Seasonal dynamics of nitrification in the water column of marine Lake Grevelingen

Coastal ecosystems serve as vital connectors between land and ocean, and their nitrogen cycle and ammonium removal can be affected by various factors. During our seasonal sampling campaign in 2021, the eutrophic marine Lake Grevelingen exhibited high ammonium concentrations and low nitrification rates in the water column, except for a brief period in late summer. Our study revealed ammonium accumulation in the anoxic middle and bottom water layers due to restricted transport caused by water column stratification. Only when ammonium reached the oxic part of the water column, was a short-lived peak in nitrification activity and nitrifier abundance observed at the end of August. Amplicon sequencing indicated very low abundances of Nitrosococcus and Nitrospira (<0.2%) in March, while Nitrosomonas and Nitrospina peaked at the end of August with relative abundances of 2.5% and 1.3%, respectively. Archaea and archaeal ammonium oxidizers were found in very low abundances. Anammox 16S rRNA genes were not detected. Together, these observations suggest a limited role for nitrification in ammonium removal in marine Lake Grevelingen.