Nitrogen cycling in a turbid, tidal estuary

Abstract

In this thesis I investigated nitrification, dissolved inorganic and organic nitrogen uptake, and the relative importance of nitrification and ammonium assimilation. I have also investigated exchange with marshes and sediments.

Nitrification, oxidation of ammonium to nitrate is the first step for removal of nitrogen from a system. Evaluating a suitable method to quantify nitrification in estuarine systems was the subject of the first study. One method quantifying nitrification as nitrifier activity was compared to another method using growth of nitrifying bacteria in the Scheldt estuary over a salinity gradient. Measurements were made during 4 seasons using 15N enriched ammonium and 14C labeled carbon incorporation. Our study demonstrated that the conversion of growth rates to nitrifying activity induces uncertainty because activity and growth of nitrifiers may be uncoupled.

Uptake of dissolved ammonium, nitrate, nitrite, urea and amino acids was the topic of the second study. It was studied in the Scheldt estuary during the same cruises mentioned above. Urea and amino acids constituted up to 43 and 29 % of total nitrogen uptake, respectively, and were of similar importance as inorganic substrates. In January, April and November amino acids constituted a source of both nitrogen and carbon while urea mainly constituted a source of carbon. During the summer months amino acids were used mainly as a source for nitrogen, while urea was a source for both carbon and nitrogen; urea was rarely used as nitrogen substrate alone.

The subject of the third study was production of N2. Sediment nitrogen removal via production of N2, depends on a number of factors. This study showed that the interaction between two factors, temperature and nitrate concentration in the overlying water, governs benthic production of N2, whereas these factors separately have a minor effect. Increasing temperatures above ambient levels, both during winter and summer, caused the flux of ammonium out of the sediment to increase.

After investigating the abiotic factors, temperature and nitrate concentration on N2 production, attention turned in the fourth study to biotic factors, such as macrofauna. We successfully defaunated the sediment of a tidal flat in situ and were able to follow the effect of recolonization on biogeochemical processes. The experiment did not last long enough for the sediment to return to natural conditions. Nevertheless, a strong effect of faunal community on biogeochemical processes were found. N2 production rates were up to 70 % lower in defaunated sediment compared to control sediment.

The fifth study was a whole-ecosystem whole-ecosystem labelling experiment in a freshwater marsh. 15N-NH4+ was added to the floodwater entering a tidal marsh area, and marsh ammonium processing and retention were traced in six subsequent tide cycles. Data were presented for the water phase components of the marsh system, in which changes in concentration and isotopic enrichment of NO3-, NO2-, N2O, N2, NH4+ and suspended particulate nitrogen were used in a mass balance study. 31 % of added 15N-NH4+ was retained or transformed. NO3- was the most important pool for 15N, with nitrification accounting for 30 % of 15N-transformation. Whole-ecosystem nitrification rates were four to nine times higher than those in the water column alone, implying a crucial role for the large reactive marsh surface area in N-transformation.