Role of dissolved organic matter in hypolimnetic mineralization of carbon and nitrogen in a large, monomictic lake

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 km2, 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. Hypolimnetic mineralization of organic carbon and nutrients in stratified waterbodies of large lakes represents an important biogeochemical process that affects the magnitude and spatiotemporal patterns of hypolimnetic oxygen depletion and the internal loading of N and P (Cornett and Rigler 1979; Wetzel 2001). Traditionally, studies have considered that this process is primarily linked to sedimentation and the subsequent mineralization of particulate organic matter (POM; Hutchinson 1938). Little is known as yet about the role of dissolved organic matter (DOM) in the regulation of hypolimnetic metabolism of large lakes, even though DOM often represents the largest organic matter pool in lake waters (Wetzel 2001; Biddanda and Cotner 2002). Recently, McManus et al. (2003) found that hypolimnetic oxygen depletion far exceeded the value that could be accounted for by sinking fluxes of particulate organic carbon (POC) in Lake Superior, suggesting that the oxidation of dissolved organic carbon (DOC) might explain the major fraction of hypolimnetic oxygen consumption. However, rigorous testing of this hypothesis has been hampered by a paucity of data on the dynamics and consumption of DOC in the hypolimnion of large lakes. Coherent examinations of seasonal, horizontal, and depthdependent variations of DOM in lake waters could provide useful information regarding the contribution of DOM to hypolimnetic metabolism. In marine environments, investi1 Corresponding author ([email protected]). Acknowledgments We thank T. Koitabashi, T. Miyano, K. Yoshiyama, C. Yoshimizu, and T. Yokokawa for support during the field sampling. H. Ogawa provided helpful advice on DOC measurements. This study was supported by MEXT/JSPS grants 12800016 and 13308029, the Sumitomo Foundation, and the Grant for the Biodiversity Research of the 21st Century COE (A14). Financial support was also provided by the Basic Research Program (CREST-type strategic sector, R&D of Hydrological Modeling and Water Resources System) of Japan Science and Technology Agency. gators have examined seasonal variations of DOC profiles to estimate the vertical flux of DOC to the mesopelagic zone (Copin-Montégut and Avril 1993; Carlson et al. 1994). These studies have demonstrated that vertical mixing in winter introduces DOM into the mesopelagic zone where DOC is mineralized during the stratification period, suggesting that the DOC flux driven by this mechanism (the DOM pump) accounts for a significant fraction of the total downward flux of organic carbon (Copin-Montégut and Avril 1993; Carlson et al. 1994). Studies have also suggested that the downward transport of DOC is coupled with the transport of dissolved organic nitrogen (DON) and phosphorus (DOP), a process regulated by the C : N : P ratio and the stoichiometry of the decomposition of DOM (Hopkinson and Vallino 2005). Because the C : N : P ratios of bulk DOM are typically high compared with those of POM, one would expect that the DOM pump more efficiently delivers C relative to N and P (Hopkinson and Vallino 2005), yet significant amounts of nutrients could be transported depending on the lability of DON and DOP. Whether or not the emerging paradigm just described regarding the DOM pump in marine systems applies to large, monomictic freshwater lakes has yet to be examined. Our objective here was to examine, on the basis of extensive measurements of basin-scale, seasonal variations in DOM, whether DOC, DON, and DOP contribute to the mineralization of C, N, and P in the hypolimnion of Lake Biwa, a large (surface area 674 km2) tectonic lake located in the central part of Honshu Island, Japan (Horie 1984). Our study was conducted in the monomictic North basin (water volume 28.1 km3; water residence time 5.5 yr), which is deep (maximum depth 104 m), mesotrophic (annual mean levels of total phosphorus, chlorophyll a, and primary production are 0.2 mmol L21, 2 mg L21, and 104 mmol C m22 d21, respectively; Yoshimizu et al. 2001, Nishimura et al. 2005), and oxygenated (the annual minimum concentration of hypolimnetic dissolved oxygen (DO) concentration is ;4 mg L21), 71 Hypolimnetic DOM mineralization Fig. 1. Lake Biwa and sampling stations. Depth is shown in meters. providing a useful test of the hypothesis that the DOM pump operates in large freshwater lakes. Materials and methods Thermal regime of the water column in the study area— Lake Biwa is a warm monomictic lake with surface water temperatures ranging from 7.88C 6 0.58C in March to 27.68C 6 2.38C in August (average 6 SD for the period between 1990 and 2000; http://www.ecology.kyoto-u.ac.jp/ biwako/teikan/index.htm). Winter cooling combined with strong seasonal wind promotes vertical water mixing, allowing the whole water column to be included in the circulation between late January and March (overturn period). Surface water temperature starts to increase in April to establish thermal stratification (Horie 1984). Water sampling—Bimonthly samplings were carried out between June 2001 and June 2002 at six transect stations deployed along the major axis of the lake (Fig. 1). Because of rough weather, Stas. 1 and 5 were not visited in December 2001. Additional samplings were carried out at selected stations (Stas. 4 and 6) until February 2003 with a bimonthly interval and in October 2003. Each sampling was completed within 2 or 3 days to obtain quasi-synoptic data. The hydrographic structure was determined with a CTD probe (SBE911plus, Sea Bird Electronics Sealogger), and vertical sampling was conducted at predetermined depths. We used 5-liter Niskin-X bottles (General Oceanics Miami) for water sampling. The water samples near the bottom (1, 3, and 5 m above the bottom) were collected with the use of three sampling bottles sequentially attached to the wire at appropriate intervals. An echo sounder was used to help determine the position of the bottle for sampling at 1 m above the bottom. Actual sampling depths were ascertained with depth loggers attached to the sampling bottles. Before each survey, sampling bottles were cleaned by detergent followed by soaking with 0.6 mol L21 hydrochloric acid and then vigorously rinsed with Milli-Q water (Mill-Q A10 Gradient, Millipore). Sample waters for the measurement of dissolved constituents, except for dissolved oxygen, were gravity filtered through GF/F glass fiber filters (Whatman, acid-rinsed and precombusted) with in-line filtration manifolds (Advantec Co.) directly connected to the outlet of the Niskin bottles. For the measurement of DOC, the filtrate was dispensed into precombusted glass vials and sealed with acid-washed Teflon-faced silicone septa. These samples were acidified (2 mmol L21 HCl) and stored in a refrigerator until analysis (principally within 100 h). For the measurement of other dissolved constituents, the filtrate was contained in acidwashed polyethylene bottles and stored at 2208C until analysis. During sampling, plastic gloves were worn and care was taken to minimize contamination. Contamination associated with sampling was examined by measuring DOC concentrations in Milli-Q water that had been in Niskin bottles and then filtered as just described. Results showed that DOC contamination was undetectable (,1 mmol L21). Water samples for the measurement of particulate constituents were filtered through acid-rinsed, precombusted GF/F glass fiber filters (Whatman), and the filters were stored frozen until analysis. Chemical analyses—DOC concentration was determined by high-temperature catalytic oxidation with a TOC-5000A (Shimadzu) with autosampler injection after being purged with the CO2 free carrier gas for 10 min at 105 mL min21. The high-sensitivity catalyst (Shimazu) was conditioned by repeated injection (Benner and Strom 1993). Instrumental and Mill-Q water blanks were typically 2.1–3.2 and 2.7–5.4 mmol L21, respectively, accounting for ,10% of the minimum DOC concentration in our water samples. Standards were made in the range 0–166 mmol L21 with solutions of potassium hydrogen phtalate. The analytical precision (C.V.) of the replicate measurements (n 5 3–5) was ,2%. Nitrate, nitrite, and soluble reactive phosphorus (SRP) were measured spectrophotometrically with an autoanalyzer (AACS II, Bran 1 Luebbe) according to the manufacturer’s manual. Ammonium was analyzed fluorometrically after the derivatization with orthophthaldialdehyde (Holmes et al. 1999). Total dissolved nitrogen (TDN) and phosphorus (TDP) were simultaneously measured by the wet oxidation method of Pujo-Pay and Raimbault (1994). Recovery of glycine after this oxidation was 95%. Analytical precision (C.V.) was 0.7% for TDN and 2.8% for TDP. Concentrations of dissolved organic nitrogen (DON) were estimated as differences between TDN and total DIN (nitrate 1 nitrite 1 ammonium), and those of DOP were estimated as differences between TDP and SRP. Concentrations of particulate car-