Book Description

Giant kelps are among the most productive marine macrophytes and can alter surface water chemistry through photosynthesis and respiration. Along the California coast, Macrocystis pyrifera is the dominant kelp species and is commonly encrusted by the bryozoan Membranipora spp., which forms a crustose, opaque layer on kelp thalli. Reduction of primary production by this bryozoan could have implications for kelp growth and surface water chemistry via the production of oxygen and uptake of carbon within kelp forests. The effects of bryozoan encrustation on growth were measured in situ using a hole punch method. Effects on primary production were investigated by measuring dissolved oxygen production in situ and in the laboratory using sealed respirometry chambers that included artificial shading of blades to mimic bryozoan encrustation and measuring pigment concentration. Patterns of Membranipora spp. abundance and photosynthetic pigment concentration in kelp blades were also examined in a kelp forest in Santa Barbara, CA over the course of a year to understand potential impacts and patterns of Membranipora encrustation over longer periods. Growth surveys revealed that blade age, and not bryozoans, was the main factor affecting growth rates although encrusted blades showed a slightly greater decline in blade length. In lab experiments, O2 production rates of non-encrusted blades was up to 2 times greater than encrusted blades, while O2 consumption rates were up to 1.5 times greater for encrusted blades due to bryozoan respiration. Artificial shading of blades did not affect oxygen production rates, but encrusted blades showed decreased pigment concentrations at increasing levels of bryozoan encrustation. Relative abundances of Membranipora spp. were highest in the summer months, and decreased pigment concentrations were correlated with older blades and higher presence of Membranipora spp. These results suggest that bryozoan encrustation can significantly decrease primary productivity of M. pyrifera which may translate to reduced CO2 uptake. With the suggestion that kelp forests may serve to locally ameliorate against rising CO2 in surface waters and the widespread and increasing abundance of Membranipora spp. in some regions, bryozoan encrustation should be taken into consideration when evaluating the uptake potential of CO2 by giant kelp.