Above: (c) Erin Walsh for Journal of Conservation Physiology on the evolution of heat tolerance in Daphnia
“No they do not itch”, “Yes, they are super cute”, and “Yes, they live in water”. “Also in this pond?”. Behold the standard answers given during an ‘interrogatory’ conversation with a passer-by on a sunny sampling day, somewhere in a city in Flanders (Belgium). While I am standing in a pond, wearing waders and throwing a zooplankton net unexpectedly elegantly back and forth, the confusion in the eyes of the beholder markedly rises. “A flea that lives in the water? Are you sure?”. Yes I am. And there is a ton of amazing things you can do with them!
They sometimes seem to go unnoticed by people, but freshwater ponds and pools are of major biological significance in our landscapes. Underneath and above those water surfaces, water plants, macro-invertebrates, fish, zooplankton, phytoplankton, flying insects, and amphibians live and thrive. Ponds and pools are crucial for biodiversity conservation because of their local habitat heterogeneity; they often harbor endemic or unique species and thus fuel beta and gamma diversity in a pondscape (i.e. collection of ponds in a regional landscape). Compared to other habitat types, ponds can contribute a substantially larger proportion to landscape-level biodiversity. They are important for offspring deposition and food gathering, and serve as mating arenas for organisms such as amphibians, water birds, dragonflies, and damselflies. They provide stepping stones in the unsuitable terrestrial landscape for semi-aquatic and water-associated organisms, which is assumed to become increasingly important in the light of climate change and its associated species range expansions. Lastly, they are of high socio-ecological value as they play a key role in water management, rainfall interception, carbon sequestration, and nutrient retention, as well as human psychological well-being and nature education, the latter being especially relevant in densely populated areas, such as cities.
While evidence on evolution in cities is growing excitingly fast (Johnson & Munshi-South, 2017), so far freshwater ponds and pools (and river systems) have received considerably less attention in urban evolutionary ecology studies. Compared to those in rural areas, ponds and pools in cities are often closed, isolated, ecosystems; ‘blue’ remnants in a built-up landscape, or blue entities designed for specific human needs such as rain water catchment or aesthetic purposes. They are in general heavily managed and constructed with a simple morphometry such as rectangular or oval shapes, with steep bank angles. This morphometry reduces the establishment of aquatic vegetation, which is a key component structuring freshwater biodiversity and ecosystem functions related to hydrogeological water balance, nutrient uptake, and bioremediation of contaminants. Urban ponds are additionally impacted by pollution-loaded surface run-off (e.g. nutrients, pesticides, de-icing salts, metals, etc). Animals residing in ponds are often limited in dispersal capacity, which thus likely increases the importance of evolutionary trait change in response to urbanization-driven changes in this habitat type.
Water fleas (Daphnia magna) are important interactors in the aquatic food chain as they are not only a food source for larger freshwater organisms (macro-invertebrates, larvae of amphibians etc.), but additionally exert top down control on algae, thereby stabilizing the clear water phase. Their grazing efficiency is correlated with their body size; larger animals have a higher grazing rate and are thus important to reduce the formation of blooms of algae and nuisance cyanobacteria, which are expected to increase in frequency and intensity due to climate change and urbanization. Genetic differentiation for functional traits in urban D. magna might thus substantially shape ecological dynamics in city ponds, and possibly alter grazing-associated ecosystem functions via eco-evolutionary feed backs.
One of the most reoccurring phenomena associated with urbanization is the ‘urban heat island’ (UHI) effect. While profoundly studied in terrestrial landscapes, the impact of terrestrial urbanization-driven warming on water bodies remained strikingly unknown. Water buffers temperature increases and fluctuations due to its high heat capacity, and often larger water bodies (‘urban blue space’) in and around (mostly larger) cities have been shown to reduce terrestrial UHIs. Nevertheless, when strolling around cities, we noticed the majority of water bodies in regions such as Flanders (study region of this research), but also other parts of Europe, are relatively small (e.g. park ponds, ponds in school yards, public places, and economic districts, etc.). Therefore, it is not unlikely that these smaller freshwater systems and its residents might be impacted by urbanization-driven warming.
During my PhD, at the Laboratory of Aquatic Ecology, Evolution and Conservation (KU Leuven, Belgium) of Prof. Dr. Luc De Meester (De Meester Lab), I set out to test if and how urbanization might impact evolutionary trajectories of species inhabiting ponds and whether we could find signals of urban adaptations. I performed common garden experiments in which I reared multiple genetically distinct lineages (i.e. clones) of urban and rural populations of D. magna under standardized conditions. Populations were sampled in the region of Flanders (Belgium). Flanders is one of the most densely populated traffic hubs of western Europe with 7.5 billion people living on a surface area of 13.682 km². Urban land cover in this area is predicted to reach over 40% by 2050.
We especially focused on the UHI effect as a driver of urban adaptive evolution, given that we found temperature to be the most systematically differing environmental variable between urban and rural ponds out of a set of more than 20 measured variables. Urban ponds are substantially warmer compared to rural ones, especially during spring and summer (which we coined the ‘urban hot-tub’ effect, Brans et al., 2018a). The monitored temperature differences in summer ranged between 3.2 to 4 °C (Brans et al., 2017a, Brans et al., 2018a), strikingly resembling predicted average global temperature increases due to climate change (i.e. IPCC RCP8.5 scenario). Such temperature differences could drive genetic differentiation for functional traits in urban compared to rural populations of aquatic animals, especially given that the majority of animals inhabiting these ecosystems are ectotherms. Additionally, we expected oxygen limitation, which increases under warming in aquatic ecosystems, in combination with increased metabolic rates, to fuel adaptations to urban heat islands in freshwater ecosystems.
What did we find? Check out Part II of this story on Friday to read about how these differences in water temperature impact urban water fleas.
- Time for a Dive Part II: Urban Evolution in the Aquatic - January 25, 2019
- Time for a Dive Part I: An Introduction to the Water Flea Daphnia magna and Urban Aquatic Habitats - January 21, 2019