March 1 is World Seagrass Day, which celebrates the flowering plants that look like blades of grass waving in our oceans and in Puget Sound. as an opportunity “to promote and facilitate actions for the conservation of seagrasses in order to contribute to their health and development.”

, 天美影视传媒 professor of biology, studies the relationship between the environment and marine organisms, including eelgrass, the primary species of seagrass that resides in the waters in and around Washington.

In honor of World Seagrass Day, UW News asked Ruesink to explain what seagrass is and what makes the seagrasses in Washington unique.

What is seagrass and why is it important?

Jennifer Ruesink: Seagrasses are 鈥渓and plants鈥� that have moved into ocean habitats. They have roots, stems, leaves, flowers, fruits and seeds. There are only about 70 species of seagrasses, representing just 0.02% of all flowering plant species.

Seagrass matters to humans in many ways. It cycles nutrients and carbon, provides habitat for fish and decapods, and it anchors sediment in place, which contributes to shoreline stabilization. It鈥檚 a sentinel species for good water quality 鈥� in fact, impaired water quality from nutrient pollution, coastal building and erosion are its biggest threats.

Beyond these utilitarian values, seagrass is 鈥渨onderful鈥� in the truest sense of that word 鈥� the way it grows, moves and shapes the environment provides a continual source of wonder.

What makes seagrass different from seaweed and other ocean plants?

JR: In addition to seagrasses, there are many other photosynthetic organisms that live in the ocean. Collectively they provide half of our global oxygen. But the others are different from seagrasses: Seaweeds, also known as macroalgae, do not make roots or flowers. Tiny microalgae live on ocean surfaces, even on the seagrass leaves themselves. Other photosynthetic organisms, such as phytoplankton, drift as single cells or small colonies in the water.

Seagrasses are colloquially called 鈥済rasses鈥� because many have grass-like shapes with long strap-like leaves that grow from the base, and their stems move horizontally underground. From an evolutionary perspective, seagrasses do not group with the terrestrial grass family but instead have unique families or share relatives with freshwater plants.

What does seagrass look like in the ocean?

JR: If you think of a prairie on land, it is full of different plant species that grow to different heights, flower at different times, and extract light and nutrients with different efficiencies. Seagrass meadows are the prairies of the ocean, but they frequently consist of just one seagrass species. Because the number of seagrass species is so small, much of the dramatic variability occurs within single species, rather than across multiple species. Here in Washington we mostly have the same species 鈥� eelgrass, or Zostera marina 鈥� that鈥檚 found from 23-70 degrees north latitude on both sides of the Pacific and Atlantic Ocean.

Tell us about eelgrass in Washington.

JR: The remarkable thing is that there is so much of eelgrass variability present within our state. For example, some populations have shoots that replicate solely by branching, making genetic copies of themselves as they go. Other populations have shoots that never branch, but instead germinate, flower and die within a summer, overwintering as seeds. Shoots in Washington range from a diminutive 0.7 feet to nearly 6.5 feet long.

It makes sense that this diversity within a species is a product of evolving in the varied environments of Washington鈥檚 vast and convoluted shoreline. We think this variability should confer resilience to change, but that鈥檚 an ongoing exploration.

Washington also has two seagrass species other than Zostera marina: Ruppia maritima, which is a fast-growing species characteristic of brackish channels in saltmarshes, and Nanozostera japonica, which was established in the state in the 1950s after being inadvertently introduced from Japan. You can find them all growing together in a few places.

How would you suggest that someone celebrate World Seagrass Day?

JR: There are plenty of public-access shores around Seattle 鈥� including Golden Gardens and the south side of Alki Point 鈥� where you can see eelgrass growing. At this time of year, you might see nearby. These small geese feed on eelgrass to fuel their migration. To see eelgrass, you need a low tide since it can鈥檛 handle staying out of the water very long. On World Seagrass Day, good low tides occur after dark 鈥� around 9 p.m. in the Seattle area. If you do find seagrass, you can take a picture and help data collection about its distribution by uploading your information to iNaturalist or .

Any time you鈥檙e at the beach, you might find eelgrass washed up on shore: Keep an eye out for the leaves 鈥� green, flexible rectangles 鈥� especially if they’re connected to chunky brown cylinders 鈥� the stems, or . Each node on the rhizome is the scar of a former leaf. This is fun to think about because it helps demonstrate the dynamic lifestyle of this plant: Each leaf lasts a couple of months before it鈥檚 left behind on the rhizome and decays. Meanwhile the production of a new leaf every couple of weeks both turns over the biomass and moves the shoot along the sediment.

The point of 鈥淲orld Days鈥� in general is to raise awareness about global issues of concern and to celebrate accomplishments: If you pass the news about Washington eelgrass along to someone else, that鈥檚 a celebration!

For more information, contact Ruesink at ruesink@uw.edu.

Learn more about Jennifer Ruesink’s eelgrass research

Ruesink’s recent research on eelgrass delves into understanding the mechanism behind eelgrass flowering:

• (collaboration with Takato Imaizumi, UW biology professor)
• (collaboration with Kerry Naish, UW professor in the School of Aquatic and Fishery Sciences, and Takato Imaizumi, UW biology professor)
• (collaboration with Kerry Naish, UW professor in the School of Aquatic and Fishery Sciences),

By area, tidal flats make up more than 50 percent of Willapa Bay in southwest Washington state, making this more than 142-square-mile estuary an ideal location for oyster farming. On some parts of these flats, oysters grow well, filling their shells with delicacies for discerning diners. But according to experienced oyster farmers, oysters raised in other parts of Willapa Bay don’t yield as much meat.

Now, scientists may have an explanation for this variability. In a published online July 26 in the journal , researchers at the 天美影视传媒 and the University of Strathclyde report that the water washing over the Willapa Bay tidal flats during high tides is largely the same water that washed over the flats during the previous high tide. This “old” water has not been mixed in with “new” water from deeper parts of the bay or the open Pacific Ocean, and has different chemical and biological properties, such as lower levels of food for creatures within the tide flats.

The team, led by , a UW professor of biology, employed oceanographic modeling and water quality readings to show that high-tide water flowing over the Willapa Bay flats can take as many as four tidal cycles 鈥� or about two days 鈥� before it is fully replaced by “new” water. Through field experiments measuring oyster growth, they found that this slow turnover has consequences for the creatures that call Willapa Bay home.

Their findings overturn a prior assumption about tides.

“Previously, there had been this belief that when water drains off of tide flats or out of a bay, currents and wind mix that water up,” said lead author , a UW instructor in the College of the Environment who conducted this study as a doctoral student in the UW Department of Biology. “It turns out that this is not necessarily true. It takes multiple tidal cycles for this mixing to occur.”

To determine water turnover rates in Willapa Bay, Ruesink and Wheat partnered with , an oceanographer at the University of Strathclyde in Glasgow, who modeled water “residence times” and circulation in Willapa Bay using data on the bay’s depth profile, the rivers that feed into it and its outlet to the Pacific Ocean. The model predicted that high-tide waters over the flats have residence times ranging from zero to four tidal cycles 鈥� depending on location in the bay 鈥� before it is fully replaced by “new” water from deeper channels. On stretches of tidal flats more than one kilometer long, generally water over near-shore flats had longer residence times than flats farther from shore.

“It’s a bit of a paradox: We can walk across those flats at low water, so how can water stay there for more than a couple of hours between successive low tides?” said Ruesink. “Now we’ve discovered a new explanation for the quality of oyster beds, which doesn鈥檛 depend on how much time they spend under water, but rather on the history of the water that reaches them.”

The team collected data directly from the bay. They used a network of sensors 鈥� some free-floating, others at fixed positions 鈥� to collect information such as water depth, temperature, salinity and the amount of chlorophyll present. All of those water properties varied throughout the bay. Temperature varied primarily according to the tidal cycle, while variations in salinity and chlorophyll throughout Willapa Bay were more consistent with their model of water residence times. One of the key differences between “old” and “new” water is that “old” water contains less chlorophyll and is usually lower in salinity.

The team also measured oyster growth on flats in sections of the bay with “old” and “new” water. In all parts of Willapa Bay, oysters grew to approximately the same shell size. But oysters grown farther from the main channel of the bay 鈥� regions with higher levels of “old” water at high tide 鈥� had trouble filling those shells with the meaty morsel that people eat. Oysters grown on flats just half a kilometer from the main channel showed a 25 percent drop in dry tissue weight per shell height compared to oysters grown closer to the channel, where “new” water arrives faster.

“Scientists have known for a long time that water residence times increase as you go deeper into bays,” said Ruesink. “But this is the first time that both a model and field data show ‘old’ water close to shore across tidal flats.”

These findings may explain why some parts of Willapa Bay 鈥� known as “fattening grounds” by oyster farmers 鈥� are better than others for generating large-biomass oysters, according to Wheat. The study also has far-reaching implications for how scientists understand the health and well-being of all creatures in tidal ecosystems like Willapa Bay. The lower levels of chlorophyll in “old” water, for example, indicate that this water contains fewer particles for filter-feeding creatures along the flats, likely because food was already taken out of the water column during previous passes over the flats. Creatures in these parts of Willapa Bay must wait longer for the bounty brought by “new” water.

Future studies would have to look at additional consequences of these longer water turnover rates, such as how pollutants are diluted and cleared from the water column, said Wheat. The team’s findings add a layer of complexity to tidal environments and show definitively what experienced oyster farmers in Willapa Bay already knew: Not all tidal flats are created equal.

The research was funded by a National Oceanic and Atmospheric Administration grant to Washington Sea Grant at the 天美影视传媒.

###

For more information, contact Ruesink at ruesink@uw.edu.

Grant number: NA07OAR4170007