Hypolimnetic Sediments Research Articles

Mineralization of organic matter in boreal lake sediments: rates, pathways, and nature of the fermenting substrates

Abstract. The complexity of organic matter (OM) degradation mechanisms represents a significant challenge for developing biogeochemical models to quantify the role of aquatic sediments in the climate system. The common representation of OM by carbohydrates formulated as CH2O in models comes with the assumption that its degradation by fermentation produces equimolar amounts of methane (CH4) and dissolved inorganic carbon (DIC). To test the validity of this assumption, we modelled using reaction-transport equation vertical profiles of the concentration and isotopic composition (δ13C) of CH4 and DIC in the top 25 cm of the sediment column from two lake basins, one whose hypolimnion is perennially oxygenated and one with seasonal anoxia. Furthermore, we modelled solute porewater profiles reported in the literature for four other seasonally anoxic lake basins. A total of 17 independent porewater datasets are analyzed. CH4 and DIC production rates associated with methanogenesis at the five seasonally anoxic sites collectively show that the fermenting OM has a mean (± SD) carbon oxidation state (COS) value of -1.4±0.3. This value is much lower than the value of zero expected from carbohydrate fermentation. We conclude that carbohydrates do not adequately represent the fermenting OM in hypolimnetic sediments and propose to include the COS in the formulation of OM fermentation in models applied to lake sediments to better quantify sediment CH4 outflux. This study highlights the potential of mass balancing the products of OM mineralization to characterize labile substrates undergoing fermentation in sediments.

Influence of water column stratification and mixing patterns on the fate of methane produced in deep sediments of a small eutrophic lake

AbstractMethane (CH4), a potent greenhouse gas, is produced in and emitted from lakes at globally significant rates. The drivers controlling the proportion of produced CH4 that will reach the atmosphere, however, are still not well understood. We sampled a small eutrophic lake (Soppensee, Switzerland) in 2016–2017 for CH4 concentrations profiles and emissions, combined with water column hydrodynamics to investigate the fate of CH4 produced in hypolimnetic sediments. Using a mass balance approach for the periods between April and October of both years, net CH4 production rates in hypolimnetic sediments ranged between 11.4 and 17.7 mmol m−2 d−1, of which 66–88% was stored in the hypolimnion, 13–27% was diffused to the epilimnion, and 6–7% left the sediments via ebullition. Combining these results with a process‐based model we show that water column turbulent diffusivity (K z) had a major influence on the fate of produced CH4 in the sediments, where higher K z values potentially lead to greater proportion being oxidized and lower K z lead to a greater proportion being stored. During fall when the water column mixes, we found that a greater proportion of stored CH4 is emitted if the lake mixes rapidly, whereas a greater proportion will be oxidized if the water column mixes more gradually. This work highlights the central role of lake hydrodynamics in regulating CH4 dynamics and further suggests the potential for CH4 production and emissions to be sensitive to climate‐driven alterations in lake mixing regimes and stratification.

Effect of organic enrichment and thermal regime on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in hypolimnetic sediments of two lowland lakes

We analyzed benthic fluxes of inorganic nitrogen, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates in hypolimnetic sediments of lowland lakes. Two neighbouring mesotrophic (Ca' Stanga; CS) and hypertrophic (Lago Verde; LV) lakes, which originated from sand and gravel mining, were considered. Lakes are affected by high nitrate loads (0.2–0.7 mM) and different organic loads. Oxygen consumption, dissolved inorganic carbon, methane and nitrogen fluxes, denitrification and DNRA were measured under summer thermal stratification and late winter overturn. Hypolimnetic sediments of CS were a net sink of dissolved inorganic nitrogen (−3.5 to −4.7 mmol m −2 d −1) in both seasons due to high nitrate consumption. On the contrary, LV sediments turned from being a net sink during winter overturn (−3.5 mmol m −2 d −1) to a net source of dissolved inorganic nitrogen under summer conditions (8.1 mmol m −2 d −1), when significant ammonium regeneration was measured at the water–sediment interface. Benthic denitrification (0.7–4.1 mmol m −2 d −1) accounted for up to 84–97% of total NO 3 − reduction and from 2 to 30% of carbon mineralization. It was mainly fuelled by water column nitrate. In CS, denitrification rates were similar in winter and in summer, while in LV summer rates were 4 times lower. DNRA rates were generally low in both lakes (0.07–0.12 mmol m −2 d −1). An appreciable contribution of DNRA was only detected in the more reducing sediments of LV in summer (15% of total NO 3 − reduction), while during the same period only 3% of reduced NO 3 − was recycled into ammonium in CS. Under summer stratification benthic denitrification was mainly nitrate-limited due to nitrate depletion in hypolimnetic waters and parallel oxygen depletion, hampering nitrification. Organic enrichment and reducing conditions in the hypolimnetic sediment shifted nitrate reduction towards more pronounced DNRA, which resulted in the inorganic nitrogen recycling and retention within the bottom waters. The prevalence of DNRA could favour the accumulation of mineral nitrogen with detrimental effects on ecosystem processes and water quality.

Quantifying gas ebullition with echosounder: the role of methane transport by bubbles in a medium‐sized lake

In lakes and reservoirs with variable water level, gas ebullition can play a substantial role in methane transport in the water column and to the atmosphere. However, measuring methane ebullition from sediment is difficult as releases are highly heterogeneous and intermittent on macro‐ and micro‐scales. In contrast to conventional gas traps and optical methods, hydroacoustic technology allows rapid scanning over large volumes of the water column synoptically to quantify gas bubble abundance. A 120‐kHz dual beam downward‐looking echosounder was used to measure the size distributions of bubbles that do not resonate at the sonar frequency. Data obtained with this sonar permit accurate calculation and evaluation of ebullition flux from the bottom. A robust relationship was established between gas volumes and backscattering cross‐section of individual bubbles in experimental conditions, and rise velocities of bubbles were precisely measured. The volume backscattering coefficient was shown to be a good gauge of the total volume of bubbles per cubic meter of water, allowing the use of a single‐beam sonar for measuring volumetric bubble concentrations. Data obtained from hydroacoustic surveys on Lake Kinneret, where gaseous methane is emitted from randomly dispersed sediment sources, indicated that ~90% of bubbles escaping from soft sediments ranged from 1.3 mm to 4.5 mm and ~50% ranged from 2.0 mm to 3.2 mm in equivalent radius. In summer‐fall 2001, the gaseous methane fluxes from hypolimnetic sediments was ~10 mmol m−2 d−1, accounting for one‐third of the observed methane accumulation in the hypolimnion. This relatively high ebullition rate could be attributed to the gradual decreasing of the mean water level in preceding years.

Long-Term Fate of a Pulse Arsenic Input to a Eutrophic Lake

The long-term fate of a 30-year-old pulse arsenic input to a eutrophic lake was studied to determine if As has become effectively trapped in sediments or remains in active exchange with the water column. Legacy As was readily mobilized from sediments of Spy Pond (Arlington, MA), a eutrophic kettle-hole lake that was treated with 1000s kg As in the 1960s to manage excessive aquatic macrophyte growth. Arsenic was mobilized from hypolimnetic sediments during bottom-water anoxia in spring, summer, and fall, and As accumulated to maximum concentrations of 2100 nM. Mobilization of As from epilimnetic sediments was the largest source of As to the water column on a mass basis (145 mol), despite the fact that the epilimnion remains oxic year-round. Sediment cores revealed that surficial sediments contained As at 30-50 times background levels and suggested that there is contemporary As loading to hypolimnetic sediments (590 mol y(-1)). Mass balance estimates indicate that <5% of the contemporary As load comes from external inputs and that the remainder can be explained by mobilization and redistribution of legacy As, both through the water column and by vertical migration of dissolved As within sediments. These findings demonstrate that, decades after As inputs cease, As in contaminated sediments may remain labile and be mobilized to both anoxic and oxic water columns and accumulate to levels near the sediment surface and in the water column that may pose ongoing risks to ecological health.

Effectiveness and Longevity of Phosphorus Inactivation with Alum

ABSTRACT Effectiveness and longevity of alum treatments were evaluated in 21 lakes (or lake basins) across the United States; 9 were polymictic and 12 were dimictic. Effectiveness was judged from reductions in lake TP (total phosphorus) and internal loading rate, as well as chlorophyll a (chl a), both initially and over periods ranging from 4 to 20 years following treatment. Internal loading rate was reduced in six of nine polymictic lakes/basins by an average of two-thirds, and lake TP was reduced by about one-half, which persisted for 5–11 years. Internal loading rate in dimictic lakes (7 of 7 with adequate data) remained reduced by an average of 80% for 4 to 21 years (average 13 yrs). For the six polymictic lakes, in which treatment was effective, chl a decreased by an average of two-thirds initially, but was about 40% less than die pre-treatment level after 5 to 11 years. Chl a decreased in seven dimictic lakes by an average of 57% initially and 42% after 5–18 years. In some cases, response was independent of alum treatment because the trophogenic zone was affected more by external than internal P loading. At West Twin Lake, OH, nutrient diversion was more important than control of P release from hypolimnetic sediments in improving surface water quality. The significance of vertical P entrainment, as well as external loading, should be considered before treating dimictic lakes. Alum was initially ineffective in two polymictic lakes and was effective only for a few years in a third, apparently due to interference from macrophytes. Dense macrophytes may have caused uneven floe distribution or sediment P was recycled from below the floe layer through plant senescence and decay. Where macrophytes did not interfere, treatments were usually effective for at least 8 years and up to 11 years in polymictic lakes; an expectation of 10 years longevity is considered reasonable for unstratified lakes. Several dimictic lakes showed effectiveness/longevity of internal loading control for at least 13 and up to 18 to 20 years; an expectation of 15 years longevity seems reasonable for dimictic lakes. The decline in treatment effectiveness could have been due to bioturbation, or to sinking and burial of the aluminum floe over time, as evidenced by a distinct Al layer located at about 20 cm after 16 years in West Twin Lake, OH. A direct relation between dose and treatment longevity, as well as an inverse relation between dose and sediment P release rate, was apparent in the dimictic lakes.

Effects of an early fall cold front on heat, phosphorus, silica, and manganese distributions in the hypolimnion of Lake Mendota, Wisconsin

I examined the effects of an early fall (8–10 September 1977) cold front on eutrophic Lake Mendota, Wisconsin, during which ‘Lake Number’ L N (dimensionless) attained a minimum 3 h value of 0.45, and remained below 1.0 for a total of 39 h. The front accelerated heat transport downward through the main thermocline, resulting in hypolimnetic warming; the vertical eddy conductivity K z increased monotonically with depth from a minimum 0.8 m 2 day −1 at 16 m to 1.9 m 2 day −1 at 21 m. The front also affected significant lateral and vertical redistribution of three passive solute ‘tracers’: phosphate (P), silicic acid (Si) and soluble reduced manganese (Mn 2+). Lateral concentration gradients along isopycnals predating the storm were erased. For Mn, especially, the vertical redistribution within the hypolimnion agreed with modeled changes based on vertical gradients and the K z profile. Mn and P behaved quasi-conservatively within the anoxic hypolimnion (no net sources or sinks), but additional supplies of Si appeared in the hypolimnion (8.0 ± 1.5 mg m −3 day −1). This rate over 4 days agreed with the mean rate of hypolimnetic Si accumulation over the 80 day interval preceding 7 September 1977 (8.7 mg m −3 day −1). These results corroborate other environmental evidence that Mn and P both enter Lake Mendota's hypolimnion mainly during the first half of the summer, prior to the onset of bulk hypolimnetic anoxia, whereas Si is released by hypolimnetic sediments continuing into the late summer.

Sites of Methyl Mercury Production in Remote Canadian Shield

Methylation and demethylation of mercury were studied in sediments and surface waters of several remote lakes on the Canadian Shield. Radiochemical assays of mercury methylating activity, which peaked during summer, were 20–40 times faster in epilimnetic than in hypolimnetic sediments. Demethylation rates were usually highest during winter and in hypolimnetic sediments. Epilimnetic sediments were capable of producing methyl mercury 20–40 times faster than hypolimnetic sediments sampled at the same time, with methylating activity peaking during the warm summer months. Because of the opposite pattern of methylating and demethylating activity and because epilimnetic sediments often constitute most of the surface area of these lakes, most of the net methylation (M/D) occurred in the epilimnion of the lakes during summer. Mercury methylation rates were not related to average sediment mercury concentrations, or directly to rates of microbial activity (decomposition) in sediments. It appeared that increasing temperature was an important controlling factor that stimulated methylation but retarded demethylation. Specific methylation rates, M/D, and the measured concentration of methyl mercury in the surface waters of the lakes all peaked during midsummer, suggesting that the radiochemical assays used reflected changes in the relative activities of natural methylating and demethylating microorganisms.

A Successful Alum/Aluminate Treatment of Lake Morey, Vermont

ABSTRACT Lake Morey, Vermont provides a case history of a successful lake restoration effort. Decades of algal blooms and a major summer fish kill preceded a hypolimnetic alum and sodium aluminate treatment in 1986. Earlier diagnostic studies with paleolimnological analyses and a detailed phosphorus budget revealed the cause to be internal phosphorus loading from anoxic hypolimnetic sediments. The aluminum treatment of these sediments resulted in sharp declines in total phosphorus and chlorophyll a concentrations, and increased Secchi transparency. The dose rate of 44 g Al/m2 appears to have been sufficient to form an effective barrier to sediment phosphorus release, and improved conditions continue to persist after four years. Elevated dissolved aluminum levels and some possible adverse effects on the benthic invertebrates and the yellow perch population were temporarily observed following the treatment. The apparent success of the project in controlling phosphorus concentrations is the result of an accu...

Sediment biogeochemistry of iron and sulfur in an acidic lake

The biogeochemical cycling of Fe and S in the sediments of acidic Big Moose Lake, Adirondack Park, NY, U.S.A., was investigated. Sediment cores and porewater samples were collected along a depth transect from the hypolimnion (24 m water depth), metalimnion (17 m), and epilimion (8, 12 m). Four vertically distinguishable zones in the sediment environment were observed at each site: 1.1) NO3− disappearance near the sediment/water interface;2.2) accumulation of solid-phase Fe in the top 5 cm;3.3) coincident accumulation of chromium reducible sulfur (CRS), disappearance of SO4− and minima in C:S and δ34S at or slightly below maxima in oxide-bound Fe; and4.4) homogenous background concentrations in S and Fe below 30 and 10 cm, respectively. Iron and sulfur accumulations in the upper 10 cm occurred at the same depths in cores of different ages, indicating that diagenetic rather than depositional processes played a dominant role in determining near-surface Fe and S profiles. Although sediment accumulation rates varied along the transect, the four zones were located at similar depths in the sediment column at all sites. Diagenetic processes play a major role in the development of these features in Big Moose Lake.The extent of Fe enrichment was considerably greater in the metalimnetic and epilimnetic sediments. In contrast, concentrations of Fe were lower in hypolimnetic sediments, which appeared to be losing Fe to the water column. At all sites, increases in total S from a background concentration of 60–80 μmol g−1 of dry mass occurred in sediments dated approximately 1850. More recent increases to 560 μmol Sg−1 occurred asynchronously in the cores, indicating an important role of sulfate reduction in adding S to sediments. However, organic sulfur accumulation accounted for 22–56% of the recent increase in sediment S. Concentrations of iron and organic carbon were high in lake sediments and probably do not limit S storage in this lake.

A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake

The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,12C and13C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in δ13CDIC), due to preferential uptake of12C by phytoplankton and a change in the dominant CO2 source from inflow andin situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in δ13CDIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion. Mass balance calculations of DI12C and DI13C in the epilimnion for the summer (June 23–September 25) yield a mean rate of net conversion of DIC to organic carbon (Corg) of 430 ± 150 moles d-1 (6.5 ± 1.8 m moles m-2 d-1. Net CO2 invasion from the atmosphere was 420 ± 120 moles d-1 (6.2 ± 1.8 m moles m-2 d-1) with an exchange coefficient of 0.6 ± 0.3m d-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO2. About 50% of CO2 invasion and conversion to Corg for the summer occurred during a two week interval in mid-summer. DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to Corg in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The δ13C of DIC added to the deep water (-22‰.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO2 (-7‰) was produced by a combination of POC oxidation and as a by-product of methanogenesis.

The carbon cycle for Lake Washington—A stable isotope study1

We investigate the carbon cycle in Lake Washington for the year 1980 using monthly measurements of the dissolved inorganic carbon (DIC) and its 13C : 12C isotopic composition. Mass balances of DIC and 13C : 12C yield estimates of CO2 gas exchange rates and net organic carbon production rates. Between 24 June and 13 August, the calculated CO2 gas invasion rate of 0.80 × 106 mol C d−1 is nearly equal to the river DIC inflow rate. The calculated epilimnetic net organic carbon production rate is 0.68 × 106 mol C d−1, about 20–30% of primary productivity estimated from 14C‐fixation experiments and ETS‐derived respiration rates. Metalimnetic and hypolimnetic DIC increase rates and porewater DIC gradients in hypolimnetic sediments indicate that remineralization of particulate organic carbon (POC) previously deposited in the sediments is a major (0.5 × 106 mol C d−1) DIC source to the lake during summer.For the whole year, summertime CO2 gas invasion balances wintertime CO2 gas evasion and DIC and POC outflow balance DIC and POC inflow rates, implying no net carbon burial in the sediments during 1980. This contrasts with the measured long term sedimentation‐rate‐derived carbon burial rate of 0.8 × 106 mol C d−1. Year‐to‐year variability in summertime primary production rates largely determines net gains or losses of carbon via CO2 gas exchange and sedimentation.

A Radiotracer Study of Phosphorus Cycling in a Eutrophic Canadian Shield Lake, Lake 227, Northwestern Ontario

Phosphoric acid labelled with32P was added to the epilimnion of experimentally eutrophied Lake 227, northwestern Ontario, in August 1978 to trace the lake's phosphorus cycle during late stratification and fall overturn. Radiophosphate was incorporated into bacteria and microphytoplankton (&lt;10 μm in diameter) within minutes of introduction. Thereafter, the bulk of32P exchange was between the microplankton, which typically held &gt;90% of32P, dissolved phosphate, and a mobile subcompartment of "dissolved organic" phosphorus. Phytoplankton &gt;10 μm in diameter and zooplankton acquired32P label very slowly and contained &lt;5% of the radiotracer. In addition, about half of dissolved organic phosphorus failed to acquire a32P label over 17 d of incubation, indicating that this phosphorus may be functionally inert, at least at the time scale of biological phosphorus exchange. Movement of32P to the littoral zone of Lake 227 was much slower than in lakes with well-developed macrophyte communities, but losses to deep sediments were similar, about 2% of32P per day. Particles settled through the hypolimnion at a modal velocity of 28 cm∙d−1, losing 10% or less of their32P during descent. Most of the released phosphorus was reincorporated into particles and sedimented. Less than 1% of the32P that entered the hypolimnion in particles between August and October remained in the hypolimnion in late October.32P release from Lake 227′s hypolimnetic sediments during late summer and fall of 1978 was &lt;5% of the32P that sedimented.

CHANGE IN LAKE TROPHIC STATE AND INTERNAL PHOSPHORUS RELEASE AFTER ALUMINUM SULFATE APPLICATION1

ABSTRACT: Recovery of eutrophic lakes after nutrient diversion may be delayed if the lake experiences significant internal phosphorus (p) loading to the water column. A maximum dose of aluminum sulfate, defined herein, was applied to the anaerobic sediments of the hypolimnia of two dimictic Ohio lakes following septic tank diversion, with the objective of attaining long term control of the release of phosphorus to the water column from these sediments. The results were compared to a similar, downstream, untreated lake. Total phosphorus concentration declined sharply after treatment and has remained so through 1980 for both lakes, a period of 5 and 6 years of control, respectively. Internal P loading from anaerobic, hypolimnetic sediments was partially controlled by the treatment but there are other important sources, perhaps in the littoral zone, in these lakes. Algal biomass is Smaller and water transparency has increased. Both lakes became mesotrophic after treatment, as described by the Carlson (1977) trophic state index, and remain in that improved condition to date. No deleterious side effects were observed, although one lake experienced a significant decrease in diversity of planktonic microcrustacea and a lakeward extension of the macrophyte community. This method appears to be an effective and lasting means of accelerating the recovery of a eutrophic lake following nutrient diversion.

Hypolimnetic redox and phosphorus cycling in hypereutrophic Lake Sebasticook, Maine

The vertical distribution of reduced species and phosphate was monitored throughout a summer stagnation period in hypereutrophic Lake Sebasticook, Maine. The oxygen demand of reduced species released from hypolimnetic sediments was dominated by methane, followed by ammonium, and included minor contributions from Fe 2+, Mn 2+ and sulfide. Release of iron and manganese from the sediments was apparently controlled by dissolution of siderite and rhodochrosite, with the requisite acidity for the dissolution provided by methanogenesis. Redox recycling of these two metals occurs in two vertically displaced “wheels”, which enhance oxidant delivery to the hypolimnion beyond that provided by oxygen diffusion alone. Phosphorus is tightly coupled to the “ferrous wheel”, in the absence of wind events, and is scavenged with a stoichiometry consistent with Tessenow's (Tessenow U., Arch. Hydrobiol. Suppl. 47, 1–79, 1974) laboratory studies.

Recent Hypolimnetic Sediment in Lake Gjersjöen, a Eutrophicated Lake in SE Norway

During the last decades Lake Gjersjøen has developed from an oligotrophic state to a level of high eutrophy. The present state is characterized by an intense primary production in the trophogenic zone and anaerobic conditions in the lower part of the hypolimnion during periods of thermal stratification. The deterioration of the lake as a resource has resulted in research on main components of the ecosystem. Most of the limnological data about the lake are to be found in Johansen (1955), Skulberg (1968), Norwegian Institute for Water Research (1969, 1972), Holtan (1972) and Lillevold (1975). The intention of the present investigation was to reveal in which way the rapid eutrophication has been manifested in the hypolimnetic sediments and to search for processes in the sediment that could influence a suggested development towards meromixis. This investigation may provide data for further and more specific research on the sediments in Lake Gjersjøen.