Lake Malawi sediment and pore water chemistry: Proposition of a conceptual model for stratification intensification since the end of the Little Ice Age

Branchu, Philippe; Bouchet, L.; Pons‐Branchu, Edwige; Violier, Eric; Dittrich, Maria; Massault, M.; Ghaleb, Bassam

doi:10.1016/j.gloplacha.2010.01.008

Cited by 8 publications

(7 citation statements)

References 45 publications

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“…However, the water column shifts toward subsaline conditions during that period, as evidenced by the chloride content ( Fig. 1A; Branchu et al 2010). Salinity in the water column increases with evaporation while a long residence time in the water column also leads to increased internal recycling of nutrients (Howarth and Marino 2006), possibly making Nfixing cyanobacteria the major suppliers of organic N to other algae (Marcarelli et al 2006;Håkanson et al 2007).…”

Section: Microbial Signal In Om Signaturesmentioning

confidence: 97%

Paleoenvironmental conditions define current sustainability of microbial populations in Laguna Potrok Aike sediments, Argentina

Vuillemin

Ariztegui

Lücke³

et al. 2013

Aquat Sci

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Rainfall and geology of the catchment exert a dominant control on the trophic state of endorheic basins. River inflows and runoff provide nutrients, influencing primary productivity in the water column. Through time, paleoenvironmental conditions are recorded as variations within the sedimentary organic fraction. Thereafter, microbial populations settle and develop within sediments and lead to degradation processes as long as they remain active. However, their presence is generally not considered in Quaternary studies.The present study is based on the sedimentary record of the maar lake of Laguna Potrok Aike, southern Patagonia. We investigate the relationship between paleoenvironmental conditions and colonization of the corresponding sediments by microbes. Microbiological and geochemical analyses were combined to determine factors allowing microbes to sustain their activity over time. The study of Holocene sediments, containing dense and active microbial communities, provided means to evaluate the potential of microbial communities as agents of early diagenesis. We show that phosphorus released during organic matter degradation is essential for microbial growth. In highly colonized sediments, microbial communities appear capable of recycling the excreted ammonium, thus accounting for nitrogen fractionation toward high values in bulk sediment. Microbial activity in Laguna Potrok Aike still persists in 30 ka old sediments. Thus, we proposed that future lacustrine studies should include some microbial indicators to assess their impact in diagenetic processes.

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Section: Microbial Signal In Om Signaturesmentioning

confidence: 97%

Paleoenvironmental conditions define current sustainability of microbial populations in Laguna Potrok Aike sediments, Argentina

Vuillemin

Ariztegui

Lücke³

et al. 2013

Aquat Sci

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“…Therefore its value was adjusted to close the water balance in Table 2.1 . Large differences between evaporation and precipitation cause a concentration of salts in the surface waters in other East African lakes such as Lake Tanganyika and Lake Malawi (Branchu et al 2010 ) . In Lake Kivu the difference between these two components is small, and increased salt concentrations are due to inputs by subaquatic springs (Sect.…”

Section: Water Balancementioning

confidence: 99%

Stratification, Mixing and Transport Processes in Lake Kivu

Schmid

Wüest

2012

Lake Kivu

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This chapter summarizes the knowledge on mixing and transport processes in Lake Kivu. Seasonal mixing, which varies in intensity from year to year, in fl uences the top ~65 m. Below, the lake is permanently strati fi ed, with density increasing stepwise from ~998 kg m −3 at the surface to ~1,002 kg m −3 at the maximum depth of 485 m. The permanently strati fi ed deep water is divided into two distinctly different zones by a main gradient layer. This gradient is maintained by a strong in fl ow of relatively fresh and cool water entering at ~250 m depth which is the most important of several subaquatic springs affecting the density strati fi cation. The springs drive a slow upwelling of the whole water column with a depth-dependent rate of 0.15-0.9 m year −1 . This upwelling is the main driver of internal nutrient recycling and upward transport of dissolved gases. Diffusive transport in the deep water is dominated by double-diffusive convection, which manifests in a spectacular staircase of more than 300 steps and mixed layers. Double diffusion allows heat to be removed from the deep zone faster than dissolved substances, supporting the stable strati fi cation and the accumulation of nutrients and gases over hundreds of years. The strati fi cation in the lake seems to be near steady-state conditions, except for a warming trend of ~0.01°C year −1 .

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“…The warm nature of these lakes moreover increases the susceptibility to oxygen depletion: at an atmospheric pressure of 760 mmHg fully oxygen-saturated freshwater at 25°C contains 8.2 mg L )1 DO, whereas at 15°C this is 10.1 mg L )1 DO and at 5°C this is 12.7 mg L )1 DO (c. 1.55 · the concentration at 25°C). Moreover, depletion of the oxygen available to deep-water biota is accelerated because the ventilation of deeper waters has remained the same or declined (Plisnier, 1997(Plisnier, , 2000Vollmer, Weiss & Bootsma, 2002;O'Reilly et al, 2003;Verburg et al, 2003;Verburg & Hecky, 2009, Branchu et al, 2010Tierney et al, 2010). Moreover, depletion of the oxygen available to deep-water biota is accelerated because the ventilation of deeper waters has remained the same or declined (Plisnier, 1997(Plisnier, , 2000Vollmer, Weiss & Bootsma, 2002;O'Reilly et al, 2003;Verburg et al, 2003;Verburg & Hecky, 2009, Branchu et al, 2010Tierney et al, 2010).…”

Section: Discussionmentioning

confidence: 99%

“…On the other hand, biochemical reaction rates, and thus oxygen consumption, are at least doubled when temperatures increase with 10°C (Q 10 ‡ 2; Fryer, 1972a). Moreover, depletion of the oxygen available to deep-water biota is accelerated because the ventilation of deeper waters has remained the same or declined (Plisnier, 1997(Plisnier, , 2000Vollmer, Weiss & Bootsma, 2002;O'Reilly et al, 2003;Verburg et al, 2003;Verburg & Hecky, 2009, Branchu et al, 2010Tierney et al, 2010). Deep-water deoxygenation has also been demonstrated for Lake Victoria (Hecky et al, 1994).…”

Section: Discussionmentioning

confidence: 99%

“…This reduced stratification during the wet season has allowed faster erosion of the thermocline in the dry season, followed by enhanced convective mixing. Verburg et al (2003) have suggested that stratification has strengthened and mixing reduced in Lake Malawi over the last few decades, but this probably applies mainly to very deep waters (>100 m), where reduced mixing can be attributed to lesser intrusions of cool water (Vollmer et al, 2005), or to the northern part of the basin (Branchu et al, 2010) where biodiversity is lower and deep-water molluscs do not occur (Eccles, 1962). Given that the oxycline for Lake Malawi (Fig.…”

Section: Upward Displacement Of the Oxyclinementioning

confidence: 99%

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Does the decline of gastropods in deep water herald ecosystem change in Lakes Malawi and Tanganyika?

et al. 2012

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Summary 1. Ancient, deep lakes have traditionally been considered as stable, ecological islands, well buffered from environmental change because of their great depth. However, they are not immune to anthropogenic and climatic stress. Ecosystems of the permanently stratified warm Lakes Malawi and Tanganyika in the Great East African Rift are particularly delicate. Their stratification regime has historically limited the distribution of benthic biota to a ‘bathtub ring of biodiversity’, namely substrata in the upper, oxygenated water layer. 2. We use historical data on the endemic deep‐water molluscs of these lakes to assess present‐day stress on their benthic ecosystems. During the 20th century, these molluscs have probably decreased in abundance and migrated to shallower water. 3. These apparent trends have a significance beyond species‐based conservation, foremost because deep‐water organisms heavily rely on the position and temporal stability of the oxycline and therefore provide an early warning of large‐scale changes in the distribution of dissolved oxygen. Oxygen demands have increased in the East African Great Lakes over the last century whereas ventilation of deep water has remained the same or declined. 4. The combination of these factors is resulting in a narrowing of the ring of biodiversity and a changed nutrient flux through this ring. Reduction in the habitat available inevitably puts biota at risk, whereas changes in nutrient flux may cause shifts in the entire ecosystem or the collapse of parts of it. 5. Considering the socioeconomic value of these lakes and the potentially grave implications for their faunal biodiversity and entire ecosystems, existing evidence of faunal decline, especially in taxa that depend strongly on the stratification regime, is of great concern. Moreover, because the factors responsible are widespread and include surface‐water warming, increased run‐off and eutrophication, respiration stress may also develop in other tropical and subtropical lakes.

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Lake Malawi sediment and pore water chemistry: Proposition of a conceptual model for stratification intensification since the end of the Little Ice Age

Cited by 8 publications

References 45 publications

Paleoenvironmental conditions define current sustainability of microbial populations in Laguna Potrok Aike sediments, Argentina

Paleoenvironmental conditions define current sustainability of microbial populations in Laguna Potrok Aike sediments, Argentina

Stratification, Mixing and Transport Processes in Lake Kivu

Does the decline of gastropods in deep water herald ecosystem change in Lakes Malawi and Tanganyika?

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