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.

Study System

We studied three common intertidal kelp species (Hedophyllum sessile, Postelsia palmaeformis, and Egregia menziesii) 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 P. palmaeformis 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.

Macroalgal Transect Surveys

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 m² 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 were kelp percent cover, density, and maximum length of the longest individual in each quadrat, or for P. palmaeformis, maximum stipe and frond length.

In situ Macroalgal Growth and Breakage

We quantified growth of H. sessile and E. menziesii 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 P. palmaeformis 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 H. sessile and E. menziesii, growth rates were quantified using the hole-punch method. Monthly H. sessile growth 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 E. menziesii growth rate was determined by punching a hole in the longest vegetative blade 5 cm below the intercalary meristematic region. 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 E. menziesii. 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. H. sessile experienced no blade breakage throughout the survey obviating the need for its estimation.

Elemental Composition

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 H. sessile blades, five fronds of P. palmaeformis and 5-cm sections of E. menziesii 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 H. sessile samples taken each July from 2016 to 2018 were analyzed.

Environmental Parameters

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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