Seeking alternative stable states in a deep lake

Lancet, 2011

Climatic change is recognized as an important factor capable of influencing the structural properties of aquatic ecosystems. Lake ecosystems are particularly sensitive to climate change. Several long time-series studies have shown close coupling between climate, lake thermal properties and individual organism physiology, population abundance, community structure, and food-web structure. Understanding the complex interplay between climate, hydrological variability, and ecosystem structure and functioning is essential to inform water resources risk assessment and fisheries management. The purpose of this paper is to present the current understanding of climate-induced changes on lake ecosystem phenology. We first review the ability of climate to modulate the interactions among lake hydrodynamics, chemical factors, and food-web structure in several north temperate deep lakes (e.g., Lake Washington, Lake Tahoe, Lake Constance, Lake Geneva, Lake Baikal, and Lake Zurich). Our aim is to assess long-term trends in the physical (e.g., temperature, timing of stratification, and duration of ice cover), chemical (e.g., nutrient concentrations), and biological (e.g., timing of the spring bloom, phytoplankton composition, and zooplankton abundance) characteristics of the lakes and to examine the signature of local weather conditions (e.g., air temperature and rainfall) and large-scale climatic variability (e.g., ENSO and PDO) on the lake physics, chemistry and biology. We also conducted modeling experiments to quantify the relative effect of climate change and nutrient loading on lake phenology. These modeling experiments focused on the relative changes to the major causal associations underlying plankton dynamics during the spring bloom and the summer stratified period. To further understand the importance of climate change on lakes, we propose two complementary directions of future research. First, additional research is needed to elucidate the wide array of in-lake processes that are likely to be affected by the climate change. Second, it is essential to examine the heterogeneity in responses among different water bodies. The rationale of this approach and its significance for dealing with the uncertainty that the climate signals cascade through lake ecosystems and shape abiotic variability and/or biotic responses have been recently advocated by several other synthesis papers.

Freshwater Biology, 2012

ABSTRACT 1. As long-term observational lake records continue to lengthen, the historical overlap with lake sediment records grows, providing increasing opportunities for placing the contemporary ecological status of lakes in a temporal perspective. 2. Comparisons between long-term data sets and sediment records, however, require lake sediments to be accurately dated and for sediment accumulation rates to be sufficiently rapid to allow precise matching with observational data. 3. The critical role of the sediment record in this context is its value in tracking the changing impact of human activity on a lake from a pre-disturbance reference through to the present day. 4. Here, we use data from a range of lakes across Europe presented as case studies in this Special Section. The seven sites considered all possess both long-term observational records and highquality sediment records. Our objective is to assess whether recent climate change is having an impact on their trophic status and in particular whether that impact can be disentangled from the changes associated with nutrient pollution. 5. The palaeo-data show clear evidence for the beginning of nutrient pollution varying from the mid-nineteenth century at Loch Leven to the early and middle twentieth century at other sites. The monitoring data show different degrees of recovery when judged against the palaeo-reference. 6. The reason for limited recovery is attributed to continuing high nutrient concentrations related to an increase in diffuse nutrient loading or to internal P recycling, but there is some evidence that climate change may be playing a role in offsetting recovery at some sites. If this is the case, then lake ecosystems suffering from eutrophication may not necessarily return to their pre-eutrophication reference status despite the measures that have been taken to reduce external nutrient loading. 7. The extent to which future warming might further limit such recovery can be evaluated only by continued monitoring combined with the use of palaeo-records that set the pre-eutrophication reference.

Ecology, 2013

While considerable insights on the ecological consequences of climate change have been gained from studies conducted on remote lakes, little has been done on lakes under direct human exposure. Ecosystem vulnerability and responses to climate warming might yet largely depend on the ecological state and thus on local anthropogenic pressures. We tested this hypothesis through a paleolimnological approach on three temperate large lakes submitted to rather similar climate warming but varying intensities of analogous local forcings (changes in nutrient inputs and fisheries management practices). Changes in the structure of the cladoceran community were considered as revealing for alterations, over the time, of the pelagic food web. Trajectories of the cladoceran communities were compared among the three study lakes (Lakes Geneva, Bourget, and Annecy) over the last 70-150 years. Generalized additive models were used to develop a hierarchical understanding of the respective roles of local stressors and climate warming in structuring cladoceran communities. The cladoceran communities were not equally affected by climate warming between lakes. In Lake Annecy, which is the most nutrient-limited, the cladoceran community was essentially controlled by local stressors, with very limited impact of climate. In contrast, the more nutrient-loaded Lakes Geneva and Bourget were more sensitive to climate warming, although the magnitude of their responses and the pathways under which climate warming affected the communities varied between the two lakes. Finally, our results demonstrated that lake vulnerability and responses to climate warming are modulated by lake trophic status but can also be altered by fisheries management practices through changes in fish predation pressure.

Freshwater Biology, 2010

1. Palaeolimnology and contemporary ecology are complementary disciplines but are rarely combined. By reviewing the literature and using a case study, we show how linking the timescales of these approaches affords a powerful means of understanding ecological change in shallow lakes.2. Recently, palaeolimnology has largely been pre-occupied with developing transfer functions which use surface sediment-lake environment datasets to reconstruct a single environmental variable. Such models ignore complex controls over biological structure and can be prone to considerable error in prediction. Furthermore, by reducing species assemblage data to a series of numbers, transfer functions neglect valuable ecological information on species’ seasonality, habitat structure and food web interactions. These elements can be readily extracted from palaeolimnological data with the interpretive assistance of contemporary experiments and surveys. For example, for one shallow lake, we show how it is possible to infer long-term seasonality change from plant macrofossil and fossil diatom data with the assistance of seasonal datasets on macrophyte and algal dynamics.3. On the other hand, theories on shallow lake functioning have generally been developed from short-term (<1–15 years) studies as opposed to palaeo-data that cover the actual timescales (decades–centuries) of shallow lake response to stressors such as eutrophication and climate change. Palaeolimnological techniques can track long-term dynamics in lakes whilst smoothing out short-term variability and thus provide a unique and important means of not only developing ecological theories, but of testing them.4. By combining contemporary ecology and palaeolimnology, it should be possible to gain a fuller understanding of changing ecological patterns and processes in shallow lakes on multiple timescales.

2016

A comparative study of independent geochemical and biological proxies was carried out on a short (83 cm) sediment core collected in 2011 from the deepest point of a small subalpine Lake Ledro (Trentino, N-Italy). The aim of the study is to compare the capability of subfossil photosynthetic pigments, diatoms and Cladocera in tracking lake ecological transitions and tipping points related to major environmental perturbations occurred during the last three centuries, i.e. after the culmination of the Little Ice Age in the Alpine region. In relation to the sparse neo-limnological and climate data available for the lake, the study aims also at defining of the lake trophic and ecological reference conditions, at improving the reconstruction of the nutrient enrichment process during the last decades, and at evaluating the effects of restoration measures initiated in the 1990s. The analysis of the selected proxies outlined a pronounced sensitivity of Lake Ledro to hydrological variability throughout the whole time span considered, especially during the 18 th and 19 th century, and revealed two major stages in the ecological evolution of the lake, which were mainly controlled by climate related hydrological variability and lake nutrients. The results largely agree with the hypothesis that responses of sediment biological proxies to different natural and human stressors may differ in type, timing and magnitude. Subfossil pigments, diatoms and Cladocera showed a comparable capability in tracking ecological transitions and tipping points related to lake hydrology and nutrient variability, while only diatoms demonstrated a certain capability to track changes in water temperature of the lake studied. The strong response of planktonic organisms to hydrological variability depends on the peculiar catchment and lake morphology, and confirmed that planktonic organism principally respond to climate variability in an indirect way. The reconstruction of the trophic development of Lake Ledro during the last decades revealed that the vulnerability of the lake toward climate and land use driven hydrological variability is congenital for the lake, though at present it is masked by nutrients. This stresses the necessity to maintain and improve the control of nutrient inputs also in reoligotrophicated subalpine lakes, in relation to the present context of human use and climate change, and paying particular attention to the lake-specific sensitivity to local forcings.

Understanding the long-term dynamics of ecological communities on the centuries-to-millennia scale is important for explaining present-day biodiversity patterns. Placing these patterns in a historical context could yield reliable tools for predicting possible future scenarios. Paleoarchives of macro+ and micro-fossil remains, and most importantly biomarkers such as fossil pigments and ancient DNA present in various sedimentary deposits, allow long term changes in ecological communities to be analysed. We use recent compilations of data including fossil pigments, metabarcoding of sedimentary ancient DNA and microfossils together with data analysis to understand the impact of gradual versus abrupt climate changes on a lake`s ecosystem status over the last 14.5 kyr. We give examples of hypotheses that need long-term data, which can be addressed using well-established paleoproxy variables. These variables describe the climate, together with vegetation change and the appearance of anthro...

Archiv für Hydrobiologie, 2004

We used long-term monitoring data to assess causes behind a recent shift from a clear to a turbid water state in Lake Tåkern, Sweden. The lake has a previous record of shifts between clear-water and turbid states, but the causes behind these shifts are not well known. During the recent shift, which occurred in 1995-1997, submerged vegetation subsequently declined after a 30-year period of clear-water and abundant vegetation. Among the possible explanations we identify several processes unlikely to have contributed to the recent shift from clear to turbid conditions, including long-term changes in external input of phosphorus, fluctuations in water level, and changes in zooplankton grazing pressure. Instead, likely scenarios to have contributed to the macrophyte decline, and thereby to the shift were: (1) a series of mild winters with short ice cover and absence of winter-kills of fish, leading to high biomasses of benthivorous and planktivorous fish before the shift, and thereby increased bioturbation and internal nutrient recycling, (2) unusually cool and windy springs the years before and during the shift, leading to unfavourable conditions during the establishing phase of submerged macrophytes. Both shorter periods of ice cover and high wind velocity in winter and spring were associated with climate, approximated by the North Atlantic Oscillation (NAO). We argue that none of these processes alone can force the lake from the clear to the turbid state, but that several stress factors in concert are necessary to initiate a shift. Hence, we conclude that climate variability is likely to have contributed to a multi-causal stress, reducing the resilience of the clear-water state and finally triggering the shift through inter-year dependent changes in biomass of submerged macrophytes and fish, organism groups known to have key roles in the dynamics of shallow lakes.

Scientific Reports

After strong fertilization in the 20 th century, many deep lakes in Central Europe are again nutrient poor due to long-lasting restoration (re-oligotrophication). In line with reduced phosphorus and nitrogen loadings, total organismic productivity decreased and lakes have now historically low nutrient and biomass concentrations. This caused speculations that restoration was overdone and intended fertilizations are needed to ensure ecological functionality. Here we show that recent re-oligotrophication processes indeed accelerated, however caused by lake warming. Rising air temperatures strengthen thermal stabilization of water columns which prevents thorough turnover (holomixis). Reduced mixis impedes down-welling of oxygen rich epilimnetic (surface) and up-welling of phosphorus and nitrogen rich hypolimnetic (deep) water. However, nutrient inputs are essential for algal spring blooms acting as boost for annual food web successions. We show that repeated lack (since 1977) and complete stop (since 2013) of holomixis caused drastic epilimnetic phosphorus depletions and an absence of phytoplankton spring blooms in Lake Zurich (Switzerland). By simulating holomixis in experiments, we could induce significant vernal algal blooms, confirming that there would be sufficient hypolimnetic phosphorus which presently accumulates due to reduced export. Thus, intended fertilizations are highly questionable, as hypolimnetic nutrients will become available during future natural or artificial turnovers.

Hydrobiologia, 2015

Frontiers in Ecology and Evolution, 2015

! Varved lake sediments provide opportunities for high-resolution paleolimnological investigations that may extend monitoring surveys in order to target priority management actions under climate warming. This paper provides the synthesis of an international research program relying on >150 years-long, varved records for three managed perialpine lakes in Europe (Lakes Geneva, Annecy and Bourget). The dynamics of the dominant, local human pressures, as well as the ecological responses in the pelagic, benthic and littoral habitats were reconstructed using classical and newly developed paleo-proxies. Statistical modelling achieved the hierarchization of the drivers of their ecological trajectories. All three lakes underwent different levels of eutrophication in the first half of the XX th century, followed by re-oligotrophication. Climate warming came along with a 2°C increase in air temperature over the last century, to which lakes were unequally thermally vulnerable. Unsurprisingly, phosphorous concentration has been the dominant ecological driver over the last century. Yet, other human-influenced, local environmental drivers (fisheries management practices, river regulations) have also significantly inflected ecological trajectories. Climate change has been impacting all habitats at rates that, in some cases, exceeded those of local factors. The amplitude and ecological responses to similar climate change varied between lakes, but, at least for pelagic habitats, rather depended on the intensity of local human pressures than on the thermal effect of climate change. Deep habitats yet showed higher sensitivity to climate change but substantial influence of river flows. As a consequence, adapted local management strategies, fully integrating nutrient inputs, fisheries management and hydrological regulations, may enable mitigating the deleterious consequences of ongoing climate change on these ecosystems.

Global Change Biology, 2007

Shallow lakes, the most abundant lake type in the world, are very sensitive to climatic changes. The structure and functioning of shallow lakes are greatly impacted by submerged plants, and these may be affected by climate warming in various, contrasting, ways. Following a space-for-time substitution approach, we aimed to analyse the role of aquatic (submerged and free-floating) plants in shallow lakes under warm climates. We introduced artificial submerged and free-floating plant beds in five comparable lakes located in the temperate zone (Denmark, 55-571N) and in the subtropical zone (Uruguay, 30-351S), with the aim to study the structure and dynamics of the main associated communities. Regardless of differences in environmental variables, such as area, water transparency and nutrient status, we found consistent patterns in littoral community dynamics and structure (i.e. densities and composition of fish, zooplankton, macroinvertebrates, and periphyton) within, but substantial differences between, the two regions. Subtropical fish communities within the macrophyte beds exhibited higher diversity, higher density, smaller size, lower relative abundance of potentially piscivores, and a preference for submerged plants, compared with otherwise similar temperate lakes. By contrast, macroinvertebrates and cladocerans had higher taxon richness and densities, and periphyton higher biomass, in the temperate lakes. Several indicators suggest that the fish predation pressure was much stronger among the plants in the subtropical lakes. The antipredator behaviour of cladocerans also differed significantly between climate zones. Submerged and free-floating plants exerted different effects on the spatial distribution of the main communities, the effects differing between the climate zones. In the temperate lakes, submerged plants promoted trophic interactions with potentially positive cascading effects on water transparency, in contrast to the free-floating plants, and in strong contrast to the findings in the subtropical lakes. The higher impact of fish may result in higher sensitivity of warm lakes to external changes (e.g. increase in nutrient loading or water level changes). The current process of warming, particularly in temperate lakes, may entail an increased sensitivity to eutrophication, and a threat to the high diversity, clear water state.

Freshwater Biology, 1993

1. Lake Takem and Lake Krankesjon, two moderately eutrophic, shallow lakes in southern Sweden, have during the past few decades shifted several times between a clear-water state with abundant submerged vegetation and a turbid state with high phytoplankton densities. 2. Between 1985 and 1991, Lake Takem was in a clear state, whereas Lake Krankesjon shifted from a turbid to a clear state. During this shift, the area covered by submerged macrophytes expanded, followed by an increase in water transparency, plant-associated macroinvertebrates, and piscivorous fish. Nutrient concentrations, phytoplankton biomass and abundance of planktonic cladocerans decreased. 3. In both lakes, water level fluctuations were the most common factor causing shifts, affecting submerged macrophytes either through changes in light availability or through catastrophic events such as dry-out or mechanical damage by ice movement. 4. Our data give further support for the existence of two alternative stable states in shallow lakes maintained by self-stabilizing feedback mechanisms.

Ecological Engineering, 2020

The over-enrichment of shallow lakes in nutrients has emerged as one of the main causes of water quality deterioration, and is today a major focus of water quality studies worldwide. In the present work, changes in trophic conditions over three decades (1985-2017) in the largest shallow freshwater lake in Central Europe, Lake Balaton, are assessed using the time series of 10 water quality variables measured at 4 sites, one in each basin of the lake. Using combined cluster and discriminant analyses, and assessing each of the four basins of the lake separately, it was possible to divide the history of the lake into three time intervals. Principal component and Sen's slope analyses highlight the fact that the oligotrophization of the lake took place at a different pace in each of these three major time intervals (1985-1994; 1995-2003; 2004-2017) along the lake's major axis. A significant decrease in the concentration of parameters indicating trophic conditions (e.g. chlorophyll-a and soluble reactive phosphorus) was first observed in the western basins, in the proximity of the main water input to the lake, followed by the eastward spread of this phenomenon. At the same time, the importance of external total phosphorus input to the lake was found to decrease easttwards, thereby diminishing its capacity to explain the variance of the water quality parameters in the lake. Over the time period covered by this study, various measures were taken to reduce the nutrient loads to the lake. These were, in the main, successful, as may be seen in the decade-by-decade overview of the lake's trophic state presented here. A brief review of similar cases from around the world only serves to reinforce the conclusion that a drastic reduction in external phosphorus loads arriving in similar shallow lakes will result in their oligotrophization, albeit with a timelag of at least ten years.

Nature Communications, 2021

One of the most important physical characteristics driving lifecycle events in lakes is stratification. Already subtle variations in the timing of stratification onset and break-up (phenology) are known to have major ecological effects, mainly by determining the availability of light, nutrients, carbon and oxygen to organisms. Despite its ecological importance, historic and future global changes in stratification phenology are unknown. Here, we used a lake-climate model ensemble and long-term observational data, to investigate changes in lake stratification phenology across the Northern Hemisphere from 1901 to 2099. Under the high-greenhouse-gas-emission scenario, stratification will begin 22.0 ± 7.0 days earlier and end 11.3 ± 4.7 days later by the end of this century. It is very likely that this 33.3 ± 11.7 day prolongation in stratification will accelerate lake deoxygenation with subsequent effects on nutrient mineralization and phosphorus release from lake sediments. Further mis...