Oxygen in Ecosystems

Hello readers. Since you decided to read this article a few minutes ago, you have probably taken two or three breaths. Through breathing, humans get oxygen from the air. Without oxygen, our bodies would be unable to produce energy from the food we eat, and you would probably not be able to continue reading this article. Like humans, other animals also need oxygen to digest their food and survive. Aquatic animals like fish use oxygen that has dissolved in water. However, in many of Israel’s coastal streams, the concentration of dissolved oxygen is low, making it difficult for these creatures to live there comfortably.

Oxygen is a gas found in Earth’s atmosphere, and it is produced by plants and bacteria in a process called photosynthesis. Photosynthesis is a biochemical reaction in which light, a gas called carbon dioxide, and water are converted into oxygen and sugar—the source of energy for animals. Photosynthesis is the “secret of life” because the bodies of plants, which are created through photosynthesis, are the food for all other organisms [1]. Organisms that perform photosynthesis are green and grow better when fertilized. That is why farmers and gardeners fertilize the soil. The fertilizers they use often contain phosphorus and nitrogen.

We hope that you (like us) go out with your family sometimes to see flowers, climb trees, and enjoy the variety of plants. Seas and rivers usually do not contain large trees or even shrubs like those found on land. Most of the plants growing in the water are tiny creatures called phytoplankton. Phytoplankton drift with the water currents, and their bodies are the food that most aquatic animals feed on—they are the basis of the food web.

Estuaries and Water Stratification

In Israel, rainwater drains into streams, most of which flow from the mountains to the Mediterranean Sea. Thirteen streams flow along Israel’s coast. Many of the streams change when they reach the coastal plain (Figure 1A). The amount of freshwater flowing through the streams may not be the same throughout the year—instead it transitions from a slow stream to a steady stream during the rainy season in winter. Measurements we conducted showed that, in the stream section closest to the coast, the water’s width and depth are greater, and the flow speed is lower.

The part of the stream close to the sea is called an estuary, and the water there has unique characteristics. Through the mouth of the estuary, where the stream meets the sea, seawater enters the stream, making the water saltier [2]. The unique characteristics of the water in an estuary lead to changes in the plant and animal species that live there. Marine organisms enter the estuary from the sea and live there; some of them even use it as a home for their offspring, which is why estuaries are a very diverse and rich habitat.

The water in estuaries is divided into two layers: the top layer contains fresher water, and the lower layer contains saltier, denser water. Because salt water is “heavier”, it sinks and does not mix with the upper layer [3]. This is called stratification (Figure 1B). It may sound unlikely that deeper water and surface water would not mix, but anyone who has tried to mix water and oil knows that it is difficult to mix liquids with different densities. In addition, the waves in the sea create a barrier of sand on the shoreline that limits the flow of seawater at the mouths of estuaries (Figure 1), so the water in the depths of the estuary remains there for a long time (up to 20 days or more).

Water and Fertilizers in Rivers

People use water for many purposes, from cooking and showering to flushing toilets. Therefore, the water that comes out of our homes does not smell good and is called sewage. Sewage must be treated in wastewater treatment facilities to prevent the spread of diseases. Israel is a world leader in the treatment and recycling of sewage, using approximately ninety percent of the treated wastewater (also known as sewage effluent) to irrigate fields. Both wastewater and effluent contain large amounts of fertilizers, such as phosphorus and nitrogen [4], and some of it reaches the streams that flow into estuaries.

The upper layer of water in estuaries contains a high concentration of fertilizers (Figure 1C). This layer is exposed to sunlight, allowing phytoplankton to multiply significantly, resulting in a green appearance (Figure 1D). Some of the phytoplankton sink to the deep, salty waters. Most living creatures obtain their food from materials that were created by phytoplankton during photosynthesis and is called particulate organic matter. Estuaries contain a significant amount of this material, allowing animals and bacteria in the deep water to feed on it and thrive. These animals breathe oxygen and convert it into carbon dioxide (Figure 1E).

Oxygen in water comes from two sources: photosynthesis and diffusion from the atmosphere. Once oxygen reaches the water, it dissolves into the water and flows with the currents. The amount of oxygen in the water decreases when organisms use it for respiration, for example when bacteria breathe oxygen to break down sunken phytoplankton.

The strong sun in Israel, together with the high concentration of fertilizers in estuaries, increases the rate of photosynthesis in the surface water. As a result, the rate of oxygen use increases. In the depths of estuaries, the water is dark (due to a high concentration of particles in the water that do not allow light to penetrate). Additionally, the rate of water and oxygen exchange in deep water is low. The darkness and low exchange cause the oxygen concentration to be very low over long periods of time (Figure 2).

In contrast, on the well-lit surfaces, the water flows faster and it is easier for oxygen from the atmosphere to dissolve. Therefore, the oxygen concentration is higher (Figure 2). Additionally, photosynthesis on the surface is high during the day, when sunlight is present, and lower at night when it is dark. Therefore, oxygen concentrations during the day can be very high and drop significantly at night (Figure 3). In rivers, the water flow is fast, and there are no differences between the surface water and the deeper water; therefore, the transfer of oxygen from the surface water to the depths is possible. In contrast, in estuaries, oxygen does not flow downward, so the deep water suffers more from a lack of oxygen (Figure 2). Remember: when there is no oxygen, animals cannot survive (imagine that you had to stop breathing every night until the next morning!).

Sometimes, the bottom water experience long periods of oxygen deficiency, up to weeks straight. Which makes the situation even worse, because toxic substances are formed in the estuary water (Figure 1E). The combination of oxygen deficiency and the release of poisonous [5] substances almost completely prevent animals like snails and clams from living at the bottom of estuaries, where they were once very common.

What Can we do to Increase Oxygen Levels?

In summary, we have explained that water in estuaries in Israel suffers from a lack of oxygen, causing damage to the natural environment. Three main things can be done to overcome the lack of oxygen.

What we found is simple: lack of oxygen is a huge problem in Israel’s estuaries, but because these systems are so small, we can fix them quickly. Everything that causes the oxygen to disappear—from the fertilizers in the river water to the sandbar blocking the sea—is an issue we, as humans, can control. By stopping sewage from reaching the streams, managing river flow, and even removing the sand barriers [2], we can bring oxygen back to the deep water. This will make the estuaries healthy again for the snails, clams, and fish that used to live there. These small, fragile ecosystems need your help to thrive!

The next time you are near a coastal stream, remember that protecting the water is one of the quickest ways to save a vital natural treasure.

Glossary

Photosynthesis: ↑ The process by which organisms use light, carbon, and water to produce chemical energy and the building blocks for their bodies.

Phytoplankton: ↑ Tiny organisms that drift with the water currents and are capable of photosynthesis. Most of the organic matter in the sea is produced by phytoplankton.

Estuary: ↑ A waterbody found where a river or stream meets the sea, creating a mixture of saltwater and freshwater. The deeper water is generally saltier than the water at the surface.

Stratification: ↑ A phenomenon that occurs when two flows (liquids or gases) of different densities meet. In such a situation, the denser flow settles below the less dense flow.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We, the researchers at the Estuarine Research Center, would like to thank the Yad Hanadiv Foundation and the Jewish National Fund for their funding of this research. Thanks also to the many people who participated in the research: Lee Shayish, who is now doing more important things; Tal Amit and Merav Gilboa, who, when they find time to get out of the water or get off the ship, may be able to read the story of oxygen to the children they have had in the meantime; Reuven Rosenblatt, who we know will not find time to get out of the water and will therefore read to his grandchildren underwater. And for those who have forgotten – take comfort in the fact that we love you. The funding for this research was by a grant from KKL-JNF and another one from Yad-Hanadiv.

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