DOI: 10.25349/d9v61r
title: Coastal upwelling may strengthen the controls of herbivory and light over the population dynamics of Hedophyllum sessile in the Oregon rocky intertidal

description: Survival of early life history stages is critical to the successful
 establishment of benthic populations. Although light availability and
 herbivory are likely to influence passage of marine macroalgae through a
 “recruitment bottleneck” at the sporeling stage, the interactive effect of
 these factors on subsequent community patterns of macroalgae is not well
 studied. We experimentally tested the effect of light and grazing on
 sporelings of the common intertidal kelp Hedophyllum sessile. Studies were
 conducted at two sites along the Oregon coast: Strawberry Hill (an
 intermittent upwelling region) and Cape Blanco North (a persistent
 upwelling region). Herbivory and light availability were manipulated and
 kelp performance metrics (density and length) were measured monthly from
 May to November 2019. We found that the effects of herbivory and light
 availability were pronounced at Cape Blanco North but negligible at
 Strawberry Hill. At Cape Blanco North, herbivory had strong but opposing
 effects on density and length of H. sessile. Kelp density was higher in
 treatments without herbivores while kelp length was greater in treatments
 with herbivores. Responses also differed with life history stage.
 Herbivory had negative effects on juvenile kelp but positive effects on
 adult kelp while light availability had opposing effects on length of
 juvenile and adult kelps. Length of juvenile kelps was higher in shaded
 treatments while length of adult kelps was higher in unshaded treatments.
 Our study highlights the potential importance of coastal geophysical
 processes (and subsequently, nutrients) in modifying herbivore and light
 effects on population dynamics of H. sessile, and how these dynamics may
 be further influenced by different characteristics of the kelp (i.e.,
 demographic traits and life history stages).
description: Study
 System We
 tested the effect of herbivory and light availability on established
 individuals and new recruits of Hedophyllum sessile, a common intertidal
 kelp species, at two sites along the Oregon coast. Strawberry Hill is on
 Cape Perpetua [central Oregon] (44.25°N, 124.12°W) and Cape Blanco North
 is on Cape Blanco [southern Oregon] (42.84°N, 124.57°W). Kelp performance
 metrics (length and density) were measured monthly from April 2019 to
 November 2019 at Strawberry Hill, and, because of delays due to logistical
 and weather difficulties, from June 2019 to November 2019 at Cape Blanco
 North. These capes differ in physical, biological, and
 geological features. Cape Perpetua is bordered by a relatively wide
 continental shelf, has retentive currents, experiences more intermittent
 upwelling, has lower dissolved inorganic nitrogen and higher phytoplankton
 levels, and is dominated by sessile invertebrates and non-canopy and
 turf-forming algae in the low zone. Cape Blanco’s continental shelf is
 relatively narrow, experiences more persistent upwelling, has higher
 dissolved inorganic nitrogen levels, and is dominated by macrophytes in
 the low zone. Experimental Design
 At both sites, 20 x 20 cm plots
 were established in the low zone where H. sessile and coralline species
 (e.g., Bossiella plumosa, Corallina vancouveriensis) were abundant. We
 specified this association because H. sessile relies on geniculate
 corallines to provide structural complexity for settlement. The
 fully-crossed randomized block experiment consisted of two treatment
 factors with four replicates or blocks (light availability and herbivory).
 Each treatment factor had two levels: light availability (no shade/shade)
 and herbivory (present/absent). Light level (shade) was manipulated using black
 plastic mesh (¼-in mesh
 openings, neutral spectral
 density). The mesh imitated natural shading by phytoplankton in the water
 column and attenuated light levels reaching benthic surfaces. The
 mesh was attached to a PVC
 frame using cable ties. Stainless steel all-thread support rods were
 inserted approximately 15 cm into holes drilled into the bedrock, and the
 rods were cemented into the holes with Z-spar marine epoxy (Koppers Splash
 Zone A-788 kit). The shading mesh was elevated approximately 20 cm over
 the rock surface using the rods to minimize wave resistance but allow
 access to the plots and reduce abrasion of the algae by the mesh at high
 tide. The mesh was also approximately 10 cm2 wider than the plot to
 minimize edge effects. To quantify light levels in shaded vs. unshaded
 plots, at each site we placed four HOBO light/temperature sensors (ONSET
 Computer Corp., Part #: UA-002-64) underneath the mesh in the shaded
 treatments, and four sensors were placed near unshaded treatments.
 Light/temperature sensors recorded continuously at thirty-minute intervals.  Using the resulting data,
 we estimated that during a full tidal cycle (including immersion and
 emersion periods), the mesh attenuated light level an average of 64% and
 59% (i.e., 100 – [mean shaded light intensity / mean unshaded light
 intensity] * 100) throughout the experiment at Strawberry Hill and Cape
 Blanco North, respectively. Herbivory was manipulated by coating a square band
 of Z-spar marine epoxy (Koppers Splash Zone A-788 compound) placed around
 each plot with copper-based antifouling paint (Pettit Trinidad SR
 Antifouling Bottom Paint). Previous research has shown that such paint
 excludes “flat” grazers such as limpets and chitons (Sousa 1979, Cubit
 1984, Paine 1984, Farrell 1988, Menge 2000, Menge et al. 1999).
 The experiments were
 monitored monthly, when damaged shades were repaired and fouling organisms
 were removed. To maximize time available for experimental setup and
 monitoring at the low intertidal zone of these experiments, we did not
 establish paint controls. Since prior experiments in this system that did
 include such controls never found a paint artifact, we believe this
 omission did not affect the robustness of our
 results. Response variables in the experiment were monthly
 estimates of kelp density and length of each individual kelp thallus.
 Density was measured by counting all individual kelp present in the plots,
 including both the established ones and new recruits. Length of each
 individual was measured as the distance between the holdfast and the tip
 of a longest blade. Kelp density and length increments were standardized
 by subtracting the
 initial mean value at the beginning of the experiment (T0) from the mean
 value at subsequent time points (Tn) for each replicate. These replicates
 then were averaged for each increment (Tn - T0).
 At both sites, we measured
 chlorophyll-a levels every month during the sampling period using bottle
 samples taken from the surf zone (Menge et al. 1997). We also quantified
 abundance of small mobile species using the transect-quadrat method.
 Surveys were conducted annually in the low intertidal zone (i.e., tidal
 height ± ~0.7 m) in June or July. The method involves visual sampling in
 10 haphazardly chosen 0.25 m2 quadrats per each of two to three sectors
 per site. Quadrats were spaced at ~three-meter intervals along 30 m
 transects placed in the center of each zone parallel to the water’s edge.
 Mobile organisms were then counted in each 0.25 m2 quadrat to provide
 density estimates. We present data from 2016-19.