Effects On Water Environment Of An Off-shore Wave-elimination Weir In Riparian Restoration

With the rapid growth of economic development, people demand more and more resources in recent years. Predatory development and the unreasonable use caused the wetland ecosystem, especially the riparian ecosystem has been severely degraded. A series of riparian restoration research and engineering were launched. However, evaluation of the recovery effect is still relatively lacking. A research in an ecological restoration area of Gonghu Bay in Taihu Lake was taken to study the ecological restoration effects.Riparian restoration engineering with offshore wave-elimination weir is an engineering measure with pile wave-elimination weir which is some meters away from the shore. It dissipates waves and promotes sediment deposition in riparian which have hard dam and destroyed vegetation, and creates an artificial semi-closed bay to restore vegetation. The engineering forms3habitat gradient zones:emerged vegetation area, submerged vegetation area and open water area. Field investigations were undertaken in the three different habitats in the restoration area during June2010-May2011. The results showed that:(1) With the comparison of the annual average of the major water quality indicators, we found that there is a certain trend.in the gradients, though the difference between the gradient is not significant. The water inside the weir generally had less nitrites, nitrates, Orthophosphate and total phosphorus but higher ammonia and total nitrogen concentrations than that outside the weir. It shows that the implementation of the project to some extent, reduce concentrations of nitrite, nitrate, orthophosphate and total phosphorus in the weir, but also increases the pollution load of ammonia nitrogen and total nitrogen.(2) With the comparison of the water quality indicators in growing season and non-growing season, we found that the water inside the weir generally had less nitrites, nitrates in the two periods, but more significant in growing season. The water inside the weir generally had more ammonia in growing season, but less in non-growing season. Phosphorus has the greater improvement in non-growing season.(3) Ammonia nitrogen, total nitrogen, nitrate, nitrite, orthophosphate and total phosphorus decreased by sequence of the coefficient of variation of the major water quality indicators throughout the year. The amplitude of the indicators in the three gradient zone are quite different. The amplitude of nitrate and orthophosphate inside the weir is less than the that outside the weir weir, while the amplitude of ammonia and total nitrogen inside the weir is much larger than that outside the weir outside.(4) The highest values of nitrite, nitrate and orthophosphate inside the weir are smaller than that outside the weir. It shows that the project contribute to the suppression of the emergence of high values of nitrite, nitrate and orthophosphate. However, the highest values of ammonia and total nitrogen inside the weir are bigger than that outside the weir. It means that the restoration engineering exacerbated, to some extent, the deterioration of ammonia and total nitrogen at the end of the growing season.(5) Aquatic plants control, to some extent, the nutrient elements and heavy metal contents of sediments in the project area. Emergent plants have stronger ability to remove organic matter, nitrogen and heavy metals than submerged plants.The research shows that riparian restoration engineering with offshore wave-elimination weir on the whole, conducive to the decline of some indicators of water quality and sediment, but also easily lead to high concentration of ammonia and local hypoxia conditions. So some measures must be taken, such as expanding the area of open water in the weir, increase the dissolved oxygen of water to promote nitrification; salvaging residues of aquatic plants in non-growing season, to reduce the secondary pollution caused by decomposing.