Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m^–3 could have occurred even though 90% or more of the biogenic silica production is recycled.     

Biogenic silica as an estimate of siliceous microfossil abundance in Great Lakes sediments

Biogenic silica concentration (BSi) in sediment cores from the Great Lakes is evaluated as an estimate of siliceous microfossil abundance. A significant linear relationship was found between measured BSi and diatom valve abundance for sediment cores from the Bay of Quinte, Lake Ontario, Lake Erie, Lake Michigan and Lake Superior and between measured BSi and diatom biovolume for Lake Erie, Lake Michigan, and Lake Superior but not for Lake Ontario. Diatom silica predicted from diatom species abundance and an estimated silica content per cell in the Lake Erie cores accounted for 117% and 103% of measured BSi, respectively. By contrast, predicted diatom silica could only account for 28% of measured BSi in the Lake Michigan core and only 25% in the Lake Superior core. A few large diatoms with a large silica content per cell comprised a major portion of predicted diatom silica in all cores. The discrepancy between chemically measured BSi and the silica predicted from diatoms in the Lake Michigan and Lake Superior cores was partially due to the inability of the regression model, used to estimate diatom silica content, to account for different degrees of silicification in the diatom asemblages from the more dissolved silica rich Lake Michigan and Lake Superior.     

Freshwater marshes as dissolved silica recyclers in an estuarine environment (Schelde estuary, Belgium)

Compared to knowledge about N and P processing in the aquatic continuum of lakes, wetlands and estuaries, knowledge concerning transport and cycling of Si is only fragmentary. Furthermore, Si research in estuaries has mainly been focused on subtidal benthic sediments and uptake and recycling by diatom communities. The biogeochemical cycling of Si in tidal wetlands, which can contain large amounts of Si, has thus far been neglected. We have conducted several whole ecosystem Si mass-balances on a freshwater marsh located in the Schelde estuary (6 tidal cycles, 2 with BSi included). Our measurements show that the freshwater marsh acts as an important source of dissolved Si to the main river (1–18% more export than import, on average 0.114 g m^–2). This export is compensated by import of amorphous silica into the marsh (19–55% more import than export). The marsh was shown to act as silica recycler, resupplying biologically available dissolved Si to the estuarine ecosystem. Extrapolations show that during summer and spring months, when dissolved silica is depleted due to diatom growth, almost half of the total dissolved silica load in the main river channel could result from marsh recycling.     

NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.     

Silica fluxes and trapping in two contrasting natural impoundments of the upper Mississippi River

Human activities have altered riverine silica cycling and diminished the supply of silica to the oceans, but few rivers have been intensively monitored to evaluate the magnitude of these changes. In this study we measured dissolved silica (DSi) and amorphous silica (ASi) fluxes into and out of two large, culturally-impacted natural impoundments of the upper Mississippi River, Lakes St. Croix and Pepin, USA. ASi sedimentation rates and sediment–water fluxes of DSi were calculated for each lake, and a mass-balance approach was used to determine in-lake ASi production. ASi from terrestrial phytoliths in the lake sediments was determined to be only partially available to biotic recycling, and in-lake ASi dissolution was small relative to the total silica budgets. The river reaches upstream of the two lakes were found to have abundant DSi, and riverine diatom production was found to contribute significant amounts of ASi to each lake. The average total phosphorus concentration in Lake Pepin is four times that in Lake St. Croix but ASi production in Lake Pepin is only 2.3 times higher than in Lake St. Croix, indicating that diatom growth in Pepin is limited by factors such as turbidity. Lake St. Croix currently traps about 10% of the inflowing total bioavailable silica (TSi_b = DSi + ASi) while Lake Pepin traps closer to 20% of its inflowing TSi_b, clearly demonstrating the importance of silica retention in lakes and reservoirs along the land–ocean continuum.     

Biogenic silica record in the sediments of Little Round Lake,Ontario

The biogenic silica (BSi) record has been determined in the sediments of Little Round Lake, Ontario in order to review its postglacial development and study the relationship between BSi and diatoms. BSi concentrations in the sediment stratigraphy were found to correspond for the most part to the trophic history of Little Round Lake. Calculation of accumulation rates for BSi improved the correspondence of the BSi profile to the trophic history. Thus, BSi is a valuable paleoindicator when concentration and flux profiles are considered concurrently. Regression analysis of BSi and diatoms revealed that the concentration of BSi in the sediments was not a simple function of diatom numbers or biovolume, but that factors such as a correction for the rate of dissolution and abundance of chrysophycean scales and cysts were also important.     

Examination of the role of dreissenids and macrophytes in the phosphorus dynamics of Lake Simcoe,Ontario, Canada

Our study examines the relative importance of the causal linkages between exogenous total phosphorus (TP) loading and internal nutrient recycling with the water quality conditions in Lake Simcoe, Ontario, Canada. We enhance the mechanistic foundation of a simple TP mass-balance model, originally developed to guide the eutrophication management in the system. The structural improvements include the incorporation of macrophyte dynamics, the explicit representation of the role of dreissenids in the system, and the improved portrayal of the interplay between water column and sediments. Our model provides good agreement with the observed TP variability in the system during the study period (1999–2007). Consistent with empirical evidence, our model predicts that macrophyte uptake from the interstitial waters is responsible for a significant loss of P from the sediments. Our model also suggests that dreissenids filter a considerable amount of particulate P from the water column, but the effective clearance rate is significantly lower with a substantial amount of the filtered particles (> 85%) returned into the water column as faeces, pseudofeces or other metabolic excreta. P diffusive fluxes from the sediments account for about 30–35% of the exogenous P loading in Lake Simcoe. The sediments in the main basin are mostly driven by fast diagenetic processes of settling organic matter from the epilimnion, suggesting an internal P loading of 9.2 tonnes yr^− 1. Finally, our study attempts to explain the lack of distinct decreasing trends in ice-free TP concentrations after the invasion of dreissenid mussels, suggesting that the presence of active nutrient recycling pathways, potentially magnified by the particular morphological features and hydrodynamic patterns of Lake Simcoe, could counterbalance the direct effects of dreissenid filtration.     

Laurentian Great Lakes Phytoplankton and Their Water Quality Characteristics,Including a Diatom-Based Model for Paleoreconstruction of Phosphorus

Recent shifts in water quality and food web characteristics driven by anthropogenic impacts on the Laurentian Great Lakes warranted an examination of pelagic primary producers as tracers of environmental change. The distributions of the 263 common phytoplankton taxa were related to water quality variables to determine taxon-specific responses that may be useful in indicator models. A detailed checklist of taxa and their environmental optima are provided. Multivariate analyses indicated a strong relationship between total phosphorus (TP) and patterns in the diatom assemblages across the Great Lakes. Of the 118 common diatom taxa, 90 (76%) had a directional response along the TP gradient. We further evaluated a diatom-based transfer function for TP based on the weighted-average abundance of taxa, assuming unimodal distributions along the TP gradient. The r² between observed and inferred TP in the training dataset was 0.79. Substantial spatial and environmental autocorrelation within the training set of samples justified the need for further model validation. A randomization procedure indicated that the actual transfer function consistently performed better than functions based on reshuffled environmental data. Further, TP was minimally confounded by other environmental variables, as indicated by the relatively large amount of unique variance in the diatoms explained by TP. We demonstrated the effectiveness of the transfer function by hindcasting TP concentrations using fossil diatom assemblages in a Lake Superior sediment core. Passive, multivariate analysis of the fossil samples against the training set indicated that phosphorus was a strong determinant of historical diatom assemblages, verifying that the transfer function was suited to reconstruct past TP in Lake Superior. Collectively, these results showed that phytoplankton coefficients for water quality can be robust indicators of Great Lakes pelagic condition. The diatom-based transfer function can be used in lake management when retrospective data are needed for tracking long-term degradation, remediation and trajectories.     

Grazers: biocatalysts of terrestrial silica cycling

Silica is well known for its role as inducible defence mechanism countering herbivore attack, mainly through precipitation of opaline, biogenic silica (BSi) bodies (phytoliths) in plant epidermal tissues. Even though grazing strongly interacts with other element cycles, its impact on terrestrial silica cycling has never been thoroughly considered. Here, BSi content of ingested grass, hay and faeces of large herbivores was quantified by performing multiple chemical extraction procedures for BSi, allowing the assessment of chemical reactivity. Dissolution experiments with grass and faeces were carried out to measure direct availability of BSi for dissolution. Average BSi and readily soluble silica numbers were higher in faeces as compared with grass or hay, and differences between herbivores could be related to distinct digestive strategies. Reactivity and dissolvability of BSi increases after digestion, mainly due to degradation of organic matrices, resulting in higher silica turnover rates and mobilization potential from terrestrial to aquatic ecosystems in non-grazed versus grazed pasture systems (2 versus 20 kg Si ha⁻¹ y⁻¹). Our results suggest a crucial yet currently unexplored role of herbivores in determining silica export from land to ocean, where its availability is linked to eutrophication events and carbon sequestration through C–Si diatom interactions.     

汕头南澳养殖区表层沉积物中生源要素的分布及其污染评价

于2010年9月采集南澳海域鱼类养殖区、贝类养殖区、大型藻类栽培区和对照区4个功能区表层沉积物样品,分析沉积物中生物硅(BSi)、总有机碳(TOC)、总氮(TN)和总磷(TP)4种生源要素的含量。结果表明,南澳海域生源要素含量与国内外养殖区相比属于中等水平。鱼类养殖区TOC、TN、TP的含量最高,而BSi的最高含量出现在藻类栽培区(平均含量为0.30%)。鱼类养殖区BSi、TOC、TN和TP的平均含量分别为0.24%、0.89%、0.13%和0.097%。根据沉积物中TOC/TN比值分析,发现鱼类养殖区的TOC主要来源于水生,贝类、藻类和对照区TOC则主要来源于陆源。生源要素污染评价表明,4个功能区的TN均属于Ⅱ类污染,鱼类和贝类养殖区的TP属于Ⅱ类轻度污染。     

Assessment of nutrient limitation in Rijeka Bay, NE Adriatic Sea, using miniaturized bioassay

In this paper we consider the trophic state and ambient nutrient limitation by the trophic index (TRIX) calculation and nutrient enrichment bioassay using the diatom Phaeodactylum tricornutum at two differently eutrophicated NE coastal Adriatic locations (in the Rijeka Bay area). The first station in Rijeka Port had significantly higher concentrations of dissolved inorganic nutrients and TRIX indices (up to 7.53) than the station near Opatija. The DIN/TP ratios in Rijeka Port were lower (11.03 to 44.47) than those in Opatija (38.67 to 82.00), indicating P limitation in Opatija. The nutrient enrichment bioassays revealed P as the key limiting nutrient for the diatom growth at both sites. In addition, silica limitation was found in all water samples with the exception of the surface water samples in Rijeka Port. Nitrogen limitation was rarely evident.     

Seasonal sedimentation trends in a mesotrophic lake: Influence of diatoms and implications for phosphorus dynamics

To quantify the extent to which biomass and phosphorus in particular is removed from an aquatic system via sedimentation as well as to identify factors that influence sedimentation of nutrient elements, various characterizations of suspended and settling particulate matter were made in Trout Lake, Wisconsin, USA. The proportion of water column phosphorus reaching sediment traps showed a seasonal component with a minimum during late summer. Biogenic silicon analysis indicated that relatively high rates of phosphorus removal were associated with the sedimentation of siliceous algae (diatoms) from the water column. Estimates of the impact of nutrient removal through diatom sedimentation indicate that this process can reduce primary production by decreasing the amount of nutrient remineralization in the water column during the stratified period.     

Historic Trends in Lake Michigan Silica Concentrations

Historic data on soluble silica in the offshore waters of Lake Michigan were compiled and analyzed to determine whether the data supported a decrease in silica concentration which had been hypothesized previously on the basis of other studies. Although the data base was limited and no data were obtained for offshore waters (depths >40 m) prior to 1954, the available data support the conclusion that the silica concentration decreased rapidly after 1954. The thesis is developed that rapid silica depletion occurred in the 15-year period from 1954 to 1969 when the winter maximum concentration decreased from approximately 4.4 to 1.4 mg SiO₂ · 1⁻¹ and the summer minimum decreased from approximately 2.2 to <0.2 mg SiO₂ · 1⁻¹. The decrease in silica concentration is attributed to increased production and sedimentation of diatoms that resulted from increased anthropogenic phosphorus loading. Total phosphorus in offshore waters was probably <10 μg P · 1⁻¹ during the period of rapid silica depletion. The rapid decrease in silica concentration with relatively small phosphorus enrichment demonstrates that the tight coupling of biological and geochemical processes drastically affected the biogeochemistry of silica in a large, aquatic system within a relatively short period of time.     

Transition from P‐ to N‐limited phytoplankton growth in an artificial lake on flooded cutaway peatland in Ireland

Question: Which nutrient limits primary production in a lake created by flooding industrial cutaway peatland? Location: Clongawny Lake (53°10’N, 07°53’W), County Offaly, Ireland Methods: Nutrient concentrations in lake water and the dynamics of phytoplankton populations were monitored over a 38‐month period. The ratio of dissolved inorganic nitrogen to total phosphorus (DIN:TP) and nutrient enrichment bio‐assays were used to investigate temporal changes in nutrient limitation. Results: Primary production in the new lake was phytoplankton‐driven due to the scarcity of recolonizing macrophytes. Phytoplankton growth was initially phosphorus‐limited. The runoff of phosphate fertilizer from an adjacent coniferous forestry plantation raised the TP concentration of lake water 5.5‐fold. Consequently, the biovolume of phytoplankton increased 30‐fold, and chlorophyll‐a concentrations increased eightfold, reaching hyper‐eutrophic levels. A concurrent depletion of nitrogen in lake water reduced the DIN:TP ratio from 17.8 to 0.6, and phytoplankton growth rapidly became nitrogen‐limited. Phytoplankton composition shifted from dinoflagellates to minute, unicellular chlorophytes, with a coincident decline in species diversity. Cyanobacteria did not proliferate, most likely due to the acidic nature of the lake. Conclusions: Results illustrated the vulnerability of newly created cutaway peatland lakes to developing severe phytoplankton blooms and coincident secondary nitrogen limitation in the presence of moderate external phosphorus inputs.     

Potential of diatom consortium developed by nutrient enrichment for biodiesel production and simultaneous nutrient removal from waste water

Because of the decreasing fossil fuel supply and increasing greenhouse gas (GHG) emissions, microalgae have been identified as a viable and sustainable feedstock for biofuel production. The major effect of the release of wastewater rich in organic compounds has led to the eutrophication of freshwater ecosystems. A combined approach of freshwater diatom cultivation with urban sewage water treatment is a promising solution for nutrient removal and biofuel production. In this study, urban wastewater from eutrophic Hussain Sagar Lake was used to cultivate a diatom algae consortium, and the effects of silica and trace metal enrichment on growth, nutrient removal, and lipid production were evaluated. The nano-silica-based micronutrient mixture Nualgi containing Si, Fe, and metal ions was used to optimize diatom growth. Respectively, N and P reductions of 95.1% and 88.9%, COD and BOD reductions of 91% and 51% with a biomass yield of 122.5 mg L^?1 day^?1 and lipid productivity of 37 mg L^?1 day^?1 were observed for cultures grown in waste water using Nualgi. Fatty acid profiles revealed 13 different fatty acids with slight differences in their percentage of dry cell weight (DCW) depending on enrichment level. These results demonstrate the potential of diatom algae grown in wastewater to produce feedstock for renewable biodiesel production. Enhanced carbon and excess nutrient utilization makes diatoms ideal candidates for co-processes such as CO₂ sequestration, biodiesel production, and wastewater phycoremediation.     

沉积物生源要素对水体生态环境变化的指示意义

受人类活动影响,输入到湖泊、河口中的营养盐剧增,导致水体富营养化、食物链结构改变、底质季节性缺氧等生态环境变化。这些环境变化会在沉积物中留下记录,沉积物中生源要素及其稳定同位素的变化是指示水域古生产力、营养盐水平的有效指标。总结归纳了沉积物中生源要素(碳、氮、磷、硅)指标对水体环境中初级生产力、物质来源、营养盐水平3方面变化的指示作用。沉积物中TOC、TN、δ13C、CaCO3和BSi可以反映水体沉降有机质含量和浮游植物的生长状况,是指示水域初级生产力水平的有效指标。根据不同类型植物来源的有机质中δ13C、δ15N和C/N的差异,可以追踪沉积物中有机质的来源。有效地区分有机质来源对于研究人类活动对水体环境的影响及水体富营养化具有重要的价值。沉积物中TN、δ15 N、TP和非磷灰石磷(NAIP)含量的升高直接反映了陆源输入氮、磷的增加。BSi在指示浮游植物生长状况的同时,还反映了水体溶解硅浓度水平和富营养化状况。水体中的生源物质在沉降和埋藏的过程中,会受到早期成岩作用、水动力搬运等众多因素的作用,使沉积物记录的环境变化信息发生改变,从而干扰其对水体环境演变的重建作用。因此综合分析各生源要素指标反映的环境变化信息及其可能的干扰因素,方能正确地反映水域的环境变化过程。     

Effect of intensive catchment and in-lake restoration procedures on phosphorus concentrations in a eutrophic lake

Lake Okaro is a small, warm monomictic lake in central North Island, New Zealand, which progressed from oligotrophic to eutrophic through the 1960s. Trends in phosphorus (P) concentrations in the lake are linked to multiple restoration efforts over a 5-year period (2003–2008). The restoration procedures include a 2.3 ha constructed wetland established in February 2006 and riparian margin protection to reduce external loading, as well as an Alum application in December 2003 and sediment capping using modified zeolite in September 2008 to reduce internal loading. The annual average total phosphorus (TP) concentration in the lake decreased by 41% from 2004–2005 to 2007–2008. Two predictive models based on external P loading data generally underestimated the measured TP concentrations in the water column due to internal P loading. The relatively rapid response of TP concentrations after reduction of the internal loading using modified zeolite suggests that this technique can effect a rapid decrease in lake water TP concentrations though the trophic state of Lake Okaro showed high resilience to the reduced P loading. It is concluded that the combined effect of all restoration procedures resulted in a relatively rapid decrease in TP concentrations in Lake Okaro, which may be prolonged by continued external load reduction.     

Surficial sediment phosphorus fractions along a biogeochemical gradient in Nyanza (Winam) Gulf, northeastern Lake Victoria and their possible role in phosphorus recycling and internal loading

Different phosphorus fractions and metal element composition of surficial sediments were measured on three occasions in 2005 and 2006 along a transect between Nyanza Gulf and offshore Lake Victoria, in order to assess the potential for sediments to contribute to the water column P concentrations in Lake Victoria. Total phosphorus (TP), apatite phosphorus (AP), inorganic phosphorus (IP) and organic phosphorus (OP) increased in sediments along the gulf towards the main lake while the non-apatite inorganic phosphorus (NAIP) increases were less defined. The longitudinal gradient of sediment TP and its fractions in Nyanza Gulf is a result of high rates of terrigenous input and resuspension and transport of the light, phosphorus rich inorganic and organic matter towards the main lake. TP in the sediment ranged from 812.7 to 1,738 mg/kg dry weight (DW) and was highest in the Rusinga Channel, the exchange zone between the gulf and the main lake. AP was the most important TP fraction, contributing between 35 and 57.3% of TP. Ca content in the sediment was strongly associated with TP and AP in the sediment (r² = 0.92 and 0.98, respectively) in the gulf and the channel, indicating the importance of apatite in controlling P availability in these zones. In the gulf and the Rusinga Channel, the less bioavailable apatite phosphorus dominated, whereas in the deeper main lake OP was the major fraction illustrating the importance of anaerobic release of P from sediments and acceleration of internal P loading in the main lake.     

Phragmites australis and silica cycling in tidal wetlands

Tidal marshes have recently been shown to be important biogenic Si recycling surfaces at the land–sea interface. The role of vegetation in this recycling process has not yet been quantified. In situ and ex situ decomposition experiments were conducted with Phragmites australis stems. In a freshwater tidal marsh, litterbags were incubated at different elevations and during both winter and summer. Biogenic Si (BSi) dissolution followed a double exponential decay model in the litterbags (from ca. 60 to 15 mg g⁻¹ after 133 days), irrespective of season. Si was removed much faster from the incubated plant material compared to N and C, resulting in steadily decreasing Si/N and Si/C ratios. Ex situ, decomposition experiments were conducted in estuarine water, treated with a broad-spectrum antibiotic, and compared to results from untreated incubations. The bacterial influence on the dissolution of dissolved Si (DSi) from P. australis stems was negligible. Although the rate constant for dissolved Si dissolution decreased from 0.004 to 0.003 h⁻¹, the eventual amount of BSi dissolved and saturation concentration in the incubation environment were similar in both treatments. P. australis contributes to and enhances dissolved Si recycling capacity of tidal marshes: in a reed-dominated small freshwater tidal marsh, more than 40% of DSi export was attributable to reed decomposition. As the relation between tidal marsh surface and secondary production in estuaries has been linked to marsh Si cycling capacity, this provides new insight in the ecological value of the common reed.     

Nutrient metabolism (C,N, P,and Si) in the trophogenic zone of a meromictic lake

The dynamics of seston and dissolved elements in a meromictic lake with high concentrations of manganese and iron in the monimolimnion were studied through an annual cycle. This publication presents results for assimilation, sedimentation and recovery of nutrients (C, N, P, and Si) in the trophogenic zone. Phosphorus deficiency kept the productivity of the diatom dominated phytoplankton at an oligotrophic level. High concentrations of iron in influent streams and redistribution followed by precipitation of iron during periods of partial turnover removed phosphorus from the water. High concentrations of manganese and sulfate did not have the anticipated fertilizing effect, and recovery of nutrients from the depth of the lake was negligible. Mass balance calculations indicate that liberation of phosphorus from the sediments in the trophogenic zone was most important for the maintenance of primary production. 75% of carbon, 80% of nitrogen and 25% of phosphorus assimilated by the phytoplankton was mineralized in the trophogenic zone. Silica was effectively regenerated from the littoral zone during the decline of diatom blooms. Nitrogen and silica retention was 45% of the external load compared to 66% for phosphorus.Dept. of Limnology University of Oslo