Long-term Eutrophication Research Articles

Long-term Circulation and Eutrophication Model for Tolo Harbour, Hong Kong

Since the early 1980s excessive discharges of organic waste into Tolo Harbour have created serious problems through nutrient enrichment. A number of eutrophication related problems have been reported which incurred financial losses of billions of dollars and serious ecological imbalance. This work addresses the development, verification and application of a water quality model to synthesize the available large database of water quality and to study water quality management issues of Tolo Harbour. Since detrimental water quality problems usually occur during summer when there is stratification and the water temperature is high, the tidally-averaged hydrodynamics and the relative contribution of gravitational circulation and tidal exchanges in Tolo Harbour have been studied. Gravitational circulation is found to be the dominant mixing process for most of the year, accounting for 70 percent of the mixing. A simple and tractable predictive two-layer mass transport and diagenetic dynamic eutrophication model has been developed. The model computes daily variation of key water quality variables in the water column: algal biomass, dissolved oxygen, organic-nitrogen, ammonium-nitrogen, nitrate-nitrogen, and carboneous oxygen demand. In addition, to study the response and impact of the seabed to the overall eutrophication process a sediment sub-model is developed. The diagenetic sediment sub-model computes explicitly the amount of nutrient recycled and the sediment oxygen demand exerted on the water column. The calibrated model has been validated against a 20 year water quality data base under a wide range of hydro-meteorological and environmental conditions. Both spatial and seasonal variation of observed water quality variables are reproduced. The verified model shows that a significant reduction of total nitrogen loading would be required to meet the water quality objectives, with a recovery time of three months for water column and more than two years for sediment.

Long-term eutrophication development in five coastal lakes of the Bulgarian black sea region

Referring to data for orthophosphate phosphorus and soluble inorganic nitrogen consisting of NO31− and NH41+ ions, an attempt was made to estimate the trophic status and to follow up trophic changes over 15 to 30 year periods. The PO43−-P concentrations alone put most lakes permanently in the highest trophic state (hypereutrophic level) according to the Likens (1975) scale, set for total phosphorus and nitrogen concentrations. Only the Ezerets and Shabla lakes presented an exception in the 1963-1967 period. The nitrogen concentration covered the range from higher than oligomesotrophic to hypereutrophic levels. The average values of the N/P ratio of Dourankoulak and Bourgas lakes (the more shallow lakes) varied around the optimum, and the lakes have been tending to become slightly nitrogen limited, while Ezerets, Shabla and Mandra lakes (the deeper lakes) turned from phosphorus into clearly nitrogen limited lakes. The dominance changes in phytoplankton composition of Shabla lake corresponded to the variations in the N/P ratio. The NH41+ -nitrogen/ NO31− -nitrogen ratio shows higher values and a greater increase in the more shallow lakes than in the deeper lakes, which reveals the differences of eutrophication development in lakes of different depths.

Long-term eutrophication development in five coastal lakes of the bulgarian black sea region

Referring to data for orthophosphate phosphorus and soluble inorganic nitrogen consisting of NO 3 1− and NH 4 1+ ions, an attempt was made to estimate the trophic status and to follow up trophic changes over 15 to 30 year periods. The PO 4 3−-P concentrations alone put most lakes permanently in the highest trophic state (hypereutrophic level) according to the Likens (1975) scale, set for total phosphorus and nitrogen concentrations. Only the Ezerets and Shabla lakes presented an exception in the 1963–1967 period. The nitrogen concentration covered the range from higher than oligomesotrophic to hypereutrophic levels. The average values of the N/P ratio of Dourankoulak and Bourgas lakes (the more shallow lakes) varied around the optimum, and the lakes have been tending to become slightly nitrogen limited, while Ezerets, Shabla and Mandra lakes (the deeper lakes) turned from phosphorus into clearly nitrogen limited lakes. The dominance changes in phytoplankton composition of Shabla lake corresponded to the variations in the N/P ratio. The NH 4 1+ -nitrogen/NO 3 1− -nitrogen ratio shows higher values and a greater increase in the more shallow lakes than in the deeper lakes, which reveals the differences of eutrophication development in lakes of different depths.

Carbon to chlorophyll-a ratio in modeling long-term eutrophication phenomena

Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions, including solar radiation and water temperature. With the dynamic CCHL, an unsteady two-layered, two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (IS) is determined as weighted average of the light intensity for previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxic condition during summer in Tolo Harbour, Hong Kong is successfully reproduced by the present method. Otherwise, the simulation with a constant CCHL gave a wrong result.

Carbon to chlorophyll-a ratio in modeling long-term eutrophication phenomena

Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions, including solar radiation and water temperature. With the dynamic CCHL, an unsteady two-layered, two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (IS) is determined as weighted average of the light intensity for previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxic condition during summer in Tolo Harbour, Hong Kong is successfully reproduced by the present method. Otherwise, the simulation with a constant CCHL gave a wrong result.

The environmentally-sound management of agricultural phosphorus

Freshwater eutrophication is often accelerated by increased phosphorus (P) inputs, a greater share of which now come from agricultural nonpoint sources than two decades ago. Maintenance of soil P at levels sufficient for crop needs is an essential part of sustainable agriculture. However, in areas of intensive crop and livestock production in Europe and the U.S.A., P has accumulated in soils to levels that are a long-term eutrophication rather than agronomic concern. Also, changes in land management in Europe and the U.S.A. have increased the potential for P loss in surface runoff and drainage. There is, thus, a need for information on how these factors influence the loss of P in agricultural runoff. The processes controlling the build-up of P in soil, its transport in surface and subsurface drainage in dissolved and particulate forms, and their biological availability in freshwater systems, are discussed in terms of environmentally sound P management. Such management will involve identifying P sources within watersheds; targeting cost-effective remedial measures to minimize P losses; and accounting for different water quality objectives within watersheds. The means by which this can be achieved are identified and include developing soil tests to determine the relative potential for P enrichment of agricultural runoff to occur; establishing threshold soil P levels which are of environmental concern; finding alternative uses for animal manures to decrease land area limitations for application; and adopting management systems integrating measures to reduce P sources as well as runoff and erosion potential.

Ecological modelling at Osaka Bay related to long-term eutrophication

Based upon the ecological model presented by Andersen & Ursin (1977), we constructed an adequate model which will quantatively evaluate the overall impact of industrial large-scale development on fisheries at Osaka Bay. It is expected that the patterns of changes in biomass of dominant species, which is caused by eutrophication, will be explained by the model we have established. The simulated results for two years, 1956–1957, are as a whole comparable to the actual variations. However, the results for several species, including sea bass, flat fish, crab, mantis shrimp, and cuttlefish, differ significantly from observed data. It was attributed to the lack of accurate parameters of anabolism and catabolism for younger-aged stages of these species.

Long-term eutrophication of the Northern Adriatic Sea

In the Northern Adriatic Sea, the occurrence of anoxic events near the bottom has considerably increased in frequency during the last 15 years. Although it has been suggested that increasing nutrient inflow from land-based sources has caused the phenomenon, there has been no direct evidence for this. An analysis of the oxygen distribution in the Northern Adriatic Sea between 1911 and 1982 clearly demonstrates that the probability of anoxic events near the bottom has increased in time. This process is related to the increasing primary production near the surface. There is strong evidence that the long-term nutrient enrichment of the Northern Adriatic Sea forces the above changes.