Denitrification and nitrogen fixation dynamics in the area surrounding an individual ghost shrimp (Neotrypaea californiensis) burrow system

Bioturbated sediments are thought of as areas of increased denitrification or fixed-nitrogen (N) loss; however, recent studies have suggested that not all N may be lost from these environments, with some N returning to the system via microbial dinitrogen (N(2)) fixation. We investigated denitrification and N(2) fixation in an intertidal lagoon (Catalina Harbor, CA), an environment characterized by bioturbation by thalassinidean shrimp (Neotrypaea californiensis). Field studies were combined with detailed measurements of denitrification and N(2) fixation surrounding a single ghost shrimp burrow system in a narrow aquarium (15 cm by 20 cm by 5 cm). Simultaneous measurements of both activities were performed on samples taken within a 1.5-cm grid for a two-dimensional illustration of their intensity and distribution. These findings were then compared with rate measurements performed on bulk environmental sediment samples collected from the lagoon. Results for the aquarium indicated that both denitrification and N(2) fixation have a patchy distribution surrounding the burrow, with no clear correlation to each other, sediment depth, or distance from the burrow. Field denitrification rates were, on average, lower in a bioturbated region than in a seemingly nonbioturbated region; however, replicates showed very high variability. A comparison of denitrification field results with previously reported N(2) fixation rates from the same lagoon showed that in the nonbioturbated region, depth-integrated (10 cm) denitrification rates were higher than integrated N(2) fixation rates (～9 to 50 times). In contrast, in the bioturbated sediments, depending on the year and bioturbation intensity, some (～6.2%) to all of the N lost via denitrification might be accounted for via N(2) fixation.