Bivalve aquaculture is increasing globally. As bivalves feed, they transport large quantities of particulate matter from the water-column to the benthos, possibly stimulating biogeochemical processes. It remains unknown how this pressure changes biogeochemical cycles within coastal ecosystems over time. Using an in situ approach we measured rates of nitrogen (N) cycling across an oyster aquaculture chronosequence in a temperate coastal lagoon (Ninigret Pond, RI, USA) over a two year period. Water samples were collected and analyzed for fluxes of dinitrogen (N2), nitrous oxide (N2O), and ammonium (NH4+) and nitrate (NO3-), using membrane inlet mass spectrometry, gas chromatography, and colorimetric methods, respectively. We hypothesized that the total amount of N exchanged between sediment and water column would be higher beneath oyster aquaculture regardless of the length of time aquaculture had been in place, but that fluxes of individual N-species would change. We expected initial high release of NH4+ due to remineralization, followed by a switch to denitrification (sediment N2 release) as rates of nitrification increased, and more NO3- became available in the sediment. Similarly, we expected a pulse of N2O release immediately following implementation of aquaculture, then a return to pre-aquaculture levels as sediment N cycling processes became more efficient.
Results/Conclusions
We found that bare sediment switched from being an N consumer (-14.41 µmol N m-2 hr-1) to an N producer (553.57 µmol N m-2 hr-1; p = 0.003), a trend that remained constant regardless of aquaculture age. Similarly, overall rates of denitrification (i.e.,N2 fluxes) were greater beneath aquaculture than from bare sediment (p = 0.038), but this was driven by the two year old site. We found no difference in NH4+ flux between bare sediment and oyster aquaculture (p = 0.462), but measured a range of high uptake and release events. There was no change in N2O or NO3- flux, and we did not find any relationships between fluxes and sediment properties. Our findings indicate that loading of particulates to the sediment drove N cycling processes to chaos, by stimulating higher rates of remineralization and nitrification, after which there was intense competition for NO3- between denitrifiers and ammonification via DNRA. This theory is supported by unpredictability and variance of dominant N fluxes, and monthly changes in the dominant flux at the same sampling location. Our results demonstrate that sediment N cycling processes are dynamic and dominated by chaotic processes under the pressure of increased resource availability from bivalve aquaculture.