Food quality of algae and water plants affected by climate change
Climate change has many consequences for ecosystems. Some effects are immediately visible, others are less obvious. In the case of underwater plants and algae, one thing that may change is their role in maintaining the important 'carbon balance'. NIOO-researcher Mandy Velthuis recently completed her PhD thesis on aquatic systems under global change.
"To grasp a vast problem such as climate change, it may be best to home in on minute details first", says Mandy Velthuis. Plants form the basis of the food chain, or food web. They produce organic material from CO2 and water, which makes them essential for maintaining the carbon balance. "I started at the beginning. For water ecosystems, that means the basic composition of submerged aquatic plants and algae. Any changes to the composition of these plants and algae may have major consequences for the animals that feed on them.”
Worldwide environmental changes - higher concentrations of the greenhouse gas CO2 in the atmosphere, higher average temperatures, enrichtment with nutrients (eutrophication) or any combination thereof - could easily wreak havoc. Water ecosystems are a cornerstone of our planet's carbon cycle, and algae and water plants are instrumental as they fix carbon (the C in CO2) in order to grow, and then become a source of food for aquatic animals themselves. They take up carbon from the air, and effectively feed the underwater food web.
Basic 'recipe'
At the end of the day, it's all about relationships. Not between animals or plants but between elements, the basic components of everything on earth and in the universe. Carbon - represented by a capital 'C' - is one of the most common elements underpinning life on our planet. Generally followed by phosphorus (capital P, just think of phosphates) and nitrogen (capital N).
In ecology, the ratio of C to P and C to N is a key measurement. It indicates the carbon balance, and represents a kind of basic chemical formula for all life on earth. Each organism can be defined as its own unique recipe containing specific proportions of carbon and the other two nutrients. The greater the proportion of C in a plant as compared to P and N, the poorer its food quality for the animals that feed on it.
Qualitywise, the best 'recipe' is the one closest to the proportions that exist in the animal itself: a ratio of 600 C to 1 P, for example, wouldn't be much good; 100 C to 1 P would be better. This guarantees that waste is at a minimum, and that the animal's diet is balanced, "totally zen."
Disrupted
In a lake, climate change can seriously disrupt these proportions. How? "The conclusion of our research is that the carbon balance will be affected by climate change", says Velthuis. "For plants and algae - what we biologists call the 'primary producers' - the stock of biomass is changing, and so is the ratio of C to P and N."
Interactions with aquatic animals are affected too. "Also, the kind of 'primary producer' can be a determining factor in how much carbon ends up in the sediment."
Velthuis' research demonstrates that the food quality of algae and submerged aquatic plants decreases when the amount of CO2 in the atmosphere increases, but also that eutrophication leads to better food quality. So the effects of global change are by no means unequivocal.
a) represents the 'normal' situation: 'primary production' means the growth of submerged aquatic plants and algae, which involves the fixation of carbon (C) in CO2. This results in a specific ratio of carbon to nutrients such as phosphorus and nitrogen in those plants and algae.
b) shows the decrease of carbon stocks due to eutrophication, and an increase in the consumption of algae by tiny aquatic animals. With more CO2 in the atmosphere as shown in c), the algae will have a higher proportion of carbon, which affects their quality as food.
And in d) the effects of global warming are shown to be far from unequivocal, with the rate of algae consumption accelerating and water plants growing faster, resulting in more plant residues sinking to the sediment. Higher temperatures lead to accelerated decomposition of the organic material, releasing more greenhouse gases.
Being eaten
“Personally, I'm most interested in trophic interactions, that is to say the relationships between plants and herbivores, prey and predator", says Velthuis. In other words: eating and being eaten, which is always and inevitably part of nature. These interactions, too, can change. Small aquatic animals, in particular, react strongly to climate change. "Because of global warming, tiny grazers such as water fleas reach their peak numbers earlier in the season and generally have to eat more. Because of this, the smallest plants - algae - end up being eaten earlier as well, which keeps them in check. So you end up with less algae in the water."
But in addition to the direct effects in terms of food quality and the indirect effects through small grazers, something else is happening too. "In water with mostly aquatic plants, more organic material sinks to the sediment than in water with mostly algae. The water bed retains plant residues, and this sequesters carbon. That carbon is only released when the organic material is eaten or when it decomposes", adds Velthuis. "Global warming makes water plants grow faster and more abundant, and more carbon sinks into the water bed in the form of plant residues, while at the same time more algae are eaten by an even bigger army of tiny grazers."
So the specific 'identity' of the plant or alga is key to what happens when carbon is sequestered in the water. This is of vital importance, as it reduces the amount of CO2 in the atmosphere.
Mini-lakes & greenhouse gases
Obviously, there is plenty to ponder here before the true impact of climate change can become clear. For her thesis at Utrecht University, Mandy Velthuis did four years of research financed by the Gieskes-Strijbis Fund in aquariums, outdoor mesocosms and 1000-litre indoor 'mini-lakes' inside the NIOO-laboratory.
Coming to grips with all the different effects proved to be quite a puzzle. But the impact of climate change on aquatic plants and algae - and therefore on the carbon balance - is expected to be large. Velthuis: "Climate-driven changes in the water can lead to increased greenhouse gas emissions, with serious consequences for our future climate."
Thesis: Mandy Velthuis, Elements of carbon cycling: primary producers in aquatic systems under global change (Utrecht University 2018). Promotor: Prof. dr. Ellen van Donk (NIOO-KNAW), co-promotors dr. Liesbeth Bakker and dr. Dedmer van de Waal (NIOO-KNAW).- Images: Mandy Velthuis (diagram), Cees Mooij (photograph)
