Global warming is causing a change in shallow aquatic ecosystems

Scientists from the Chinese Academy of Sciences have demonstrated that increasing temperatures in shallow lakes induce a change from clear water conditions dominated by macrophytes to cloudy (cloudy or disturbed) conditions. This leads to the loss of important ecosystem functions.

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This change in diet is due to behavioral changes in juvenile common carp (Cyprinus Carpio), a benthivorous (bottom-feeding) fish.

Environmental stressors such as eutrophication contribute to the decline of macrophyte biomasses in shallow lakes. Bentivorous fish increase in number during eutrophication, increasing sediment disturbance (and turbidity) at the bottom of lakes. This has a detrimental effect on phytoplankton, periphyton and macrophytes.

A brief explanation of eutrophication

Macrophytes are aquatic plants large enough to be seen with the naked eye. They can be floating, submerged or emergent. They are a key part of the aquatic food chain and can also provide cover for this aquatic life, as well as produce oxygen. They also affect soil chemistry at the bottom of lakes and slow the flow of water, thereby trapping sediments and pollutants. Some macrophytes can even absorb these pollutants into their tissues.

Eutrophication occurs when the growth elements necessary for photosynthesis – such as sunlight, fertilizers and carbon dioxide – are present in excess. This happens naturally in lakes as they fill with sediment over centuries, but human activities have dramatically accelerated this process. Thus, discharges of phosphorus, nitrogen and other elements into aquatic ecosystems have contributed to declining water quality and fisheries.

Floating leaf macrophytes are unaffected by shading caused by phytoplankton (microorganisms producing oxygen through photosynthesis), periphyton (mixtures of cyanobacteria, algae and other organisms) and sediments in suspension. Submerged macrophytes are however affected by the increase in turbidity caused by an abundance of these organisms, itself linked to eutrophication.

Additionally, as these algal blooms die, increased microbial decomposition depletes dissolved oxygen in the water, creating a hypoxic (low oxygen) environment unable to support aquatic organisms.

The effects of warming on the growth of macrophytes

Chinese scientists studied the combined effects of warming on the behaviors of benthivorous fish, floating and submerged leaf macrophytes, and water turbidity.

Floating-leaved common water chestnut (Trapa bispinosa) and curly-leaved pondweed (Potamogeton Crispus) have been used as model macrophyte species. Common carp (Cyprinus Carpio) modeled benthivorous fish – that is, fish that feed on the bottom of lakes. It is a freshwater fish found in most parts of the world.

The team constructed 24 isolated mesocosms – 2500 liters in volume, 1.5m in diameter and 1.4m in height each – to simulate a shallow lake ecosystem. Each mesocosm was filled with 10 cm of sediment and 1800 liters of water. A computer-controlled heating element was installed on each mesocosm. The water temperature was increased by 4.5°C based on estimates of temperature increases at the end of the century.

Structural equation models were used to model the effects of warming and benthivorous fish on macrophyte, phytoplankton and periphyton populations, as well as water turbidity. This showed that warming and carp populations affected macrophyte abundance both directly and indirectly.

In the absence of carp, warming had a negligible effect on phytoplankton populations, competition for nutrients and light with macrophytes being greater. The presence of carp has considerably reduced the population of submerged macrophytes. This was most likely the result of increased turbidity due to more sediment disturbed by fish and their predation on zooplankton (i.e. less phytoplankton eaten by zooplankton).

This Chinese study is the first experimental evidence of a change in clear water conditions in shallow lakes induced by an increase in water temperature. Thus, in an increasingly warming global environment, it may be beneficial to control benthivorous fish populations in shallow lakes.

References and further reading

Xu, J. et al.(2021) Warming alters the effects of juvenile carp on macrophytes, leading to a shift to turbid conditions in freshwater mesocosms. Journal of Applied Ecology, [online] Available at: https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2664.14040

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