Physical parameters

The mean spring tidal range (absolute water level rise from lower LW until higher HW) at the upper, middle and outer estuarine site was 169 cm (range: 138 - 220), 169 cm (range: 130 - 200), and 152 cm (range: 1.35 - 1.77), respectively. Overall, salinity, DO content, and pH increased towards the coast (Fig. 2). Surface water temperature displayed no consistent longitudinal gradient but was clearly influenced by season with lower temperatures in March/April (ca. 26°C) than in June-September (ca.

30°C). Water transparency also increased towards the sea, however, since the outer estuarine site was very shallow, vertical Secchi depth readings usually equaled water depth and did not cover the potential visibility range, which usually exceeded 2 m.

Fig. 2: Medians with 25-75% quartile and min-max of surface salinity, dissolved oxygen content, pH, water temperature, and Secchi depth for each spring tide cycle sampled at the upper (grey), middle (dotted) and outer (white) estuarine site in 2007, 2008, and 2009.

n.d.: no data.

Fig. 3: Examples of spring tidal variations in physical parameters at the three fixed estuarine sites (a-c) of the Wenchang/Wenjiao estuary (East Hainan) in Mar/Apr 2009.

Grey columns indicate night. Filled symbols: surface water; open symbols: bottom water 0.5 m above the ground. Heavy rainfall in (a) occurred from 16:15 - 16:45 h, and in (c) from 19:00 - 21:00 h, and from 05:00 - 06:00 h. n.d.: no data.

During the spring tide cycles, physical parameters usually oscillated regularly with the tide irrespective of season and sometimes values also varied in interaction with the diel cycle (Fig. 3). At the upper estuarine site, surface and bottom water

salinity usually co-varied with water level, except after strong rainfall as witnessed during the tidal cycle in April 2009, when surface salinities stayed permanently low (Fig.

3a). Despite a water depth of only 2 to 4 m, the water column was highly stratified with bottom water salinities often being >10 units higher than the salinity at the surface.

Surface water temperatures were always highest around midday and the early afternoon and lowest in the morning. After sunrise, water temperature often continued to fall and only rose with a time delay. At night, surface water temperatures could intermittently be colder than the more saline bottom water (not observed during the tidal cycle shown in Fig. 3a, but in the first and second 2007 and in the first 2008 tidal cycle; see Electronic supplementary material 1). DO contents co-varied with water level and were usually higher at the bottom. Maximum values always occurred around the higher HW. This was the only period when DO at the surface could exceed values at the bottom, likely due to increased in situ phytoplankton activity in the upper water column during maximum insolation (see Electronic supplementary material 1). This effect is not visible in Fig. 3a because run-off from a thunderstorm at the beginning caused a rapid decrease and permanently low water transparencies throughout the tidal cycle. The pH was stratified with higher pH values at the bottom. Higher values were recorded during the higher HW period, when the landward excursion of the tide reached its maximum. Secchi depth was usually lowest at lower LW and highest after the higher HW at the onset of ebb tide. The current was bi-directional, upstream (20°) at flood and downstream (195°) at ebb tides. Flood current speeds reached a maximum of 0.3 ms^-1, while ebb tide current speeds could exceed 0.6 ms^-1. Around slack lower LW in the upper and middle estuarine site there was outflowing surface water despite rising water levels, indicating that at early flood tide marine water entered the system along the bottom.

In the middle estuarine site, salinities also co-varied with water level (Fig. 3b).

The water was also stratified, often in the range of 5 units. Around slack lower LW, salinities always displayed an overall cycle minimum, likely due to water flowing off the Wenchang River and temporally influencing the lagoon. Bottom water temperatures were usually 1.5°C cooler than surface water temperatures. Lowest bottom water temperatures were recorded during the higher HW (maximum intrusion of the tide).

Surface water temperatures were lowest in the morning and reached a maximum in the afternoon during lower LW. DO contents were usually higher at the bottom than at the surface, especially during the higher HW (maximum intrusion of the tide). Surface DO contents were usually highest at late ebb tide after the ebb tide minimum in Secchi depth and when lower LW still occurred at daylight; DO then decreased through the night. Intermittently, lower DO values around midday were likely due to photoinhibition.

pH co-varied with the changes in DO. Secchi depth was usually lowest at lower LW and highest after the higher HW at the onset of ebb tide. The currents were also bi-directional, upstream at flood (325°) and downstream (120°) at ebb tide, and of similar strength as at the upper estuarine site.

In the outer estuarine site, salinity also co-varied with water level (Fig. 3c).

Another maximum at lower LW was likely due to evaporation given water depths <1 m and maximum daily water temperatures during this tidal stage. The site was characterized by the smallest range of salinity values during tidal cycles, i.e. relatively high and stable salinities over time. Yet, the water column of the back-reef area displayed a minor stratification, with slightly higher salinities (<1 unit) and lower temperatures (<1°C) close to the ground as evidenced by samples taken manually with the sample bottle a few centimeters above the substrate. This was not always detectable in our standard measurements, which were taken ca. 0.5 m above the ground. Water temperatures usually varied inversely to water level. Intense rains from 19:00 to 21:00 on Mar 29 caused a delayed fall in surface salinity and a rise in water temperature. DO contents were extremely high (>10 mg l^-1) during the lower LW phase or even increased after sunset with slightly rising water levels. Minimum values were recorded in the morning and increased again with a time delay when the higher HW occurred around sunrise. The range in DO contents was the greatest of all estuarine sites. Again, pH co-varied with the changes in DO but displayed a low pH-spike due to a second thunderstorm between 05:00 and 06:00 on Mar 30. Secchi depth was always equal to water depth, except for periods of stronger ebb tide currents or wave-induced sediment resuspension. The currents were usually alongshore (340° at flood and 130°

at ebb tide), generally weak, and rarely exceeding 0.2 ms^-1. However, wind and waves could significantly alter surface current directions at this site.

The mean net tidal excursion based on surface current measurements at the upper, middle and outer estuarine site was 7.3 km downstream (±2.6 SD), 3.6 km downstream (±4.7 SD), and 0.9 km westward (±0.6 SD), respectively. This indicates that the lagoon was clearly an ebb-dominated system at spring tides.