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Perspectives on freshwater ecosystems in a changing climate

By Brigitte Melly, Postdoctoral Researcher, SAEON Fynbos Node
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The recent drought in southern Africa has reached news pages across the world, especially when Cape Town prepared for Day Zero in 2018. There is even a Wikipedia page dedicated to the “Southern Africa Drought”*.

Recent droughts in the region, and across the world, have reminded people of the impacts of global changes on our water resources, and how important it is to understand, monitor and protect these ecosystems.

Global monitoring and research

My recent attendance at the 11th Symposium for the European Freshwater Sciences (SEFS) held in Zagreb, Croatia, suggested that the regional drought is part of a worldwide phenomenon of increasing extreme events. Approximately 500 delegates from six continents and 42 countries attended the event and presentations covered a wide range of topics.

Multiple factors are placing stress on freshwater ecosystems across the globe. Recent extreme events exacerbate already existing problems in these systems that are due to the direct and increasing influence of anthropogenic activities, and rising temperatures and CO2 levels.

Many of these impacts were reported on during a special session at the conference on climate change and freshwater ecosystems, illustrating the current eminence of these problems. Conference attendees reported on plankton, macroinvertebrate and vegetation community shifts, as well as changes in nutrient dynamics in freshwaters that can be directly attributed to changes in climate.

For example, rivers and lakes in Europe that have experienced a steady increase in average temperatures, have seen phytoplankton and macroinvertebrate community shifts, due to the thermal stress of the original communities being reached. These changes can be gradual or more sudden and result in new species moving in that are more tolerant of the new range in temperatures.

There were also reports on increases in extreme events such as the prevalence of wildfires, heat waves and droughts, which have affected freshwater ecosystems and their associated biotic community structures in the USA, Europe, South Africa (my research) and Australia. It is in this special session that I presented on the impact of the recent drought on wetland inundation patterns on the Agulhas Plain.

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View of Zagreb, Croatia, from the conference venue (image: Brigitte Melly)

Approximately 500 delegates from six continents and 42 countries attended the symposium (image courtesy of SEFS 11 photographer)

Key issues

The size and geographical spread of the conference meant that there was an amazing diversity of talks and topics to listen to. Two key issues stood out that can be related to the current status of eco-hydrological research in South Africa:

Firstly, many of the Western European countries have basins and rivers that have been monitored extensively over the last 30 to 40 (sometimes 50) years and are showing steady and significant drops in base level flows, and drastic losses in biodiversity.  

Similar findings have been observed in larger river basins in South Africa, but we lack an extensive spatial and temporal coverage of this type of data that covers the diversity of our freshwater systems. It is only over a longer period that we can start to understand the patterns and potential causes of persistent hydrological and biological changes in these freshwater ecosystems.

South Africa needs to emphasise developing and maintaining long-term hydrological and ecological datasets so that we can adequately protect our water resources, and the associated communities, as global change impacts continue to place pressure on these systems.

Secondly, there were several studies reporting on the impact of drought and climate change on hydrological ecosystems across the world. Several talks addressed recent ecological shifts in systems that have become ephemeral/intermittent** for the first time in recent recorded history due to drought.

For example, Alpine streams in Italy have become intermittent, and this has had an impact on the aquatic communities, which aren’t adapted for coping with the resultant hydrological and physico-chemical extremes.

This presents a similar scenario to the Agulhas Plain, where extensive and permanent wetlands dried during the recent drought. This affected the surrounding agricultural activities by reducing freshwater availability in quantity and quality, the latter due to increases in salinity and lowered oxygen levels (associated with the lower water levels).

This highlights the global nature of the problem and the need to tackle the effects of these changes if they become persistent, or more regular, in the future.

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Brigitte explores the oldest national park in Croatia during a field trip

Field trip

A mid-conference field trip to Plitvice Lakes provided an amazing opportunity to see large karstic lakes (something that is not found at such a scale in South Africa), and a chance to learn from some of the Croatian ecologists who conduct research in these systems.

The lakes are found in the oldest national park in Croatia, which is also a UNESCO World Heritage area, due to its unique and beautiful tufa deposits and highly diverse aquatic and terrestrial ecosystems. The main feature of the park is the 16 lakes that have formed on a dramatic landscape underlain by both dolomites and limestones.

These lakes are surrounded by cascading waterfalls and over 100 caves that support the freshwater and terrestrial biodiversity in the region.

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View of the Plitvice Lakes (images: Brigitte Melly). The karst relief forms when the carbon dioxide enriched water dissolves into the underlying carbonate substrate (limestone and dolomite), creating the karst surface, such as these sunken lakes (see picture). The calcium carbonate precipitates out and accumulates along the edges, creating tufa barriers that are incrusted with algae, mosses and aquatic bacteria. These barriers result in the formation of lakes, cascades and waterfalls.

More details on the conference, and the abstract book can be found at the following link: http://www.sefs11.biol.pmf.hr/

* Wikipedia 2019: 2018–19 Southern Africa drought. https://en.wikipedia.org/wiki/2018%E2%80%9319_Southern_Africa_drought. Accessed September 2019. 

** An ephemeral or intermittent river flows for only part of the year, during the rainy season. In the case of a wetland, it is when it is inundated with water.

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