The role of dissolved organic and inorganic nitrogen for growth of macrophytes in coastal waters of the Baltic Sea

Macrophytes and phytoplankton compete for inorganic nitrogen during growth, even in eutrophied coastal waters containing relatively high nitrogen concentrations. In this study we investigated, whether dissolved organic nitrogen (DON) serves as an additional nitrogen source for rooted submerged macrophytes in several key species in the nutrient-rich inner coastal waters of the Darss-Zingster-Bodden chain, located in the southern Baltic Sea. The uptake and translocation of 15N-labeled dissolved inorganic nitrogen (DIN, ammonium and nitrate) and DON (amino-acid mixture) were measured for three common species: Chara aspera, Chara tomentosa, and Stuckenia pectinata. A two-compartment-device was used to discriminate between the roles of roots and shoots in N uptake. The results showed that DON and DIN were taken up by all species, but ammonium (mean 0.116%15N mg DW− 1 h− 1) was preferred over amino acids (mean 0.024%15N mg DW− 1 h− 1) which were preferred over nitrate (mean 0.007%l15N mg DW− 1 h− 1). To our knowledge, this is the first study to demonstrate the uptake of DON in charophytes and the submerged angiosperm S. pectinata. Both nitrate and ammonium, as DIN, were translocated in the basipetal and acropetal directions in Characeae, which was unexpected given the lack of vascular bundles in these species. By contrast nutrient transport was below the detection limit in the vascular macrophyte S. pectinata. The translocation of DON was not observed in any species or in any direction. Our findings suggest that rooted plants have an advantage over phytoplankton based on their ability to assimilate and transport nutrients not only from the water column but also from the sediments, whereas phytoplankton can only use nutrients of the water column.