3. Results
3.1. Hydrodynamics
In 2008, the water level (WL) of the Daning River ranged from
144.69 m to 172.77 m, with higher flow in the flood season than in the
dry season (Fig. 3a). At station D4, the relative stability of the water column
(RWCS) in the upper (15 m) was strongly associated with the
hydrologic regime, reached its maximum of 166 in July 2008, and approximately
zero from October 2008 to February 2009 (Fig. 3b). According to
variation of the RWCS, mixing regime could be identified: stratification
(May–September 2008, March–April 2009) and mixing (October 2008–
February 2009).
3.2. Physical and chemical characteristics
The physical and chemical characteristics of the Daning River showed
significant spatial differences (estuary, midstream and upstream) and
temporal differences (flood season and dry season) (Fig. 4a–h).Although water temperature was significantly lower in the upstream
than in the midstream and estuary during the flood season (pb0.05),
there were no significant spatial differences during the dry season
(p>0.05) (Fig. 4a). Water temperature was at its highest in August and
lowest in February.
Euphotic zone (Zeu) and light availability (Zeu/Zmix) were higher,
and turbidity (Turb) was lower, in the midstream than in upstream and
estuary during the flood season (pb0.05); there were no significant spatial
differences during the dry season (p>0.05) (Fig. 4b, d, and c
respectively).
The annual mean of total phosphorus (TP) was higher in the estuary
than in the midstream and upstream (pb0.05) (Fig. 4e). There were
significant seasonal variations in TP levels only in the estuary, where
they were higher during the flood season than in the dry season
(pb0.05). Dissolved orthophosphate (P–PO4) was significantly higher in
the estuary than in the midstream and upstream during the flood season
(pb0.05); there were no significant differences among the three sampling
regions after the impoundment of the reservoir during the dry season
(p>0.05) (Fig. 4g). Total nitrogen (TN) and dissolved nitrate (N–NO3)
showed similar changes along the length of the river, with highest
concentrations in the estuary and lowest concentrations upstream
(pb0.05); there were no significant seasonal variations (p>0.05)
(Fig. 4f and h)