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July 2014
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The Decline of Seagrass Meadows

Zostera! Eelgrass, Zostera marina, is a flowering, marine vascular plant that remains submerged all the time. This is quite a feat for vascular flowering plants, and only a few dozen species world wide are capable of growing completely submerged in a marine environment. Eelgrass creates and extremely important habitat, its upright structures and complex root system create a 3-D living space for many different types of animals. It is (or was) the dominant habitat forming SAV (submerged aquatic vegetation) throughout much of the coastal waters in the northeastern United States. Unfortunately, for various reasons, eelgrass meadows have seen drastic declines, and in many locations eelgrass only exists in a mosaic of small patches. This is extremely bad news as many of the important, and formerly important, commercial and recreational fisheries of the northeast US are dependent on Zostera at some part of their life cycle as a nursery and foraging ground. Some of the species are finfish like tautog, bluefish, fluke, winter flounder, porgies, while others are shellfish such as blue mussels, hard clams, oysters, bay scallops, and blue crabs. Many of the aforementioned species support or once supported vibrant fisheries. Many of those fisheries have collapsed, also for various reasons. However, is it possible there is a link between the crash of the fisheries, the decline of Zostera and the failure for recovery on both ends?

Bay Scallop on Eelgrass

Argopecten on Zostera! Bay Scallops, Argopecten irradians , have developed a very close relationship with eelgrass, Zostera marina. As larvae, they are passively transported, and tend to settle in eelgrass meadows when the current is dampened by the 3D structure of the seagrass. This same 3D structure provides post-set juvenile scallops a spatial refuge from predation. Even as larger juveniles and adults, scallops are capable of, and have been shown to, actively select eelgrass habitats.

Other species also use eelgrass

grass shrimp A number of other species utilize eelgrass as a habitat. Included are grass shrimp, like the Palaemonetes pugio, other decapods such as blue crabs, bivalves such as hard clams, gastropods (snails), and numerous fish species, including winter flounder, tautog and cod.

Functional Extinction

Recently, a few articles started appearing about the dramatic loss of oysters throughout the world, and how in many areas, they are “functionally extinct.”  The article from ScienceBlogs talks about the findings of an international research team lead by Dr. Mark Luckenbach of the Virginia Institute of Marine Science.  In over 70% of the 144 estuaries studied, current oyster levels are at 10% or less of historic levels.  They estimate that over 85% of the world’s oyster reefs have been lost.  The amount of loss exceeds any other shallow water benthic marine habitat that has been similarly studied.  Obviously, this can cause problems.

The Underwater Times article mentions the term “functionally extinct” when referring to current oyster populations – that in some areas, oyster populations are less than 1% of historic levels, mainly due to overharvesting, disease, and invasive species introductions.  But what does “functionally extinct” mean? In this sense of the term, it is when a species experienced such a reduced population that said species no longer plays a role in the functioning of an ecosystem.  Obviously, the loss of any players in an ecosystem can be devastating.  But oysters are a foundation species, providing a variety of ecosystem functions that renders them more important to their estuarine ecosystems than many of the other species. Oysters create biogenic 3-D structure in the forms of reefs, which build up from the seafloor and in many locations emerge from the water, particularly at low tide.  This structure provides a plethora of microhabitats and niches for a variety of species to live.  In addition, since oysters are filter feeders, they play an important role in nutrient cycling in estuaries, packaging things in the water column (plankton, particulates) and delivering them to the bottom where they are consumed and utilized.  During this process, oysters actively clear the water column, increasing light penetration to the bottom and potentially allowing valuable submerged macrophytes to grow, adding structure to the reef and surrounding area, creating even more habitat.  A number of species depend on this habitat for food ad shelter, as they are valued nursery and feeding grounds for numerous estuarine species.  This function is vital to fisheries, as many finfish spend a portion of their lives foraging around oyster reefs.  So when the articles suggest that oysters are becoming functionally extinct, it has serious repercussions for the ecosystem as a whole.

Understanding oyster reef ecosystem services are important as researchers and managers move forward in trying to save oysters worldwide.  Oysters have been subject to considerable research, and more recently, a variety of restoration efforts have been conducted in the Chesapeake, Delaware Bay, North Carolina, and, more recently, the Hudson River.  The Hudson River project in particular is focusing on restoration of ecological function and not for the fishery.  In fact, due to their filtration capacity, landscape architects have proposed using oysters in the Greater New York City Area to clean up some highly polluted canals.

Clearly, the loss of oyster reefs are problems both economically and ecologically.  However, some research suggests all is not lost.  Stricter harvesting laws, fewer baymen, and no-take sanctuaries have helped maintain oyster populations, albeit low populations, in some areas.  Better and more successful management is the first step towards saving oysters, and the report made the following suggestions for restoring and maintaining oyster reefs:

  • The prohibition of harvests where oyster populations constitute less than 10% of their prior abundances, unless it can be shown that dredging and other harvest methods do not substantially limit reef recovery.
  • New thinking and approaches to ensure that oyster reefs are managed not only for fisheries production but also as fundamental ecological components of bays and coasts that provide invaluable ecosystem services.
  • Steps to ensure that harvests, particularly those carried out by dredging, do not damage the remaining reefs.
  • Regular monitoring of reef conditions.

There is plenty of other relevant information out there about oyster reefs, research, and the issues facing them.  I particularly recommend the blog In the Grass On the Reef, which focuses on research underway by Florida State researchers on salt marshes and oyster reefs.  In particular, they update posts about their research in ways which are easy to understand with great visual aids including photos and videos.  Definitely check that one out.

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