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).

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.