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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.

Fishes respond poorly to seagrass loss
Well it has been a few weeks since I’ve posted on some research articles. But then the Journal of Experimental Marine Biology and Ecology published a manuscript about cod responses to expanding seagrass meadows. In addition, a paper out of Japan earlier this year talks about the loss of fish species with the loss of an eelgrass meadow. Combined, these point out the obvious, many finfish are dependent on seagrasses for habitat. However, its not just typical seagrass-associated species that are affected by the loss of seagrass.

First, what happens when seagrasses disappear? There is a wealth of literature that suggests disappearing seagrasses has many negative consequences for both resident and transient species. Many species, including numerous commercially important species, utilize seagrass as a habitat for at least some portion of their life cycle. A paper by Yohei Nakumura examining seagrasses next to l reefs demonstrated that seagrass loss has an impact on the abundance and diversity of fishes, including reef associated species. A series of disturbances, particularly typhoons, decimated a seagrass meadow near a reef, to the point where in 2009, the seagrass meadow had totally disappeared. This caused a 80% reduction in the number of species and a 90% reduction in the total number of individual fish along transects at the same site before and after the disappearance. In addition, they monitored a nearby undisturbed site as a reference, and there was no difference in the abundance or diversity of fishes over the same time period. Many of the fishes that disappeared weren’t just seagrass residents, but also coral dwellers. In fact, the only species that didn’t seem affected were some gobies. The reason for the loss of fish might not be the eelgrass itself, although the habitat does provide shelter from predators, but could also be due to loss of food for many of the fish – tiny crustaceans that live amongst the seagrass.

A more recent paper involves the increase in abundance of juvenile cod in areas where seagrass is recovering and expanding. First, I know what you are all thinking, I love cod and eelgrass associations! And second, it is great news to hear that seagrass is recovering in some areas (I can talk more about this later). Apparently, there are seagrass meadows in Newfoundland, Canada, that are recovering and expanding over the past decade. These habitats are nursery grounds for both Atlantic cod and Greenland cod. So, one might imagine that an increase in seagrass would be beneficial to these species. Using biweekly seines to monitor changes in fish abundance, Warren and others were able to demonstrate dramatic increases in young of the year cod in the seagrass habitats, in particularly in those “recovering” habitats. This increase also occurred rapidly with expanding seagrass meadows. This suggests that these fish are capable of recovering quite quickly if enough suitable habitat exists. However, it also suggests that since juvenile cod might respond so rapidly, that any negative changes in seagrass cover can be detrimental to stocks. Combined with the Japanese study, the literature indicates that fish populations may lack resiliency to seagrass loss, and illustrate the need for water quality monitoring and management, as well as seagrass restoration. Otherwise, the news that cod stocks might recover, might be just internet fodder.

Nakamura, Y. (2010). Patterns in fish response to seagrass bed loss at the southern Ryukyu Islands, Japan Marine Biology DOI: 10.1007/s00227-010-1504-7

Warren, M., Gregory, R., Laurel, B., & Snelgrove, P. (2010). Increasing density of juvenile Atlantic (Gadus morhua) and Greenland cod (G. ogac) in association with spatial expansion and recovery of eelgrass (Zostera marina) in a coastal nursery habitat Journal of Experimental Marine Biology and Ecology DOI: 10.1016/j.jembe.2010.08.011

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