Juha Sarkkula

Impact of the Mekong River Flow Alteration on the Tonle Sap Flood Pulse

Abstract Rapid development in the upper reaches of the Mekong River, in the form of construction of large hydropower dams and reservoirs, large irrigation schemes, and rapid urban development, is putting water resources under stress. Recent studies have concluded that these developments will lead to flow alterations in the Mekong River. These flow alterations would threaten the sensitive ecosystems downstream, particularly Tonle Sap River, Tonle Sap Lake, its floodplain, and its gallery forest and protected areas, by changing the flood-pulse system of the lake. This article estimates the changes in parameters of the Tonle Sap flood pulse due to the aforementioned flow alterations. The impacts on the flooded area and loss of gallery forest and protected areas were analyzed using geographic information system–based methods. Relatively small rises in the dry-season lake water level would permanently inundate disproportionately large areas of floodplain, rendering it inaccessible to floodplain vegetation and eroding the productivity basis of the ecosystem. It is highly important to maintain the natural hydrological pattern of the Mekong River, particularly the dry-season water levels, to preserve Tonle Sap Lakes ecosystem productivity.

Ecosystem Management of the Tonle Sap Lake: An Integrated Modelling Approach

The monsoon floods of the Mekong River are a key driver of the Tonle Sap Lake ecosystem. This pulsing system together with a large floodplain, rich biodiversity and high annual sedimentation and nutrient fluxes from the Mekong makes the lake one of the most productive fresh water ecosystems in the world. The livelihoods of people living in and around the Tonle Sap are strongly dependent on the lakes natural resources. An integrated modelling system, supported with primary data collection and analysis, has been developed for the Tonle Sap to assess the impacts of planned developments on the lakes ecosystem and riparian communities. Understanding the ecosystem processes and tools for predicting the development impacts are essential for Integrated Water Resources Management, as well as for sustainable basin-wide planning, and national and regional policy-making.

Sediment: Curse or Blessing for Tonle Sap Lake?

Abstract It has been claimed that Tonle Sap Lake is rapidly filling with sediment as a result of increasing sediment yields from the catchment. Infilling of the lake basin would have serious implications for the magnitude of flooding in central Cambodia and the Mekong Delta region and threaten the lakes unique ecosystem. In this article, we synthesize the results of radiocarbon dating of sediment cores and hydrodynamic modeling results to provide an empirically based assessment of this issue. We find that current sedimentation rates within the lake basin proper are low and have been for several millennia. However, sedimentation at the lake margin and in its floodplain is relatively high, which presents a range of issues for riparian communities.

A physically based model to predict hydraulic erosion of fine‐grained riverbanks: The role of form roughness in limiting erosion

Hydraulic erosion of bank toe materials is the dominant factor controlling the long-term rate of riverbank retreat. In principle, hydraulic bank erosion can be quantified using an excess shear stress model, but difficulties in estimating input parameters seriously inhibit the predictive accuracy of this approach. Herein a combination of analytical modeling and novel field measurement techniques is employed to improve the parameterization of an excess shear stress model as applied to the Lower Mekong River. Boundary shear stress is estimated using a model (Kean and Smith, 2006a, 2006b) for flow over the irregular bank topography that is characteristic of fine-grained riverbanks. Bank erodibility parameters were obtained using a cohesive strength meter (Tolhurst et al., 1999). The new model was used to estimate annual bank erosion rates via integration across the Mekongs annual flow regime. Importantly, the simulations represent the first predictions of hydraulic bank erosion that do not require recourse to calibration, thereby providing a stronger physical basis for the simulation of bank erosion. Model predictions, as evaluated by comparing simulated annual rates of bank toe retreat with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery, indicate a tendency to overpredict erosion (root-mean-square error equals ±0.53 m/yr). Form roughness induced by bank topographic features is shown to be a major component (61%–85%) of the spatially averaged total shear stress, and as such it can be viewed as an important factor that self-limits bank erosion.

Modelled phytoplankton dynamics in the Gulf of Finland

Abstract The three-dimensional water quality and flow model was used to simulate phytoplankton biomass and dissolved nutrient concentrations in the Gulf of Finland. In the model, transport of nutrients is based on calculated wind and river flow-induced currents. Water quality data from two points off the Finnish coast were used for the model calibration. Nitrogen is the limiting nutrient in most parts of the Gulf of Finland, but in the eastern Gulf phosphorus plays a more important role. According to the model results, load reductions are needed everywhere along the coast in order to counteract eutrophication. Nitrogen reduction is more important for the whole Gulf of Finland, while phosphorus reduction is most effective in coastal regions, particularly in the Neva estuary. A sensitivity analysis was made to the water quality model to study the effects of parameters and nutrient loads on calculated phytoplankton biomass. The influence of some parameters was found to be widely varying in different areas of the Gulf depending on the limiting nutrient. Also the dynamics of the phytoplankton growth—e.g. timing of the spring bloom peak—was strongly dependent on model parameter values.

Three-dimensional water-quality-transport model compared with field observations

Abstract An ecological model, coupled with a hydrodynamic one, is applied to Lake Nasiselka, the southernmost sub-basin of Lake Nasijarvi, Central Finland. Its surface area is 93 km 2 , mean depth 15 m, maximum depth 61 m, and through-flow 64 m 3 /s. The transport velocities to the ecological model are computed with a three-dimensional hydrodynamic sub-model. Its results are compared with the flow velocities measured in 1981. In the ecological model the transport velocities change according to the weather conditions, through-flow and temperature stratification. The ecological mdoel computes the dissolved oxygen, total phosphorus, phytoplankton biomass, biological oxygen demand and sodium lignosulphonate taking into account their decay, settling, mutual reactions, loading and transport with water flow. The model validity is tested with extensive field observation data of the year 1982 and more sparse data of the summer 1983. The results show reasonable agreement with the measured values. The model is used to test the effects of human activities (industrial loading and through-flow regulation) on the water quality. Industrial loading affects the oxygen and algal distributions in the lake whereas the executed regulation has not appreciably changed the natural conditions in the water body.

Patterns of ecosystem metabolism in the Tonle Sap Lake, Cambodia with links to capture fisheries.

The Tonle Sap Lake in Cambodia is a dynamic flood-pulsed ecosystem that annually increases its surface area from roughly 2,500 km2 to over 12,500 km2 driven by seasonal flooding from the Mekong River. This flooding is thought to structure many of the critical ecological processes, including aquatic primary and secondary productivity. The lake also has a large fishery that supports the livelihoods of nearly 2 million people. We used a state-space oxygen mass balance model and continuous dissolved oxygen measurements from four locations to provide the first estimates of gross primary productivity (GPP) and ecosystem respiration (ER) for the Tonle Sap. GPP averaged 4.1±2.3 g O2 m−3 d−1 with minimal differences among sites. There was a negative correlation between monthly GPP and lake level (r = 0.45) and positive correlation with turbidity (r = 0.65). ER averaged 24.9±20.0 g O2 m−3 d−1 but had greater than six-fold variation among sites and minimal seasonal change. Repeated hypoxia was observed at most sampling sites along with persistent net heterotrophy (GPP<ER), indicating significant bacterial metabolism of organic matter that is likely incorporated into the larger food web. Using our measurements of GPP, we calibrated a hydrodynamic-productivity model and predicted aquatic net primary production (aNPP) of 2.0±0.2 g C m−2 d−1 (2.4±0.2 million tonnes C y−1). Considering a range of plausible values for the total fisheries catch, we estimate that fisheries harvest is an equivalent of 7–69% of total aNPP, which is substantially larger than global average for marine and freshwater systems. This is likely due to relatively efficient carbon transfer through the food web and support of fish production from terrestrial NPP. These analyses are an important first-step in quantifying the resource pathways that support this important ecosystem.

Tonle Sap Pulsing System and Fisheries Productivity

1 Finnish Environment Institute (SYKE), Mechelininkatu 34a, 00260 Helsinki, Finland juha.sarkkula@environment.fi 2 World Fish Center, IFReDI, PO box 582, Phnom Penh, Cambodia – e.baran@cgiar.org 3 Inland Fisheries Research and Development Institute (IFReDI), Cambodia – chhengp@yahoo.com 4 Laboratory of Water Resources, Helsinki University of Technology, P.O. Box 5200, FIN-02015 HUT, Finland marko.keskinen@hut.fi & matti.kummu@iki.fi 5 EIA Ltd., Tekniikantie 21B, 02150 Espoo, Finland – koponen@eia.fi

Linking the growth of filamentous algae to the 3D-ecohydrodynamic model of the Gulf of Finland

Abstract One of the most visible symptoms of eutrophication in the Gulf of Finland is blooms of unattached filamentous algae. This decomposing algal biomass causes serious nuisance problems for recreational uses of the coastal zone, particularly when cast ashore. The nutrient availability for these macroscopic filamentous algae is regulated by a superior competitor, the phytoplankton. Nutrients are left for the weaker competitor only when the conditions are not suitable for the growth of phytoplankton. This happens during vertical mixing of the water column. The lack of stratification prevents the formation of phytoplankton blooms, but does not limit the growth of filamentous algae when still growing attached to the bottom. A simple growth model was developed to describe the growth and biomass of filamentous algae. In the absence of suitable nutrient measurements, the model was linked to a 3D-ecohydrodynamic model which generated the nutrient input data. The model was calibrated with three-year monitoring data of filamentous algal biomass at one location. Validation was carried out with one-year monitoring data from an adjacent location. The model was able to describe the number of biomass peaks and their timing with good accuracy. After further development, the filamentous algal model will be used as one of the management tools for the evaluation of the sustainable nutrient load to the Gulf of Finland.

Spatial and temporal variability of turbidity, dissolved oxygen, conductivity, temperature, and fluorescence in the lower Mekong River–Tonle Sap system identified using continuous monitoring

Continuous monitoring of turbidity, dissolved oxygen (DO), conductivity, temperature, and fluorescence was done at five locations on the Tonle Sap Lake and the Mekong–Bassac Rivers near Phnom Penh, Cambodia, between 2004 and 2010 using autonomous datasondes. Seasonal, daily, and spatial trends were clearly identified in the data and were related to the annual monsoon rainy season–dry season cycle, system metabolism, system hydraulics, and in some cases, localized phenomena such as waste discharges. The datasondes were particularly useful to track the oxygenation of anoxic black water areas in the flooded forest fringe of the Tonle Sap that occurred during the rainy season freshwater pulse. A strongly developed vertical variation of turbidity, DO, and conductivity in the flooded forest fringe may be related to a combination of factors, including dissolved material release from bed sediment and a floating organic-rich particulate layer near the bottom of the lake. Grab samples for total suspended solids (TSS) were collected at the Preak Leap (PL) site (Mekong River) in 2009 and 2010. An excellent relationship was established between daily mean turbidity and TSS concentration for the PL site, with r 2 = 0.95. Autoregressive, integrated, moving average models adequately forecast water level and water quality data one month ahead.