Seagrass meadows are among the most productive marine habitats. They filter nutrients, pathogens and sediments from the water, fix carbon and offer a home for thousands of animal and algae species, all around the globe. Yet in many regions, they are in decline. Can this troubling trend be stopped? A brief introduction
Seagrass meadows are among the most valuable and productive marine habitats. They protect coasts by slowing waves and holding the sandy ground together with their roots. They offer a source of shelter and food for thousands of juvenile fish, clams, crabs and other marine fauna, contributing to marine biodiversity in the process. Accordingly, they also help ensure that millions of people around the globe have enough fish and seafood to eat.
Moreover, seagrass meadows and potentially, their associated microbiome filter pollutants and pathogens out of seawater and fix large quantities of the greenhouse gas carbon dioxide. They chiefly store this carbon in their roots, making a fundamental contribution to limiting global warming. In light of the worsening climate and biodiversity crisis on our planet, healthy seagrass meadows are growing more important by the day. But which organism actually forms these impressive underwater meadows?
Seagrasses are often confused with large algae. But unlike sugar kelp (Laminaria saccharina) and other brown and red algae, seagrasses, with their long, grass-like leaves, are true plants and related to land-based flora like meadow grasses, palms and irises. Just like them, seagrasses produce flowers, seeds and a dense network of roots, which firmly anchors them in the ocean floor and supplies them with water and nutrients from the ground. In addition, just like trees, flowers and shrubs, they use sunlight for photosynthesis and produce energy by converting carbon dioxide and water into sugar and oxygen. Consequently, sufficient sunlight is just as important for them as it is for their light-hungry cousins on land.
However, there is one difference between seagrasses and land-based plants: seagrasses have no leaf pores that they can open and close to regulate water and gas exchange using the surface of their leaves. Instead, their leaves are covered by a thin membrane that allows gases and nutrients to move directly back and forth between the seawater and leaves.
By the way, beachcombers can easily tell washed-up seagrasses from algae by the smell: seagrasses don’t stink when broken down by microorganisms; the majority of algae do, most likely because they contain certain sulfurous components that produce foul-smelling gases when broken down.
Since seagrasses rely on photosynthesis, they need sufficient sunlight, carbon dioxide and nutrients to grow and form dense, healthy underwater meadows. But since sunlight can of course only come from above, the deeper you go, the less there is. If the seawater is also turbid because there are many algae or sediment particles in the water column, it allows even less light to reach the depths. As a result, seagrasses only grow in shallow coastal areas with clear sea- or brackish water – chiefly in lagoons, quiet bays, and river deltas around the globe.
Eelgrass (Zostera marina) for example needs access to at least 15 percent of the sunlight at the ocean’s surface. When this sunlight penetrates to the ocean floor, it accelerates the plant’s growth. In the Swedish Baltic Sea, for example, researchers have observed substantial plant growth at 18 percent or higher. In the shallowest and therefore sunniest seagrass areas of the Baltic, they have counted up to 2,000 plants per square meter of seafloor.
Seagrass meadows cover only 0.1 percent of the ocean floor worldwide but can be found in 159 countries and on six continents. As such, they are one of the world’s most widespread ecosystems. Their total area amounts to more than 317,000 square kilometers, nearly the size of Germany. In fact, some meadows are so vast that astronauts have photographed them from space with the aid of telephoto lenses (photo).
In the Baltic Sea, and on other coasts around the world, seagrass meadows can most often be found at depths of one to three meters. However, they can also be found much deeper: Caribbean seagrass (Halophila decipiens) has been spotted at depths of up to 58 meters. There are roughly 65 different seagrass species worldwide; experts can’t agree on the exact number. The species differ in terms of form, size and distribution areas. While only one species (Zostera marina, eelgrass) has been identified in the Baltic and only two in the North Sea, in many tropical waters of the Indian and Pacific Oceans, as many as 14 different seagrass species can grow together.
There are no seagrasses in the Antarctic Ocean, though they can be found in certain marginal zones of the Arctic Ocean. For example, they flourish on the northern and western coasts of Iceland, the western coast of Alaska and the northern coast of Norway.
©Levi Westerveld/GRID-Arendal (2019), Projektion: Goode Homolosine
Seagrass meadows grow both vertically and horizontally. Their leaves grow up toward the sunlight; their roots spread under the seafloor. In addition, the individual plants are connected to one another by rootlike, horizontally oriented shoots. This rhizome spreads laterally just under the ocean floor, penetrating new habitat. As a clearly visible result, new grass sprouts begin to grow outside the actual seagrass meadow. Experts refer to this as asexual reproduction.
But seagrasses also reproduce sexually, i.e., by forming flowers and seeds. The ocean itself takes care of the pollination; its currents carry pollen from the male to the female flowers, allowing egg and sperm cells to merge and combine. The eelgrass (Zostera marina) in the Baltic blossoms in late May or early June. Four to five weeks later, it releases its seeds, with a diameter of three to four millimeters, into the water column.
The seeds of many seagrasses don’t sink right away; they float in the water column and are often carried kilometers away by the current. Eventually they do settle on the ocean floor, where they begin to germinate, provided the conditions are favorable.
Over the last century, Earth lost at least 30 percent of its seagrass meadows; as a result, of the 65 known seagrass species, 22 are now in acute decline. In Europe alone, the total extent of seagrass meadows declined by nearly 35,700 hectares from 1869 to 2016 – an area roughly half the size of Hamburg. In the meantime, some European underwater meadows are recovering. Nevertheless, total global seagrass stocks are now decreasing by 7 percent a year.
The reasons for this seagrass die-out were and are chiefly a surplus of nutrients, the wide-spread urbanization and increasing human use of coastal areas, as well as temperature feedbacks produced by climate change.
Seagrass meadows are above all suffering from the rising input of nutrients in coastal waters around the globe. When heavy rains flush fertilizers from crop fields into rivers and carry this nutrient-rich water, together with untreated wastewater, refuse and other waste from cities and factories to the sea, the result is often extensive algal blooms in coastal waters. These algae carpets pose a direct threat to seagrasses, as they cut off the plants on the ocean floor from sunlight, their only energy source. As a result, the grasses wither and only survive in those areas where sufficient sunlight still reaches them. Whereas eelgrass (Zostera marina) used to grow down to twelve meters in the Baltic, today, due to the nutrient load in the water, it only grows down to six or eight meters.
The overfertilization of coastal waters and resultant algal growth also explain why e.g. in the first five months of 2021, more than 760 manatees (Trichechus manatus latirostris) starved in the lagoons of Florida, USA. The sea cows could no longer find enough food, since the seagrass meadows hadn’t been able to survive in the murky water.
Cartographer: Hisham Ashkar / GRID-Arendal (2020), Source: https://www.grida.no/resources/13583
Today, well over a billion people live in the direct vicinity of seagrass meadows; i.e., in a radius of 100 kilometers or less. This urbanization of coastal zones comes at a price, since wherever levees, industrial areas, houses, streets, harbors or aquaculture facilities are erected, the zone’s overall makeup is changed, and with it, the water quality and local conditions – often to such an extent that the seagrass meadows die off. Equally destructive for seagrass meadows: intensive boating and intensive fishing. Anchors and trawling nets can tear the delicate seagrasses from the ocean floor, often leaving behind nothing but bare, sandy ground.
Currently, only 26 percent of all known seagrass meadows enjoy protections, as they lie within clearly defined marine protected areas. For coral reefs and mangrove forests, which perform similarly important functions for humans and the ocean, the numbers are 40 and 43 percent, respectively. Accordingly, seagrass meadows are often referred to as a “forgotten habitat.”
Climate change is seriously affecting seagrasses, especially in the form of marine heat waves. If the water temperature is unusually high for prolonged periods, the grasses die off.Eelgrass (Zostera marina) for example can tolerate temperatures up to 26 degrees Celsius on a short-term basis. But if the water stays this warm for several days or grows even warmer, the meadows rapidly wither away.
The enormous damage that extreme heat can do became apparent ten years ago, in Australia’s Shark Bay. The bay, which is home to one of the largest and most diverse seagrass meadows in the world, lost more than 1,000 square kilometers of its original size in the summer of 2010/2011. The reason: water temperatures suddenly rose up to 4 degrees Celsius over their normal summer highs and didn’t meaningfully decline again for more than two months. Over the past four decades, on Sweden’s Skagerrak coast and in the Polish Baltic, a combination of changed local conditions, heat waves and disease have wiped out between 60 and 100 percent of native Zostera seagrass meadows.
Marine biologists often compare the loss of seagrasses with the loss of trees in a forest: it’s not just the plants themselves that are lost, but all of the vital functions and services that the seagrass meadow habitat provides for marine and coastal biotic communities and human beings alike.
Consequently, the meadows’ decline not only needs to be stopped; ideally, it needs to be reversed. The first prerequisite for doing so is to reduce major local stress factors like overfertilization, urbanization of the coasts, and fishing. If this difficult first step can be taken, then not only will the remaining seagrasses have a chance to recover and, over the course of decades, regain their original size. What’s more, we can then consider targeted efforts to actively restore the underwater meadows.
There have already been numerous attempts to transplant seagrass meadows. In fact, the first projects were in the 1950s and 1960s, though they attracted little interest. Shortly thereafter, when the global decline of seagrass meadows accelerated, more and more researchers and marine conservationists began asking themselves what measures could achieve new growth in the seagrass meadows.
The number of new restoration projects grew, with
more than two-thirds based in temperate and subtropical waters. Though half
focused on planting eelgrass (Zostera
marina), 25 other species were also included. In most cases, the project
organizers chose to use seagrass seedlings rather than to sow seeds. This could
be due to the fact that initial efforts to sow seeds had met with little success.
Today, there is a wealth of studies, best practice examples, and practical guidelines on the reintroduction of seagrass meadows. But transplantation attempts, especially for eelgrass (Zostera marina), often fail. These failures indicate that, at least for this species, no truly effective restoration method has been found, and our grasp of its preferred living conditions is still inadequate.
The goal of the joint German project SeaStore is to close these gaps in our knowledge. Through their research, the participating experts hope to lay the groundwork for the robust and scientifically sound reintroduction of eelgrass in the southern Baltic Sea. In the following, you’ll learn exactly how they plan to do so.