Mats of the nonnative macroalga, Gracilaria vermiculophylla, alter net denitrification rates and nutrient fluxes on intertidal mudflats

Abstract

We hypothesized that mats of a nonnative macroalga, Gracilaria vermiculophylla, which is often found incorporated several centimeters into intertidal mudflat sediments, would increase net denitrification rates relative to bare sediments. At moderate densities (∼ 40 g dry weight [dry wt] m−2), net denitrification rates in June (182.37 ± 16.87 µmol N‐N2 m−2 h−1), July (213.19 ± 16.30 µmol N‐N2 m−2 h−1), and September (124.82 ± 11.17 µmol N‐N2 m−2 h−1) were higher than rates previously documented with macroalgal mats. Compared with rates from bare sediment in June (25.48 ± 15.09 µmol N‐N2 m−2 h−1) and September (46.47 ± 15.79 µmol N‐N2 m−2 h−1), net denitrification was significantly higher when G. vermiculophylla was present. Rates measured on bare sediment in July (254.81 ± 19.86 µmol N‐N2 m−2 h−1) were not significantly different from G. vermiculophylla counterparts, most likely because of highly reduced conditions in G. vermiculophylla cores, which could have limited nitrification. July incubations also demonstrate that at higher densities (∼ 120 g dry wt m−2 G. vermiculophylla), denitrification rates can drop, suggesting a potential biomass threshold for macroalgal enhancement of denitrification.