Threats to Eelgrass Habitat

Eelgrass is influenced by both natural and anthropogenic (man-made) factors. Boating activities, fishing practices, habitat loss, coastal development, and global climate change may individually or together negatively impact this habitat. In extreme cases, eelgrass can disappear quickly during a particularly rainy season with severe storms. On the other hand, eelgrass habitat may decline slowly (over years or decades) as a result of the regular discharge of nutrients, sediments, and contaminants that accumulate from urban and stormwater runoff entering rivers, streams, and estuaries and resulting in continued poor water quality and clarity.

: Dredging barge moving through Newport Bay

It is because eelgrass grows in shallow subtidal zones of coastal areas that it is available for use by juvenile fish, crabs, waterfowl, and other fisheries resources. Unfortunately, because it grows at this elevation range, eelgrass is particularly vulnerable to land-based activities, such as coastal development, boating, aquaculture and fishing, and urban runoff. Eelgrass is resilient and typically recovers from natural disturbance and has for millions of years. However, because of the accelerated growth of the human population, eelgrass and other seagrasses may not be able to adapt quickly enough to rapidly changing environmental conditions, particularly climate change. Humans have caused the most dramatic and lasting changes to eelgrass habitat.

Physical damage

Scarring from boat propellers, anchors, and mooring chains, and activities that modify intertidal and subtidal environments (e.g., dredging and aquaculture activities) can diminish eelgrass populations.

Loss of Light

1. Sedimentation

Eelgrass requires more light than most marine plants (approximately 20-25% of the total amount of sunlight hitting the surface of the water). Small changes in light availability can greatly influence the quality of eelgrass habitat. Factors that affect turbidity (water clarity or the amount of suspended particles in the water) or the amount of light reaching eelgrass beds include:

Natural Factors

Storms – erodes land sediment and churns up bottom sediment in the water
Tidal flushing – the regular movement of water, nutrients, sediments, and other suspended particles with the tides. Low flushing rates may result in the suspension of particles in the water for sustained periods of time and constantly reducing light penetration.

Anthropogenic Factors

Discharge of excess sediments from urban areas
Coastal development – may increase the rate of natural sediment erosion on land entering streams, rivers, and estuaries
Dredging activities – removal of underwater sediment and movement of large barges through channels, stirring up sediment
Shading by docks and boats
Eutrophication

2. Eutrophication

Eutrophication, or “nutrient pollution”, is a process where bodies of water receive increased amounts of nutrients (e.g., phosphates and nitrates) that stimulate excessive plant growth, especially algae. Such nutrients are naturally occurring and essential to life in certain amounts but, in excess, may negatively impact a body of water, resulting in a decrease in overall water quality, oxygen in the water, and marine species populations.

: Source: James Douglass (click photo to link to his blog!)

Human activities tend to accelerate the rate at which nutrients, both organic and inorganic, enter ecosystems. High amounts of chemical nutrients from urban, industrial and business areas are washed through storm drains by overwatering of lawns, driveways, golf courses etc. Rain events also move nutrients, litter, and other pollutants through off streets, parking lots, and driveways into storm drains. This water accumulates and eventually drains straight to a common body of water, such as the ocean.

In coastal marine environments, for example, algal growth is limited by the amount of nutrients in the water. For example, when nitrate levels increase – often called “nitrogen loading” or “nitrogen enrichment” — there is an increase in the growth and abundance of algae. The excessive growth of algae is often referred to as an “algal bloom” or “red tide”. The algae may out-compete other aquatic plant species such as eelgrass for nutrients, space, and sunlight.

When the algal blooms eventually die, they sink to the bottom where the organic material is consumed by decomposers. Decomposers consume oxygen in the process, resulting in a hypoxic (low oxygen) state, which causes increased fish and other species mortality.

Humans are impacted as well because eutrophication decreases the resource value of rivers, lakes, and estuaries. Recreational activities such as fishing, swimming, boating as well as aesthetic enjoyment are all affected . Poor water quality and the potential threats of aquatic life toxicity (e.g., the increase in harmful toxins in shellfish and other fish and invertebrate species that humans consume) may lead to bacterial diseases or poisoning in humans.Eelgrass is directly affected by eutrophication as large algal blooms on the surface of the water block the amount of sunlight reaching eelgrass beds. Algae may also grow on the eelgrass blades, further preventing the plant from performing photosynthesis and delivering oxygen to the surrounding water and sediment. One of the largest and most significant examples of the effects of eutrophication resulting from land-based human activities is the dead zone in the Gulf of Mexico, which stretches over 22,126 square kilometers (8,543 mi²). Nutrient inputs, mainly from the large amount of agricultural production along the Mississippi River, are the primary cause of this huge loss of life and habitat.

Wasting Disease

Not nearly as common or devastating on the west coast as on the east coast where, in 1931, it ravaged the eelgrass population by up to 90%, it still poses a significant potential threat to eelgrass beds in Orange County waters. Caused by a species of slime mold, Labyrinthula zosterae, the mold infects the host causing cell death and is spread from plant to plant by direct contact. The blade will appear to have brown blotches and streaks. Other threats may contribute to the effects of Wasting Disease by lowering the plants immune response, putting it at greater risk.

Predation and Bioturbation

Polychaetes (a class of worm in the phylum Annelida), green crabs, and sea urchins can damage eelgrass when they are present in high numbers due to over grazing. Furthermore, some species living underneath the soil such as crabs can cause damage to the plant by uprooting them.

Non Native Species Competition: A Case of Caulerpa

Caulerpa invading an eelgrass bed. Photo by Rachel Woodfield, Merkel & Associates

It is well known that invasive species are especially devastating to native ecosystems as they tend to have no natural predators and some are able to outcompete native species. The problem surrounding invasive species is that dredging and water quality problems can be reduced or even reversed whereas it is nearly impossible to remove an invasive species. Caulerpa taxifolia is a type of algae not native to California but has been found in certain waters along the Southern California coast since 2000. It readily adapts to certain environments, especially those waters in which eelgrass thrives, and competes with eelgrass for space, light, and nutrients. Without any natural predation and necessary plant species such as eelgrass to support life, Caulerpa taxifolia alters habitats and those species able to live within them, causing the ecosystem to become unbalanced. Losses in native eelgrass and algae would displace or kill populations of invertebrates, fish, and waterfowl dependent on such habitats. The state of California has taken up an active campaign to prevent this species from establishing itself in coastal waters

Habitat Loss

Eelgrass habitat is defined as a wetland area by both California and federal definitions and is protected by a no net loss wetlands policy. Since the 1850s, 90% of the California’s coastal wetland acreage has been destroyed, and the remaining 10% is continuously exposed to increasing sedimentation from eroding watersheds, raw sewage spills, and urban run-off pollutants. Additional causes include coastal land development, dredging, bulkhead, pier, and dock construction and maintenance, and extreme storm events. Loss of habitat may also increase habitat fragmentation, which increases the area between populations and may reduce genetic diversity as the transfer of genetic material becomes more and more limited. Genetic diversity is beneficial because it increases resistance to other stressors, including climate change.

Global Climate Change

Although seagrasses, including eelgrass, have experienced considerable environmental changes in sea level, carbon dioxide levels, and temperature throughout their 100 million years in existence, these changes had been gradual (Orth et al., 2006, A Global Crisis for Seagrass Ecosystems). Such environmental changes, including sea level rise, warmer temperatures, and extreme weather patterns and storm events, are occurring much more rapidly today because of increased human activity and we do not know how this will affect eelgrass populations as well as populations of numerous species around the world (Short and Neckles, 1999).