Takehiko Fukushima

Dissolved organic carbon in a eutrophic lake; dynamics, biodegradability and origin

The seasonal and spatial changes in dissolved organic carbon (DOC) in Lake Kasumigaura, a shallow, eutrophic lake, were analyzed and the lability of DOC was tested by long-term incubations. There was a nearly 1 mgCl−1 downstream increase in refractory DOC in the lake; at the center it fluctuated little seasonally. The characteristic UV-absorbance: DOC ratios were determined for samples from the influent rivers (pedogenic: used interchangeably with “allochthonous”) and outdoor experimental ponds (autochthonous) during incubations. These ratios were then used to calculate the proportion of total measured lake water DOC in each of four components: pedogenic-refractory (PR), pedogenic-labile (PL), autochthonous-refractory (AR) and autochthonous-labile (AL). PR was uniform (around 1.5 mgCl−1) or diminished very slightly over time. AR increased from nearly zero at the station closest to an influent river to 1 mgCl−1 at the lake center. PL declined downstream from 0.3 mgCl−1 to zero. AL was virtually constant at 0.8 mgCl−1 except at the station closest to the influent river. The constancy of the UV-absorbance: DOC ratio during the biodegradation process was confirmed for Lake Kasumigaura; hence a two-component model (pedogenic-autochthonous) could be applied here without consideration of DOC lability. However, this assumption is not always met for other water bodies, and therefore it should be checked before applying a two-component model elsewhere.

Short- and long-term control of external and internal phosphorus loads in lakes—A scenario analysis

In this paper the structure and validation of the deterministic eutrophication model SALMOSED is discussed. It is applied to two lakes of different depths with a long history of cultural eutrophication. The simulation results are used to assess the short- and long-term efficiency of alternative measures for eutrophication control considering external and internal phosphorus loads. In the case of the shallow Lake Mueggelsee the control of the phosphorus release from the sediment by sediment dredging seems to be the only efficient method. To consolidate a mesotrophic steady state of Lake Mueggelsee in a long-term perspective, 10-year simulation runs show that single sediment dredging should be followed by a reduction of the external phosphorus load by at least 50%. The eutrophication of the medium-deep Lake Yunoko can be controlled by a significant reduction of the effluent phosphorus concentration. Additionally it is recommended to control the considerable internal phosphorus load of Lake Yunoko by artificial aeration/destratification or the more costly sediment dredging.

Diffusive exchange of linear alkylbenzenesulfonates (LAS) between overlying water and bottom sediment

Linear alkylbenzenesulfonate (LAS) was detected in a 0–30 cm deep sediment column collected in Lake Teganuma (one of the most polluted lakes in Japan). The range of the LAS concentration in sediments was between 0.1 and 500 µg g−1 (C11–C14 homologs per dry solid) and its vertical profile showed a seasonal variation. A mathematical model, which includes a diffusion term and a biodegradation term, was used to simulate the temporal variation of LAS in the sediment column and to calculate the diffusive flux rate of LAS across the sediment/water interface. An averaged diffusion coefficient of 2.4 × 10−5 cm2 s−1 for the sediment interstitial water was obtained from sediment core samples located in Lake Teganuma. The biodegradation rate constant (0.002 d−1) of LAS in the sediment obtained from the model analysis was considerably less than that reported for LAS in anaerobic waters. These results confirm that a model describing diffusive transport and biodegradation of LAS in the sediments can simulate the temporal variation of LAS in near surface sediments. The diffusive flux rate from overlying water to bottom sediment was calculated to be between −0.20 and 0.52 (C11–C14 LAS) mg m−2 h−1 and the annual net flux rate was 0.7 g m−2 y−1.

EFFECT OF OMNIVOROUS FISH ON THE PRODUCTION OF LABILE AND REFRACTORY DISSOLVED ORGANIC CARBON BY ZOOPLANKTON EXCRETION IN A SIMULATED EUTROPHIC LAKE

Abstract Experiments using six large, outdoor, ponds were conducted from 2 August 1993 to 10 September 1993 to examine whether the presence of omnivorous fish can affect the production of labile (DOC L ) and refractory dissolved organic carbon (DOC R ) by zooplankton excretion in a shallow eutrophic lake. Addition of goldfish ( Carrasius auratus ) to duplicate ponds at two densities (high density: 14.0xa0g C l −1 ; low density: 1.6xa0g C l −1 ) resulted in lower densities of large zooplankton (>100xa0 μ m) and higher chlorophyll- a concentrations than in reference ponds without goldfish. Rapid density increases in numbers of large zooplankton in reference ponds were significantly correlated with decreases in chlorophyll- a concentration, and were followed by increases in total dissolved organic carbon (DOC T ). While no relationship between density of large zooplankton and chlorophyll- a concentrations in high- or low-density ponds was observed due to reduced feeding pressure on large zooplankton by goldfish, DOC T concentrations remained at levels 30–220% higher than those in reference ponds after the 15th day. Although DOC T and DOC L concentrations in apparent steady state in high- and low-density ponds were significantly higher than those in reference ponds ( P R concentration was observed. Estimates suggest that the enhanced DOC T production rates are mainly attributable to excretion by small zooplankton (100xa0 μ m>), which may also enhance DOC T production in eutrophic lakes.

Carrying capacity and vulnerability of four Asian lakes: A comparative study

Abstract To present axes for evaluating the water resources, lake environment and water use in a lake and its watershed, a comparative study was done for four East Asian lakes; lake Kasumigaura (Japan), Lake Laguna (Philippines), Lake Songkhla (Thailand) and Lake Dianchi (China). Two axes were then determined from the relationship between: (1) water use and water supply rate, and (2) water use and water quality. To clarify the meanings ‘carrying capacity’ and ‘vulnerability’, the maximum population density in the lake basin and total nitrogen concentration in the lake water were calculated and discussed to decide the direction of environmentally sound management of the drainage basin.