Nitrogen recycling in coastal waters of southeastern U.S. during summer 1986

Summer-time pelagic nitrogen recycling using an 15NH4+ tracer technique was studied for important biological pathways, sinks, and residence times in the microbial food web of coastal Georgia, U.S.A. Results showed that estimated rates of NH4+ regeneration by a variety of microheterotrophs and microzooplankton balanced rates of NH4+ assimilation by the microbial community, i.e., phytoplankton, bacteria and other microheterotrophs, in surface waters. In bottom water below the 1% light level, NH4+ regeneration exceeded community NH4+ assimilation by 3.5 times. However, over a period of 2 weeks, high NH4+ concentrations rarely occurred in bottom waters, and this was attributed to rapid mixing of the water column by winds and tides. Estimated mixing times from time-dependent numerical models were on the order of NH4+ turnover times of 5 to 10 hours. Overall, rates of community NH4+ assimilation exceeded rate estimates of phytoplankton N demand by 1.8 to 2.7 fold, which were made from rates of 14CO2 incorporation into proteins. In bottom samples where phytoplankton were light limited, rates of NH4+ assimilation exceeded the phytoplankton N demand by 3.6 to 11.1 fold. Calculations concerning the role of dissolved organic nitrogen in N cycling suggest that only 10% is recycled to NH4+ daily. This N could support 12 to 29% of the estimated phytoplankton N demand. Residence times of particulate nitrogen pools, based on rates of NH4+ regeneration, were on the order of 3-10 days. During summer-time hydrographic conditions and despite other losses, particulate nitrogen could recycle as often as 100 times before autumn removal processes occur off southeastern U.S.