Does water life die out under the ice in winter?

życie w wodzie

Are you familiar with the theme from a children’s book or cartoon depicting an angler on a stool over a break? Amazed, he pulls out a block of transparent ice with a fish frozen inside. This picture illustrates our conflicting expectations of what happens in the lake in winter – on the one hand, we know that life must go on there despite the cold and darkness, but on the other, we expect it to be miserable, basically frozen over.

life in the water
Does water life die out under the ice in winter? 1

The myth of frozen life in the lake in winter is reflected in the scientific literature on the composition and functioning of freshwater organism communities under ice in our geographic zone. There are two reasons for this – first, a practical one, stemming from the difficulty of studying the aquatic environment in winter, since it is cold and gets dark quickly, it is difficult to break through the ice, and sometimes it is dangerous to step on it. The second reason is precisely something of a scientific bias or myth. Namely, it’s about the iconic 1986 Plankton Ecology Group (PEG) model, for many years shaping the study of the ecology of aquatic organisms.

The model described the seasonal variation of phytoplankton and zooplankton in relation to physical abiotic conditions, the availability of nutrients for phytoplankton and food for zooplankton in the form of phytoplankton, and consumer and predator pressures. The model depicting 24 stages of succession assumed that in winter, in ice-covered lakes, the activity of primary and secondary producers is very limited (if any). Going hand in hand with this model, the widespread use of the term “growing season,” which describes how organisms function in summer, reflects the prevailing view of winter as a period of dormancy and almost complete inactivity.

However, the gradual reduction in the duration of ice cover on lakes, accompanying a warming climate, and the attempt to determine what consequences this may have for aquatic ecosystems in the future, has drawn scientists’ attention to how little we know about what is happening under the ice. The PEG model was revised in 2012. by Sommer and colleagues, who also pointed out the need to expand studies of freshwater ecosystems to include winter observations and suggested the possibility of more abundant plankton under the ice than previously thought. As a result of increased research by scientists working on different groups of organisms, we are now tempted to conclude that: life under the ice is surprisingly lush and …colorful.

The specific physical characteristics of water, namely the temperature-dependent change in its density and ability to dissolve oxygen, enabling organisms to function in freshwater during cold winters. The density of water decreases as the temperature increases, so in summer near the surface the water is warm, but the deeper you go, the cooler it is. At temperatures between 0o C and 4o C, water reaches its highest density at 4o C, consequently sinking to the bottom of the tank. Ice, on the other hand, being lighter than water, floats on the surface, insulating the deeper layers from the direct impact of low temperatures. It turns out that in a lake deeper than 1 m, the water hardly freezes to the very bottom, allowing fish to survive the winter.

The ice cap and the snow that lies on it, while insulating the water from very low temperatures, act as a barrier to light, needed for photosynthesis, and to oxygen from the atmosphere. Therefore, the O2 concentration in the lake under the ice decreases gradually during the winter, although the higher solubility of oxygen in low-temperature water compensates for this problem to some extent. Unfortunately, sometimes, especially in shallow lakes, but also in deeper lakes, during a very cold and long-lasting winter, the oxygen concentration under the ice can be too low, causing the so-called winter fish die-off. On the other hand, the 10-cm-thick snow cover on the ice reduces the availability of light so much that it not only effectively inhibits photosynthesis, but also convective mixing, which affects the suspension of algae and the concentration of nutrients for phytoplankton in the photic zone.

Activity and diversity of organisms in water in winter

Phytoplankton are microorganisms that float freely in the water, carry out photosynthesis and form the basis of the trophic network in aquatic ecosystems, thus providing food for zooplankton, which in turn are the food base of plankton-eating fish or juvenile predatory fish. Studies of freshwater ecosystems in winter, summarized in Hampton et al. (2017), indicate, contrary to well-established beliefs, that phytoplankton biomass in frozen lakes, although lower than in the ice-free period, is nevertheless higher than expected. The average winter concentration of chlorophyll a was found to be approx. 43% of the average summer concentration, and the biovolume (i.e., the total volume of phytoplankton, calculated on the basis of cell abundance and size) is about 16%. Why the difference? Under light-limiting conditions, algae synthesize more chlorophyll a per unit of biomass, and conversely at very high light intensities, phytoplankton cells contain less chlorophyll a. Therefore, chlorophyll a concentration measurements are an indirect measure of phytoplankton biomass and are good when supported by analysis of cell abundance and/or biovolume. It is true that ice and snow cover effectively inhibit light, but already less than 10 cm of snow, and especially the lack of snow, make the conditions for photosynthesis quite good, especially when the ice insulates the water from too low temperatures. Therefore, in some lakes, such as. Simcoe in Canada, Scharmützelsee in Germany or Fish Lake in the U.S., which were studied over a period of more than a decade, even higher concentrations of chlorophyll a were recorded under the ice than in summer! However, it is difficult to say unequivocally what the typical composition of phytoplankton in winter is, as it varies greatly from one reservoir to another. Analyses of 110 lakes have shown that, on average, there are far fewer cyanobacteria in winter than in summer, but there are more diatoms, a surprisingly large number of green algae, and there are also quite numerous myxotrophic cryptophytes and gold-formers (chrysophytes) that give the water a greenish or golden color. Although, when averaged, the differences between winter and summer do not seem dramatic, in any particular lake the phytoplankton varies greatly between the two seasons, there is still not enough data.

Another misconception concerns zooplankton in winter. According to earlier assumptions, this important element in the trophic network, transferring organic carbon and biogenes from producers to higher levels, was to disappear from the water column into the sediment in the fall, undergoing diapause. However, studies have shown that some representatives of various zooplankton taxonomic groups are active in winter and do not enter diapause (in the case of crustaceans, this is the formation of surviving forms). These include various species ofcopepods (Copepoda), such as Leptodiaptomus minutus, Eudiaptomus graciloides and Cyclops scutifer. Taxa that occur only in winter and go into diapause in summer are also known. Also, some of the paddlefish, such as Daphnia cucullata and D. pulicaria actively overwintered in the water column, as demonstrated by my UW colleagues Pijanowska (1990) and Slusarczyk (2009), among others, who were the few who dealt with winter zooplankton. Typical representatives of this group produce survival eggs in autumn, which overwinter in thick, chitinous sheaths (epiphyses) in bottom sediments. As a result, the average density of zooplankton in the water in winter can be about ¼ of the average summer density. This assemblage in winter, like in summer, is dominated by copepods, but the proportion of paddlefish under the ice is apparently smaller than in summer.

And now it’s time for color

Intuition would tell us that in winter everything is either white or dark and gray, including underwater. And here’s a surprise! It turns out that the copepods mentioned above, Leptodiaptomus minutus and C. scutifer, are intensely red in winter. This color is given to them by karetonoids accumulated in the cells. This raises questions about why and why they are so colorful. One of the more important known functions of these pigments in algal cells is to protect against excessive solar radiation. Also, some Daphnia, found in high mountain fishless lakes, are red for the same reason. But is zooplankton under the ice threatened by too much sunlight? Rather not. The reason is different. In order for zooplankton to survive the winter under the ice, they store a lot of spare materials in their bodies in the form of lipids (fats), some of which are polyunsaturated fatty acids. However, they are prone to decomposition through peroxidation during the long winter, and it is only when they are combined with pigments such as carotenoids that this process slows down, protecting the body from oxidative stress, and the fats, as spare material, can be used by cells when they are needed. Conventional thinking, however, would dictate that such conspicuous coloring is not evolutionarily beneficial to plankton because it makes them more visible to fish. And these, as predators that use their eyesight, catch well-visible prey first. And indeed, in summer the same species are much more modestly colored. In winter, however, light under ice and snow is scarce enough that copepods can take on a fiery red coloration with relative impunity, protecting lipids and adding color to life under the ice.

Fish, too, have many characteristics to survive winter in our climate. So the angler in our cartoon did not wait in vain, although an ice cube is obviously an exaggeration. These adaptations include migrating, stockpiling, reducing activity and thus oxygen requirements, and going into numbness, a type of hibernation called stupor ( torpor in English).

And so some fish species, such as goby and catfish, like amphibians, sink to the bottom of water bodies and bury themselves in sediment to wait out the winter. Others, however, remain in the water column, albeit with much slower activity. Plankton-eating fish tend to stick closer to the surface, protecting themselves from predators in shallower water layers, between submerged vegetation. There, they move slowly (but do not stop, as only by swimming, the fish can breathe) and rarely feed on the passing zooplankton. Predatory fish, on the other hand, travel deeper, away from the cold layers just below the ice. However, to eat, they swim closer to the surface, where they unhurriedly hunt plankton-eating fish. Everything takes place a bit in slow motion. However, this is not freezing, but a different pattern of bodily functioning, characteristic of low temperatures.

Life for fish in near-zero water is possible because they accumulate omega3 acids in their cells, which keep their cell membranes flexible and allow them to function normally. In addition, the fish, like zooplankton, benefit from the fat reserves stored in the cells in summer, compensating for the low concentration of food in winter. Therefore, at the end of winter they are on average leaner than in summer and autumn.

And finally, let’s go back to the picture from the beginning of the article, i.e. For fish in a block of ice. The joke? Of course, but not exactly. It turns out that in special cases frozen fish, after slow thawing, are able to “come to life”. It seems improbable, and yet it is possible. In 2016. YouTube has circulated a video showing the revival of a frozen fish. He was not a gimmick and had a scientific explanation. There are proteins in the cells of some fish that slow down the formation of bonds in water molecules, preventing ice crystallization in the cells. It was because of them that, after the film fish partially froze, its cells remained undamaged and, after slowly warming up, it was able to return to activity. Thanks to these proteins, fish in the Arctic also survive the winter in the freezing sea.

In conclusion, lake life under the ice not only does not die out, but can even sometimes bloom with green or golden phytoplankton or beautiful red zooplankton.

The author is a PhD in biological sciences, professor at the University of Warsaw. He works at the Institute of Environmental Biology in the Department of Ecology and Environmental Protection, and specializes in phytoplankton ecology, as well as cyanobacteria ecology and toxicity. President of the Polish Hydrobiology Society.

Używamy plików cookie, aby zapewnić najlepszą jakość korzystania z Internetu. Zgadzając się, zgadzasz się na użycie plików cookie zgodnie z naszą polityką plików cookie.

Close Popup
Privacy Settings saved!
Ustawienie prywatności

Kiedy odwiedzasz dowolną witrynę internetową, może ona przechowywać lub pobierać informacje w Twojej przeglądarce, głównie w formie plików cookie. Tutaj możesz kontrolować swoje osobiste usługi cookie.

These cookies are necessary for the website to function and cannot be switched off in our systems.

Technical Cookies
In order to use this website we use the following technically required cookies
  • wordpress_test_cookie
  • wordpress_logged_in_
  • wordpress_sec

Cloudflare
For perfomance reasons we use Cloudflare as a CDN network. This saves a cookie "__cfduid" to apply security settings on a per-client basis. This cookie is strictly necessary for Cloudflare's security features and cannot be turned off.
  • __cfduid

Odrzuć
Zapisz
Zaakceptuj

music-cover