Turf wars: Evaluating competition between calcifying and non-calcifying macroalgae in a coastal upwelling region

Background/Question/Methods

On the Oregon coast, the low rocky intertidal zone is dominated by a diverse mosaic of turf-forming algae, including calcifying coralline algae and non-calcifying fleshy algae. These two algal types are expected to respond differently to ocean acidification, an increase in surface-seawater CO₂ levels. While coralline algae are predicted to be negatively impacted by ocean acidification, fleshy algae may be less sensitive to, or potentially benefit from, CO₂ enrichment. The direct effects of ocean acidification may also be amplified by species interactions. In shallow volcanic CO₂ vent systems, competitive shifts between coralline and fleshy algae have been demonstrated, as coralline algae were overgrown by fleshy algae in zones with higher CO₂. The Oregon coast is another naturally acidified ecosystem, where upwelling draws up high-CO₂ water from depth. To evaluate spatial competition between coralline and fleshy turfs, we conducted a manipulative field experiment at two sites. At each site, competition was evaluated between Corallina vancouveriensis/Bossiella plumosa (corallines) and either Dilsea californica (fleshy) in wave-exposed areas or Neorhodomela larix (fleshy) in wave-protected areas. The three treatments were both turfs present, coralline present/fleshy removed, and coralline removed/fleshy present. Plots were tracked through time with photographs, and percent cover of each turf was determined.

Results/Conclusions

No spatial competition was observed between coralline and fleshy turfs over the 10-month period from June 2014 to April 2015. In treatments where coralline algae were removed, fleshy turf cover either remained steady or declined, corresponding to the outcomes in the control where neither turf was removed. In treatments where fleshy algae were removed, coralline turf cover remained steady, also similar to the control. Neither coralline nor fleshy turfs increased their occupancy when space was cleared. This lack of apparent competition suggests that competitive strengths between coralline and fleshy turfs were comparable under the conditions of the experiment, contributing to the maintenance of a diverse algal mosaic in the low rocky intertidal zone. Although current observations do not support the dynamics predicted under ocean acidification, this study provides a baseline for future investigations as global change accelerates.