Climate warming may increase algal blooms caused by sediment nutrient release in lakes – management must adapt

As in many lakes, the pressure of growing human populations and fertiliser run-off from intensive agriculture (among other stressors such as industrialisation and species invasion) has previously caused eutrophication in Balaton – the largest lake in Central Europe. This problem, where nutrients such as nitrogen and phosphorous reach high levels, can lead to extensive growth of algae, upsetting the underlying ecosystem through blocking out light, for example. Climate change exacerbates the issue, through effects such as nutrient loading from agricultural run-off and altering the temperature structure of water bodies.

In 2019, after 25 years of successful management avoiding previously serious eutrophication, Balaton developed an unexpected, record-breaking algal bloom. The event was surprising as it would have required substantial nutrient input (estimated at 12.4 tonnes of phosphorous within about three weeks), but no external source was linked to the emergence of the bloom. In this new study, researchers hypothesised that exceptional amounts of phosphorous must have been released from sediment, which can occur when lake water is oxygen-depleted near the bottom, due to complex physical and chemical factors.

To explore this, the researchers analysed data on lake conditions recorded at a near-shore observatory over 10 years, and used modelling to explore the likeliness of oxygen depletion in deep water in 2019. They found that average levels of oxygen in summer 2019 were among the lowest in previous decades.

In summer 2019, the lake water was turbid (not clear due to an intense precipitation of lime) and warm, and management kept the water level relatively high in anticipation of possible drought. A sudden increase in air temperature warmed the water in late July and, together with insufficient mixing, led to different layers of temperatures in the lake water (stratification) and low oxygen in deeper waters (below 2.5 m depth). Similar phenomena occurred during two periods in August, and the algal bloom began to develop in the third week of that month.

Balaton is a shallow, polymictic lake, which means that apart from when the surface freezes in winter, the water is continuously mixed and the temperature is similar throughout. However, the lake is deep enough to occasionally show vertical temperature heterogeneity (dissimilarity). Based on their findings, the researchers conclude that the precursor of the bloom was internal eutrophication, caused by bottom hypoxia (depleted deep-water oxygen) and a related release of sediment phosphorous. This was due to insufficient mixing in the water column – a function of weather sequences (temperature patterns) and water depth – providing favourable conditions for the growth of certain algae. Analysing data on both from 2009–2020, the researchers found that the probability of stratification increased when daily mean air temperature rose rapidly in summer months.

Alterations in thermal structure and mixing are the most direct impacts of climate warming on lakes, and the likelihood of stratification events lasting two or more days in Balaton may increase five-fold by 2100 under global warming¹. Such conditions may limit the effect of external nutrient control on eutrophication.

The researchers say it is, therefore, vital to use not only external (from inputs such as fertiliser run-off) but also internal nutrient-control strategies, to manage eutrophication. Such strategies may include water-level regulation², as the likelihood of bottom hypoxia increases non-linearly with increased water level. The researchers suggest that water-level regulation focusing solely on water quantity should be replaced with adaptive regulation that optimises levels based on both preventive drought security (requiring water storage and high-water level) and eutrophication management (requiring a relatively low-water level in summer).

Additionally, the algal species present in the bloom – Ceratium furcoides and Aphanizomenon flos-aquae – differed from previous events in the 1980s. C. furcoides is a native, but previously rare, species; and is not usually co-dominant with A. flos-aquae. This indicated a change in the ecology of the lake. The researchers warn that such ecological surprises may become the norm for lakes under a warming climate.

Footnotes

1. i.e. from 10–13% to 50%, under the present high-water level of +1.20 m.
2. The water level of Lake Balaton is regulated through an outflow weir connected to the Danube River. During droughts, the level can drop significantly. Since extreme droughts in the early 2000s, the water authority increased the regulation level by 20% in 2013.

Source:

Istvánovics, V., Honti, M., Torma, P. and Kousal, J. (2022) Record‐setting algal bloom in polymictic Lake Balaton (Hungary): A synergistic impact of climate change and (mis) management. Freshwater Biology, 67(6): 1091–1106.

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

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The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.