What is Eelgrass and why is it so important?

Eelgrass is a vital component of coastal ecosystems. It provides habitat, food, shelter and buffering from shoreline erosion, and contributes greatly to the overall health and functioning of marine environments.

Understanding the dynamics of eelgrass meadows and the benefits they provide, is key to understanding why and how to preserve this important seagrass species, and marine biodiversity overall.

Eelgrass (Zostera marina) is a marine true plant which relies on both vegetative (rhizome elongation where shoots grow up along the rhizome and expand the area of the eelgrass bed), and reproductive shoots that break off to float in currents until landing in suitable areas, and sexual (seed) propagation for maintaining existing beds and colonising new areas. It has high colonization potential for new habitats due to its dispersal mechanisms. Long-distance dispersal by both reproductive shoots and seeds are facilitated by tidal currents and wind influences.

Eelgrass provides critical habitat and refuge from physical and biological threats and disturbances, increases the amount (density) and variety (richness) of marine species.

Zostera marina is an engineering species capable of modifying benthic habitat structurally and metabolically. It increases the structural complexity of the substrate and supports habitat for a variety of species, thereby stimulating biodiversity. Meadows are highly productive and hence support secondary production which has a major effect on nutrient and carbon cycling.

Eelgrass nutrient cycling is critical for ecosystem resilience and biodiversity. Nutrient cycling refers to the way nutrients move from living organisms back to the environment, and vice versa. For example, the accumulation of organic matter on the sea floor then decomposition of matter into the substrate or by organisms utilizing the organic matter for food.

Eelgrass is also an important refuge and feeding area for juvenile salmonids as they travel from fresh water streams to the ocean stages of their lives. Many species rely on the out migrating salmon smolts as food. (Blue Herons, merganser and other diving birds, many larger fish species.) Crabs flourish within and near the eelgrass beds, and squid spawn within the protection of the meadows.

Eelgrass is also a keystone component of nearshore and subtidal ecosystem function, providing numerous ecosystem services such as sediment stabilisation, pollutant filtration, carbon storage, and release of oxygen through photosynthesis.

Eelgrass has always been important to coastal First Nations. Traditionally, rhizome roots and stems were dried and used as a supplementary food source. It was also used in baskets for weaving, and for mattress or pillow stuffings. The meadows themselves serve as a prime habitat for many foods that were a mainstay to the coastal Nations. Many fish and crustaceans that were food for the Nations lingered in the eelgrass to feed and to find safe harbour, making it an easily accessible food source.

Threats to Eelgrass

There are a number of factors which threaten eelgrass health, including low light conditions, increased nitrogen from vegetative decomposition, pollution from point sources, phosphorus loading from non point-sources such as polluted rainwater, and sediment from logging in nearshore or streamside areas which flow into the sea. Multiple studies from around the world have shown that, worldwide, seagrasses are declining at a rate of 7% per year. Eelgrass meadows in the Pacific Northwest have experienced significant declines over the past several decades. Some studies suggest that eelgrass beds have decreased by as much as 30-50% in certain areas over the last century.

The decline of eelgrass is caused by a number of factors, including human uses, disease, and extreme climatological events. Understanding and addressing these threats is key to curbing the loss of and restoring eelgrass meadows.

Human Uses:

Overwater structures, such as piers, docks and aquaculture infrastructure, can impact eelgrass health through shading. Shading is a major concern as it reduces the amount of light available for photosynthesis, leading to decreased growth and productivity, and eventually death. Shading can also lead to increased macroalgal growth, further reducing eelgrass biomass through shading effects. Shading by large masses of algae in the water has also been found to be detrimental to new eelgrass shoots. Lower photosynthetic rates caused by shading can lead to less and smaller eelgrass shoots.

Anchoring/ buoys which are dropped near or on eelgrass meadows can cause significant harm through physical damage, disruption of eelgrass reproduction, and sedimentation and alteration of the ocean floor. Physical damage can occur through the direct application of anchors, often with attached chains for weight, which drag across the seafloor and scour, uprooting eelgrass plants. The alteration of the seafloor from scouring can create physical barriers that impede growth and expansion of eelgrass beds. They can also stir up the seafloor causing sediment which smothers eelgrass leaves, reduces light, and inhibits growth.

Water Quality contributes to eelgrass losses by means of excess nutrients, and high inorganic nitrogen content. There are several ways water quality can affect eelgrass growth, such as introduced toxins, nutrient pollution, and over sedimentation.

Chemical pollutants such as heavy metals, pesticides, and petroleum hydrocarbons can accumulate in eelgrass tissues and sediments, potentially causing toxicity and impairing the physiological processes of eelgrass. Chronic exposure to pollutants can weaken eelgrass beds and make them more vulnerable to other stressors.

Excess nutrients often from agricultural runoff, sewage discharge, or stormwater runoff, can lead to eutrophication. Eutrophication promotes the growth of algae, which can shade out eelgrass and compete with it for light, space, and nutrients. This reduces the availability of light for eelgrass photosynthesis, hindering its growth and survival.

Sedimentation can occur in eelgrass beds from poor land management practices, such as deforestation, urbanisation, and construction activities. These events can increase sediment runoff into coastal waters. Excessive sedimentation can smother eelgrass leaves, reducing light penetration and inhibiting photosynthesis. It can also bury eelgrass shoots and rhizomes, preventing their growth and reproduction.

Log booms may also effect eelgrass.

Climate Change

Ocean warming, extreme climate events and disease are also large factors contributing to eelgrass mortality. Temperature and salinity changes caused largely by climate change can adversely affect the growth of eelgrass meadows.

Changes in ocean patterns are of concern for eelgrass beds.Rising sea levels can increase vulnerability to erosion and wave action in areas that are limited by anthropomorphic influences, such as hardening by sea walls or rip rap. Similarly, altered storm patterns leading to greater intensity and frequency of storms can cause physical damage to eelgrass beds through wave action, excessive sedimentation, and habitat alteration.

Changes in water temperature and salinity, often associated with climate change or freshwater inputs from rivers and streams, can stress eelgrass populations. Eelgrass is sensitive to fluctuations in environmental conditions, and extreme temperatures or salinity levels can negatively impact its growth, reproduction, and overall health. Climate change has been seen to cause changes in precipitation patterns, leading to alterations in freshwater inputs into the ocean. This can increase nutrients in coastal ecosystems and potentially affect eelgrass health. Increased nutrients and water temperatures may increase the abundance of ephemeral algal mats due to eutrophication and increased water temperatures have been linked to the decline of eelgrass beds during the summer, contributing to trophic cascades that promote algal growth and hinder eelgrass recovery.

Ocean acidification – referring to increased levels of carbon dioxide (CO2) in the atmosphere from the burning of fossil fuels – can impair the ability of eelgrass to absorb calcium carbonate, a vital component for the growth of its leaves and rhizomes. This can weaken eelgrass beds and make them more susceptible to physical disturbances and erosion.

Microplastics also pose a threat to eelgrass health, through both physical and chemical impacts. Microplastics can accumulate on the surface of the ocean and impede growth by blocking light and altering the composition of the ocean floor in texture, composition, and porosity. Microplastics can also get entangled with eelgrass rhizomes (underground stems) and prevent the expansion of eelgrass beds. Chemically, microplastics can release chemical pollutants, such as heavy metals and organic contaminants, which can interfere with reproduction when eelgrass takes up the contaminants and can have toxic effects. 

Mapping Eelgrass - Why does it Matter?

Eelgrass mapping plays a vital role in conservation, management, and research efforts related to eelgrass ecosystems. It provides valuable data for monitoring changes in eelgrass distribution, assessing the impacts of environmental stressors, and guiding conservation strategies to protect these critical marine habitats.

Accurate mapping of eelgrass meadows is essential for baseline data to support eelgrass conservation and management measures, providing valuable insights into the presence, density, and characteristics ( ie. form, substrate type) of eelgrass beds. It is crucial for mitigation of damage that may be caused by modifications and structures on the water, assessing the impacts of human disturbances on eelgrass distribution and density, as well as for studying the spatial and temporal patterns of eelgrass meadows.

Through mapping, we can assess sustainable eelgrass conservation and management plans which aid in habitat restoration and ecosystem management efforts for species which use eelgrass as habitats and shelters. The mapping of eelgrass habitats aids in larger scale mapping initiatives, such as establishing baselines for regional-scale monitoring and integrating satellite imagery with field survey data to improve map accuracy.

Mapping also contributes to understanding the ecological importance of eelgrass meadows as habitats and shelters for many ocean species, aiding in habitat restoration and ecosystem management efforts.

 

SCCA Eelgrass Mapping Program

The SCCA’s Freshwater & Marine liaison Dianne Sanford, has been working for decades to inform, educate, and engage the public on the importance of eelgrass, to ensure that our Sunshine Coast shorelines are mapped and monitored for the absence or presence of eelgrass. 

Dianne began mapping eelgrass beds in Gibsons in 2001 using the protocols developed by Cynthia Durance of Precision Identification - Methods for mapping and Monitoring Eelgrass. In ensuing years Dianne has worked with a range of groups. She leads the Friends of Forage Fish Citizen Science Group, collaborates closely with the SeaChange Marine Conservation Society, and leads projects for the SCCA. 

In 2018 Dianne mapped the main waters of Sechelt Inlet for the absence or presence of eelgrass. Porpoise Bay was mapped in 2019 and Roberts Creek in 2022-2023. We will be undertaking mapping in Narrows Inlet and Salmon Inlets in 2024-2025.

 

Need for Restoration

The need for restoration is evident with the decline of seagrasses worldwide by an average of 7% per year.  Restoring eelgrass sites can be challenging due to a range of factors, cost being a major factor, as monitoring would ideally take place for up to five years post transplanting. Dredging, oil and chemical spills, and structures built within or close to ideal eelgrass habitat necessitates restoration activity as well.  Challenges in eelgrass restoration include local climate regime shifts that prevent natural recovery and restoration of lost eelgrass beds, 

The cost of restoring eelgrass beds in subtidal environments and the difficulty in protecting transplants from bioturbating (disturbing sediment) organisms, such as crabs, clams, and shrimp, have led to the development of new methods that do not require SCUBA. Green crabs, and other introduced invasive species, are a great risk to eelgrass, often digging up (bioturbating) newly planted areas, and damaging new shoots on established beds. 

Despite the challenges, efforts to restore eelgrass habitats continue, with initiatives focusing on site suitability assessments, conservation mooring systems, and predicting optimal restoration sites using species distribution modelling through mapping. Current research efforts aim to address the complexities associated with eelgrass restoration and enhance the success of restoration projects.

Eelgrass & Carbon Sequestration

Eelgrass plays a significant role in carbon storage in marine ecosystems. Studies have shown that eelgrass meadows, such as those found on the Sunshine Coast , are important for blue carbon storage.

Eelgrass beds help trap and accumulate organic matter from the ocean, and can contribute to the processing and storage of nutrients and carbon associated with organic matter. Since eelgrass is a photosynthetic plant, it also stores carbon and releases oxygen, helping with water filtering and oxygen production. Research has shown that eelgrass meadows can store a lot of carbon and nutrients.

Eelgrass has been identified as a valuable habitat for juvenile Pacific salmon, where juvenile salmon feed on invertebrates which are attracted to eelgrass meadows. When juvenile salmon leave the stream to enter the ocean, eelgrass meadows provide a safe shelter for the fish to become accustomed to their new saltwater home and prepare for their open ocean phase. 

Overall, eelgrass plays a crucial role in carbon sequestration by storing organic matter in sediments, contributing to the cycling of nutrients, and providing habitat, nurseries, and refuge for other marine organisms. The preservation and restoration of eelgrass meadows are essential for maintaining blue carbon stocks and supporting the ecological balance of coastal ecosystems.