1 Sediment Resuspension Rate

The limnology of shallow lakes is strongly influenced by the dynamics of sediment resuspension, possibly resulting in high turbidity, promotion of internal loading of nutrients, and phytoplankton dominance. The upper layer of the sediments in shallow lakes is subjected to the exchange of free oxygen and substances contained in sediment (i.e., nutrients) with the water column through physical, chemical, and biological processes (Blom et al. 1992), thereby being related to the overall structure of the lake ecosystem.

Influenced by the monotonal flood pulse, sediment processes in TSL exhibit strong seasonality (see Chaps. 17 and 18). Sedimentation was dominant during the high-water period (e.g., Sep.–Dec.), whereas the dominant process shifts to resuspension during the low-water period (e.g., Mar.–Jun.). The resuspension rate (RR) in TSL is much higher than the gross sedimentation rate (SR) during the low-water period (Siev et al. 2018). Although the inflow of sediments to TSL during the high-water period contributed to sedimentation flux in TSL, most of the sedimentation flux in TSL was derived from the lake bed (approximately 90%) through resuspension in the low-water period. High resuspension rates in the lake during the low-water period indicate that little of the material that settles during the high-water period is permanently deposited. Therefore, sedimentation and resuspension in TSL are almost in balance (Siev et al. 2018), explaining the reason for the very low annual sedimentation rate (0.05–2.55 mm/year) in the TSL (Kummu et al. 2008; Penny et al. 2005; Tsukawaki 1997).

In our survey, RR was determined in September (n = 25) and December (n = 8) in 2016 and in March (n = 34) and June (n = 6) in 2017 in four floodplain areas around TSL (i.e., Kampong Phluk, Prek Toal, Kampong Luong, and Chhnok Tru) using sediment traps following Gasith (1975). Kampong Phluk and Prek Toal are located in the northern part of TSL, whereas Kampong Luong and Chhnok Tru were located in the southern part. The observed RR indicated a significant seasonal variation (three-way ANOVA, p < 0.001, F = 9.77) and spatial variation (three-way ANOVA, p < 0.01, F = 4.74). Higher RR was observed during the low-water period (March–June). The average water depths (±standard error) at the 39 sampling locations across the lake between 2016 and 2017 were 0.9 ± 0.6 m in September, 2.7 ± 0.6 m in December, 3.9 ± 1.4 m in March, and 4.7 ± 0.8 m in June, corresponding to average resuspension rates of 115 ± 66 g/m2/day, 93 ± 128 g/m2/day, 260 ± 246 g/m2/day, and 348 ± 227 g/m2/day, respectively. The highest resuspension rate was observed at Kampong Phluk (412 ± 278 g/m2/day), an area located in the northern part of the lake, followed by Kampong Luong (245 ± 192 g/m2/day), Chhnok Tru (191 ± 198 g/m2/day), and Prek Toal (78 ± 119 g/m2/day) (Fig. 19.1).

Fig. 19.1
figure 1

Seasonal changes in resuspension rates measured. The average concentration of TSS measured at Kampong Luong (TSSKL, 1995–2010)2, monthly average water volume in the lake (1995–2010)2,3, whole lake average concentration of TSS (TSSlake) calculated as the average concentrations of TSS from all the sampled points in September and December 2016 and in March and June 2017 (2016–2017)1, and the sediment resuspension rate measured with 73 traps installed at different locations in TSL (2016–2017)1. 1Siev et al. (2018); 2MRC database; 3Kummu et al. (2014). The error bars represent the standard deviations

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The water depth seasonally changed, resulting in seasonal variation of the dynamic ratio and water volume of the lake. The concentrations of total suspended solids (TSS) in TSL were positively correlated with RR (r = 0.45, p < 0.01) during the low-water period, whereas they showed no significant correlation (r = 0.11, p > 0.05) during the high-water period (Siev et al. 2018). This indicates that sediment resuspension in the TSL strongly contributed to TSS concentration in the water column during the low-water period. As resuspension-induced TSS concentrations in the TSL increase greatly during the low-water period, light availability is more likely to be a limiting factor for the pelagic primary production. In addition, during the low-water period, sediment resuspension can also affect nutrient cycling in TSL. For instance, phosphorus concentration in the lake water significantly depends on the interactions between the sediment and water because the phosphorus pool in the sediment is generally over 100 times higher than that in the lake water (Søndergaard et al. 2003). High TSS concentration would also be a constraint for the treatment for water supply, requiring high doses of flocculants and coagulants (e.g., aluminum and ferric salts; Siev et al. 2018).

The resuspension rate in TSL was found to be higher than those in some small shallow lakes such as Wingra Lake, Hiidenvesi Lake, Lammijarv Lake, and Peipsi Lake but lower than that in Taihu Lake (Fig. 19.2, Siev et al. 2018). A variety of factors influence sediment resuspension (e.g., lake shape, water depth, wind-wave action, vegetation; Zhu et al. 2015; Siev et al. 2018), as elaborated in the following sections.

Fig. 19.2
figure 2

Relation between the dynamic ratio and the resuspension rate in lakes in the world (modified from Zhu et al. 2015, Siev et al. 2018). The plots of TSL include results from the four sampling periods (September and December 2016 and March and June 2017). The gray area represents the range of the dynamic ratio of TSL calculated using data from MRC’s database and Siev et al. (2018). Markermeer Lake (not plotted in the above figure) has a dynamic ratio < 10 and a resuspension rate of ~1000

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2 Sediment Resuspension in Relation to Flood Pulse

The seasonal flood pulse distinguishes hydrodynamic conditions in TSL from those of typical shallow lakes, driving seasonal changes in lake bathymetry and thus its surface area and depth distribution. Such changes significantly affect sediment processes, particularly resuspension (Siev et al. 2018). The dynamic ratio has been used in analyzing the relationship between potential wave disturbance and sediment resuspension, with a higher ratio generally relating to higher RR (Bachmann et al. 2000; Zhu et al. 2015; Siev et al. 2018). For example, Fig. 19.2 indicates a positive correlation between the dynamic ratio and the resuspension rate in lakes in the world and TSL. As TSL is a shallow and flat-bottomed lake, the seasonal variation in the water depth between the low- and high-water periods directly corresponds to the lake volume and surface area. Consequently, the mean monthly dynamic ratio of the lake also changes seasonally from 14 in the high-water period to 128 in the low-water period, corresponding to the increment in TSS concentration (Fig. 19.3).

Fig. 19.3
figure 3

Monthly average TSS concentration at Kampong Luong against the dynamic ratio of TSL. The surface area of the TSL was calculated using the method in Kummu et al. (2014) based on the database of the Mekong River Commission. The error bars represent standard errors

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The bathymetry of TSL is a key factor for the higher sediment resuspension than that of the other shallow lakes mentioned above because sediment resuspension is likely to occur in lakes with low water depths and large open areas (Evans 1994; Zhu et al. 2015; Siev et al. 2018). In general, wave generation, which affects the intensity of resuspension, depends on the energy transfer from the wind to the water surface, which is a function of lake exposure to the water surface, e.g., fetch (the unobstructed distance over which the wind can blow), wind characteristics (e.g., speed, direction, and duration), and water depth (Evans 1994; Gloor et al. 1994; Fagherazzi and Wiberg 2009). However, RR in TSL during the low-water period (March and June) is lower than in Taihu Lake (dynamic ratio, 25; RR, ~400 g/m2/day) and Markermeer Lake (dynamic ratio, <10; RR, ~1000 g/m2/day) although those lakes have smaller dynamic ratios, indicating that other factors affect resuspension. For instance, while the dynamic ratio is smaller, the estimated annual sediment RR in Markermeer Lake is higher than that in Taihu Lake, which was due to stronger near-bottom currents in Markermeer Lake (Zhu et al. 2015). In TSL, which is influenced by the flood pulse, seasonal variations in water depth appear to be a moderator determining the wave-imposed hydrodynamic shear stress on the lake bed.

3 Factors Affecting Sediment Resuspension

In addition to water level fluctuations and the dynamic ratio, there are some other factors affecting resuspension in TSL, namely, wind, vegetation, bioturbation, and human activities, and their relative importance varies with water level as discussed below.

3.1 Wind

Wind-induced water disturbance is considered to be one of the most important factors affecting resuspension in shallow lakes. The simultaneous action of wind-induced currents and surface waves leads to an increase in the bottom shear stress, which may exceed the critical shear stress and cause resuspension (Churchill et al. 2004; Hawley et al. 2004). According to Sheng and Lick (1979), wind-induced waves account for up to 70% of sediment resuspension in shallow lakes. Wind also affects phosphorus dynamics in numerous typical shallow lakes that have no substantial seasonal variation in water depth (e.g., Taihu Lake, China). In a shallow lake characterized by the flood pulse such as TSL, wind-induced turbulence accounts for resuspension during the low-water period, whereas it is less important during the high-water period. The shallowness and large area of TSL during the low-water period, as indicated by the dynamic ratio (Fig. 19.3), tends to provide sufficient wind fetch to generate turbulence in the bottom layer, thereby promoting sediment resuspension and increasing turbidity.

Wind data on the lake were not available. Wind speeds and wind directions in the wet and dry seasons at Siem Reap Airport (2006–2018), approximately 15 km away from TSL, showed the wind condition to be predominately moderate (wind speed: 2–6 m/s, accounting for >72% of the time), with occasional strong winds (wind speed: >6 m/s, <5% of the time). Wind speed, however, did not differ between those two periods, although the wind direction reversed its path governed by two monsoon periods characterized by two distinct seasons (i.e., south-west monsoon from mid-May to early October and north-east from November to March; Uk et al. 2018; see also Chap. 6 regarding climate and rainfall in Cambodia).

TSS concentration in TSL has been found to be an exponential function of wind speed and water depth, indicating a stronger effect of wind on TSS concentration during the low-water season (Sato et al. 2021). For example, higher TSS concentrations tend to be observed under higher wind speed and a shallower condition. However, such empirical relationships between the wind speed and TSS concentration differ spatiotemporally over the lake. In general, during the low-water period in the whole TSL, TSS was related to wind speed (W) and water depth (h) by the empirical equation 𝑇𝑆𝑆 = f(Wm, h-n) (m > 0, n > 0). Wind speeds as low as 3 m/s were found to cause resuspension in Lake Filso (mean depth: 1.03 m), whereby resuspension increased with higher wind speed. In Lake Taihu (mean depth: 1.9 m), long-term moderate wind (2–6 m/s) keeps sediment suspended in the water column (Chao et al. 2017). Because wind speed and water depth in TSL vary from 2 to 6 m/s and from 1 to 10 m, respectively, TSS is found to spatiotemporally vary in TSL, with generally higher TSS concentration during the low-water period than during the high-water period.

3.2 Vegetation

Vegetation in lakes mitigates the effects of wind-induced waves and thus reduces RR in vegetated areas of TSL compared to that in areas of open water (Siev et al. 2018 and citations therein). Suspended solids in TSL are trapped by vegetation and settle out on the floodplain, up to 80% of the annual sediment influx (Kummu et al. 2008). Aquatic vegetation can potentially increase friction and resistance against wind and flow depending on its structure, density, height, and stem diameter.

The ecosystem of TSL is composed of at least eight subsystems: permanent waterbody, tributary, seasonally inundated forest, shrubland, grassland, receding and floating rice fields, seasonally flooded crop field, and marsh swamp (Lamberts 2001, Koponen et al. 2010). In addition to the natural vegetation, several other crops are cultivated in the floodplain, for example, rice, vegetables, lotus, and corn (Koponen et al. 2010). The TSL’s floodplain consists of grassland (49.1%), shrub (30.2%), inundated forest (1.5%), water and soil (19.1%), and urban area (0.1%) (Baran 2005).

The average sediment rate and its proportion to the average gross sediment rate depend on vegetation types in TSL (Table 19.1). Compared to forest-dominated areas, open water has higher RR or SR, thereby confirming the effect of vegetation on resuspension in TSL. For example, the inundated forest reduces the resuspension of sediment by up to 26%, which is the highest reduction of sediment resuspension among those of other vegetation types.

Table 19.1 Average sediment resuspension rate (RR) in TSL and its proportion to average gross sedimentation rates (RR/SR) in different vegetation types compared with the open water. (Adopted from Arias et al. (2013) and Siev et al. (2018))
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3.3 Bioturbation

Sediment resuspension in TSL might also be caused by aquatic organisms to some extent. For example, benthivorous fish and burrowing benthic animals, which feed and move on the surface layer of sediment, can cause resuspension and increase TSS concentration (Scheffer et al. 2003). In TSL, the low-water period is the time of massive fish migrations from floodplain habitats into the permanent water body and from the lake to Tonle Sap River. The extensive and diverse migration between TSL and Mekong River has been reported for more than 200 fish species (Lamberts 2001). Additionally, experimental results from Scheffer et al. (2003) suggested that the presence of benthivorous fish may help prevent consolidation of lake sediment that, in the absence of fish, would be sufficiently stabilized during the period with little wave action. Consolidation of sediment during such quiet periods may apparently allow the sediment to become firm enough to resist the shear stress caused by waves during windy periods. Such potential effect of fish on sediment resuspension has not been investigated in TSL, but it should not be ignored and needs further investigation.

3.4 Human Activity

Human activities could potentially affect the resuspension of sediment although, to date, no clear data are available to show how it affects RR in TSL. The relevant activities include fishing, boat for transportation, tourism, and selling goods (Fig. 19.4). Population growth in the TSL basin has resulted in an increase in food demand and exerts pressure on the lake’s natural resources (Uk et al. 2018; see also Chap. 22). According to Hortle (2007), TSL is surrounded by one of the most densely populated regions in Cambodia. Approximately five million people live around the lake (Ministry of Planning 2013), with livelihoods relying on resources and services provided by the TSL ecosystem. In addition, this ecosystem suffers from indiscriminate fishing at the largest scale in the world (McCann et al. 2015). Thus, these human activities are possibly causes of sediment resuspension where and when the water depth is shallow enough for people to create turbulence on the sediment surface.

Fig. 19.4
figure 4

Running boats in TSL causing sediment resuspension. (Photos taken by authors)

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Key Points

  • Sediment resuspension in TSL is a seasonal process, which occurs dominantly during the low-water period.

  • Sediment in TSL and its floodplain serves as a major source of suspended solids during the low-water period, contributing to the increment of TSS concentration in the water column.

  • During the low-water period when the lake is shallow, as well as in large open areas, the wind-induced wave would create sufficient turbulence in the bottom layer to stir the lake bed, resulting in resuspension.

  • Flood pulse substantially influences sediment dynamics in TSL, especially resuspension, as a distinct seasonal process, which makes TSL a unique ecosystem.

  • Water level and floodplain vegetation significantly affect sediment resuspension.

  • To date, no clear evidence is available to show how wind, bioturbation, and human activities affect sediment resuspension in TSL; therefore, further investigation is needed.