Abstract
Abstract. Seawater microbes as well as those associated with macrobiota are increasingly recognized as a key feature affecting nutrient cycling. Tidepools are ideal natural mesocosms to test macrofauna and microbe interactions, and we quantified rates of microbial nitrogen processing using tracer enrichment of ammonium (15NNH4) or nitrate (15NNO3) when tidepools were isolated from the ocean during low intertidal periods. Experiments were conducted during both day and night as well as in control tidepools and those from which mussels had been removed, allowing us to determine the role of both mussels and daylight in microbial nitrogen processing. We paired time series observations of 15N enrichment in NH4+, NO2− and NO3− with a differential equation model to quantify multiple, simultaneous nitrogen transformations. Mussel presence and daylight increased remineralization and photosynthetic nitrogen uptake. When we compared ammonium gain or loss that was attributed to any tidepool microbes vs. photosynthetic uptake, microbes accounted for 32 % of this ammonium flux on average. Microbial transformations averaged 61 % of total nitrate use; thus, microbial activity was almost 3 times that of photosynthetic nitrate uptake. Because it accounted for processes that diluted our tracer, our differential equation model assigned higher rates of nitrogen processing compared to prior source–product models. Our in situ experiments showed that animals alone elevate microbial nitrogen transformations by 2 orders of magnitude, suggesting that coastal macrobiota are key players in complex microbial nitrogen transformations.
Highlights
Nitrogen cycle processes are carried out by a diversity of taxa, from microbes to macrofauna, that can all reside in the same habitat
Ammonium concentration was overall greater with mussels and during the day, and oxygen, temperature and pH all tended to be greater during the day
Previous analysis of ammonium uptake in this system indicated that suspended particles in tidepool seawater account for a negligible amount of ammonium uptake and microbial activity in tidepool seawater was an order of magnitude less than benthic microbial activity (Pather et al, 2014)
Summary
Nitrogen cycle processes are carried out by a diversity of taxa, from microbes to macrofauna, that can all reside in the same habitat. Most studies tend to focus on characterizing and/or measuring the rate of only a single transformation at a time (e.g., nitrification or nitrate reduction), despite the co-occurrence of a diversity of nitrogen processes, including those leading to loss or retention. Given an anthropogenic doubling over the past century of the supply rate of biologically available nitrogen to ecosystems (Galloway et al, 2008; Fowler et al, 2013) simultaneous with accelerated harvest of animals that recycle nitrogen (Worm et al, 2006; Maranger et al, 2008), it is essential that we understand how microbes and macrobiota interact to influence nitrogen cycling. The paradigm of productivity driven by upwelled nitrate has been challenged by studies quantifying the effects of animal excretion and regeneration (Dugdale and Goering, 1967; Aquilino et al, 2009; Pather et al, 2014). It is well known that nitrogen regeneration is quantitatively significant in a variety of ecosystems (Schindler et al, 2001; Vanni, 2002; Allgeier et al, 2014; Subalusky et al, 2014), to make a significant contribution to productivity, uptake of animal excreted ammonium by photoand chemolithotrophs needs to be sufficiently rapid to retain nitrogen locally to avoid dispersion into the larger environment
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