [SAT], Multivariate El Niño
 Southern Oscillation Index [MEI v2], North Pacific Gyre Oscillation
 [NPGO], Biologically Effective Upwelling Transport Index [BEUTI], and
 significant wave height [SWHT]) for the sampling months were provided by
 the Menge laboratory or the National Oceanic and Atmospheric
 Administration (NOAA). Daily SST and SAT were measured at every site using
 HOBO TIDBIT and/or Pendant temperature loggers (Onset, Bourne,
 Massachusetts, USA) held to the rock with small stainless-steel cages. The
 loggers sampled at 5-min intervals in the low intertidal at all sites. A
 detiding program was used to separate air from water temperatures. Monthly
 Chl-a and DIN were extracted from bottle samples taken from the surf zone
 at every site and measured using the protocol in Menge et al. (1997).
 Monthly SWHT was measured by NOAA buoys 20 nautical miles west of the
 Oregon coast at 42°N (Station 46015) and 45°N (Station 46050) latitudes
 (https://www.ndbc.noaa.gov/). For months when these buoys were
 inoperative, we used the wave data from the next closest buoy and fitted a
 regression line to estimate the missing values (R2 = 0.82). BEUTI data
 were measured offshore between 31°N and 47°N latitudes at 1o resolution
 (https://oceanview.pfeg.noaa.gov/products/upwelling/intro). MEI v2) data
 were obtained from NOAA’s Physical Sciences Laboratory
 (https://psl.noaa.gov/enso/mei/) and NPGO data were obtained from Georgia
 Institute of Technology (Di Lorenzo et al. 2008;
 http://www.o3d.org/npgo/).
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