The stunted growth of coral reefs.

 
New York City shelters its 8.3 million residents within a complex, three-dimensional matrix of concrete and steel. Construction crews must work on the integrity of the city’s structural skeleton constantly just to keep up with growth and age. Like a living organism, the city is expanding up, down and outwards while repairing and replacing decrepit trusses and frames with new, high quality materials.

It is a common big-screen fantasy to imagine the physical decay of New York City if these construction efforts were to suddenly cease in some kind of post-apocalyptic madness. However, the structural collapse that would bring New York City back to nature is actually occurring within nature, and you don’t need CGI graphics to imagine it. Just last week a team of scientists shared new findings that warming oceans are reducing the size and strength of coral reefs in two different parts of the world.

A coral reef is not unlike a city. Reefs are massive underwater edifices that support a stunning diversity and density of organisms. But instead of using building materials like concrete and steel, coral reefs house their residents within and upon a skeleton of calcium carbonate. This calcium carbonate is continually laid down by the reef's construction crew – growing colonies of coral polyps.

Individual coral polyps are actually small, soft-bodied animals that grow affixed to a hard surface. For protection they secrete calcium carbonate near their base. This calcium carbonate accumulates as the polyps grow, clone themselves, and multiply into large colonies. Over many generations, the calcium carbonate left behind becomes the skeleton of the reef, which still consists of a living colony of coral polyps on the surface.

The ability of coral polyps to produce calcium carbonate, called calcification, depends on a host of environmental factors. For example, seasonal differences in water temperature cause calcification to increase in the summer and decrease in the winter, resulting in alternating layers of high and low calcium carbonate density. It creates a pattern similar to the annual growth rings of a tree. The scientists used this pattern to measure the extent of growth and the density of calcium carbonate produced every year by coral colonies. They focused on two genera, Porites and Montastraea, from the Great Barrier Reef and Mesoamerican Barrier Reef. They compared these annual growth data to existing records of warming sea surface temperatures spanning at least a decade.

One of the coral genera, Porites, showed the greatest sensitivity to warming temperatures regardless of location. In both the Great Barrier Reef and the Mesoamerican Barrier Reef, Porites experienced sharp declines in calcification rate as sea surface temperatures increased. If climate change models are correct, at the given rate of decline, Porites in the Great Barrier Reef will stop laying down calcium carbonate entirely by 2100. In the warmer Mesoamerican Barrier Reef, calcification by Porites will cease in 2060. The genus Montastraea has also experienced reduced rates of calcification, although not as extreme. Montastraea in the Mesoamerican Barrier Reef will experience a 40% reduction in calcification by 2100.

The consequences of reduced calcification will manifest differently in the two genera. As temperatures rise, Porites produces calcium carbonate at the same density, but compensates for reduced calcification by not extending as far. Therefore, Porites reefs will grow more slowly and could be outcompeted for space by other organisms. Montastraea continues to extend in warmer temperatures, but it does so at the expense of calcium carbonate density. Much like osteoporotic bones, a Montastraea reef with reduced calcium carbonate density is more susceptible to physical and biological damage. Warming water temperatures will compromise both types of reefs in their ability to support biodiversity, either in terms of space or strength.

As grim as they seem, these predictions are likely conservative. The scientists mention that they don’t consider other factors that affect calcification such as coral mortality, coral bleaching, disease, and the negative consequences of pollution, erosion and other environmental concerns. The slow decline of coral reefs may not be fodder for a disaster flick, but piles of stunted, brittle coral reef will be utterly disastrous for the world’s oceans in a time that is already considered a biodiversity crisis.

Carricart-Ganivet JP, Cabanillas-Tera´n N, Cruz-Ortega I, Blanchon P (2012) Sensitivity of Calcification to Thermal Stress Varies among Genera of Massive Reef-Building Corals. PLoS ONE 7(3): e32859. doi:10.1371/journal.pone.0032859 

Photo: A view of the Mesoamerican Barrier Reef, Belize en.mesoamericanreef.org

Wetland restoration: Is it worth the effort?

If forests are the lungs of our planet, then wetlands are surely the liver. Wetlands filter overland runoff through a unique suite of plants and soil microorganisms that break down harmful compounds, ensuring that waters reaching streams and coastlines are contaminant-free. Wetlands also store a massive amount of carbon in their soils, serve as a nursery ground for many fisheries, and protect shorelines from floods and storm surges.

Despite these benefits and more, humans allowed worldwide wetland degradation to continue unchecked for two hundred years. Instead of protecting existing wetlands, we gamble on restoration efforts, investing over $70 billion to bring back functional wetlands in North America alone. Until now the overall outcome remained unclear. But after investigating 621 wetland restoration projects around the world, a team of scientists led by Dr. David Moreno-Mateos and renowned ecologist Dr. Mary Power of the University of California Berkeley determined that wetland restoration is generally a losing bet. They published their results last month in PLoS Biology.

The team combed the scientific literature for studies that compare restored and newly created wetlands to adjacent undisturbed wetlands. They compiled all the data from these studies to find out how three characteristics of the restored or created wetlands recover over time: hydrology, biological community, and biogeochemistry.

Wetland hydrology includes water level, flooding patterns, and water storage. It recovers quickly because it depends on the surface features engineered by restoration teams. Before they place a single plant in the ground, restoration teams manipulate soil levels and permeability to achieve ideal patterns of wetland hydrology. The biological community, however, is much harder to recreate. Even after 100 years, biological communities in restored wetlands only recovered to 77% of their undisturbed counterparts. Moreover, plant communities recovered more slowly than animals.

Biogeochemistry, the movement and transformation of nutrients in a system, only recovered to 74% after 50 to 100 years. A healthy, functioning wetland stores large amounts of carbon and organic matter, because their flooded soils deprive decomposing microorganisms of necessary oxygen. If wetlands dry out during a disturbance, oxygen reaches decomposers, allowing them to break-down carbon compounds and larger pieces of organic matter. The decomposers eventually release carbon dioxide to the atmosphere as a byproduct of this process. Once restoration teams restore the proper wetland hydrology, decomposition shuts down and carbon begins to build up in the soil once again. However, even after twenty years, the amount of carbon and organic matter stored in wetland soils remains significantly less than levels in undisturbed wetlands.

Dr. Power's team suggests that restored wetlands do not resemble their original condition because they have shifted to alternative states. Much like building a house, successful ecosystem restoration requires that all the right pieces come together in the correct order. If a construction crew lays out the wrong foundation or neglects to build the second floor, the house will look completely different from the blueprints. The result of this botched construction project is like an alternative state. In ecosystems, the presence or absence of certain organisms determines which organisms come next. If a restoration team fails to introduce an important organism, or does so at the wrong time, a restored wetland may give rise to an alternative state that differs from the goal.

When it comes to wetland restoration, we may know which plant species to add, but the proper foundation remains unknown. Wetland plant communities and biogeochemistry depend on soil microorganisms. If the microorganism community shifts during wetland degradation, or if restoration requires imported soils, the soils may lack the microorganisms necessary for native wetland plants to grow. These unseen changes underground have cascading effects through the wetland ecosystem, producing an alternative state. Alternative states often host different organisms, function at reduced levels, and cannot confer the ecosystem benefits we depend on. Unfortunately, the only way to fix it is to start again from scratch.

Moreno-Mateos D, Power ME, Comı´n FA, Yockteng R (2012) Structural and Functional Loss in Restored Wetland Ecosystems. PLoS Biol 10(1): e1001247. doi:10.1371/journal.pbio.1001247

Photo: Vermont Department of Forests, Parks and Recreation

Hospitality Returning to the Bottom of the Bay

We must consider many factors before settling our lives into a new place. Whether it is the school system, safety or convenience of transportation, people have a list of requirements for their suitable habitat. It is the services and infrastructure after all that distinguish our comfortable communities from hard life on the frontier. Whether dispersing from their population or remaining exactly where they were born, organisms seek to fill similar needs when settling into their habitat. Some plants and animals, known as foundation species, are specially equipped to satisfy these requirements for many different species. By virtue of their physical characteristics, foundation species provide three-dimensional habitat for entire ecological communities, and in doing so dramatically enhance biodiversity in a relatively small area. Reef-building organisms like corals and oysters, trees that form a dense canopy layer, and the impenetrable underwater forests of kelp are all examples of foundation species that host robust ecosystems.

As we have learned from the worldwide destruction of coral reefs, declines in foundation species pose an enormous threat to their associated, denizen species. For the same reason, the protection and restoration of foundation species could be the most important step to maximize the conservation of global biodiversity. This conservation strategy is currently underway in San Francisco Bay, as collaborative effort between Audubon California, Save the Bay, NOAA, and San Francisco State University strives to restore and enhance one of the most important foundation species in San Francisco Bay – Zostera marina, or eelgrass.

Eelgrass is a type of seagrass – a marine flowering plant (not a seaweed!) that grows submerged in shallow water areas worldwide. Seagrasses form vast meadows in coastal environments that resemble terrestrial grasslands, their dense root system and tall leaf canopy creating complex habitat in areas that are otherwise unvegetated. Seagrass beds provide food, cover, and spawning ground for a wide array of invertebrates, fishes, birds, and mammals. Found in tropical as well as temperate regions, seagrasses play host to hundreds of associated species including green sea turtles, manatees, seahorses, and countless species of fish. As a biodiversity hotspot, seagrass beds also attract hordes of predators that come to feed on their residents. Unfortunately, seagrasses are declining worldwide at an astounding rate. Like all foundation species, the destruction of seagrasses can have severe impacts on the many associated species that rely on seagrass bed habitat. A recent study by A. Randall Hughes of UC Davis found that nearly 15% of all seagrass species worldwide are currently listed as threatened in some portion of their range. For every species of seagrass, there is at least one associated species of concern, and in total there are 74 species of concern that are associated with seagrasses worldwide. These results come from a preliminary study, however, and the true conservation costs of seagrass declines have likely been underestimated. Moreover, there is a general lack of awareness of the importance of seagrass bed ecosystems on biodiversity.

Over the past few decades within the San Francisco Bay, the size and number of eelgrass beds has been steadily declining, mostly due to reduced light availability. Like all plants, eelgrass requires sunlight for photosynthesis; however, conditions within the Bay have severely limited the depth to which light can penetrate the Bay water. Dredging operations in the Bay destroy eelgrass beds either by physical disturbance or by stirring up sediments to increase turbidity. Construction activities in the Bay watershed release sediments to streams that eventually reach the Bay, smothering eelgrass beds. Even if there is enough light for the plants to growth, excess nutrients can accelerate algae growth beyond the feeding rates of grazers, allowing the algae to over take eelgrass leaves and block out light.

Biologists of the eelgrass restoration team are putting in a great effort to enhance existing eelgrass beds and restore this habitat to the fullest extent of its San Francisco Bay range. Richardson Bay in North San Francisco Bay is the ideal location for eelgrass restoration. It harbors the second largest eelgrass bed in the estuary, with plants that have the greatest genetic diversity of all beds sampled. Given its sheltered location and distance from dredging operations, Richardson Bay has the model environmental conditions for large eelgrass beds, but the genetic diversity of its plants also gives hope for successful transplanting to other sites in the Bay. Using a variety of techniques, biologists are hoping to discover the restoration method with the highest success rate. Both mature plants and seedlings are transplanted to sites at four different depths where they are either hand-planted by divers or tied to special grid-like frames which sink to the muddy bottom. At the same time the team is modeling Bay circulation patterns to better understand the potential for seed dispersal from existing beds. Divers are collecting mature eelgrass flowers from donor beds and using them for targeted seed dispersal at other sites in the Bay. Seeds can be deployed either by hand or from mesh bags attached to buoys which hold the flowers and distribute the seeds in a circular pattern, moving with the current.

The restoration team and Bay Area naturalists alike hope that these efforts will reverse the Bay’s long-term decline in native biodiversity, giving us an example of the multi-layered ecosystem that this foundation species once supported. At the base of its complex food web, dense mats of eelgrass roots hold sediments in place and keep them well-oxygenated to support the growth of important bacteria. Eelgrass leaves provide substrate for algae and epiphytic plants which are grazed upon by a number of invertebrates. Even dead eelgrass plants that settle within the bed, develop a film of bacteria, fungus and detritus, which also feeds small invertebrates. During low tides, eelgrass beds hold moisture, so that these small organisms are protected. As a result, waterfowl arrive in droves to feast on a surfeit of invertebrate prey. The Pacific Herring, the largest commercial fishery in the Bay which has seen recent population declines, depends on eelgrass beds for spawning and cover. The herring lay their sticky eggs on eelgrass leaves so that the young will be protected until they reach maturity. The success of these restoration efforts could enhance the Pacific herring population, allowing for increased takes by local fishermen. Birders would also be happy to see the return of the diversity of native and migratory waterfowl that hunt the in eelgrass beds. To learn more about the progress of the San Francisco Bay eelgrass restoration, visit www.tiburonaudubon.org

Cited: Hughes, A. Randall, Susan L. Williams, Carlos M. Duarte, Kenneth L. Heck Jr., and Michelle Waycott. 2009. Associations of concern: declining seagrasses and threatened dependent species. Frontiers in Ecology and the Environment. 7:242-246

Photo credit: www.ceoe.udel.edu