Extreme hydro-climatic changes associated with floods and drought greatly influence the ecosystem dynamic of tropical lakes in Malaysia, resulting in negative impacts on their water quality and trophic status. This study examined the seasonal changes in water quality and trophic state connected to hydro-ecological changes in a natural lake and two reservoirs. Water levels in the three water bodies varied with changes in rainfall regime, and water regulation for irrigation or water supply. Multivariate analysis of variance showed a significant relationship between season and temperature, pH, conductivity, total nitrogen and chlorophyll-a (p < 0.05). Drought events and the associated period of low water levels were characterized by high temperature, conductivity and chlorophyll-a levels in these shallow lakes, which probably led to high photosynthetic rates with concomitant increase in dissolved oxygen and pH and decrease in total nitrogen concentration. Elevated chlorophyll-a and total phosphorus concentrations were also associated with drought events and the decrease in water level in Bukit Merah Reservoir, possibly due to the release of phosphorus from internal loading resulting from sediment re-suspension by wind. Salinity increase in Durian Tunggal Reservoir during dry season was possibly due to seawater intrusion into river. During the rainy season, transparency in Bera Lake and Bukit Merah Reservoir was strongly influenced by the influxes of the main river that transported elevated levels of suspended solids and nutrients.

Hydrodynamic simulations using three-dimensional numerical model were carried out in three different shallow tropical lakes to understand the characteristics of water movement in the respective water bodies. The models were based on meteorological data from the nearest stations and calibrated with current measurement, temperature, or water-level data. The results show good agreement between measured and simulated velocities and/ or temperature at certain depth. This study found that the major driving forces of the hydrodynamic pattern were different in the three lakes. Hydrodynamic simulations showed that Bukit Merah and Durian Tunggal reservoirs were more sensitive to winddriven motion. Floodplain lakes, such as Bera Lake, are more sensitive to flood inflow by the main river during the monsoon season. Convective motion driven by water temperature gradient was important for Bukit Merah and Bera Lake.

This paper assessed and compared physicochemical profiles and biodiversity indices of two shallow lakes namely the natural Bera Lake and Bukit Merah reservoir in Peninsular Malaysia. Variations in relation to hydro-meteorological variables, including rainfall, wind speed and air temperature were also measured during the field sampling. The results showed clear distinctions between natural and man-made lakes in terms of physical and chemical properties of lake waters and their relationships to the hydro-meteorological dynamic. Persistent oxygen and temperature differences between the epilimnion and hypolimnion were clearly detected in Bukit Merah reservoir under low winds and strong heat. Oxycline also persist during flooding or high turbidity levels indicating their enriched bottom waters. On the other hand, the natural Bera lake showed temporary stratification during the dry season with low winds but the water columns were thoroughly mixed during the wet season when the water temperature was lower and wind speed was higher. Plankton diversity in Bera lake was significantly higher (p < 0.05) than the Bukit Merah reservoir, probably due to the much longer history of the natural lake compared to the man-made one. Due to the longer residence time of the reservoir compared to the natural lake, the former tends to be more vulnerable to eutrophication if subjected to uncontrolled nutrient enrichment.