Occurrence of organic colloids in the stratified estuary of the Krka River (Croatia)

Along a vertical profile of a stratified estuary, we determined organic carbon in the following fractions: low molecular weight (LMW-OC<0•02 μm), colloidal (0•02 μm 0•7 μm). The results showed abundant concentrations of organic colloids accounting for: 40% of total organic carbon (TOC) in the overlying brackish water (salinity 2), 22% in the underlying marine water (salinity 38) and 19% (±5%) for intermediate salinity samples (salinity 3–31). Upon contact with seawater (in the salinity range 2–5) both DOC and COC concentrations decreased drastically (44 and 47 μM respectively), whereas POC slightly increased (11 μM), being consistent with previous laboratory and field studies, and indicating that at low salinities, the colloidal fraction is actively involved in the DOC physicochemical aggregation. In the halocline, we observed that organic colloids and relatively degraded particles accumulated together in the lowest part of the interface (salinity 31). Because previous studies in the same area have shown accumulation of degraded cells and fragments in the bottom of the halocline, we suggest that large colloids (COC in this study) at the interface probably originated from fragmentation of non-living organisms or aggregates. Furthermore, it is likely that these processes which are enhanced in a highly stratified estuary, are partly responsible for the DOC non-conservative dilution observed along this profile.