Testing effects of bottom-up factors, grazing, and competition on New Zealand rocky intertidal algal communities

Abstract

Top-down and bottom-up factors and their interaction highlight the interdependence of resources and consumer impacts on food webs and ecosystems. Variation in the strength of upwelling-mediated ecological controls (i.e., light availability and herbivory) between early and late succession stages is less well understood from the standpoint of influencing algal functional group composition. We experimentally tested the effect of light, grazing, and disturbance on rocky intertidal turf-forming algal communities. Studies were conducted on the South Island of New Zealand at Raramai on the east coast (a persistent downwelling region) and Twelve Mile Beach on the west coast (an intermittent upwelling region). Herbivory, light availability, and algal cover were manipulated and percent cover of major macroalgal functional groups and sessile invertebrates were measured monthly from October 2017 to March 2018. By distinguishing between algal functional groups and including different starting conditions in our design, we found that the mosaic-like pattern of bare rock intermingled with diverse turf-forming algae at Twelve Mile Beach was driven by a complex array of species interactions, including grazing, predation, preemptive competition and interference competition, colonization rates, and these interactions were modulated by light availability and other environmental conditions. Raramai results contrasted with those at Twelve Mile Beach in showing stronger effects of grazing and relatively weak effects of other interactions, low colonization rates of invertebrates, and light effects limited to crustose algae. Our study highlights the potential importance of an upwelling-mediated 3-way interaction among herbivory, light availability, and preemption in structuring contrasting low rocky intertidal macroalgal communities.

Keywords: algae; coastal upwelling; competition; disturbance; grazing; light; rocky intertidal; top‐down/bottom‐up.

FIGURE 1

A summary of upwelling‐mediated effects on intertidal macroalgal diversity. Intermittent to persistent upwelling [colored blue]. Intermittent to persistent upwelling brings a large amount of nutrients to the surface (depicted by a solid line, indicating a positive effect). Top path: Increased nutrients will promote higher grazing activity on macroalgae, which in turn will increase the algal diversity. Bottom path: Increased nutrients will promote phytoplankton blooms and reduce the amount of light reaching the benthos, which in turn will reduce macroalgal diversity (depicted by a dashed line, indicating a negative effect). Little to no upwelling (or downwelling) [colored orange]. Little to no upwelling brings a limited amount of nutrients to the surface. Top path: Limited nutrients will decrease grazing activity on macroalgae, which in turn will reduce the algal diversity. Bottom path: Limited nutrients will allow a high amount of light to reach the benthos, which in turn will increase macroalgal diversity. Disturbance also plays a role in influencing macroalgal diversity by initiating algal succession. The shaded region represents the unknown effects of a three‐way interaction among grazing, light availability, and disturbance (or preemption) on macroalgal communities, and the 3‐way interaction is the central question in our study.

FIGURE 2

Map of the two study sites, Raramai (−42.46° S, 173.55° E) and Twelve Mile Beach (−42.82° S, 171.82° E) on opposite coasts of the South Island of New Zealand. Stars = our study sites and circles = other long‐term monitoring sites (Menge et al., 1999, 2003).

FIGURE 3

Average abundance of free space (bare rock plus crustose algal cover), “short” fleshy algae (turf‐forming algae, filamentous algae, sheet‐forming algae), and sessile invertebrates (mostly mussels and barnacles) at six New Zealand study sites. Site codes are RR = Raramai, BT = Box Thumb, SP = Shag Point, WB = Woodpecker Bay, TMB = Twelve‐Mile Beach, and JH = Jackson Head. Data are mean ± 1 standard error of the mean, and are overall averages across zones from periodic surveys taken between 1995 and 2010.

FIGURE 4

Average percent cover (all treatments combined) of macroalgal functional groups across months in the cleared plots in Raramai and Twelve Mile Beach. All values are arithmetic means ± standard error. Each panel corresponds to a macroalgal functional group. The site abbreviations and line type are: RR = Raramai (solid line) and TMB = Twelve Mile Beach (dashed line). Macroalgal functional group line codes are: Sheet = green, Filamentous = blue, Coarsely Branched = purple, Jointed Calcareous = red, Crustose = orange, Invertebrates = gray.

FIGURE 5

Average percent cover (all treatments combined) of macroalgal functional groups across months in the uncleared plots at Raramai and Twelve Mile Beach. All values are arithmetic mean ± standard error. Each panel corresponds to a macroalgal functional group. Site and color codes are as in Figure 4.

FIGURE 6

Average percent cover of macroalgal functional groups at Raramai throughout the experiment for eight different treatments. All values are reported using arithmetic mean ± standard error. Each panel corresponds to a treatment. The top row shows uncleared plot (−Cl) results, and the bottom row shows cleared plot (+Cl) results. Herbivore effects are shown by comparing the first two columns (+He) to the last two columns (−He). Shading effects are shown by comparing the first column (−Sh) to the second (+Sh), and the third (−Sh) to the fourth (+Sh). Macroalgal functional group line codes are: Sheet = green, Filamentous = blue, Coarsely Branched = purple, Jointed Calcareous = red, Crustose = orange, Rock = black. Functional groups with cover <5% for all months are not shown.

FIGURE 7

Average percent cover of macroalgal functional groups in Twelve Mile Beach throughout the experiment for eight different treatments. All values are reported using arithmetic mean ± standard error. Each panel corresponds to a treatment. Layout with respect to treatments is the same as that in Figure 6. Macroalgal functional group line codes are the same as in Figure 6 caption with the addition of Invertebrates = gray.

FIGURE 8

Average percent cover of sheet algae at Twelve Mile Beach throughout the experiment for eight different treatments. All values are reported using arithmetic mean ± standard error. Each panel corresponds to a treatment. The treatment layout is the same as in Figure 6. Sheet phylum group line codes are: Green Sheet = green solid, Brown/Red Sheet = orange dot‐dash.