We examined the chemical composition of the water column of Lake Matano, Sulawesi Island, Indonesia, to document how the high abundances of Fe (hydr)oxides in tropical soils and minimal seasonal ...temperature variability affect biogeochemical cycling in lakes. Lake Matano exhibits weak thermal stratification, yet a persistent pycnocline separates an oxic epilimnion from anoxic meta- and hypolimnions. The concentration of soluble P in the epilimnetic waters is very low and can be attributed to scavenging by Fe (hydr)oxides. Chromium concentrations in the epilimnion are high (up to 180 nmol ${\rm{L}}^{{\rm{ - 1}}} $), but below U.S. Environmental Protection Agency guidelines for aquatic ecosystems. The concentration of chromium decreases sharply across the oxic-anoxic boundary, revealing that the hypolimnion is a sink for Cr. Flux calculations using a one-dimensional transport-reaction model for the water column fail to satisfy mass balance requirements and indicate that sediment transport and diagenesis play an important role in the exchange of Fe, Mn, P, and Cr between the epilimnion and hypolimnion. Exchange of water between the epilimnion and hypolimnion is slow and on a time scale similar to temperate meromictic lakes. This limits recycling of P and N to the epilimnion and removal of Cr to the hypolimnion, both of which likely restrict primary production in the epilimnion. Owing to the slow exchange, steep concentration gradients in Fe and Mn species develop in the metalimnion. These concentration gradients are conductive to the proliferation of chemoautotrophic and anoxygenic phototrophic microbial communites, which may contribute a significant fraction to the total primary production in the lake.
Dissimilatory sulfate reduction (DSR) is a major carbon mineralization pathway in aquatic sediments, soils, and groundwater, which regulates the production of hydrogen sulfide and the mobilization ...rates of biologically important elements such as phosphorus and mercury. It has been widely assumed that water-column sulfate is the main sulfur source to fuel this reaction in sediments. While this assumption may be justified in high-sulfate environments such as modern seawater, we argue that in low-sulfate environments mineralization of organic sulfur compounds can be an important source of sulfate. Using a reaction-transport model, we investigate the production of sulfate from sulfur-containing organic matter for a range of environments. The results show that in low sulfate environments (<500μM) the in-sediment production of sulfate can support a substantial portion (>50%) of sulfate reduction. In well-oxygenated systems, porewater sulfate profiles often exhibit sub-interface peaks so that sulfate fluxes are directed out of the sediment. Our measurements in Lake Superior, the world’s largest lake, corroborate this conclusion: offshore sediments act as sources rather than sinks of sulfate for the water column, and sediment DSR is supported entirely by the in-sediment production of sulfate. Sulfate reduction rates are correlated to the depth of oxygen penetration and strongly regulated by the supply of reactive organic matter; rate co-regulation by sulfate availability becomes appreciable below 500μM level. The results indicate the need to consider the mineralization of organic sulfur in the biogeochemical cycling in low-sulfate environments, including several of the world’s largest freshwater bodies, deep subsurface, and possibly the sulfate-poor oceans of the Early Earth.
The productivity of aquatic ecosystems depends on the supply of limiting nutrients. The invasion of the Laurentian Great Lakes, the world's largest freshwater ecosystem, by dreissenid (zebra and ...quagga) mussels has dramatically altered the ecology of these lakes. A key open question is how dreissenids affect the cycling of phosphorus (P), the nutrient that limits productivity in the Great Lakes. We show that a single species, the quagga mussel, is now the primary regulator of P cycling in the lower four Great Lakes. By virtue of their enormous biomass, quagga mussels sequester large quantities of P in their tissues and dramatically intensify benthic P exchanges. Mass balance analysis reveals a previously unrecognized sensitivity of the Great Lakes ecosystem, where P availability is now regulated by the dynamics of mussel populations while the role of the external inputs of phosphorus is suppressed. Our results show that a single invasive species can have dramatic consequences for geochemical cycles even in the world's largest aquatic ecosystems. The ongoing spread of dreissenids across a multitude of lakes in North America and Europe is likely to affect carbon and nutrient cycling in these systems for many decades, with important implications for water quality management.
Animal excretion provides nutrients for primary productivity and can be a crucial component of ecosystem nutrient cycling. The concentrations of carbon (C), nitrogen (N), and phosphorus (P) in an ...animal’s excretion are strongly influenced by the C:N:P stoichiometry (molar ratios) of its body and of the food it eats. We measured the C:N:P ratios of quagga mussel (
Dreissena rostriformis bugensis
) tissues and excreta and of seston across wide environmental and spatial gradients in the upper Laurentian Great Lakes. We then investigated how mussel excretion rates were impacted by stoichiometric mismatch—the difference between the C:P ratios of mussel tissues and the C:P ratios their food. Quagga mussel internal C:N:P stoichiometry varied significantly across sites and seasons, driven primarily by changes in tissue P concentrations. When mussel tissues had substantially lower C:P ratios than seston (that is
,
strong stoichiometric mismatch), mussels excreted significantly less N and P relative to C. Excretion C:N ratios varied by nearly threefold, while C:P ratios varied by tenfold. The effect of the stoichiometric mismatch on excretion stoichiometry was more dramatic in the spring, when mussels had higher tissue P concentrations, than in the summer. This suggests seasonality in mussel P demand. Our results challenge the assumption of strict internal homeostasis in consumers and demonstrate that food and tissue stoichiometry need to be considered to predict consumer excretion stoichiometry. These findings help to better understand the impact of consumer-driven nutrient cycling in aquatic environments and quagga mussel contributions to the nutrient budgets of invaded ecosystems.
We investigated the response of nutrient cycling in the Lower St. Lawrence Estuary (LSLE) to perturbations, using a linear three-box model that reflects summer stratification. The model is used to ...(i) test the sensitivity of each layer’s nutrient concentration (fixed-nitrogen, phosphorus, and silica) to perturbations in nutrient and water volume inputs to the LSLE, (ii) compute the response time of the system to a new steady state following a perturbation, and (iii) estimate the amount of oxygen consumed by respiration as bottom waters are advected through the LSLE. We find that the system adjusts rapidly to perturbations (half a year to reach 90% of the new steady state under a doubling of river input nutrient concentration). Most of the dissolved nutrients (60% of the fixed-nitrogen, 85% of the soluble reactive phosphate) that reach the surface waters in the LSLE originate from deeper waters. This dampens the effect of nutrients of anthropogenic origin on eutrophication (a doubling of river input nutrient concentration will lead to less than a doubling of bottom-water respiration rates). Our nutrient budget suggests that the Lower St. Lawrence Estuary acts as a nutrient pump for the Gulf of St. Lawrence and nitrate appears to be the limiting nutrient to surface productivity in the LSLE. This model can be used to test the impact of natural or anthropogenic perturbations on nutrient in the LSLE and oxygen concentrations in its bottom waters.
•We present a box model of nutrient cycling in the Lower St. Lawrence Estuary (LSLE).•We test the impact of natural or anthropogenic perturbations on nutrients and oxygen.•50% riverine and 25% deep concentrations increase result in 30% more eutrophication.•The effect of nutrients of anthropogenic origin is dampened by upwelling.•The Lower St. Lawrence Estuary (LSLE) adjusts rapidly to perturbations.
•Widespread ferruginous conditions existed during OAE1a (∼120 Ma).•OAE1a ferruginous oceans are only possible with low seawater sulfate concentrations.•Seawater sulfate concentrations likely dropped ...to <600 μM, and possibly <100 μM.•Expansion of global S-sinks promoted the development of ferruginous conditions.•Ferruginous conditions may have promoted photoferrotrophy in the Phanerozoic oceans.
Seawater sulfate is one of the largest oxidant pools at Earth's surface today and its concentration in the oceans is generally assumed to have varied between 5 and 28 mM since the early Phanerozoic Eon. Intermittent and potentially global Oceanic Anoxic Events (OAEs) are accompanied by changes in seawater sulfate concentrations and signal perturbations in the Earth system associated with major climatic anomalies and biological crises. Ferruginous (Fe-rich) ocean conditions developed transiently during multiple OAEs, implying strong variability in seawater chemistry and global biogeochemical cycles. The precise evolution of seawater sulfate concentrations during OAEs, however, is uncertain and thus models that aim to mechanistically link oceanic anoxia to broad-scale disruptions in the Earth system remain incomplete. We use analyses of Fe-speciation and redox sensitive trace metals in sediments deposited in the Tethys and Pacific oceans to constrain seawater sulfate concentrations and underlying dynamics in marine chemistry during OAE1a, ∼120 Ma. We find that large parts of the global oceans were anoxic and ferruginous for more than 1 million years. Calculations show that the development of ferruginous conditions requires that seawater sulfate concentrations drop below 600 μM and possibly below 100 μM, which is an order of magnitude lower than previous minimum estimates. Such a collapse of the seawater sulfate pool in less than one-hundred thousand years is a key and previously unrecognized feature of Phanerozoic Earth surface redox budgets. This sensitivity of the Earth system to changes in seawater sulfate concentrations illustrates potential for dramatically altered global biogeochemical cycles with corresponding climate impacts on remarkably short timescales.
Organic sulfur plays a crucial role in the biogeochemistry of aquatic sediments, especially in low sulfate (< 500 μM) environments like freshwater lakes and the Earth's early oceans. To better ...understand organic sulfur cycling in these systems, we followed organic sulfur in the sulfate‐poor (< 40 μM) iron‐rich (30–80 μM) sediments of Lake Superior from source to sink. We identified microbial populations with shotgun metagenomic sequencing and characterized geochemical species in porewater and solid phases. In anoxic sediments, we found an active sulfur cycle fueled primarily by oxidized organic sulfur. Sediment incubations indicated a microbial capacity to hydrolyze sulfonates, sulfate esters, and sulfonic acids to sulfate. Gene abundances for dissimilatory sulfate reduction (dsrAB) increased with depth and coincided with sulfide maxima. Despite these indicators of sulfide formation, sulfide concentrations remain low (< 40 nM) due to both pyritization and organic matter sulfurization. Immediately below the oxycline, pyrite accounted for 13% of total sedimentary sulfur. Both free and intact lipids in this same interval accumulated disulfides, indicating rapid sulfurization even at low concentrations of sulfide. Our investigation revealed a new model of sulfur cycling in a low‐sulfate environment that likely extends to other modern lakes and possibly the ancient ocean, with organic sulfur both fueling sulfate reduction and consuming the resultant sulfide.
We used a diagenetic model to test the hypothesis that manganese-rich layers in gas hydrate-bearing Arctic Ocean sediments are reliable time markers for interglacial periods. In the model, diagenesis ...is fuelled by two sources of reactive carbon: particulate organic carbon settling to the sediment surface, and methane diffusing up from deep gas hydrate deposits. The model includes oxidation of organic carbon and soluble reduced manganese by oxygen supplied continuously from an invariant bottom-water oxygen reservoir; reduction of particulate manganese by hydrogen sulfide generated through anaerobic methane oxidation; transport of dissolved oxygen and manganese by diffusion; and advective transport of particulate components by burial. Particulate organic matter and particulate manganese are only supplied to the sediment during interglacials. Sulfate reduction is not modeled explicitly; instead, the effect of anaerobic methane oxidation on Mn reduction is simulated at the lower boundary of the model by prescribing that particulate manganese is reduced there to soluble Mn(II). The soluble reduced Mn then diffuses upward and is oxidatively precipitated to Mn(IV) by downward diffusing oxygen. The upward flux of soluble Mn(II) is thus a function of the rate at which particulate manganese is advected into the Mn-reduction layer at the bottom of the model; it is not synchronous with events at the sediment–water interface. Model runs reveal that, under idealized but realistic conditions for the Arctic Ocean, oxidation of upward-diffusing Mn(II) generates post-depositional manganese enrichments that cannot readily be distinguished from the manganese-rich sediment layers that accumulate during interglacials. This compromises the use of manganese-rich layers as proxies for interglacial periods. In contrast, manganese-rich layers may be used as first-order markers of interglacial periods in sediments where gas hydrates or other forms of reactive carbon are absent.
•Mn- enriched layers in gas hydrate-bearing Arctic Ocean sediments are not reliable time markers for interglacial periods.•Diagenetic Mn-enrichments cannot readily be distinguished from primary Mn-rich layers accumulating during interglacials.•Diagenesis in gas hydrate-bearing sediments compromises the use of Mn-rich layers as proxies for interglacial periods.•Mn-rich layers may be used as first-order interglacial markers in sediments where gas hydrates are absent.
In the > 590-m deep, tropical Lake Matano (Indonesia), stratification is characterized by weak thermal gradients (< 2°C per 500 m) and weak salinity gradients (< 0.14‰ per 500 m). These gradients ...persist over seasons, decades, and possibly centuries. Under these nearly steady-state conditions, vertical eddy diffusion coefficients (K
z
) cannot be estimated by conventional methods that rely on time derivatives of temperature distributions. We use and compare several alternative methods: one-dimensional k-ε modeling, three-dimensional hydrodynamic modeling, correlation with the size of Thorpe instabilities, and correlation with the stability frequency. In the thermocline region, at 100-m depth, the K
z
is ~ 5 × 10⁻⁶ m⁻²s⁻¹, but, below 300 m, the small density gradient results in large (20 m) vertical eddies and high mixing rates (K
z
~ 10⁻² m² s⁻¹). The estimated timescale of water renewal in the monimolimnion is several hundred years. Intense evaporation depletes the surface mixed layer of ¹⁶O and ¹H isotopes, making it isotopically heavier. The lake waters become progressively isotopically lighter with depth, and the isotopic composition in the deep waters is close to those of the ground and tributary waters. The vertical distribution of K
z
is used in a biogeochemical reaction-transport model. We show that, outside of a narrow thermocline region, the vertical distributions of dissolved oxygen, iron, methane, and phosphorus are shaped by vertical variations in transport rates, rather than by sources or sinks.
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