To evaluate the effect of the tidal cycle on the pore water nitrate dynamics in intertidal sediment, concentrations of inorganic
nitrogen in water and sediment were monitored during tidal cycles in the mud flat of Tama Estuary, Japan. During submergence,
nitrate concentration was highest in the overlying water and decreased monotonically with increasing depth in the sediment,
suggesting that the primary source of nitrate in the sediment was nitrate transported from the overlying water. Pore water
nitrate decreased remarkably during the initial 3–4 hours after the onset of exposure. Thereafter, it was constant or slightly
increased until tidal flooding.In situ accumulation of nitrate at the end of exposure, however, did not exceed the nitrate concentrations in the overlying water.
The inhibition of nitrate reduction and the stimulation of nitrification would explain the change of nitrate concentration,
both consistent with the input of oxygen into the sediment following a 10 mm drop of the water table. In Tama Estuary sediments,
the effect of the tidal cycle on the removal of combined nitrogen is rather negative, because high nitrate concentrations
in the overlying water canceled the positive effect of nitrate accumulation by nitrification during exposure, while tidal
oxygen intrusion have an inhibitory effection sedimentary denitrification. 相似文献
Bioassesment by the use of the macroalga, Ulva lactuca L., was carried out in the Limfjord, Denmark, to assess the significance of nitrogen and phosphorus as limiting factors for primary production during 1985, 1993, 1994 and 1995 and for the detection of changes in eutrophication levels.
Minimum and critical tissue concentrations for nitrogen and phosphorus in macroalgae were identified. The concentrations of nitrogen were generally below the critical concentration level in June–October in 1985, 1993, and 1995 but in 1994 nitrogen was only limiting for primary production in short periods. Only in early spring in 1985 and 1993 were the tissue concentrations of phosphorus below the critical concentration level, whereas in 1994 up to 3–4 months showed phosphorus limited growth, indicating that significant changes in limitation patterns can occur between different years.
It was concluded that the use of biomonitoring techniques is well suited as a bioassessment method for direct detection and for providing a time-integrated measure of nutrient availability in coastal waters, and thus for assessing ecosystem health with regard to eutrophication. It is recommended that biomonitors and the concept of critical tissue concentrations should be used in environmental management and incorporated in future monitoring programmes. 相似文献
Generally speaking,the convection activities are inactive over western Pacific warm pool and tropical cyclone (TC) activity seldom occurs over the offshore of East Asia during the period of Meiyu rainfall.However,if a TC is active in this area,the Meiyu rainfall will often weaken or end up.Based on a statistical study with the data from 1980 to 1995,it is found that about 91% of 23 TC activities affected the intensity of Meiyu rainfall,and 50% of the end-up of Meiyu events were related to the active TCs and the change of subtropical high.The present paper simulates the effect of TC on Meiyu circulation by using MM4 model,and the results agree with the observations.From the point of view of vapor and energy transport,the landing of TC cuts not only the transport of the water vapor to Changjiang-Huaihe River basin from the Bay of Bengal but also the conversion of the mean flow energy to the Meiyu circulation because of the TC forcing to the zonal circulation.These two effects make the convection and perturbation existing in Meiyu region lack the supply of the vapor and energy for their maintenance and lead to the end of Meiyu rainfall. 相似文献