A long-term multi-proxy record of varved sediments highlights climate-induced mixing-regime shift in a large hard-water lake ~5000 years ago

This study reviews the monsoonal Yangtze and the arid Nile deltas with the objective of understanding how the process–response between river-basin modifications and delta-estuary ecological degradation are interrelated under contrasting hydroclimate dynamics. Our analysis shows that the Yangtze River had a long-term stepwise reduction in sediment and silicate fluxes to estuary due to dam construction since the 1960s, especially after the Three Gorges Dam (TGD) closed in 2003. By contrast, the Nile had a drastic reduction of sediment, freshwater, and silicate fluxes immediately after the construction of the Aswan High Dam (AHD) in 1964. Seasonal rainfall in the mid-lower Yangtze basin (below TGD) complemented riverine materials to its estuary, but little was available to the Nile coast below the AHD in the hyper-arid climate setting. Nitrogen (N) and phosphate (P) fluxes in both river basins have increased because of the overuse of N- and P-fertilizer, land-use changes, urbanization, and industrialization. Nutrient ratios (N:P:Si) in both delta-estuaries was greatly altered, i.e., Yangtze case: 75:1:946 (1960s–1970s), 86:1:272 (1980s–1990s) and 102:1:75 (2000s–2010s); and Nile case: 6:1:32 (1960s–1970s), 8:1:9 (1980s–1990s), and 45:1:22 (2013), in the context of the optimum of Redfield ratio (N:P:Si = 16:1:16). This led to an ecological regime shift evidenced by a long-term change in phytoplankton communities in the Yangtze estuary, where silicious algae tended to lose dominance since the end of the 1990s, when more toxic dinoflagellates began to emerge. In the Nile estuary, such a regime shift was indicated by the post-dam dramatic reduction in zooplankton standing crop and fish landings until the early 2000s when biological recovery occurred due to nutrient inputs from anthropogenic sources. Although the Yangtze had higher human impacts than the Nile in terms of population, industrialization, and fertilizer application, N concentrations in the Nile estuarine waters surpassed the Yangtze in recent decades. However, eutrophication in the Yangtze estuary is much more intensive than in the Nile, leading to the likelihood of its estuarine water becoming more acidic than ever before. Therefore, ecological degradation in both delta-estuaries does not follow a linear trajectory, due not only to different climate dynamics but also to human forcings. The comparative insights of this study should be incorporated into future integrated coastal management of these two important systems.

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In situ methane production in a small, hypereutrophic, hard-water lake: Loss of methane from sediments by vertical diffusion and ebullition 1

Limnology and Oceanography ◽

10.4319/lo.1978.23.6.1201 ◽

1978 ◽

Vol 23 (6) ◽

pp. 1201-1206 ◽

Cited By ~ 51

Author(s):

Richard F. Strayer ◽

James M. Tiedje

Keyword(s):

Methane Production ◽

Hard Water ◽

Vertical Diffusion ◽

Water Lake

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Longer Growing Seasons Cause Hydrological Regime Shifts in Central European Forests

Forests ◽

10.3390/f12121656 ◽

2021 ◽

Vol 12 (12) ◽

pp. 1656

Author(s):

Petr Kupec ◽

Jan Deutscher ◽

Martyn Futter

Keyword(s):

Regime Shift ◽

Hydrological Regime ◽

Annual Runoff ◽

Order Effects ◽

Runoff Generation ◽

Central European ◽

European Forests ◽

Water Limitation ◽

Growing Seasons

In this study, we present evidence for a hydrological regime shift in upland central European forests. Using a combination of long-term data, detailed field measurements and modelling, we show that there is a prolonged and persistent decline in annual runoff: precipitation ratios that is most likely linked to longer growing seasons. We performed a long term (1950–2018) water balance simulation for a Czech upland forest headwater catchment calibrated against measured streamflow and transpiration from deciduous and coniferous stands. Simulations were corroborated by long-term (1965–2018) borehole measurements and historical drought reports. A regime shift from positive to negative catchment water balances likely occurred in the early part of this century. Since 2007, annual runoff: precipitation ratios have been below the long-term average. Annual average temperatures have increased, but there have been no notable long term trends in precipitation. Since 1980, there has been a pronounced April warming, likely leading to earlier leaf out and higher annual transpiration, making water unavailable for runoff generation and/or soil moisture recharge. Our results suggest a regime shift due to second order effects of climate change where increased transpiration associated with a longer growing season leads to a shift from light to water limitation in central European forests. This will require new approaches to managing forests where water limitation has previously not been a problem.

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Multiple Dating of a Freeze Core from Lake 227, an Experimentally Fertilized Lake with Varved Sediments

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/f94-231 ◽

1994 ◽

Vol 51 (10) ◽

pp. 2274-2285 ◽

Cited By ~ 13

Author(s):

Brent Wolfe ◽

Hedy J. Kling ◽

Gregg J. Brunskill ◽

Paul Wilkinson

Keyword(s):

Algal Blooms ◽

Seasonal Abundance ◽

Sedimentation Rates ◽

Experimental Lakes Area ◽

Varved Sediments ◽

Varve Chronology ◽

Freeze Core ◽

N Loading ◽

Planktonic Community

A freeze core taken from Experimental Lakes Area Lake 227 in 1988 contained 321 rhythmically paired, dark and light laminations in the upper 60.7 cm. Tape peels revealed cyclic, seasonal abundance peaks in organic and inorganic remains, which suggested that the couplets are varves. However, comparison between varve chronology and 22 yr of experimental changes in phosphorus (P) and nitrogen (N) loading and their influence on the planktonic community confirmed that the most recent varve-year estimates were 5 or 6 yr too old; this was caused by irregular sedimentation and multiple algal blooms resulting from experimental fertilization since 1969, and indistinct laminations that hampered precise couplet identification and separation. Dated horizons determined from biostratigraphic markers were used to generate compatible profiles between 1-cm slices of Lake 227 137Cs flux and reference fallout records. Nutrient concentration profiles were less helpful, as increases in carbon, N, and, P were gradual and no distinct horizon was identified as a clear marker of eutrophication. Long-term assessment of the varve chronology using 210Pb was hindered by experimental additions of 226Ra to the lake in 1970, although similar sedimentation rates from varve years 1860–1934 suggested that the varve and the deep part of the 210Pb chronologies were comparable.

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Identification of Regime Shifts and Their Potential Drivers in the Shallow Eutrophic Lake Yilong, Southwest China

Sustainability ◽

10.3390/su12093704 ◽

2020 ◽

Vol 12 (9) ◽

pp. 3704

Author(s):

Lei Zhao ◽

Mingguo Wang ◽

Zhongyao Liang ◽

Qichao Zhou

Keyword(s):

Regime Shift ◽

Eutrophic Lake ◽

Regime Shifts ◽

Nutrient Reduction ◽

Eutrophic Lakes ◽

Test Analysis ◽

Key Drivers ◽

High Possibility ◽

Cage Fish

Regime shifts in shallow lakes can lead to great changes in ecosystem structures and functions, making ecosystem management more complicated. Lake Yilong, located in Yunnan Province, is one of the most eutrophic lakes in China. Although there is a high possibility that this lake has undergone regime shift one or more times, the presence of regime shifts and their drivers remain unknown. Here, we employed the sequential t-test analysis of regime shifts to detect the regime shifts based on the long-term (1989–2018) dataset of the lake. We further determined their potential drivers, and explored the nutrient thresholds of regime shifts and hysteresis. The results showed that during the testing period, three regime shifts occurred in 1996 (restorative type), 2009 (catastrophic type) and 2014 (restorative type). The potential key drivers for the first two regime shifts (1996 and 2009) were both related to aquaculture. The abolition of cage fish culture may have led to the restorative regime shift in 1996, and the stocking of crabs and excessive premature releasing of fry possibly caused the catastrophic regime shift in 2009. However, the third regime shift, which occurred in 2014, was possibly related to the drought and succedent hydration. These results indicate that adjustments of aquaculture strategy and hydrological conditions are critical for the lake ecosystem’s recovery. Moreover, the total phosphorus thresholds were identified to be lower than 0.046 mg/L (restorative type) and higher than 0.105 mg/L (catastrophic type), respectively. In addition, an obvious hysteresis was observed after 2014, suggesting that nutrient reduction is important for this lake’s management in the future.

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