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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.

Here today, gone tomorrow!
A complete and utter failure? You decide. Last fall I had the bright idea to track overwinter survival and subsequent spring conditioning for scallops released in different habitats (eelgrass – their preferred habitat, see Thayer and Stuart 1974; Codium fragile - an introduced macroalgae which we think might serve as a suitable alternate substrate in the absence of eelgrass; and unvegetated, as a baseline for comparison). I have examined these habitats for growth in juvenile scallops and have already published some short term survival data (Carroll et al 2010, see earlier posts for a link). So my bright idea was to free release a fairly large number of scallops into these habitats at two field sites, one in Shinnecock Bay and one near Sag Harbor, NY. This is a method we have used in the restoration efforts, and a method that has been successful, so I figured that it would be no problem. I planted ~2500 scallops in each habitat at the 2 sites (~15,000 total scallops planted), not an insignificant number, at least not in my opinion back in November/December of 2009. In other planting sites, we typically don’t monitor again after planting until the spring, so staying true to form, I did not actually go check on these scallops until last week. Much to my surprise (well, maybe not totally surprised) I didn’t recover a single scallop in Shinnecock Bay. I surveyed all the habitats around my planting zone and didn’t find a single live scallop, save for a couple natural 2 year olds. My dreams of having some uncaged growth and condition data failed! There wasn’t even evidence of major predation, because I didn’t even find empty or crushed shells. They were just all gone! A couple things are likely – either burial or transport – see Powers and Peterson’s 2000 manuscript on scallop movement. Both are equally possible scenarios, as these sites in Shinnecock Bay were relatively shallow (~1m deep) and we had quite a crazy winter in terms of storms. Its just a shame. The only positive here is that at least my marker buoys were still firmly anchored at the sites! In fact, this picture is about the most exciting thing from the Shinnecock dives:

(thats right, its my pink lemonade by the throttle while the boat is tied to the dock. Nice, right?)

I was hoping Sag Harbor would be slightly better. I mean, this was a deeper site, so things had to work out, right? Actually, I originally expected there to be scallops at Shinnecock (typically low energy sites) and not in Sag Harbor (strong tidal currents). In Sag Harbor, the scallops already drifted slightly down current from the planting area the date of planting, and despite the scallops looking good at the bottom,

I figured strong currents and potential predation

would essentially eliminate them. Luckily, there was scallops to be found in Sag Harbor, although in much reduced densities. So, not enough scallops to monitor with enough replication to have confidence in the results, but at least all of the 7500 scallops here were not lost. What was lost, you ask? The Codium! It was all gone! My Codium planting area in Sag Harbor was completely devoid of Codium! So despite the semi-success of overwintering some of the scallops in Sag Harbor, my experiment here still failed! Awesome! I guess there’s always NEXT year, I just don’t know how my committee feels about that!
I guess this season, I will just have to run juvenile growth experiments again, as soon as I get the juveniles.

At least I saw some cool stuff at Sag Harbor:

Spider crab crawling out from a cinder block.

Red beard sponge in eelgrass.

Surviving bay scallop, cryptically hidden on the bottom, save for its blue eyes!

Juvenile lady crab.

Thayer, GW, & Stuart, HH (1974). The bay scallop makes its bed of seagrass Marine Fisheries Review, 36, 27-30

Powers, SP, & Peterson, CH (2000). Conditional density dependence: The flow trigger to expression of density-dependent emigration in bay scallops Limnology and Oceanography, 45, 727-732

1 comment to Here today, gone tomorrow!

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