What happens when plants dominate lakes?

How does water change the dynamics of plant life? This is a puzzle tackled by Kaj Sand-Jensen and colleagues in a review “Shallow plant-dominated lakes – extreme environmental variability, carbon cycling and ecological species challenges. They note that while there’s been a lot of study of plant distributions, there’s been a lot less work on physiochemical dynamics and biological challenges.

An example would be the daily cycle of photosynthesis. During the day plant life will produce oxygen in abundance, but as night falls, the cycle turns. As plants respire the oxygen in the lake is used, reducing the availability for other species. How do algae and plants cope with these changes?

There isn’t just variability over the day. You also have variability in space too, with different conditions at different depths, as Professor Sand-Jensen explained: “We have taken the study a step further by following/reviewing recurring stratification and mixing as well as calcium carbonate precipitation and dissolution, and these aspects are important to aquatic physicists and chemists as well.”

“The highly variable temperatures, oxygen and pH with time and depth are extremely important for small animals and fish and, therefore, to a very wide group of animal ecologists as well as amateur entomologists and anglers.”

The daily cycle of photosynthesis causes significant chemical changes. Near the top, in the sunlight, the output of oxygen causes the creation of carbonates, lime, from carbon dioxide in the water. This reaction locks away carbon dioxide that the plants would otherwise use. The lime sinks, pulling the carbon within it down. If this were the end of the tale, Sand-Jensen says that photosynthesis would stop within a few days. However, deeper in the lake a different process is happening.

In the shade, there is no photosynthesis, only respiration. In this process carbon dioxide is released as organic matter decomposes. Carbon dioxide causes the water to become more acidic. This allows it to attack the falling lime, releasing the carbon dioxide back into the water. The fall of the cooling waterat night drives round the circulation of the lake, pushing the refreshed carbon dioxide-rich water back to the top of the lake.

A lake is a complex ecosystem, but a lot of it is also a hidden ecosystem. What led Prof Sand-Jensen to look beneath the surface? “As a schoolboy, I saw the decline of marine eelgrass in the fjord close to where I lived and went swimming and fishing. My plan as a student was to examine why some terrestrial plants grow here and not there, being particularly interested in sedges. However, I could not find a supervisor with the sufficient experimental and ecological background on sedges. Consequently, I decided to do my master thesis examining: The growth dynamics of eelgrass and critical steps in its life cycle. However, being immediately hired as an assistant professor at the University of Copenhagen’s freshwater institute, I shifted from marine plants to studies of freshwater plants in lakes and streams. Simultaneously, I have from time to time studied marine macroalgae and seagrasses and more recently even terrestrial plants. Presently I co-author a book on ‘biology and identification of freshwater plants of northern Europe’.”

One of the lines that struck me from the abstract was Small plant-dominated lakes function as natural field laboratories…. It highlights one of the attractions of small lakes for study, but also a question. What makes a lake small? Prof Sand-Jensen said: “Small lakes are usually shallow and submerged plants can cover most of the lake bottom and, thereby, dictate the processes in the entire lake ecosystems. Being also a plant physiologist, it is fascinating to bring the results of controlled laboratory experiments to “real test in the truly relevant environment – the lake – in which environmental conditions vary naturally controlled by the day and night, sunny or overcast sky, the wind and the existence and processes of the plants. Did anyone think in advance that the critical oxygen conditions may occur during the day when oxygen production by photosynthesis is running fast? Well, no.”

Prof Sand-Jensen said that these dynamic changes are important. They occur because even small lakes are not homogenous. “This is the case because oxygen depletion develops by respiration in the dark bottom waters below the surface canopy as vertical mixing is stopped by surface heating forming light surface water floating on heavy-dense bottom waters. At night, surface cooling induces vertical mixing bringing new oxygen to the bottom waters and the basal parts of the plants for respiration.”

“The physical, chemical dynamics have surprised me a lot in small lakes (and we may find the same conditions in shallow plant beds in larger lakes). Well, we did not know because we did not look for it earlier. And, the development of continuously operating sensors has opened this world. Before we needed to do – night and day – point sampling.”

The challenges now are to find how water moves horizontally and vertically through lakes. There is also the interaction with animal life to map. Prof Sands-Jensen said: “We would love to track on line the movement vertically and horizontally of fish and small animals along with the dramatic changes of environmental conditions. Do they move in accordance with changes of environmental conditions? Or do they stay put and tolerate say high surface temperatures or anoxia in bottom waters for many hours?” He also identifies automatic sensors for phosphorus and calcium as a necessary development. These would add to information ecologists already have for oxygen and pH.

With sensors like these Sand-Jensen would be able to investigate one of the environmental puzzles that interests him. “I would like to test whether plants are cleaning the environment for themselves by precipitating calcium carbonate and co-precipitating phosphate bound to calcium carbonate.”

The addition of water to the environment doesn’t just make life wetter. It makes it much more chemically diverse and dynamic. The review by Sand-Jensen and colleagues provides a platform for many possible projects.