Contemporaneous nitrogen fixation and denitrification in intertidal microbial mats: rapid response to runoff events

We examined the contemporaneous responses of N2 fixation and denitrification to inorganic nitrogen-ennched runoff in 2 lntertldal mlcroblal mats in Tomales Bay (California, USA) Prior to runoff N, fixation rates averaged 3 mm01 N m-' d.' Denltrificatlon rates were lower, corresponding to 0 to 25 % of N2 flvatlon rates After the initiation of runoff N2 flxatlon rates decreased approach~ng zero at both sites In contrast denltrlflcation rates Increased by an order of magnltude We developed a simple model to examine the magnltude of N removal from creek water by mat denltnficatlon before, dunng and after runoff Model results show that during peak runoff N removal was llmlted by the residence time of creek water in the intertidal region As runoff volumes decreased and residence tlmes increased N removal became llmited by the supply of organlc matter Our results illustrated the r a p ~d metabolic adaptation of m ~c r o b ~a l mats to altered N fluxes Changes occurring in the mat N cycling after the initiation of runotf led to 2 fundamental ecological changes (1) mats became sinks for rather than sources of futed N and (2) denitnfication became limited by the availability of organic carbon rather than nitrate