Seagrass Meadows and Their Role in Healthy Marine Ecosystems

Seagrass meadows are found along soft-bottomed, shallow, marine coastlines of every continent except Antartica. Their abundance and the important roles they play earn them the title of third most valuable ecosystem on the planet after estuaries and wetlands. These extensive meadows are made up of a group of flowering plants that are unique in their ability to thrive submerged in salty seawater. Tossed about by the tides, they feed and harbor an incredibly diverse world of marine life and help protect neighboring ecosystems by stabilizing sediments and mitigating pollution.

Seagrasses are often confused with seaweed, but they are very different organisms. Seaweed is algae. Seagrasses are plants that at one point in their evolutionary history lived on land but then retreated back into the waters of their ancient ancestors. They are rooted in the sediment of the sea floor and, depending on the species, can reproduce both sexually (submerged flowers are pollinated with the help of moving water) and/or asexually (via rhizomes). Although many of them have a grass-like appearance, none of them are in the grass family (Poaceae); instead, the approximately 72 different species belong to one of four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae, or Cymodoceaceae).

Seagrass meadow in Wakaya, Fiji (photo credit: wikimedia commons)

Seagrass meadows can be composed of a single seagrass species or multiple species, with some meadows consisting of a dozen species or more. Seagrasses depend on light for photosynthesis, so they generally occur in shallow areas. How far seagrass meadows extend out into the ocean depends on light availability and the shade tolerance of the seagrass species. Their presence at the shoreline is limited naturally by how exposed they become at low tide, the frequency and strength of waves and associated turbidity, and low salinity from incoming fresh water.

Seagrass meadows benefit life on earth in many ways. As ecosystem engineers they create habitat and produce food for countless species, sequester a remarkable amount of carbon, and help maintain the health of neighboring estuaries, mangroves, coral reefs, and other ecosystems. They are home to commercial fisheries, which provide food for billions of people. Like many ecosystems on the planet, they are threatened by human activity. Pollution, development, recreation, and climate change jeopardize the health and existence of seagrass meadows. Thus, it is imperative that we learn as much as we can about them so that we are better equipped to protect them.

Turtle grass (Thalassia testudinum) growing in an estuary on the coast of San Salvador Island, Bahamas (photo credit: wikimedia commons)

In a report published in a February 2017 issue of Science, researchers examined the ability of seagrass meadows in Indonesia to remove microbial pathogens deposited into the sea via wastewater. When levels of the bacterial pathogen Enterococcus were compared between seagrass meadows and control sites, a three-fold difference was detected, with the seagrass meadows harboring the lowest levels. When other potential disease-causing bacteria were considered, the researchers found that “the relative abundance of bacterial pathogens in seawater” was 50% lower in both the intertidal flat and the coral reefs found within and adjacent to the seagrass meadows compared to control sites.

This has implications for the health of both humans and coral reefs, the latter of which face many threats including bacterial diseases. Two important coral reef diseases, white syndrome and black band disease, as well as signs of mortality associated with bleaching and sediment deposition “were significantly less on reefs adjacent to seagrass meadows compared to paired reefs,” according to the report.

Cushion sea star in seagrass meadow (photo credit: wikimedia commons)

The researchers note that “seagrasses are valued for nutrient cycling, sediment stabilization, reducing the effects of carbon dioxide elevation, and providing nursery habitat for fisheries.” The results of this study demonstrate the potential for seagrass meadows to “significantly reduce bacterial loads,” benefiting “both humans and other organisms in the environment.” Yet another reason to care about and conserve this vital ecosystem.

Additional Resources on Seagrass and Seagrass Conservation:

And if that’s not enough, check out this fun YouTube video:

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When Sunflowers Follow the Sun

Tropisms are widely studied biological phenomena that involve the growth of an organism in response to environmental stimuli. Phototropism is the growth and development of plants in response to light. Heliotropism, a specific form of phototropism, describes growth in response to the sun. Discussions of heliotropism frequently include sunflowers and their ability to “track the sun.” This conjures up images of a field of sunflowers in full bloom following the sun across the sky. However cool this might sound, it simply doesn’t happen. Young sunflowers, before they bloom, track the sun. At maturity and in bloom, the plants hold still.

What is happening in these plants is still pretty cool though, and a report published in an August 2016 issue of Science sheds some light on the heliotropic movements of young sunflowers. They begin the morning facing east. As the sun progresses across the sky, the plants follow, ending the evening facing west. Over night, they reorient themselves to face east again. As they reach maturity, this movement slows, and most of the flowers bloom facing east. Over a series of experiments, researchers were able to determine the cellular and genetic mechanisms involved in this spectacular instance of solar tracking.

Helianthus annuus (common sunflower) is a native of North America, sharing this distinction with dozens of other members of this recognizable genus. It is commonly cultivated for its edible seeds (and the oil produced from them) as well as for its ornamental value. It is a highly variable species and hybridizes readily. Wild populations often cross with cultivated ones, and in many instances the common sunflower is considered a pesky weed. Whether crop, wildflower, or weed, its phototropic movements are easy to detect, making it an excellent subject of study.

Researchers began by tying plants to stakes so that they couldn’t move. Other plants were grown in pots and turned to face west in the morning. The growth of these plants was significantly stunted compared to plants that were not manipulated in these ways, suggesting that solar tracking promotes growth.

The researchers wondered if a circadian system was involved in the movements, and so they took sunflowers that had been growing in pots in a field and placed them indoors beneath a fixed overhead light source. For several days, the plants continued their east to west and back again movements. Over time, the movements became less detectable. This and other experiments led the researchers to conclude that a “circadian clock guides solar tracking in sunflowers.”

Another series of experiments helped the researchers determine what was happening at a cellular level that was causing the eastern side of the stem to grow during the day and the western side to grow during the night. Gene expression and growth hormone levels differed on either side of the stem depending on what time of day it was. In an online article published by University of California Berkeley, Andy Fell summarizes the findings: “[T]here appear to be two growth mechanisms at work in the sunflower stem. The first sets a basic rate of growth for the plant, based on available light. The second, controlled by the circadian clock and influenced by the direction of light, causes the stem to grow more on one side than another, and therefore sway east to west during the day.”

The researchers observed that as the plants reach maturity, they move towards the west less and less. This results in most of the flowers opening in an eastward facing direction. This led them to ask if this behavior offers any sort of ecological advantage. Because flowers are warmer when they are facing the sun, they wondered if they might see an increase in pollinator visits during morning hours on flowers facing east versus those facing west. Indeed, they did: “pollinators visited east-facing heads fivefold more often than west-facing heads.” When west-facing flowers where warmed with a heater in the morning, they received more pollinator visits than west-facing flowers that were not artificially warmed, “albeit [still] fewer than east-facing flowers.” However, increased pollinator visits may be only part of the story, so further investigations are necessary.

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I’m writing a book about weeds, and you can help. For more information, check out my Weeds Poll.

Concluding the Summer of Weeds

“Most weeds suffer from a bad rap. Quite a few of the weeds in your garden are probably edible or even medicinal. Some invasive plants, including horsetail and nettle, are rich in minerals and can be harvested and used as fertilizer teas. Weeds with deep taproots, such as dandelions, cultivate the soil and pull minerals up to the surface. … Weeds are nature’s way to cover bare soil. After all, weeds prevent erosion by holding soil and minerals in place. Get to know the weeds in your area so you can put them to use for rather than against you.” — Gayla Trail, You Grow Girl

Great Piece of Turf by Albrecht Dürer (photo credit: wikimedia commons)

With summer drawing to a close, it is time to conclude the Summer of Weeds. That does not mean that my interest in weeds has waned, or that posts about weeds will cease. Quite the opposite, actually. I am just as fascinated, if not more so, with the topic of weeds as I was when this whole thing started. So, for better or worse, I will much have more to say on the subject.

In fact, I am writing a book. It is something I have been considering doing for a long time now. With so many of my thoughts focused on weeds lately, it is becoming easier to envision just what a book about weeds might look like. I want to tell the story of weeds from many different angles, highlighting both their positive and negative aspects. There is much we can learn from weeds, and not just how best to eliminate them. Regardless of how you feel about weeds, I hope that by learning their story we can all become better connected with the natural world, and perhaps more appreciative of things we casually dismiss as useless, less quick to jump to conclusions or render harsh judgments about things we don’t fully understand, and more inclined to investigate more deeply the stories about nature near and far.

Of course, I can’t do this all by myself. I will need your help. If you or someone you know works for or against weeds in any capacity, please put us in touch. I am interested in talking to weed scientists, invasive species biologists, agriculturists and horticulturists, edible weed enthusiasts, plant taxonomists, natural historians, urban ecologists, gardeners of all skill levels, and anyone else who has a strong opinion about or history of working with weeds. Please get in touch with me in one of several ways: contact page, Facebook, Twitter, Tumblr, or by commenting below.

Another way you can help is by answering the following poll. If there is more than one topic you feel particularly passionate about, feel free to answer the poll as many times as you would like; just wait 24 hours between each response. Thank you for your help! And I hope you have enjoyed the Summer of Weeds.

Quick Guide to the Summer of Weeds: