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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.

When invasion isn’t such a bad thing…

New species get introduced into novel habitats almost like clockwork in the modern era.  These are termed introduced or exotic species.  Typically, these introductions are the effect of anthropogenic activity.  Sometimes, these species become nuisances – spreading in their new habitats via natural processes, and creating problems for native species.  These nuisance exotics are called invasive species.

Green Crab - Invader!

In the marine realm, there are numerous invaders from all taxa – from plants and algae, to tunicates, crabs, mollusks, and all the way up to fish and birds.  Some of the species have been here for centuries- such as the green crab, Carcinus menas, on the east coast, and the mute swan, Cygnus olor.  Still, others are much more recent, such as the Asian shore crab, Hemigrapsus sanguineus,  the Chinese mitten crab, Eriocheir sinensis, the lionfish, Pterois volitans,(which I’ve recently blogged about many times), and the tunicate Didemnum vexillum (which I blogged about here).  Regardless of species, and length of time since invasion, these species have potential to be harmful to their new environments.  The East Coast of the US is particularly hard hit by these species.

Map of invasive problem regions


Map of origins of marine invaders

So how do they get here? From a variety of ways, but perhaps most famously via ships’ ballast dumping.  Ballast is simply material used by ships to control and maintain buoyancy and stability.  Typically this is water, pumped into ballast tanks from the port the ship is sitting in at the time.  This ballast water gets pumped in or out depending on the weight of the cargo on this ship, and so you can imagine how water could be transferred across whole oceans, bringing with it any species that happened to get sucked in to the ballast tank.  This is a major source of marine invaders – including the now infamous zebra mussel, Dreissena polymorpha, which has become especially problematic throughout fresh waters of the Mississippi River, the Great Lakes, and the east coast.  However, invasives can also come from aquaculture gear and species – especially as native species are fished out and replaced with non-natives to keep up food production.  In addition to these non-native species used in aquaculture, other species hitch rides on them.

How Ship's Ballast works

However, as you can see above, it is possible that all invasive species are not created equal – that is, maybe not all invasives are so bad.  Green fleece, known as Codium fragile, has been introduced to the east coast of America for decades.  Originating from Japan, it has typically viewed as bad – it is a buoyant species which needs hard substrates to attach, including living shellfish.  It got the nickname “oyster thief” since it would attach to oyster shells, and whenever a storm or strong current event occurred, the buoyant macroalgae would be swept away, dislodging oysters and taking them away from reefs and culture sites. It is clear why this is considered a problematic species.

And yet, some recent research has shown that maybe Codium isn’t all that bad.  Research which I have participated in has demonstrated that Codium may act as a viable alternative habitat for native bay scallops.  Why? Bay scallops have evolved a strong association with seagrasses, and the Codium canopy likely provides the same upright structure to scallops.  We observe scallops frequently in association with Codium in Long Island bays, and a study conducted showed that survival of free-released and tethered scallops was the same in eelgrass and Codium, suggesting that the invader offers a similar predation refuge.  This was published last year in Marine Biology (See Carroll et al, 2010, below).

From Carroll et al 2010

In addition, I have taken the research further.  The aforementioned paper talked about survival on a relatively short time span – 1 week.  In order to examine the longer term effects on growth I conducted a caged field experiment the past two summers at 2 field sites with eelgrass, Codium, and unvegetated sediments in close proximity to each other.  The general findings have been that scallops in Codium grow at rates similar to scallops in eelgrass, however, there are site-specific differences.  There are also no differences in mortality between the habitats – suggesting that dense stands of Codium aren’t having as detrimental impact of low dissolved oxygen as I originally thought.  This work isn’t published yet, as I am working on a method to find the stoichiometry of the tissues, but some of the results are in the presentation I gave at CERF 2009 here: Thursday_SCI-045_1115_J.Carroll

Moon snail crawling over Codium

However, I am not the only one who sees “positive” impacts of Codium.  In the most recent issue of Marine Ecology Progress Series, a team of Canadian researchers, led by Annick Drouin, higher abundance and diversity of the faunal community in eelgrass meadows invaded by Codium fragile.  Using a variety of sampling methods and field manipulations, the team demonstrated higher abundance and diversity of invertebrate organisms on Codium, and in eelgrass meadows invaded by Codium, than those without Codium. The pattern of fish abundance and diversity was not different – likely because they are highly mobile and can move easily between structured habitats.  It is likely that Codium just generates MORE habitat, as it is branching and canopy forming.  The important thing here is the ecological implication – the lack of a negative effect on native species by the presence of this “invader.”  Perhaps Codium might not be so bad after all, especially as eelgrass is declining in many regions.

Figure from Drouin et al 2011

It is possible, then that “invasive” vegetation species in the marine environment may not always be bad.  In many cases, invasives may be beneficial.  Numerous studies (including the ones above with Codium) have demonstrated a positive effect of invasive algal species on native fauna.  Typically, the vegetation is habitat forming, and invades areas where native habitat forming vegetation has already been lost.  In essence, it is replacing a lost habitat, and creating a new habitat which is functionally similar to the species which declined/disappeared.  That being said, invasive algal species can be detrimental to native macrophytes through competition.  However, the benefit is in enhancing native fauna, which has potential fisheries ramifications.  This requires further investigation, but it is entirely possible that non-native macroalgal species might have a positive effect on a number of native fauna. 

Mud crab in Codium canopy

Pipefish chillin' in Codium canopy

The above photos, and the one of the moon snail farther up the page, are all illustrations of native species of Long Island associating with the invasive Codium fragile.  Now, again, there are certainly detrimental effects of invasive species, so I am not trying to be too much of an apologist for them here.  However, in the absence of eelgrass, it is entirely likely that the upright, canopy forming structure of Codium creates a habitat suitable to many seagrass associated fauna.  As eelgrass is declining, invasive macrophytes might be important replacement habitats for a variety of native species.  Understanding how these species affect native species will be key for management of estuaries moving forward.  Particularly, once established, invasives becoming increasingly expensive and difficult to remove.  If some invaders might be of benefit, that relationship needs to be well understood. Hey, invasives could help bring back the bay scallop in NY (and likely is having an impact), providing a habitat as eelgrass has disappeared from many Long Island areas. Who knows where else they might be beneficial.

There will be those of you out there who disagree.  I don’t blame you.  Calling an “invader” beneficial certainly goes against conventional wisdom.  When we first introduced the idea of Codium as a potential scallop habitat to a shellfish crowd, we were scoffed at.  However, the data don’t lie.  And more research points to cases where invasives may actually facilitate natives. Drouin, A., McKindsey, C., & Johnson, L. (2011). Higher abundance and diversity in faunal assemblages with the invasion of Codium fragile ssp. fragile in eelgrass meadows Marine Ecology Progress Series, 424, 105-117 DOI: 10.3354/meps08961
Carroll, J., Peterson, B., Bonal, D., Weinstock, A., Smith, C., & Tettelbach, S. (2009). Comparative survival of bay scallops in eelgrass and the introduced alga, Codium fragile, in a New York estuary Marine Biology, 157 (2), 249-259 DOI: 10.1007/s00227-009-1312-0

7 comments to When invasion isn’t such a bad thing…

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