A new practical short-term forecast system on the nutrients and phytoplankton concentrations in winter season was developed, which handled the issue of Porphyra bleaching (discoloration of cultured Nori), and applied to Harima-Nada. eastern Seto Inland Sea, Japan. A nonsteady state simulation usually needs too much time to get the calculation results for prompt reporting. However, this system can calculate a prediction for around 10 days on a PC immediately, depending on the case, by using tidal current, drift current (48cases), density current (5 cases) and tidal residual current which were compiled into a database made by a steady calculation using a multi-level flow model (Nakata et al., 1983). Model validation about dissolved inorganic nitrogen (DIN). dissolved inorganic phosphate (DIP) and phytoplankton concentration were examined respectively, and showed a result that the distribution tendency could reappear well. Although at the case of phytoplankton blooming, the prediction value showed the tendency that was slightly high on the DIN and DIP. and low on the plankton concentration for the observed value. Therefore a practical revision method concerning the nutrient concentrations was devised by using the simple estimation of biochemical processes such as the nutrient uptake of phytoplankton.
The diatom Eucampia zodiacus is one of the harmful species which cause, through nutrient depletion, the discoloration of cultivated ”Nori” (Porphyra thalli). In order to predict the short-term changes of nutrients in Nori-culture ground, the relationships between the distributions of E. zodiacus and environmental factors were investigated in Harima-Nada from January to April in 2004.The results showed that E. zodiacus increased near the bottom layer of shallow water in the north-west region of Harima-Nada at the beginning stage of appearance, and extended to the surface layer and other regions broadly when the west or north wind blew continuously. At the end stage of the appearance, E. zodiacus decreased to non-detection level after consuming near the bottom layer of the same region of the beginning stage again. Since some close relations were observed between the rise and fall of the number of E. zodiacus cells and changes in a physical condition of the field according to the wind and the precipitation, it became clear that a forecasting model that uses flow model is effective for short-term forecasting of the number of E. zodiacus cells. We confirmed that large numbers of E. zodiacus cells are maintained in spite of very low concentrations of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (DIP). Moreover, it was shown that DIN concentrations
decreased as the standing crop of E. zodiacus cells increased. This correlation was thought to be able to use for simple revision of the forecast of DIN concentrations when the species appeared dominant.
We investigated a relationship of condition factor Japanese littleneck clams (Ruditapes philippinarum) with water temperature and feeding environment for the clam, as evaluated by the abundance of chlorophyll a and pheo pigment, in eastern Ise Bay. The survey for those parameters was conducted from October to March for four years (1999-2002). Multiple regression analysis for the condition factor (X) of the clam in a given month with that in the previous month (c), water temperature (a) and the feeding environment(b) revealed that the condition factor can be described by an equation as: X = - 0.462a x 0.052b x 0.861 c x 8.169.
The condition factor of the clam estimated from the equation fitted well to measured values. The analysis also indicates that water temperature has the biggest impact on the condition factor during winter; the higher water temperature is, the lower the condition factor is. This is attributed to the effect of temperature on metabolism such as respiration.
The current velocity field in and near a borrow pit was examined with Full 3D hydrodynamic model. The water exchange between a borrow pit and an environment was analyzed as an indicator of the degree of flow stagnation inside a pit. It is found that there exists an optimum current speed outside borrow pit to exchange the water effectively through a boundary between a borrow pit and an ambient water.
Sandmining for commercial construction aggregate and for landfilling projects has left borrow pits on the bottom of several coastal regions of Japan. A nation-wide survey revealed that these borrow pits can be classified geomorphologically into two types: flattened-out or a local depression. Most of the sandmining pits in the Seto Inland Sea, which are classified as the flattenedout type, are estimated to have caused loss of eelgrass beds and habitat for economically valuable fishes. However, these areas seemed to have less oxygen depletion. On the contrary, borrow pits that are the local depression type often cause severe deterioration of water quality that can manifest as anoxia and blue tides. The local depression type of borrow pit are widespread in Tokyo Bay, Osaka Bay, Mikawa Bay and other eutrophic embayments of Japan. Recontouring (raising the bottom) is expected to be an effective method to restore damaged ecosystems. Although only a few geomorphological restoration projects have been implemented to date, actions to recontour the borrow pits and assess their effectiveness, including monitoring activities,
have started in Mikawa Bay. In order to promote effective restoration and to minimize negative effects, we conducted several research tasks including an evaluation of: the procedures for assessing the effect of restoration on water quality and coastal ecosystems; and the technical methods available to prevent the dispersion of turbid water masses produced by placing material into borrow pits. We also assessed the effects of fine chemicals contained or accumulated in the borrow pits, and the methods available to manage dredged sediment and/or develop material other than dredged sediment.
The dissolved oxygen concentration in Mikawa Bay was investigated at 10-minute intervals over 40 days in a borrow pit in the north-eastern part of the bay where the sea bottom had been dredged to reclaim land. We also observed the vertical distribution of dissolved oxygen concentrations at nine stations at and around the dredged area. Although dissolved oxygen concentrations around the borrow pit was sufficient to sustain life, concentrations within the hollow were extremely insufficient during summer. The borrow pit readily became oxygen deficient but did not recover as readily as in the neighboring area, ft is inferred that the borrow pit is a source of oxygen-deficient water in Mikawa Bay and upwelling of this stagnant water caused the niga-shio (bitter tide) , and that is what killed a large number of short-necked clams, Ruditapes philippinarum, in the summer of 2002.
Dredging to reduce nutrient loads from sediments is still performed in Japan especially in eutrophic lakes. No Held data, however, so far have shown a long term reduction in nutrient efflux from sediments due to dredging. Rather, disturbance to the benthic ecosystem, due directly to dredging or embankments of dredged sludge to protect reclaimed land, seems to reduce the natural water quality control. A case study at Lake Suwa. Japan, showed a negative correlation between fisheries yield and the cumulative amount of dredged sludge. A case study at Lake Nakaumi. Japan, showed that the potential amount of nutrient removal by shallow coastal ecosystem is as large as dredging. Therefore, dredging should be more carefully performed when its purpose is to reduce nutrient loading.
Mikawa Bay is a typical estuary with an approximate area of 600 km2, and very shallow water at an average depth of 9 m. Usually, shallow sea water is abundant in dissolved oxygen and rich in benthic fauna such as bivalves. However. Mikawa Bay has faced a serious environmental problem recently. In and around Rokujou-gata which is one of the most important tidal flats areas of the north-eastern region of Mikawa Bay, very large quantities of the little-necked clam, Ruditapes philippinarum. were killed by a bitter tide (niga-shio) in the summers of 2001 and 2002. Niga-shio occurs when oxygen-deficient water masses in the bottom layer upwell to the shallows due to wind etc. Incidents of mass mortalities of benthic fauna due to the bitter tides were reported in newspapers as a serious public issue. Our research unexpectedly identified two dredged borrow pits from reclamation works as the one of main causes of the oxygen-deficient water mass that led to this biological decimation. In this area of Mikawa Bay, restoration of these dredged borrow pits was carried out within six months of the initial dredging. Generally, restoration like this can' t be carried out so quickly, but the regulators of harbor areas could act to rapidly start rehabilitation of the borrow pits. This report demonstrates why restoration was so rapidly put in operation by considering the progress.
Abundance of macrobenthos relative to dissolved oxygen was investigated at weekly intervals during summer, when the water temperature is high, in a sea area around subaqueous borrow pits in the north-eastern part of Mikawa Bay, central Japan. The concentrations of dissolved oxygen remained at almost 0 % in the borrow pit during summer. In contrast, these concentrations varied frequently over short periods in the shallower area around the borrow pits such that an oxygen-deficient environment did not remain for a long period. Very few species were found in the borrow pit although a few macrobenthos species, such as Paraprionospio sp. type A, occurred. Moreover, the densities and biomass of macrobenthos were also very low in the borrow pit. On the other hand, arthropoda, mollusca, cnidaria and annelida inhabited the shallower area around the borrow pits. The densities and biomass of arthropoda and mollusca immediately decreased as the concentration of dissolved oxygen decreased in early summer, and then gradually increased as the concentration recovered. In contrast, cnidaria and annelida hardly decreased in the oxygen-deficient environment. Macrobenthos, particularly arthropoda and mollusca, are greatly influenced by dissolved oxygen in a sea area around the borrow pits in Mikawa Bay.
Borrow pits (huge depressions) that remain after dredging in various parts of Japan' s coastal waters are generally steepsided and deeper than the ambient seabed. These characteristics can cause stratification of the water column, reduce vertical circulation, and bring about the occurrence of oxygen-deficient bottom water. Thus, it is considered that recontouring these areas would be an effective measure to rehabilitate the water quality and damaged ecosystem.
To predict the beneficial effects of recontouring using a numerical model, it is necessary to evaluate the local flow field precisely. For this purpose, a 3-D non-hydrostatic pressure model (M1-3D model) was developed. In this study, the model was calibrated for the borrow pit off Mito in Mikawa Bay based on existing field data for the year 2002. To examine the effectiveness of the model, the numerical results were compared with those of a conventional hydrostatic pressure model. The comparison proved the full- 3D model to be much more adequate at simulating the stratification structure of the pit. Moreover, it turned out that the full-3D model also reproduces reasonably the vertical mixing process under strong wind conditions.