Community assembly in subtidal macroalgal communities: The importance of environmental gradients




Mucciarelli, Valerie

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As human activity along coastlines increase, degradation and destruction of coastal marine ecosystems around the globe will increase at an alarming rate. In an effort to mitigate degradation and destruction of coastal marine ecosystems, artificial reefs have been used in restoration and enhancement projects. As artificial reefs are the main method of restoring diversity to a degraded area, it is important to know the mechanisms that drive marine community assembly and diversity on those reefs. Understanding community assembly patterns of foundational species, in particular, may provide insight to community assembly patterns at higher trophic levels. Subtidal macroalgae are commonly seen as foundational species in marine environments and both deterministic and stochastic processes play a role in their assembly. Environmental gradients, which are deterministic processes, play a significant role in structuring subtidal macroalgae communities. Depth, which is negatively correlated with light, is the main driver structuring subtidal macroalgal communities, however, other gradients such as water flow, and distance to a propagule source also impact their assembly. This study sought to determine which environmental gradients play a prominent role in subtidal macroalgal community assembly. To study subtidal macroalgal community assembly, 92 artificial reef units called Reef Balls were deployed east of the Ogden Point Breakwater in Victoria, BC in June 2009. Two years passed to allow for macroalgal growth and early successional processes to occur prior to sampling the communities on thirty Reef Balls via underwater collection in July 2011. Algae were sorted by genus and dry weight was measured. To determine effects of environmental gradients on community assembly light, depth, water flow, distance to the nearest Reef Ball and distance to the breakwater were measured at each Reef Ball. A redundancy analysis revealed that depth was the most significant environmental gradient shaping algae communities and had the greatest effect on upper canopy algae. Spatial plots reveal a depth and coastline zonation of algae genera comprising the canopy. While depth was found to significantly structure algae genera found in the canopy, there was a high degree of unexplained variation in the model. This suggests that unmeasured variables such as colonization and priority effects may be driving algal community structure in the lower canopy. Differences in community structure between upper and lower canopy reveal that multiple mechanisms are responsible for shaping subtidal algal communities. Further study is required to determine the importance of stochastic colonization events and priority effects.



Environmental gradients, Community assembly, Macroalgae, Subtidal, Artificial reef, Marine ecosystems