Increased supply of ambient nitrogen has minimal effect on salt marsh bacterial production

We examined the role of chronic low-level nutrient enrichment on the productivity of heterotrophic marsh bacteria via a marsh fertilization experiment in which we mimicked the conditions of widespread coastal eutrophication by enriching entire salt marshes to approximately 153 background nutrient concentrations. We measured the uptake of tritiated leucine, as a proxy for bacterial production, in both low and high marsh habitats in four salt marshes, two of which were enriched with nutrients. We hypothesized that adding nitrogen in these detritus-rich systems would directly stimulate bacterial decomposition of marsh peat. Contrary to our expectations, we found no response to added nutrients in high marsh habitats, where there is a significant supply of organic matter from marsh vegetation. Bacterial production did increase in the low marsh habitats, where fertilization increased the standing stock of benthic chlorophyll. Fertilization did not directly increase bacterial production by providing added nutrients that could be used to decompose organic matter derived from nutrient-poor marsh grasses. Rather, bacterial productivity was indirectly stimulated by the concomitant increase in labile benthic microalgae in low marsh habitats. Decomposition of salt marshes may therefore have a greater resilience to the threat of chronic eutrophication than has been previously recognized.