We used flow cytometry to examine seasonal variations in basin-scale distributions of bacterioplankton in Lake Biwa, Japan, a large mesotrophic freshwater lake with an oxygenated hypolimnion. The bacterial communities were divided into three subgroups: bacteria with very high nucleic acid contents (VHNA bacteria), bacteria with high nucleic acid contents (HNA bacteria), and bacteria with low nucleic acid contents (LNA bacteria). During the thermal stratification period, the relative abundance of VHNA bacteria (%VHNA) increased with depth, while the reverse trend was evident for LNA bacteria. Seasonally, the %VHNA was strongly positively correlated (r ‫؍‬ 0.87; P < 0.001) with the concentration of dissolved inorganic phosphorus, but not with the concentration of chlorophyll a. The growth of VHNA bacteria was significantly enhanced by addition of phosphate or phosphate plus glucose but not by addition of glucose alone. Although the growth of VHNA and HNA bacteria generally exceeded that of LNA bacteria, our data also revealed that LNA bacteria grew faster than and were grazed as fast as VHNA bacteria in late August, when nutrient limitation was presumably severe. Based on these results, we hypothesize that in severely P-limited environments such as Lake Biwa, P limitation exerts more severe constraints on the growth of bacterial groups with higher nucleic acid contents, which allows LNA bacteria to be competitive and become an important component of the microbial loop.In pelagic ecosystems, heterotrophic bacteria comprise the most important trophic level for processing dissolved organic matter (DOM) and consume a substantial fraction (40 to 60%) of the primary production (1, 6, 7, 31). Generally, previous studies have treated bulk bacterial communities as a homogeneous pool (1, 6, 7, 31), even though they consist of diverse subgroups that differ in metabolic state (14), DOM use (9), growth rate (10, 46), susceptibility to grazing (17), and phylogenetic affiliations (15). One of the contemporary challenges for aquatic microbial ecology is to clarify variations and regulation of different bacterial subgroups in order to facilitate establishing ecologically useful functional units with which the internal dynamics of the bacterioplankton "black box" can be better understood.Flow cytometry has become a powerful tool for discriminating bacterial subgroups based on cellular nucleic acid content (13,26,37). Recent studies using flow sorting have revealed that bacteria with high nucleic acid contents (HNA bacteria) and bacteria with low nucleic acid contents (LNA bacteria) differ in phylogenetic composition (49), although Servais et al. (36) found that common phylotypes are present in both subgroups. Some investigators have suggested that HNA and LNA bacteria represent active and less active components, respectively (14, 22), leading to the proposition that the percentage of HNA bacteria relative to the total bacterial abundance (%HNA) can be an indicator of the fraction of active cells in bacterial communities (14,19). ...

We tested the hypothesis that dissolved organic matter (DOM) is delivered to deep layers by convective mixing in winter, where it contributes to the mineralization of C, N, and P in the oxygenated hypolimnion of large (surface area 674 km 2 , maximum depth 104 m), monomictic Lake Biwa. Basin-scale, seasonal measurements of DOM concentrations revealed that dissolved organic carbon (DOC) and nitrogen (DON) accumulated in the epilimnion during warm seasons and were redistributed into the deeper layer during winter overturn. Hypolimnetic DOC and DON decreased in concentration over the stratification period, indicating mineralization; the contributions of DOM to the total mineralization of C and N were 8% and 30%, respectively. Higher contribution of N relative to C suggests that the N-rich DOM was mineralized at depth. We failed to detect a significant contribution of dissolved organic phosphorus to P mineralization in the hypolimnion, which could be explained by substantial depletion in P relative to C and N of DOM; C : P and N : P ratios were 1,978 and 147, respectively. The data suggest that convective mixing in this monomictic basin delivers DOM to hypolimnetic depths, where it is mineralized during subsequent periods of stratification.

Semi-labile dissolved organic carbon (DOC) plays an important role in the transport and hypolimnetic remineralization of carbon in large freshwater lakes. However, sources of semi-labile DOC in lakes remain unclear. This study used a carbon stable isotope approach to examine relative contributions of autochthonous and allochthonous sources to semi-labile DOC. Vertical and seasonal variations in the concentration and carbon stable isotope ratio (d 13 C) of DOC were determined in large (surface area 674 km 2 ; maximum depth 104 m), monomictic Lake Biwa. A sharp vertical gradient of d 13 C of DOC (d 13 C-DOC) during the stratification period [mean ± standard error (SE) -25.5 ± 0.1 and -26.0 ± 0.0% in the epi-and hypolimnion, respectively] indicated the accumulation of 13 C-rich DOC in the epilimnion. Vertical mixing explained the intermediate values of d 13 C-DOC (-25.7 ± 0.0%) measured throughout the water column during the overturn period. Both DOC concentration and d 13 C-DOC decreased in the hypolimnion during stratification, indicating selective remineralization of 13 C-rich DOC. Using a two-component mixing model, we estimated the d 13 C value of semi-labile DOC to be -22.2 ± 0.3%, which was close to the d 13 C of particulate organic carbon collected in the epilimnion during productive seasons (-22.7 ± 0.7%) but much higher than the d 13 C-DOC in river waters (-26.5 ± 0.1%). Semi-labile DOC appeared to be mainly autochthonous in origin, produced by planktonic communities during productive seasons. The spatiotemporal uncoupling between production and remineralization of semi-labile DOC implies that hypolimnetic oxygen consumption may be affected by pelagic primary production during productive seasons of the preceding year.Keywords Semi-labile dissolved organic carbon Á Autochthony and allochthony Á Mixing model Á Carbon stable isotope ratio Á Large lake

Although the benthic nepheloid layer (BNL) in freshwater and marine systems is known to be an active site of material transformations, information is limited regarding the extent of and mechanisms underlying phosphorus regeneration in the BNL. We found that potential activity of particle-bound alkaline phosphatase (APase) was remarkably high in the BNL of a large (surface area: 674 km 2 ; maximum depth: 104 m), mesotrophic lake (Lake Biwa, Japan) during periods of thermal stratification. The enhancement in the BNL of other ectoenzyme activities was not as pronounced as that of the APase; the ratios of the particle-bound potential activities in the BNL relative to those in the upper hypolimnion were 2.6 ± 1.0 (mean ± SE, n = 22) and 2.0 ± 0.6 for β-glucosidase and leucine aminopeptidase, respectively, whereas the corresponding ratio for APase was 4.5 ± 2.3. The APase activity increased in the BNL with increasing concentrations of soluble reactive phosphorus, inconsistent with a general notion that phosphate represses microbial synthesis of APase. Incubation experiments revealed that the net regeneration of P proceeded in waters collected in the BNL at rates comparable to the in situ accumulation rate of soluble reactive phosphorus in the hypolimnion of the basin. Our data suggest that the enzymatic cleavage of the P moiety of organic phosphorus contributes to the active regeneration of P in the oxic BNL of Lake Biwa. KEY WORDS: Phosphorus cycle · Alkaline phosphatase · Benthic nepheloid layer · Lake BiwaResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 49: 303-311, 2007 thesis (Chróst & Siuda 2002). Although the mechanisms underlying the occurrence of high APase in deep oceanic waters are not entirely clear (Hoppe et al. 2002), the role of APase in P regeneration in aphotic layers clearly deserves closer scrutiny. This is particularly true in chronically P-limited large lakes, where hypolimnetic mineralization of organic matter represents a critical step in the internal loading of P that substantially affects basin-scale patterns of nutrient stoichiometry (Wetzel 2001).The present study aimed at examining the basinscale variation in the activity of APase and other ectoenzymes in the north basin of Lake Biwa, Japan, a large (surface area: 674 km 2 ; maximum depth: 104 m), P-limited, mesotrophic lake with an oxygenated hypolimnion (Kim et al. 2006). One notable feature of the physical settings of this basin is the development of the benthic nepheloid layer (BNL) during periods of thermal stratification (Kim et al. 2006). The BNL is a zone of high particle concentrations, which is formed by resuspension of particulate matter owing to the turbulence that is induced by the bottom current and internal waves (McCave 1986, Kalff 2002. Previous studies have found significant enhancement of bacterial production (Wainright 1987, Cotner et al. 2000 and ectoenzymatic activities (Chróst & Riemann 1994) in resuspended sediments, suggesting that the BNL can be an active...