Climate change and species facilitation affect the recruitment of macroalgal marine forests

Abstract

Marine forests are shrinking globally due to several anthropogenic impacts including climate change. Forest-forming macroalgae, such as Cystoseira s.l. species, can be particularly sensitive to environmental conditions (e.g. temperature increase, pollution or sedimentation), especially during early life stages. However, not much is known about their response to the interactive effects of ocean warming (OW) and acidification (OA). These drivers can also affect the performance and survival of crustose coralline algae, which are associated understory species likely playing a role in the recruitment of later successional species such as forest-forming macroalgae. We tested the interactive effects of elevated temperature, low pH and species facilitation on the recruitment of Cystoseira compressa. We demonstrate that the interactive effects of OW and OA negatively affect the recruitment of C. compressa and its associated coralline algae Neogoniolithon brassica-florida. The density of recruits was lower under the combinations OW and OA, while the size was negatively affected by the temperature increase but positively affected by the low pH. The results from this study show that the interactive effects of climate change and the presence of crustose coralline algae can have a negative impact on the recruitment of Cystoseira s.l. species. While new restoration techniques recently opened the door to marine forest restoration, our results show that the interactions of multiple drivers and species interactions have to be considered to achieve long-term population sustainability.

Figure 1

In situ mean seawater temperature at the donor population site.

Figure 2

Density (a) and size (b) of recruits of C. compressa as a function of temperature during the first experiment (96 days). The errors bars show the confidence intervals.

Figure 3

Densities of recruits on living and dead Neogoniolithon brassica-florida and on artificial substrates, as a function of temperature and pH under the different treatments. The errors bars show the confidence intervals.

Figure 4

Size of recruits on living and dead Neogoniolithon brassica-florida and on artificial substrates as a function of temperature and pH under the different treatments. The errors bars show the confidence intervals.

Figure 5

Calcification rate of Neogoniolithon brassica-florida after 64 days under different treatments. Error bars show the confidence intervals.

Figure 6

Experimental set-up used to test the effects of temperature (28 and 32 °C), pH (ambient 8.07, and low 7.8) and species facilitation (artificial, coralline and dead coralline substrates) on the recruits of C. compressa. The experimental set-up was repeated 4 times, resulting in 8 header tanks, and 48 experimental tanks in which the different types of substrates were randomly assigned.