It is on those calm, blue-sky days, when the surface of the water is like a smooth sheet of glass, that new worlds can be revealed. At these times, we marvel at the sheer vividness of the river bed or the edge of the sea shore. We are in awe of the colourful mosaic of rocks and plants that seemed to have just appeared. But not so.
Most of the time, we walk alongside any body of water with barely any thought towards what lies below the surface. It is at these moments when we spy the secrets lying below the surface and we are swept up into a majestic world filled with fronds, some long and wide, others slender and narrow all hypnotically swaying in the ebb and flow of the water. We have entered a whole new world.
There are a host of marine organisms, seaweeds or flowering plants (angiosperm), that grow along the bottom. Still kelp, a type of algae and eelgrass, a flowering plant are particularly noteworthy for the important roles that they play in the shallow water ecosystems in which they exist. Eelgrass also just so happens to be the only seagrass in Atlantic Canada. Both eelgrass and kelp quietly play a starring role in the life of Placentia bay.
Nature of Eelgrass
Eelgrass meadows, as they are known, are comprised of countless blades of these flowering plants. At about a ¼ inch in width and reaching lengths of around three feet, they generally remain completely submerged. Although, the depth at which eelgrass grows is ultimately decided by the light that is able to reach the bottom. When the tide is low, they are very distinctive with their vibrant bright green blades against the surface of the water. These plants also fare well in clear water and actually have a high minimum light requirement for their survival. The eelgrass meadows also have a long lifetime which can extend to decades and sometimes even for millennia.
Eelgrass is fairly robust, no doubt a quality that has helped this willowy plant to survive for so long. And while eelgrass possess an optimal salinity range of 20 to 26 ppt for photosynthesis, the plant is also tolerant of lower salinity levels of 5 to 35 ppt and even freshwater for a short time. However, eelgrass is unable to survive in oxygen free conditions (anoxic) or ones that are rich in mineral and organic matter, commonly referred to as eutrophic conditions. Eelgrass is equally versatile with regard to temperatures and can grow in a wide range of temperatures, from 10-25 °C and can withstand more extreme temperatures, from freezing to 35 °C. Eelgrass is a powerhouse of a plant and one that is able to share its strength with other species residing in the shallows of either rivers or other water bodies.
The blades lie at the surface when the tide has gone out. Their roots or rhizomes secure them firmly to the sandy, muddy or cobble-stoned bottoms where they reside. Thus secured to the surface, the blades of grass bend gracefully in the current. Although, despite the pleasant and placid appearance of the grass swaying in the flow, there is a functional element to the actions of the eelgrass.
Collectively, the myriad blades of eelgrass are able to stabilise the sediments and buffer shorelines. Once the eelgrass reaches a certain density, the assembly of grass blades is capable of moderating the wave energy and modifying the level of turbulence in the water. When looking at the surface of standing water, it is astonishing to see the effect of the eelgrass.
While the wind may be jostling the water elsewhere, the eelgrass acts to still the waves. By doing so, any of the sediment being held in the flowing water falls out of suspension, permitting more light to reach the eelgrass and be photosynthesised.
Photosynthesis
And photosynthesis lies at the heart of how and why eelgrass is able to perform the central function it does in the ecosystem. Eelgrass habitats actually rank amongst the most productive ecosystems on earth, a position that has it rubbing shoulders with the likes of the tropical rainforests, coral reefs, and wetlands of our planet. This is no small achievement. The eelgrass function at a high level of production, primarily due to the steady ongoing turnover of eelgrass leaves, as well as the numerous algae that grow on the leaf surfaces of the eelgrass. The algae provide one more element within the eelgrass meadows that can support fauna seeking food and protection. Linked to its high level of production, eelgrass also releases oxygen into the water column, which is a hypothetical cylinder of water stretching from the surface to the bottom of a water body. While doing so, eelgrass also performs one other function by filtering the water in the water column. These small blades of grass have a heavy workload.
As Part of the Food Web
The organisms that grow on the eelgrass are a primary food source for various species of invertebrates. Given its high level of production, eelgrass plays a pivotal role in the food web in which it exists. A part of the food web includes the numerous fish and invertebrates such as crabs of various kinds that are nestled in the dense meadow of grass. For these creatures, the eelgrass serves as a source of food, as well as protection. Thus, it is no surprise that the eelgrass is a favoured location for invertebrates in particular. Eelgrass is also an ideal nursery habitat for juvenile fish, in particular for Atlantic cod (Gadus morhua). So, who could ask for anything more? There is abundant food for the growing fish and the ample cover of the blades of eelgrass ensures that it is a place of safety. However, it is not all that safe for every creature seeking cover. Apparently while the level of predation diminishes substantially within the eelgrass bed, it actually increases along the edge. It makes sense that any organism seeking protection within the eelgrass will fare better in larger continuous expanses of eelgrass than in smaller patches. This is simply because there are far more edges in the latter than in the former. Life in the bay can be touch and go.
The high productivity of eelgrass also means the speed with which the it replaces itself leaves a lot of eelgrass fragments flowing in the current. Nothing goes to waste as this material become a source of food for a host of bacteria, fungi, and protozoans (largely single-celled organisms). Given the current, this material can be transported great distances where the eelgrass debris can feed other species.
With all these glowing words, eelgrass is like a poster child for many scientists studying this vital element of the various ecosystems in which it resides. Accordingly, eelgrass is considered to be a keystone species wherein, much as the name implies, without the eelgrass, the structure will weaken and begin to crumble.
Unfortunately, in recent years, this is exactly what scientists have been observing. During the 1930s, the eelgrass population faced a microscopic, yet grievous threat for eelgrass on both sides of the Atlantic. At this time, the eelgrass was forced to contend with something aptly known as a “wasting disease,” the result of the persistent bacteria, Labyrinthula. zosterae. The disease spreads either through direct contact or detached parts that drift in the current, an all too common phenomenon. The disease is merciless. Necrotic lesions form creating a type of slime mould that darkens the blades of grass. Eventually the mould destroys the cells responsible for photosynthesis. And without the life-giving energy of the sun, the eelgrass simply dies. Ultimately, at the time, the “wasting disease” resulted in a loss of roughly 90% of North Atlantic eelgrass, something that required several decades for the eelgrass to recover. The disease has re-appeared since the 1930s, albeit with less of a lasting impact. However, the threat remains.
Challenges to Eelgrass
This very real threat is joined by the all-too-common perils of the sea—human-related activity. Around the world, the impact of our behaviour on eelgrass has been felt by such things as human settlement. This has led to a general decline in the distribution of the eelgrass beds globally.
While human-related activities carry their challenges, yet another substantial threat for eelgrass has been the arrival of a new species—the European green crab. This species made its first appearance in Canadian waters in 1951 in New Brunswick. Since this time, its made headway into other waters of the Maritimes. Eventually, it did find its way to Newfoundland in 2007.
Source: www.nlnature.com
The European green crab can be found primarily in shallow waters such as saltwater marches, sandy beaches or rocky coasts. It is less than desirable to have around, as it is very determined in its efforts to predate and feed and will guard its territory vigorously. It is a survivor. In fact, it has an edge in the fact it can survive out of water for several days. What’s more, they can also tolerate a wide range of temperatures and salinity. Together, these qualities make the European Green crab a difficult species with which to deal.
As a voracious predator, it is actually able to change the balance between species in an ecosystem and impact their diversity. In many cases, it can out-compete the native crabs for food. In terms of the eelgrass, their impact is generally happenstance, as they tend to uproot or cut the eelgrass while digging for prey or making burrows. However, given the central role of eelgrass in the ecological habitat, this effect is less than desirable. Hence, efforts are underway to study and control the presence of European Green crabs in the waters, specifically Placentia Bay, where they were first noted.
Looking Ahead
Beginning in 2018, a five year project included a range of specialists, including research scientists, graduate students, staff from ACAP Humber Arm (a not-for-profit organization serving the Bay of Islands and Humber Valley coastal regions of western Newfoundland which involved in habitat protection and restoration). The goals of the project are to restore eelgrass at particular sites in Placentia Bay.
Eelgrass quietly features in our lives, playing a central role in helping to maintain the health and resilience of our oceans. As part of the underwater forest that is majestically arrayed along our shores, we may not always notice them. But we will know if they are gone.
