Limnology

We investigated the biodegradation process of freshwater phytoplanktonic organic matter using incubation experiments, with special reference to changes in three major biomolecules: neutral aldoses, amino acids and fatty acids. The concentration of neutral aldoses decreased drastically relative to amino acids and fatty acids during the early decomposition phase (days 0–7), owing largely to the rapid decomposition of storage carbohydrate. This resulted in a temporary decrease in the C/N ratio of organic matter. During the late phase (days 7–60), however, the rate of decrease in neutral aldoses slowed, while the considerable decrease in amino acids and fatty acids continued. The inconspicuous change in amino acid composition was probably due to the fact that no protein and/or peptide is composed of a limited species of amino acid. Although the compositional variety of organic matter among the phytoplankton was clearly observed at the start of decomposition, it became obscure in the course of 60 days. This indicates that while the organic composition of the labile fraction of phytoplanktonic organic matter varies depending on the phytoplankton groups, the refractory fraction has similar composition. The addition of bacterial organic matter is likely another reason for the similar composition of the remaining organic matter at day 60.

Quantitative and qualitative characterizations of dissolved organic matter (DOM) were carried out at the watershed level in central Japan by measuring dissolved organic carbon (DOC) concentration and the three-dimensional excitation–emission matrix (3-D EEM). DOC concentration was low (mean 37 ± 19 µM C) in the upstream waters, whereas, in general, it increased toward the downstream areas (mean 92 ± 47 µM C). Significant variations in DOC concentration were detected among rivers and channels. DOC concentration in the epilimnion of Lake Biwa increased during the summer period and decreased during the winter period. The lake hypolimnion has lower DOC concentration (mean 87 ± 7 µM C) compared with the epilimnion (107 ± 15 µM C). Fulvic acid (FA)-like substances in the DOM were directly characterized by 3-D EEM. The fluorescence peak for upstream DOM was found in regions with longer wavelengths (excitation/emission 386 ± 6/476 ± 5 nm) compared with downstream and lake DOM (351 ± 12/446 ± 15 nm and 341 ± 6/434 ± 6 nm, respectively). The DOC concentration is correlated with fluorescence peak intensity of FA-like substances in DOM in river waters. Such a relationship was not found in lake DOM. A blueshift of the fluorescence peak from upstream to lake DOM was observed. A decrease in fluorescence intensities was also detected during the summer period. These results may suggest that the degradation of FA-like substances in DOM occurs from natural solar irradiation. Protein-like fluorescence was significantly detected in the lake epilimnion during the summer period. A linear relationship between DOC concentration and protein-like fluorescence indicated that an autochthonous input of DOM gave rise to the increase in DOC concentration in the lake epilimnion during the summer. These results may suggest that the 3-D EEM can be used as a tool for the investigation of DOM dynamics at the watershed level with concurrent measurement of DOC concentration and the fluorescence properties of fulvic acid-like and protein-like substances.

Lake Monger (Perth, Western Australia) is a highly eutrophic lake, characterised by very low species richness of macrophytes with the dominance of Potamogeton crispus. Mesocosm experiments were performed using water and plants collected from the lake to determine the effects of vegetation decay on the phosphorus (P) concentrations in the overlying waters. After 2 weeks of experimental incubation of mesocosms with and without re-oxygenation, P concentrations in the water column were significantly higher, showing a quite similar effect of P. crispus on the phosphorus release in different mesocosms. The results of our study provide clear evidence that the P concentrations in overlying waters mainly depend upon the plant P content and developmental stage. Although many sources contribute to the nutrient load of Lake Monger, macrophyte harvesting, prior to its senescence, might constitute a significant in-lake measure for reducing the internal P load.

 Concentrations of dissolved organic carbon (DOC) in Lake Baikal ranged from 90 to 110 μM C, considerably higher than those in oceanic environments. The DOC concentrations in the epilimnion were higher than those in the hypolimnion. Since particulate organic carbon (POC) concentrations in the pelagic waters of Lake Baikal were <10–40 μM C in the epilimnion and 2–5 μM C in the hypolimnion, DOC constitutes a major component of the organic carbon pool in Lake Baikal, especially in the deep layers. The DOC concentrations downstream of the Barguzin and Selenga Rivers were quite high (400–500 μM C). Probably because of the high concentrations of DOC in these rivers, the DOC levels in Barguzin Bay and offshore at the mouth of the Selenga River were higher than those in the pelagic regions of the central and south basins of Lake Baikal. The relationship between DOC and electric conductivity revealed the transport of DOC from rivers to the pelagic area in Lake Baikal. The spatial distribution of DOC suggested that a major part of DOC in the lake was allochthonous (land-derived).

The Chironomidae of tropical South America are a very rich species, but are scarcely known. The range of environmental conditions under which chironomids are found is more extensive than that of any other group of aquatic insects. The objectives of this study were to carry out a diversity survey of chironomid larvae in wetland systems of the coastal plain in the south of Brazil and to analyze the effects of area, altitude, water conductivity, nitrate and phosphorus concentrations, and the life form of the dominant plant species on chironomid richness and composition. Collections were carried out from March to April in 2002. A total of 30 taxa (23 morphospecies and 7 species) distributed along 23 genera were found, and the Chironominae showed the greatest richness, followed by Tanypodinae and Orthocladiinae. The chironomid richness was higher in the emergent than in the multistratified wetland class. The wetland area, altitude, nitrate and phosphorus concentrations, and water conductivity did not influence the richness of Chironomidae. The Chironomidae genera and species were present in both the wetland classes (emergent and aquatic bed vegetation). However, while Chironominae were more frequent in the emergent than in the aquatic bed wetlands, no difference was observed for Tanypodinae. The aquatic vegetation was an important environmental predictor for chironomid larvae richness in the studied wetlands in the south of Brazil.

 The Gobi Steppe of Mongolia supports numerous temporary water bodies of various sizes. No accurate information is available on the factors that shape the structure of their fauna, partly because they have been regarded as less important environments than permanent lakes. This study represents the first comprehensive survey of the Recent large branchiopod fauna of Mongolia, which consists of at least 16 species. This faunal composition may have resulted from colonization through diversification in East Asia during the Cretaceous period of the Mesozoic era (ca. 135–65 My ago), and to some extent from immigration from Europe into Asia during the Cenozoic era. The distribution patterns of some geologically ancient taxa most probably reflect the ancient hydrosphere. Five new records from Mongolia are presented: three species of Anos-traca, Branchinectella media (Schmankewitsch, 1873), Chirocephalus mongolianus Uéno, 1940, and Chirocephalus sp. (undescribed); one Spinicaudata, Cyzicus sp. (undescribed); and one Laevicaudata, Lynceus sp. (undescribed). The three undescribed species are localized-endemic to the Gobi Steppe. The purpose of this paper is to draw attention to some of the scientific interest and value of ephemeral aquatic habitats in Mongolia, and to argue that hydrobiologists, water resource managers, and conservationists need to take greater interest in temporary habitats.

Two sulfur-mediated reactions are resulting in the eutrophication of Lake Biwa, Japan. The iron (II) phosphate mineral vivianite is dissolving in sulfide-enriched sediments that in places results in porewater concentrations of phosphate exceeding 3 mg l−1. The dissolution of phosphate is evident in profiles of total phosphorus where zones of dissolution and a zone of precipitation in the most oxic surface sediments are visible. At times sulfate reduction in these surface sediments results in pH values as high as 9.9, which can dissolve phosphate adsorbed to iron (III). This release of phosphorus from sediments is at least partially responsible for the recent appearance of blue-green algal blooms.