Estuarine environments have varying degrees of flocculation from river mouth further up the salinity gradient which is important in mediating the transport of material from terrestrial to marine environments. Salt induced flocculation of particulate nitrogen (PN) and particulate phosphorus (PP) was studied along a salinity gradient in a laboratory flocculation experiment and in an estuary of the northeastern Baltic Sea to detect and quantify biogeochemical transformation of nutrients found in estuarine environments. Measurable transformation of N and P from the dissolved pool to the particulate pool occurred at low salinities (< 2.5 PSU), during the laboratory experiment which was not observed in the field due to the high field salinities present. Flocculation at higher salinities in the field are attributed to mechanisms where dissolved P aggregates into PP in the presence of iron. This mechanism decouples N and P, promoting nutrient specific enrichment in different size fractions where PP is in the intermediate size fraction (3.0–10.0 µm), and PN is in the smallest size fraction (1.0–3.0 µm). Our results highlight that nutrient cycling in estuarine environments is complex and understanding the decoupling mechanisms between N and P during flocculation plays a key role in the fate and transformation of nutrients.
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