A shrinking glacier, a greening lake
Lake Ōkārito is a proglacial lake fed by a retreating valley glacier. A 1.2 m sediment core contains annual varves spanning 1870–2020. From 1870 to 1970, biogenic silica averages 8%, organic C/N is 14, δ13Corg is −26‰, and sediment phosphorus is 400 ppm. After 1995, biogenic silica rises to 24%, C/N falls to 9, δ13Corg rises to −23‰, and phosphorus reaches 900 ppm. Over the same period the glacier terminus retreats 800 m, summer meltwater discharge increases by 20%, lake surface temperature rises 1.8°C, and mean wind speed falls by 15%. Inflow dissolved silica increases by 30%, while inflow nitrate remains near 1 µmol/L. The lake is oligotrophic and well oxygenated; varve thickness has increased by 15%. A nearby weather station shows no trend in annual precipitation.
1. Evaluate two hypotheses for the post-1995 rise in biogenic silica: (a) increased nutrient supply from glacier retreat; (b) stronger summer stratification caused by warming and weaker wind. Which is better supported, and what role could silica play?
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Both processes could contribute, but the evidence favours nutrient supply as the trigger and stratification as an amplifier. Diatoms need dissolved silica and nitrogen/phosphorus. Inflow dissolved silica rises by 30% after glacier retreat, and sediment phosphorus rises from 400 to 900 ppm; that is consistent with enhanced nutrient delivery from freshly exposed rock and glacial meltwater. Nitrate remains low (~1 µmol/L), so the system may be nitrogen-limited or co-limited; the biogenic silica increase cannot be explained by light alone.
Warming (+1.8°C) and weaker wind (−15%) will shallow the mixed layer and increase stratification, keeping diatoms in the photic zone and extending the growing season. That can amplify a bloom, but stratification without a nutrient supply would soon draw nutrients down. The post-1995 rise is therefore best explained by a combination: glacier-derived nutrients set the ceiling, and stronger stratification allows more of that nutrient to be used.
A test would be diatom species composition and Si:N:P ratios in sediment traps. A stratified-water assemblage with high silica use would support the amplifier role, while a shift in source-water chemistry would support the glacier-nutrient hypothesis.
2. Do the C/N and δ13Corg shifts indicate more algal production, more terrestrial input, or better preservation? Explain.
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C/N falling from 14 to 9 strongly suggests a larger algal contribution, because terrestrial organic matter typically has C/N above 15 while algae are about 6–10. The δ13Corg rise from −26 to −23‰ is also consistent with increased algal productivity and/or reduced aqueous CO2, not with a simple increase in terrestrial input, which would usually keep δ13C lower. Better preservation is unlikely to be the main cause because the lake is well oxygenated and biogenic silica rises at the same time; preservation would not preferentially raise biogenic silica and phosphorus in this way. The combination points to more production and more algal organic matter burial, not just a change in supply.
3. Design one field measurement that would test whether the lake's carbon burial rate has increased since 1995, and explain how you would interpret it.
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Measure sediment organic carbon concentration, dry bulk density and varve thickness to calculate organic carbon burial rate (g C m−2 yr−1) before and after 1995, using varve counting or 210Pb for chronology. A sediment trap and biomarkers (for example, short-chain versus long-chain n-alkanes) would distinguish algal from terrestrial carbon. If burial rate rises with biogenic silica and algal biomarkers, the lake is a stronger carbon sink. If organic carbon concentration falls despite thicker varves, dilution by clastic sediment may offset the increase, so the burial rate could stay flat or decline.