Nitrogen regeneration mediates species interactions and enhances coastal productivity

Background/Question/Methods

Along the western margins of several continents, seasonal upwelling provides nutrients for coastal ocean productivity.   However, animal excretion activities that regenerate nitrogen can also contribute to local nutrient supply.  We tested whether the California mussel makes significant contributions to local primary production and whether this regenerated nitrogen is retained via microbial activity.  In separate experiments, we added enriched ¹⁵NH₄ (ammonium) or ¹⁵NO₃ (nitrate) and enriched H¹³CO₃ (bicarbonate) into tidepools with and without mussels and followed its fate as a function of animal abundance and varying environmental conditions.  We also examined natural variation in nitrogen isotope abundance over a 60 km gradient of animal abundance.  Finally, we documented the microbial composition in these experimental tidepools by analyzing 16s rRNA sequence data, looking in particular for nitrogen-transforming taxa.

Results/Conclusions

Ammonium concentrations and seaweed growth rates increased in the presence of animals and seaweeds show higher growth rates with these animals.  Microbial ammonium oxidation (nitrification) also increased with mussel presence, and was an order of magnitude higher in tidepools than in open ocean environments. The signal of added nitrogen was detectable in the water of the tidepool, even after repeated inundation, suggesting local recycling and retention.  Nitrogen transformations differed during daylight versus nighttime hours: as oxygen declined at night due to animal respiration in the absence of photosynthesis our tracer signal was transferred from nitrate to nitrite and ammonium, suggesting low oxygen requiring processes such as DNRA and denitrification.  Along our large-scale animal gradient, there is an isotopic enrichment in the tissues of plankton, surfgrass, and mussels in areas with greater animal abundance, suggesting increased nitrogen recycling in these areas.  Our analysis of the microbial taxonomy indicated a community capable of diverse nitrogen transformations, which lives both on inert substrates and in association with the animals themselves. In total, mussels and microbial associates appear to play a significant role in nearshore nutrient dynamics.