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Coastal & Estuarine Science News (CESN)

Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.

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2016 July

Contents

Eelgrass to the Rescue!
Seagrass Restoration: The Angling Angle
Grow with the Flow
Clam-oring for Nutrient Cycling Info?

Eelgrass to the Rescue!

Ammonium from oyster farming is taken up by eelgrass

Shellfish farming and seagrass beds often share estuarine real estate, so it’s important to understand how they can affect each other. Many studies have demonstrated negative impacts of shellfish aquaculture, but there is at least one way in which aquaculture and seagrasses can work well together. One potentially damaging byproduct of oyster farming is ammonium, which is directly excreted by the bivalves or produced by remineralization of their biodeposits. Eelgrass’s preferred form of N also happens to be ammonium. Can eelgrass adapt to the enhanced concentrations of ammonium near oyster farms and take up all they excrete? One study in a small, shallow embayment in Baja California where oyster farming has been conducted for 30 years suggests that the answer is yes.

Researchers examined ammonium uptake in eelgrass at sites near the oyster farm and about 1 km away. They found that plants growing at the oyster farming site had higher leaf NH₄⁺ uptake rates than those at the reference site, indicating that the plants have adapted physiologically to acquire ammonium efficiently from bivalve excretion. The investigators calculated that only about 3% of the eelgrass bed acreage present in the small embayment was needed to incorporate all of the NH₄⁺ excreted by the aquaculture operations, highlighting the biofiltering potential of the bay’s eelgrass beds.

A more complete understanding of the interactions of aquaculture operations and seagrass beds is needed, but the results of this study suggest that the biofiltering potential of seagrass beds should be considered when developing management strategies for mitigating the impacts of shellfish aquaculture practices in shallow coastal systems.

Source: Sandoval-Gil, J., A. Alexandre, R. Santos, and V. F. Camacho-Ibar. 2016. Nitrogen uptake and internal recycling in Zostera marina exposed to oyster farming: eelgrass potential as a natural biofilter. Estuaries and Coasts (May 2016). DOI: 10.1007/s12237-016 -0102-4.

Seagrass Restoration: The Angling Angle

Study models effects of habitat restoration on spotted seatrout – the fish and the fishery – in Tampa Bay

An estuarine habitat restoration project may look good – acres of wetlands or seagrass meadows might grow after replanting, for example – but it’s a bit harder to determine if the restored areas are functioning the way the original habitat did. It’s even harder to take such analyses one step further and determine whether restored habitats are providing “ecosystem goods and services,” benefits that accrue to human society from well-functioning ecosystems. A recent study examined the influence of seagrass restoration in Tampa Bay on the delivery of one such ecosystem service: recreational fishing for spotted seatrout, the most popular angler quarry in the region.

By adapting a model originally developed to describe juvenile fish habitat choice, the investigators looked at how fish moved among habitat patches based on each patch’s vegetation and physical parameters like temperature and salinity, and quantified the fishes’ resulting growth and mortality. Two seagrass cover scenario years were examined: 1950, when seagrass was relatively abundant, used as a proxy for a future date in which habitat restoration brings seagrass acreage back to 1950 levels, and 1990, when seagrass was at its lowest in Tampa Bay. Superimposed on both scenarios were shifts in temperature and salinity based on climate-driven patterns of rainfall. A linked angler submodel examined fishing success in the system; modeled anglers chose fishing spots based on depth and presence of seagrasses.

Results indicated that the effect of restoring seagrasses to 1950 levels on both the fish and the fishery is positive: the highest response to seagrass restoration (best growth and survival of fish as well as best angler success rates) occurred during average temperature and salinity conditions as compared to extreme wet or dry years. These findings show that seagrass restoration can be good for both fish and fisheries, although the extent of the response, as well as the best spots in the Bay for restoration, may be dependent on climate variability.

Source: Fulford, R. S., M. Russell, and J. E. Rogers. 2016. Habitat restoration from an ecosystem goods and services perspective: application of a spatially explicit individual-based model. Estuaries and Coasts (April 201 6). DOI: 10.1007/s12237-016-0100-6.

Grow with the Flow

Oyster reefs that grow perpendicular to flow are most likely to persist

Oysters are ecosystem engineers, building structures that serve as habitat not only for more oysters but also for a range of other species. The bivalves modify the physical environment by moderating flow and altering sediment dynamics, part of a self-perpetuating process that builds reefs. As oyster restoration becomes more popular in coastal areas around the world, it is important to note the characteristics of successful reefs so human-made reefs have the best chance of success.

One team of researchers noted that historically, reefs take one of three forms: string reefs that form perpendicular to prevailing currents, fringe reefs that parallel the shoreline, and patch reefs that form as irregularly-shaped mounds. The investigators constructed experimental reefs in these configurations in two small tributaries of the Chesapeake Bay and measured the growth of the reefs and hydrodynamic conditions around them. They determined that parallel and perpendicular reefs maintained their area whereas patch reefs shrunk over the two-year course of the study. Flow velocity and suspended sediment loads were highest adjacent to the perpendicular reefs, both factors that contribute to reef persistence by transporting food and oxygen to the oysters and removing waste products and smothering silt. Based on these results the researchers concluded that the perpendicular reef orientation is most advantageous and that circular reef configurations should be avoided. However, in order to determine optimal spacing of multiple reefs in a system additional studies are needed on the sphere of influence of an individual reef.

Source: Colden, A. M., K. A. Fall, G. M. Cartwright, and C. T. Friedrichs. 2016. Sediment suspension and deposition across restored oyster reefs of varying orientation to flow: implications for restoration. Estuaries and Coasts (April 2016). DOI: 10.1007/s12237-016-0096-y.

Clam-oring for Nutrient Cycling Info?

How does clam aquaculture affect carbon and nitrogen budgets? 

As bivalve aquaculture expands worldwide, so does interest in using these farmed filter feeders as a tool to extract nutrients from eutrophic systems. Clams, mussels, and oysters sequester carbon and filter excess nutrients and phytoplankton, and these accumulated nutrients are then removed from the system at harvest. On the other hand, they excrete carbon and nitrogen, and their biodeposits fuel microbial processes in the sediment, possibly contributing to water quality degradation. A more comprehensive understanding of the net effects of cultured bivalves on C and N cycles is needed, especially given that those effects are likely to be site-specific depending on water residence time, primary productivity, and other factors. A recent study examined the effects of hard clam aquaculture on C and N processes in a shallow tidal embayment (the aptly-named Cherrystone Inlet) of the Chesapeake, taking an ecosystem budget approach to the question.

Numbers and biomass of clams in the system were estimated using aerial photography. Clam physiological rates – filtration, respiration, and egestion – were compared to basin-wide estimates of primary production, nutrient regeneration, and respiration, and a model was used to estimate contributions of nutrients to the system from outside the embayment. Results indicated that clam aquaculture contributes to eutrophication in some ways: aquaculture resulted in large fluxes of N and C from the sediments to the water column, which were 3 and 1.5 times higher, respectively, than the amount of N and C removed by clam harvest annually. This flux was associated with enhanced rates of macroalgal and benthic microalgal production in the inlet. However, the news was not all negative: although clam beds cover only 3% of the bay’s surface area, they filtered 7-44% of the water column each day, which translates to between 2 and 14 days to filter the entire system. They also consumed the equivalent of 103% of the bay’s phytoplankton production, indicating that they rely on phytoplankton imported from the Chesapeake Bay. Thus clam cultivation can have a large influence on C and N cycling, as it did here, and it is important for managers to consider the ecological context when assessing eutrophication effects.

Source: Murphy, A. E., K. A. Emery, I. C. Anderson, M. L.  Pace, M. J. Brush, and J. E. Rheuban. 2016. Quantifying the effects of commercial clam aquaculture on C and N cycling: an integrated ecosystem approach. Estuaries and Coasts (May 2016). DOI: 10.1007/s12237-016-0106-0.