东辽河流域河湖光学吸收特性的季节变化

吸收特性是水体重要的光学特性,也是建立水色反演分析模型的基本参数.本文利用2011年6、9、10月在二龙湖和2012年10月在东辽河的实测数据,通过测定水体营养水平和颗粒物、有色可溶性有机物(CDOM)等光学活性物质的吸收系数,分析了东辽河河湖的光学物质吸收特性和季节变化.结果表明: 二龙湖水体9月处于富营养状态,6月营养水平较低,10月则处于中营养水平;东辽河随采样点位置的差异呈现不同程度的富营养化.水体各组分的吸收系数均随营养水平的增加而逐渐增大.6、10月二龙湖水体总悬浮颗粒物吸收光谱与非色素颗粒物吸收光谱相似,在总吸收中占主导作用的是CDOM;9月总悬浮颗粒物吸收光谱则与浮游植物吸收光谱类似,浮游植物的吸收对总吸收的贡献率最大.6、9月二龙湖浮游植物吸收系数[a_ph(440)]与总磷(TP)呈正相关,6月二龙湖a_ph(440)与溶解有机碳(DOC)呈正相关,非藻类颗粒物吸收系数[a_d(440)]仅与叶绿素a浓度(Chl a)具有相关性.二龙湖(9月)与东辽河a_ph(675)均与Chl a、卡尔森指数(TLI)相关性较好.东辽河流域的河湖水体光学特性存在比较明显的差异:东辽河水体的浮游植物吸收光谱曲线展现出了两种不同类型,浮游植物、非藻类颗粒物、CDOM的贡献率随采样点的不同而相差较大;营养水平较高的9月二龙湖与东辽河水体浮游植物主导了水体光谱吸收,而在二龙湖营养水平较低的月份中(6、10月),颗粒物与CDOM共同主导水体光谱吸收;6月二龙湖与10月东辽河水体CDOM的组成为大分子颗粒物,而9与10月其组成更趋向于小分子颗粒物.     

Thermal,chemical, and optical properties of Crater Lake,Oregon

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km² and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm) clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study, August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m) between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of 1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements provided additional evidence about the exceptional clarity of Crater Lake.     

Simulation of water colors in a shallow acidified lake, Lake Onneto, Japan, using colorimetric analysis and bio-optical modeling

Lake Onneto has unusually brilliant water colors, such as blue-green, green, and greenish yellow. We investigated the coloration mechanism in this lake by measuring the radiometric water color and inherent optical properties (IOPs), such as attenuation, absorption, and scattering coefficients of color-producing agents (CPAs) in the lake. The hue of the water body was determined using colorimetric analysis. In addition, the radiometric water color of the lake was simulated using bio-optical modeling. Results showed that the hues of the water body were different between two sampling stations (blue-green and green at 3.1 and 5.5 m of bottom depth, respectively). However, the total suspended solids (TSS) concentrations were almost identical between the two stations (0.47 and 0.42 mg l⁻¹, respectively). In addition, the attenuation and absorption coefficients showed that the CPAs in the water body were only inorganic suspended solids (ISS) more than 0.7 μm in diameter, and specific CPAs, such as dissolved mineral ions and aqueous colloids, were not observed in the lake. Simulation of the radiometric water color showed that the radiation around 600 nm of wavelength (yellow region) steeply increased with decreasing bottom depth, indicating that the water colors of Lake Onneto are strongly governed by the light reflection from yellowish bottom sediments.     

Carbon, nitrogen, and sulfur isotope changes and hydro-geological processes in a saline lake chain

The ionic concentrations,conductivity and pH of water in the Lake Chany complex in West Siberian Russia change from the mouth to the interior of the lake. This difference is indicative of marked evaporation of lake water from the closed water body system in the dry climate of Western Siberia. The carbon isotope composition of particulate organic matter (POM, composed mainly of phytoplankton) clearly changes, along with the pH of the water, reflecting the concentration of dissolved CO₂. Carbon and nitrogen isotope signatures of Chironomus plumosus larvae, a benthic invertebrate that may feed on bulk lake sediment, systematically increase, along with those of POM and sediment organic matter (SOM), through the lake chain. Both sulfate-sulfur and nitrogen isotope compositions of the POM and SOM increase with distance from the estuary into the Lake Chany complex. Heavier sulfur and nitrogen isotope recycling from the sediment, caused by microbial sulfate reduction and denitrification, respectively, may have led to the increased sulfate-sulfur and nitrogen isotope compositions of the POM and SOM.     

Chemical Characteristics of Particulate, Colloidal, and Dissolved Organic Material in Loch Vale Watershed, Rocky Mountain National Park

The chemical relationships among particulate and colloidal organicmaterial and dissolved fulvic acid were examined in an alpine andsubalpine lake and two streams in Loch Vale Watershed, Rocky MountainNational Park. The alpine lake, Sky Pond, had the lowest dissolved organiccarbon (DOC) (0.37 mgC/L), the highest particulate carbon (POC) (0.13mgC/L), and high algal biomass. The watershed of Sky Pond is primarilytalus slope, and DOC and POC may be autochthonous. Both Andrews Creekand Icy Brook gain DOC as they flow through wet sedge meadows. Thesubalpine lake, The Loch, receives additional organic material from thesurrounding forest and had a higher DOC (0.66 mgC/L). Elemental analysis,stable carbon isotopic compositon, and ¹³C-NMR characterizationshowed that: 1) particulate material had relatively high inorganic contentsand was heterogeneous in compositon, 2) colloidal material was primarilycarbohydrate material with a low inorganic content at all sites; and 3)dissolved fulvic acid varied in compositon among sites. The lowconcentration and carbohydrate-rich character of the colloidal materialsuggests that this fraction is labile to microbial degradation and may beturning over more rapidly than particulate fractions or dissolved fulvic acid.Fulvic acid from Andrews Creek had the lowest N content and aromaticity,whereas Sky Pond fulvic acid had a higher N content and lower aromaticitythan fulvic acid from The Loch. The UV-visible spectra of the fulvic acidsdemonstrate that variation in characteristics with sources of organic carboncan explain to some extent the observed non-linear relationship betweenUV-B extinction coefficients and DOC concentrations in lakes.     

The Role of Flow Velocity in the Vertical Distribution of Particulate Organic Matter on Moss-covered Travertine Barriers of the Plitvice Lakes (Croatia)

We investigated the distribution patterns of particulate organic matter (POM) on travertine barriers in respect to flow velocity. Research was conducted on the barrage-lake system of the Plitvice Lakes, Croatia. Four layers were distinguished within the substrate (moss mat + three travertine layers) in three hydraulic habitats at three sites. Substrate samples were collected monthly with a core sampler. The aim of the study was to explore the ability of moss mats and travertine substrate to accumulate POM; to ascertain the role of flow velocity and to produce a model of POM distribution pattern. The average of POM deposited in the 10 cm deep zone decreased significantly in the three sites along longitudinal profile of the system. Most POM was deposited in the moss mats, and the amounts decreased exponentially with depth. This was observed for coarse particulate organic matter (CPOM), ultra-fine particulate organic matter (UPOM) and total organic matter (TPOM) while fine organic matter (FPOM) deposition appeared unaffected by depth. More POM was accumulated in hydraulic habitats of low flow velocity. Correlation between flow velocity and POM accumulation was generally negative. Positive correlations between flow velocity and deposition rates were noted for CPOM in moss mats and top travertine layers; the deposition of other POM fractions was negatively influenced by the flow velocity. The influence of flow velocity decreased with increasing depth. In the deepest layers (7–10 cm) flow velocity influenced only the deposition of the smallest particles (UPOM).     

Regularities in Primary Production,Secchi Depth and Fish Yield and a New System to Define Trophic and Humic State Indices for Lake Ecosystems

General relationships between phytoplankton production, chlorophyll, total, dissolved and particulate phosphorus, Secchi depth, humic level, trophic level, fish production and latitude are described by regression equations using an extensive “Soviet” data base covering a wide domain of lake characteristics and a European data base. New systems for defining lake trophic and humic status are presented. The results may be used for more precise estimates of fundamental lake properties and for many practical issues of lake management, e.g., predictions of fish catch. We have used strict chlorophyll‐a concentrations for every trophic class and we have omitted Secchi depth from the trophic classes, since Secchi depth and other variables strongly related to water clarity (like suspended particulate matter and particulate organic carbon) depend on autochthonous production, allochthonous influences and resuspension. We have used the Secchi depth as a simple operational measure of the effective depth of the photic zone. It has also been shown that among these lakes there exist a very strong relationship between primary production and latitude. In fact, 74% of the variability among the lakes in mean summer primary production can be statistically related to variations in latitude. These data also show a strong relationship between primary production and fish yield, which can be used to address many fundamental issues in lake management, like “normal and abnormal fish production”.     

Total and hydrolizable particulate organic matter (carbohydrates, proteins and lipids) at a coastal station in Terra Nova Bay (Ross Sea, Antarctica)

We analysed quantity and quality of particulate organic matter during the austral summer 1994/1995 at a coastal station in Terra Nova Bay (Ross Sea, Antarctica). Our main aims were to investigate the origin and biochemical composition of particulate organic matter (POM), to measure its availability for consumers through the study of its digestible fraction (measured by using different enzymes separately) and to highlight the role of hydrolizable compounds in the organic matter diagenesis in the coastal waters at Terra Nova Bay. Temporal and spatial patterns of chlorophyll-a concentrations were reflected by the particulate organic carbon, nitrogen and total biopolymeric carbon concentrations, suggesting that most POM originated directly from phytoplankton. The most evident feature of POM in the coastal waters at Terra Nova Bay was the dominance of proteins (on average 57% of total biopolymeric particulate carbon), followed by carbohydrates (25%) and lipids (18%). We found that about 30% of the refractory particulate organic carbon (assumed to be present only after the complete exploitation of particulate organic nitrogen) did not originate from biopolymeric carbon (as sum of carbohydrate, protein and lipid carbon). This allows us to suggest the use of the digestible fraction of particulate biopolymeric carbon as a more accurate measure of the food availability of POM for consumers. In Terra Nova Bay coastal waters, most of the particulate protein pool was associated with large phytoplankton cells or phytodetritus. As a result, the protein pool appeared less available (i.e. less digestible) than the one present in oligotrophic waters where, conversely, most particulate organic nitrogen is sequestered into bacteria. The relative low availability of the protein pool, together with the rapid sinking of POM and the low remineralization rates of benthic heterotrophic microbes, are suggested as possible factors in determining the “inefficiency” in organic matter recycling of coastal waters at Terra Nova Bay, which behaves as a “loss type” system. Received: 17 June 1997 / Accepted 25 September 1997     

The distribution of major biochemical components in the waters of the Kuril Islands

In this paper, data are presented on the distribution of dissolved and particulate carbohydrates, proteins, lipids, and nucleic acids (NAs) in the waters of the Kuril Islands on both the Sea of Okhotsk and Pacific Ocean sides. By analyzing discrepancies in the absolute and relative concentrations of these substances, it is revealed that the waters of the Sea of Okhotsk have a strong bioproductivity. Changes in the absolute and relative conventrations of the major biochemical components of dissolved organic matter (DOM) and particulate organic matter (POM) are shown to result from the biological activity of auto- and heterotrophs of shelf and deep-sea ecosystems. Carbohydrates and proteins prove to be the major biochemical components of DOM and POM, respectively. Given high primary productivity, the carbohydrate content in POM is comparable to that of protein. In the southern Kuril area, the high concentration of dissolved and particulate NAs (which exceeds that of dissolved protein) and the high content of dissolved lipids are both strongly associated with local fish and zooplankton distribution. In the shelf and pelagic waters, biological factors dominate the horizontal and vertical distribution patterns of the major biochemical components of DOM and POM. The hydrological regime only affects these patterns in the straits of the Kuril Islands. For the first time, the biochemical composition of DOM and POM in the sea surface microlayer (SML) is characterized. In this microlayer, the concentrations of major biochemical components of DOM and POM are, to a great extent, higher than in the underlying layer.     

Seasonal and site-specific variability in terrigenous particulate organic carbon concentration in near-shore waters of Lake Biwa, Japan

Identifying sources of particulate organic matter (POM) is important for clarifying fundamental mechanisms by which lake food webs are sustained. We determined carbon and nitrogen stable isotope ratios of POM in near-shore waters of Lake Biwa, a large, meso-eutrophic lake in Japan, to estimate relative contributions of terrigenous particulate organic carbon (T-POC), plankton-derived POC (P-POC) and epilithon-derived POC (E-POC) to POC in near-shore waters. Samples were collected during different months (November, February, May and July) at 29 sites located near the mouth of tributary rivers with different discharge and catchment land use. The data revealed that POC mainly consisted of P-POC and T-POC, with relative contributions varying widely over season and among locations. E-POC generally contributed little to the near-shore POC. Path analyses revealed that the concentration of riverine POC whose isotopic signatures were similar to those of rice straws increased with a larger %paddy field area in the catchment of tributary rivers, which subsequently enhanced T-POC inputs to near-shore waters through riverine transportation. Furthermore, our results suggested that T-POC contribution was influenced, with a time lag, by wave-driven turbulence and shore topography, which appear to affect sedimentation and resuspension of T-POC.     

Satonda Crater Lake,Indonesia: Hydrogeochemistry and biocarbonates

Summary The results of detailed hydrochemical and biosedimentological studies of the sea-linked Satonda Crater Lake, Sumbawa Island/Indonesia are presented. They revealed that the mildly alkaline, mid-water stratified and species-poor lake supports growth of cyanobacterial-red algal calcareous reefs comparable with some ancient marine biocarbonates. The chemical and biotic changes during the last 4,000 years of the lake history have been reconstructed. They indicate that the chemistry of the lake evolved from initially fresh water, through highly alkaline to the modern slightly alkaline quasi-marine conditions with corresponding biotic changes. The influence of the 1815 eruption of the nearby located Tambora Volcano on the lake chemistry and resulting lithological and biotic changes is also discussed. The lake proves to be a good model for the recently proposed hypothesis of an early alkaline (soda) ocean.     

A low trophic position of Japanese eel larvae indicates feeding on marine snow

What eel larvae feed on in the surface layer of the ocean has remained mysterious. Gut contents and bulk nitrogen stable isotope studies suggested that these unusual larvae, called leptocephali, feed at a low level in the oceanic food web, whereas other types of evidence have suggested that small zooplankton are eaten. In this study, we determined the nitrogen isotopic composition of amino acids of both natural larvae and laboratory-reared larvae of the Japanese eel to estimate the trophic position (TP) of leptocephali. We observed a mean TP of 2.4 for natural leptocephali, which is consistent with feeding on particulate organic matter (POM) such as marine snow and discarded appendicularian houses containing bacteria, protozoans and other biological materials. The nitrogen isotope enrichment values of the reared larvae confirm that the primary food source of natural larvae is consistent only with POM. This shows that leptocephali feed on readily available particulate material originating from various sources closely linked to ocean primary production and that leptocephali are a previously unrecognized part of oceanic POM cycling.     

The Proportion of Empneuston and Total Atmospheric Inputs of Carbon,Nitrogen and Phosphorus in the Nutrient Budget of a Small Mesotrophic Lake (Piburger See,Austria)

Litter and wet traps were employed to determine the inputs of coarse organic matter (empneuston), total organic carbon, total nitrogen and phosphorus through dry fallout and precipitation to a soft-water, mesotrophic lake. The dispersal of airborne material over the lake surface was investigated, and a method for the calculation of total input was developed. The component of the particulate matter larger than 1 mm contained the largest proportion of organic carbon, while the dissolved and the fine particle (smaller than 1 mm) fractions made greater contributions to the inputs of nitrogen and phosphorus compounds. The importance of the large particle fraction of the airborne organic matter in the lake's nutrient budget during the autumnal fall of litter was confirmed. However, the maximum total airborne input seems to occur during spring and summer. The input of phosphorus compounds through the atmosphere to small forest lakes is large compared to that from other sources. Airborne material accounts for 39% of the total phosphorus loading.     

Particulate matter composition and bacterial distribution in Terra Nova Bay (Antarctica) during summer 1989–1990

Spatial distributions of particulate organic matter (POM) and microbes were investigated during the summer of 1989–1990 in the coastal waters of Terra Nova Bay (Antarctica). The elemental (organic carbon and nitrogen) and biochemical (lipids, proteins, carbohydrates, DNA and RNA) composition of organic matter was related to bacterioplankton abundance, and pico-phytoplankton density. The ATP concentrations were also measured to gather information about the relationships between particulate matter composition and microbial distribution in Antarctic waters. Total seston was characterized by little spatial variation and was unrelated to the distance from the coast. Suspended particulate matter included some terrestrial components but was mostly composed of autochthonous material. POM was characterized by a uniform distribution and homogeneous composition (mostly of phytoplanktonic origin), and was associated with a relatively scarce microbial community characterized at the surface by high picophytoplankton density. The increase with depth of the living carbon fraction suggested an increase in the microheterotrophic community in the deeper water layers. A significant positive relationship between total bacterioplankton density, and carbohydrate and RNA concentrations was found. Similar significant relationships between pico-phytoplankton abundance and lipids, proteins, carbohydrates and nucleic acids were observed. On the basis of the close coupling found between microbiological and chemical compartments, it seems that, in Terra Nova Bay, bacterial distribution depends on suspended matter and in particular to the labile fraction of the organic detritus.     

Geochemistry of aquatic humic substances in the Lake Fryxell Basin, Antarctica

Dissolved organic carbon (DOC) in Lake Fryxell, 10 streams flowing into the lake, and the moat surrounding the lake was studied to determine the influence of sources and biogeochemical processes on its distribution and chemical nature. Lake Fryxell is an amictic, permanently ice-covered lake in the McMurdo Dry Valleys which contains benthic and planktonic microbial populations, but receives essentially no input of organic material from the ahumic soils of the watershed. Biological activity in the water column does not appear to influence the DOC depth profile, which is similar to the profiles for conservative inorganic constituents. DOC values for the streams varied with biomass in the stream channel, and ranged from 0.2 to 9.7 mg C/L. Fulvic acids in the streams were a lower percentage of the total DOC than in the lake. These samples contain recent carbon and appear to be simpler mixtures of compounds than the lake samples, indicating that they have undergone less humification. The fulvic acids from just above the sediments of the lake have a high sulfur content and are highly aliphatic. The main transformations occurring as these fractions diffuse upward in the water column are 1) loss of sulfur groups through the oxycline and 2) decrease in aliphatic carbon and increase in the heterogeneity of aliphatic moieties. The fraction of modem¹⁴C content of the lake fulvic acids range from a minimum of 0.68 (approximately 3000 years old) at 15m depth to 0.997 (recent material) just under the ice. The major processes controlling the DOC in the lake appear to be: 1) The transport of organic matter by the inflow streams resulting in the addition of recent organic material to the moat and upper waters of the lake; 2) The diffusion of organic matter composed of relict organic material and organic carbon resulting from the degradation of algae and bacteria from the bottom waters or sediments of the lake into overlying glacial melt water, 3) The addition of recent organic matter to the bottom waters of the lake from the moat.     

Deposition,resuspension, and decomposition of particulate organic matter in the sediments of Lake Itasca,Minnesota, USA

Sediment traps were used to investigate the settling, resuspension, and decomposition of particulate organic matter in Lake Itasca, MN (USA). Traps were deployed in the epilimnion and hypolimnion of the deepest basin during June, 1988, sampled twice during stratified conditions (August, September) and once after the lake had mixed (October). The downward flux of particulate material increased from summer to fall. The net sedimentation of organic matter ranged from 0.6 to 2.3 g m^–2 d^–1 at 4 m and increased to 2.1 to 3.2 g m^–2 d^–1 two meters above the bottom sediment indicating that resuspended sediment was at least 33% of the settling mass during all periods. The C:N ratios of captured particles (6.8–9.5) were between the ratios of plankton (5.8 to 6.8) and the sediments (9.9 to 10.2) but smaller than the ratios of terrestrial organic materials (13.5 to 222). The monosaccharide compositions of the entrapped particles were similar to plankton samples and different from the distinct composition of the sediments. Capture of rebound particles similar to the primary flux and not decomposition may have been responsible for this similarity. Total monosaccharide concentrations were lower in the sediments than in entrapped particles. Individual sugars exhibited different patterns of accumulation in the sediments. Glucose was lowest in sediments when the relative concentrations were compared to those in source materials and entrapped particles. In contrast, sediments had the highest rhamnose and fucose concentrations. Bacterial biomass could only account for small portions of these sugars in the sediment. The distinct monosaccharide composition of resuspended sediments was not strongly recorded in materials captured by the sediment traps even after the lake had mixed.     

Annual productivity and phytoplankton changes between 1969 and 1974 in Gull Lake, Michigan

SUMMARY. The seasonal and depth distributions of dominant phytoplankters, in situ primary productivity, organic carbon, and major chemical variables are described for Gull Lake, Michigan for the years 1971–74. Despite efforts by residents surrounding this moderately large dimictic lake to reduce phosphorus loading, all indications are of progressive eutrophication over the period. Previously dominant Cyclotella michiganiana has almost completely disappeared while araphidinean diatoms have increased greatly; certain blue-green algae that were absent previously are becoming characteristic. The ratio of araphidinean to centric diatoms in recent sediments has also increased. Primary productivity and particulate and dissolved organic carbon concentrations have also increased. Voluntary attempts to reduce nutrient loading have apparently been inadequate to arrest increasing rates of eutrophication.     

太湖水色因子空间分布特征及其对水生植物光合作用的影响

利用太湖全湖64个采样点的数据,分析了各水色因子及真光层深度的空间分布和变化特征,并探讨了其对水生植物光合作用的影响.结果表明:叶绿素a浓度在全湖间的差异最大,其变化范围为1.67～159.94μg.L-1,均方差为41.03μg.L-1,在梅梁湾、竺山湖、夹浦港和小梅口附近湖区,叶绿素a浓度较高且空间变化明显,等值线在这些湖区密集分布;悬浮物浓度变化次之,其含量在6.47～143.47mg.L-1之间变化,均方差为31.63mg.L-1,其在大浦港和小梅口入湖口附近湖区的空间变化明显,等值线分布密集;有色可溶性有机物(CDOM)吸收系数在全湖变化较小,没有明显的空间变化特征;真光层深度受悬浮物和叶绿素的共同影响,其空间分布特征与悬浮物相反.     

The role of plankton biomass in controlling fluctuations of suspended matter in Lake Constance

The concentrations of particulate matter, expressed as dry weight (DW), particulate organic (POM), and inorganic material were measured at regular intervals in Lake Constance between February 1980 and December 1982. Maximum particle concentrations were recorded for the euphotic zone in summer (7 mg l⁻¹), while minima occurred during the early summer and in winter. Annual mean concentrations of DW within the entire water column varied between 0.6 and 0.7 mg l⁻¹. In the euphotic zone nearly 70% of DW is organic material. The inorganic particles originate either from phytoplankton (diatomaceous silicon, biogenic decalcification) or from the tributaries. Although phytoplankton biomass only comprises a relatively small proportion (i.e. 30% at maximum) of organic matter, it is the primary source of POM. Therefore, seasonal variations in phytoplankton control epilimnetic concentrations of POM in Lake Constance. Inorganic material comprises smaller proportions of suspended matter. Seasonal variations are related predominantly to fluctuations in biomass and therefore particulate inorganic matter is suggested to originate mainly from autochthonous sources. At the sampling station concentrations of inorganic particles supported by the main tributary, the Alpenrhein, only occasionally vary concomitantly with runoff.     

The Role of Iron and Dissolved Organic Carbon in the Absorption of Ultraviolet Radiation in Humic Lake Water

Absorption of solar ultraviolet radiation (UVR) in aquatic ecosystems is primarily controlled by dissolved organic carbon (DOC). The role of iron (Fe) has also been suggested to contribute to UVR attenuation either directly or by interactions with DOC. Here we present findings from three laboratory manipulations of Fe and DOC on changes to the dissolved UVR absorption (a_d,320) in a mid-latitude, dimictic, humic lake. In a laboratory simulation of lake turnover where anoxic, hypolimnetic water was oxygenated a_d,320 significantly increased from 23.3 to 81.7 m⁻¹ (p<0.0001). In a second laboratory experiment, addition of ferrous Fe to deoxygenated lake water increased a_d,320 upon reoxygenation up to a concentration of 1.0 mg l⁻¹ Fe, where a solubility saturation threshold may have been reached. In situ lake experiments were designed to simulate release of UV absorbing substances from anoxic sediments by placing 20-l carboys (open at the bottom, sealed at the top) onto the lake bottom. UV absorption at 320 nm increased over time for samples from within the experimental carboys. Finally, samples from several lake profiles and sediment experiments were analyzed for a_d,320, total Fe, and DOC. UV absorption of dissolved substances at 320 nm and total Fe concentration both increased with depth, however DOC remained relatively constant over depth. Furthermore, total Fe and spectral slope showed tight coupling up to 1 mg l⁻¹ total Fe in our survey analysis. Our results provide evidence for the importance of anoxic sediments as a source of ferrous iron and UV absorbing substances and suggest a role for ferric iron in increasing UVR and PAR absorption in lake water. We suggest that as this ferrous Fe oxidizes, its absorptive properties increase, and it may bind with dissolved organic matter, enabling it to remain in solution and thus increasing the dissolved absorption of lake water for extended periods of time.