Effects of an epiphytic bryozoan on the growth and photosynthetic production of the giant kelp Macrocystis pyrifera

Giant kelps of temperate coastal regions are among the most productive marine macrophytes and play an important role in the blue carbon cycle. Macrocystis pyrifera is the dominant kelp species on the North American Pacific coast occurring over rocky reef substrate. Encrusting bryozoans of the genus Membranipora are a common epiphyte throughout M. pyrifera’s range and form a crustose, opaque layer on kelp blades. Reduction of incident light by this layer may have negative effects on photosynthesis thus reducing primary production. This could have implications for kelp growth and surface water chemistry within kelp forests, which have been shown to alter local biogeochemistry. Effects of bryozoan encrustation on kelp primary production were measured using in situ growth surveys and photosynthesis/respiration (P/R) trials from June – August, 2019 in Monterey, CA. Blades on tagged surface fronds were measured weekly using a hole-punch method. P/R trials were conducted during the day and night using blades of varying levels of bryozoans inside sealed plastic bags with an oxygen probe. Growth surveys revealed that most blades were no longer growing by the time they were colonized by bryozoans, but encrusted blades showed a slightly higher decline in blade length compared to blades without bryozoans. Unencrusted blades exhibited marginally significant higher rates of daytime oxygen production (F_1,14 = 4.78, p = 0.046). Recent studies have shown a positive correlation between warming ocean temperatures, recruitment of Membranipora, and deforestation of kelp beds. As the oceans continue to warm, outbreaks of this bryozoan could have detrimental effects on kelp forests and the ecosystem services they provide, both ecological and biogeochemical.