Abstract
Production and bioavailability of dissolved organic matter (DOM) were followed during a year in the nutrient-rich estuary, Roskilde Fjord (RF), and the more oligotrophic strait, Great Belt (GB), in Denmark. Bioavailability of dissolved organic carbon (DOC), nitrogen (DON), and phosphorous (DOP) was determined during incubations over six months. Overall, RF had three to five times larger pools of total nitrogen (TN) and total phosphorous (TP) and five to eight times higher concentrations of inorganic nutrients compared to GB. However, the allocation of carbon, nitrogen, and phosphorous into different pools were remarkably similar between the two systems. DON and DOP contributed with about equal relative fractions in the two systems: 72 ± 13% of total nitrogen and 21 ± 12% of total phosphorous. The average bioavailability of DOM was 25 ± 15, 17 ± 5.5, and 49 ± 29% for carbon, nitrogen, and phosphorous, respectively. The observed release of DIN from degradation of DON amounted to between 0.1 (RF winter) and 14 times (GB summer) the loadings from land and contributed with half of the total input of bioavailable nitrogen during summer. Hence, this study shows that nitrogen in DOM is important for the nitrogen cycling, especially during summer. The sum of inorganic nutrients, particulate organic matter, and bioavailable DOM (the dynamic pools of nutrients) accounted for 42 and 92% of nitrogen, and phosphorous, respectively, and was remarkably similar between the two systems compared to the difference in nutrient richness. It is hypothesized that the pelagic metabolism of nutrients in marine systems dictates a rather uniform distribution of the different fractions of nitrogen and phosphorous containing compounds regardless of eutrophication level.
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Acknowledgements
We wish to thank The Danish Nature Agency for help with water sampling. We also thank Dorete Weysmann Jensen for nutrient analysis, Colin Stedmon for help with TOC/DOC analysis, and Ciarán Murray for help with Fig. 1. This study was supported by the project ‘Integrated management of agriculture, fishery, environment and economy’ (IMAGE) project (09-067259) funded by the Innovation Fund Denmark, and project 11-105450 and 09-066396 from the Danish council for independent research. SJT was supported by the VKR Center of Excellence on Ocean Life, funded by the Villum foundation.
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Knudsen-Leerbeck, H., Mantikci, M., Bentzon-Tilia, M. et al. Seasonal dynamics and bioavailability of dissolved organic matter in two contrasting temperate estuaries. Biogeochemistry 134, 217–236 (2017). https://doi.org/10.1007/s10533-017-0357-2
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DOI: https://doi.org/10.1007/s10533-017-0357-2