Solar Power

The act of eating is, without a doubt, the most enjoyable way of obtaining energy. As I sink my teeth into a juicy cheeseburger or try the first bite of an unusual new dish, I am blissfully unconcerned with the fact that I am meeting my metabolic obligation as a heterotrophic organism. My focus is exclusively on pleasing my senses of smell and taste while achieving that completely satisfied, comfortably-full feeling, not meeting my body’s caloric demand to ensure continued growth, maintenance and response. It is so easy to forget that we eat to live, that savoring a grilled cheese sandwich is merely a delicious way of staying alive.

While humans have made seeking, preparing, and consuming food a delightful means of survival, it is arguably an inefficient way to obtain energy. Heterotrophs like us must devise some scheme of capturing the appropriate food, the energy expenditure of which can be enormous. Autotrophic organisms such as plants and algae utilize solar (and sometimes chemical) energy to fix atmospheric carbon through photosynthesis and build the complex organic compounds needed for metabolism. In doing so they create their own food, skipping the expensive steps that we so enjoy. Autotrophs are the primary producers in the food web, creating the organic compounds upon which all other organisms rely for energy. Heterotrophs are the consumers, filling distant trophic levels which correspond to the number of links separating them from the sun’s energy. Humans typically eat as primary and secondary consumers, although consumption of some foods, such as large prize fish, can make us quaternary or even more distant consumers.

Over the course of evolution, organisms have spread out among different trophic levels. While humans enjoy the role of apex predator, occupying the end of the food web, some animals have moved in the opposite direction. Outdoing other primary consumers, some marine invertebrates have managed to acquire most of the energy they need without hunting or even eating. The ultimate convenience in predation, this unique trophic status is made possible by a special partnership between select invertebrate species and zooxanthellae algae. This partnership is considered an endosymbiotic relationship because the zooxanthellae live inside the body of the host, allowing both organisms to enjoy mutual benefits. (Our intestinal micro-flora and fauna are endosymbionts as well). Zooxanthellae are single-celled organisms that line the superficial tissues of the host like tiny solar panels, performing photosynthesis which provides the host with the organic compounds it requires for metabolism. In exchange the zooxanthellae receive shelter, nutrients and a constant supply of carbon dioxide – metabolic waste from the host. To maximize the benefits of this relationship, the invertebrate host seeks environments with conditions that are favorable for photosynthesis, as if it were an autotroph itself. The nudibranchs, sea anemones, jellies and reef-building corals that host zooxanthellae all live in shallow water for maximum sunlight.

Behind the scenes at the New England Aquarium, Upside-Down Jellies, Cassiopea andromeda, are raised under high-powered sun lamps in shallow tanks that re-create their natural sandy-bottom habitat. The Cassiopea rests on its flat bell, its reduced tentacles extended up towards to the light. At some point in the jelly’s lifecycle, zooxanthellae are incorporated into its tentacles, causing them to appear many different shades of blue-green. While they rest comfortably on the bottom, Cassiopea are by no means sessile. If you gently agitate the sand below their bell, they will casually swim off, still upside down, to another sunny spot. Small Cassiopea, about the size of a quarter, are held in a separate tank under the sun lamps. With a few exceptions, these young organisms do not appear to have zooxanthellae in their tentacles, and they must be provided with a normal diet. When the jellies are large enough, they are moved to the main tank where their tentacles eventually develop that typical blue-green color. It is likely that the adult jellies regularly expel excess zooxanthellae into the water in order to maintain a balanced number. The free zooxanthellae can then be absorbed by other jellies and will proliferate to colonize their new hosts. Even though they actively produce their own food, we would still feed adult Cassiopea small amounts of zooplankton to supplement their photosynthetic diet.

Similarly, reef-building corals (which, like the jellies, are members of the Phylum Cnidaria) acquire about 90% of the energy they need from zooxanthellae. Corals can capture and consume other plankton, cnidarians and sometimes even small fish, but they are completely dependent on their endosymbiotic relationship with zooxanthellae for survival. Research has shown that host-zooxanthellae associations in corals are extremely specific. This means that only one species of zooxanthellae will populate a colony of coral. As the zooxanthellae multiply, the host coral must regulate the number of individuals it holds within its cells in order to maintain its finite capacity. One way that the coral achieves this balance is by digesting excess zooxanthellae or expelling individuals into the water column. Zooxanthellae expulsion is responsive to environmental changes and often occurs when the corals are under stress caused by rising temperatures, changes in water chemistry, reduced light and disease. The expulsion of zooxanthellae is called coral bleaching, because the loss of the pigmented algae leaves the corals bone-white. If the corals survive the period of stress, zooxanthellae may be able to repopulate the colony within a few months. At first the species of zooxanthellae that repopulates the colony may be different from the species that was expelled, but over time the species will be replaced much like forest succession, eventually leading to that original inhabitant.

An endosymbiotic relationship with zooxanthellae may seem like the most efficient way for an animal to acquire energetic organic compounds, but one unique group of sea slug has managed to get even closer to autotrophic status. Sacoglossans, or solar-powered sea slugs, are small marine gastropods - the herbivorous cousin of the predatory nudibranch. These organisms are absolutely animals, yet amazingly they can perform photosynthesis on their own without the help of functional zooxanthellae. As a sacoglossan feeds on algae, it can isolate intact chloroplasts (the cellular machinery that performs photosynthesis) from its food and retain the chloroplasts within specialized cells in its body. While the other components of the algae are digested as food, the chloroplasts are incorporated into the body of the sacoglossan and perform photosynthesis the same way they did in the algae. This symbiotic phenomenon, aptly known as kleptoplasty, enables the sacoglossan to reap the energetic benefits of photosynthesis without playing host to an independent organism. While the functionality of the chloroplasts within the sacoglossan’s body is finite, the species Elysia chlorotica can maintain its chloroplast associations for up to ten months.

Sacoglossans thrive with the convenience of independently producing food within their own tissues, and even though they cannot glean as much pleasure from meeting their caloric requirements as we do, they don’t have a sense of taste either. Kleptoplasty and zooxanthellae endosymbiosis have allowed some animals to enjoy the ultimate reliable food source by acting more like plants. So the next time you are hungry enough to eat a horse, imagine yourself as a solar-powered sea slug, bathed in sunlight – warm, energized, and comfortably full.

To learn more about sarcoglossans and nudibranchs that host zooxanthellae, visit The Sea Slug Forum.



Dimond J. and E. Carrington. 2008. Sybiosis regulation in a facultatively symbiotic temperate coral: zooxanthellae division and expulsion. Coral Reefs. 27: 601-604

Toller, W. W., R. Rowan, and N. Knowlton. 2001. Repopulation of Zooxanthellae in the Caribbean Corals Montastraea annularis and M. faveolata following Experimental and Disease-Associated Bleaching. Bio.Bull. 201: 360-373

Photo: The sarcoglossan Placida dendritica. Photo by Bill Rudman

Jellies. Of course.

An aquarium crowd pleaser and anathema of the beach, jellies are by no means rare animals, yet they never cease to fascinate and terrify their human audience. Their simple design and alien appearance combine in awe-inspiring beauty – a diaphanous bag of cells operating at the most basic level of functionality. It is this simplicity of jellies that amazes. It is hard for us to believe that these are actually animals, yet they manage to thrive with the most primitive tissue structure, lacking viscera, and possessing only a rudimentary nervous system. The thick, brown sea nettles that contrast so attractively with a blue aquarium background and the heavy, dinner-plate shaped moon jellies that wash up on the beach are actually far more complex than many jellies in the ocean. Although they resemble their stinging cousins, the animals of Phylum Ctenophora, are usually classified separately from the more well-known jellies of Phylum Cnidaria.

At the New England Aquarium we distinguished the groups as true and comb jellies. The true jellies, members of Phylum Cnidaria, tend to have more robust bodies and possess venomous cells called nematocysts which can paralyze prey and sting a human interloper. The animals of Phylum Ctenophora are totally painless yet proliferate so aggressively that reaching into a tank feels like dipping your hand into a jar of, believe it or not, jelly. Comb jellies can bloom in extremely dense masses, flowing in to shore with the high tide where we would meet them with buckets, scooping them up with sore arms by the thousands. After an accidental introduction in the Black Sea, one type of comb jelly, Mnemiopsis, even caused the collapse of local fisheries by eating both fish larvae and fish prey and expanding to a devastating population size.

Comb jellies come in all shapes and sizes – from the marble sized gooseberry (Pleurobrachia) to the nearly flat, meter-long Venus’ Girdle (Cestum). The relatively few species of the phylum all have transparent bodies that appear as fine as a snowflake. The Venus’ Girdle, for instance, is so thin that divers can look through its body to see undistorted images on the other side. The only observable action of these animals is the rapid flutter of tiny cilia, organized in eight long stripes, which propel the animal through the water. Their swimming movement appears nearly effortless compared to the strenuous flex of a true jelly’s bell. The flutter of the cilia can also scatter light, creating a prismatic effect in many species. Most Ctenophore species are capable of bioluminescence, lighting up the dark ocean with flashes of blue and green. These nearly invisible oceanic lanterns add to the extraordinary experience of diving at night, which is at once strange and beautiful.

Read more about Phylum Ctenophora and one diver's encounter with the nighttime glow of Venus' Girdle here.

Photo: Mnemiopsis photographed by Herb Segars

Species Profile: Leopard Shark

Interested in San Francisco Bay wildlife? This is a species profile that I wrote on Triakis semifasciata, the Leopard Shark, for our Fall 2009 newsletter. This is the original, soon to be hacked away by the cruel hand of the redactor.

Where are Leopard Sharks found?
Leopard Sharks are found along the Pacific Coast - from Central Mexico to Oregon. One of the most common sharks in California, there is a large population of Leopard Sharks living in the San Francisco Bay. Most of them live in the Bay year round, with a few individuals migrating out in the fall. Leopard Sharks are often spotted near the bottom in the shallow waters of the sloughs and mudflats along the Bay margins. The Bay offers a safe haven for the Leopard Shark, because the water is too shallow and warm for predators, such as the Great White Shark. Plus the Leopard Shark finds abundant prey in the muddy bottom of the Bay. They often follow the high tide up to the shoreline to feed on animals in the shallow mudflats; then they move back out as the water recedes.

What do they look like?
The Leopard Shark is a slender fish with silvery-bronze skin and dark ovals arranged in neat rows across its back. Leopard Sharks are quite small. Their average length is around three to four feet, although they can grow up to seven feet. The Leopard Sharks in San Francisco Bay are more likely to be between two and three feet long.

What do they eat?
Leopard Sharks feast on small invertebrates, such as clams, worms and crabs that they find along the muddy bottom of the Bay. They also like small fish, eggs and the occasional Bat Ray. Sometimes the Leopard Shark can pluck prey right off the mud with its bottom-facing mouth. In other instances, the shark will shovel its nose into the bottom and toss the sediment away, exposing hidden clams or worms. Leopard Sharks may go to extreme lengths to eat small animals, but they will not attack humans.

How do they reproduce?
Leopard Sharks have ovoviviparous reproduction, which means that the baby sharks, or pups, develop in eggs that are retained within the mother’s body. The eggs hatch in the mother’s body, and then the pups are born live. A female Leopard Shark can have up to 29 pups in one litter.

What are the threats to Leopard Sharks in San Francisco Bay?
Leopard Sharks are fished both commercially and recreationally, with recreational fishing accounting for the majority of the catch. Even though it is an abundant species, Leopard Sharks grow so slowly that overfishing could deplete the population. Concerns about overfishing lead to the implementation of size limits by the Department of Fish and Game - Leopard Sharks smaller than 36 inches must be released. Leopard Sharks also contain high levels of mercury in their tissues. These animals have a greater exposure to mercury than other fish species because they spend so much time feeding in contaminated Bay sediments. It is unknown if the mercury is harmful to the Leopard Sharks, but it certainly exceeds the accepted safe limit for humans.

Photo credit: Peter J. Bryant

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

Get Glowing.

Scientists at the Tohoku Institute of Technology in Japan recently made an amazing discovery - we glow. That’s right. Humans actually GLOW as we emit photons of light energy. And while our photon auras are far too dim to be seen by the human eye (but certainly sensed in other ways), super-sensitive cameras at the Tohoku Institute have captured the human glow which actually changes over the course of the day. We glow the most from our faces, with peak glow occurring in the late afternoon. You can read more about the discovery of your inner light on Ed Yong’s blog, Not Exactly Rocket Science.

I must have been glowing a little bit brighter as I read this exciting news. Bioluminescence, the emission of light by a living organism, is a phenomenon that is actually quite common among creatures in the ocean, but it continues to mystify us dull humans. Shared by some of the Earth’s strangest creatures – jellies, nudibranchs, squid, and the grotesque anglerfish to name a few – bioluminescence is a trait that adds to their mysterious appeal. Bioluminescence is fascinating to many of us, but its varied mechanisms and evolutionary purpose are not well understood. Some organisms manage their shine through a series of chemical reactions, while others rely on the glimmer of symbiotic bacteria. This ability to glow can be used for communication, attraction, and camouflage. It was only recently that the flashes of fireflies, one of the few terrestrial biolumineers, were translated, earning major coverage in the New York Times (see “Blink Twice if You Like Me” by Carl Zimmer, 6/29/09). Now scientists from NOAA are taking their search for biological shine to the bottom of the ocean.

From July 20 – 30 Doctors Tamara Frank (HBOI@FAU), Sönke Johnsen (Duke), Edith Widder (Ocean Recon), Charles Messing (Nova Southeastern) and Steve Haddock (MBARI) will be studying bioluminescence on the deep-sea floor off the Bahamas. While bioluminescence in pelagic (open water) organisms is well-studied, information on benthic (living near the ocean floor) organisms in deep-sea areas is still limited due to the difficulty of collecting live specimens. To get a better look, these researchers will be deploying the Johnson Sea-Link Submersible to sit among the glimmering animals of the ocean floor. They are also baiting the deep-sea ORCA Eye-in-the-Sea camera to get up close and personal images of some voracious predators. You can follow the expedition of Bioluminescence Team 2009 on NOAA’s Ocean Explorer through daily video logs, podcasts, and amazing photographs of never before seen ocean activity. The attack by Cuban Dogfish and the shimmering Sea Pens are not to be missed!

Learn more about the organisms that really shine on NOAA's Ocean Explorer!

(Photo: Luminescing Bamboo Coral, Bioluminescence Team 2009 NOAA-OER)

The Green Sturgeon's Dangerous Diet

The enigmatic Green Sturgeon (Acipenser medirostris) patrols the benthos of the San Francisco Bay and near shore oceanic waters as a living relic of ancient seas. This large, long-lived fish species, which has persisted for millennia in evolving oceans, may have finally met its match in the environmental impacts of Bay Area development. The Southern Distinct Population Segment (DPS) of the Green Sturgeon, which is listed as threatened under the federal Endangered Species Act, swims through the San Francisco Bay to reach its only remaining spawning ground in the Sacramento River. As these animals swim through the Bay and Delta, they face deteriorating water quality, reduction of freshwater flows, potential poaching for caviar or bycatch in other fisheries, entrainment in water intake structures, and impassable upstream barriers. The principle threat to the Southern DPS is the disappearance of its spawning ground. Now restricted to a very narrow stretch of the Sacramento River, the elimination of the remaining spawning ground would mean the extinction of this genetically distinct Green Sturgeon population. The National Marine Fisheries Services (NMFS) is strengthening protective measures of the Southern DPS by proposing the same take prohibitions that are applied to species listed as endangered. Take prohibitions will make it illegal to hunt, harass, or otherwise harm the fish, including any action that degrades its critical habitat.

Unfortunately this step may not be enough to ensure the long-term survival of the population. A combination of human-influenced factors, now woven into the ecological fabric of the San Francisco Bay and Delta, has turned this estuary into a toxic environment where animals are actually poisoned by their food web. The Green Sturgeon feeds on invertebrates that it finds bottom of the Bay, including the invasive Overbite Clam (Potamocorbula amurensis). This prolific bivalve, which can be found in densities as great as 50,000 per square meter in some areas of San Francisco Bay, is likely to compose the better portion of the Green Sturgeon’s diet. The Overbite Clam itself has a voracious appetite, and it rapidly filters food particles out of the water column with amazing efficiency. Although it may be seeking tiny organisms and detritus, the clam inadvertently consumes the contaminants that pollute the water column including selenium, a bioaccumulative element that comes from oil refineries and Central Valley agriculture. The efficiency with which it incorporates selenium into its tissues makes the Overbite Clam a toxic meal for any predator it succumbs to.

The impact of selenium may not be immediately apparent in the individual adult fish, but it can cause massive reproductive failure. Selenium from a diet of contaminated Overbite Clams will bioaccumulate in fish tissues over the course of its lifetime. Although it may not harm the adult fish, selenium is transferred from a female fish to her eggs, which can cause embryonic death or fatal deformities upon hatching. These reproductive impacts can be severe enough to devastate a population. All fish species in the San Francisco Bay that feed on the Overbite Clam are at risk; however, the unique life history of the Green Sturgeon makes it more vulnerable to this poisonous prey.

In many ways the life of the Green Sturgeon is similar to that of a human. It has a remarkably long lifespan of up to 70 years, and it does not reach sexual maturity until it is at least 15 years old. As an adult it has an iteroparous reproductive strategy, meaning it allocates energy to multiple spawning efforts over the course of its lifetime as opposed to a “big bang” spawning effort once before death. The Southern DPS of the Green Sturgeon, which spends the majority of its adult life in the ocean, returns to the Sacramento River every 2 to 5 years to spawn. Even though it spends more time in the ocean than other sturgeon species, Southern DPS adults may live in the estuary for seven months of the year and juveniles can live here year-round, all the while exposed to dietary selenium.

Given its late age of sexual maturity, the Green Sturgeon will accumulate selenium in its body for at least fifteen years before it first spawns. The level of bioaccumulated selenium imparted to eggs will then increase for all subsequent reproductive efforts as the sturgeon ages. As a result the reproductive impacts of selenium are likely to be more severe for the Green Sturgeon than for a fish with a shorter lifespan and quicker maturation. This reproductive challenge can greatly reduce recruitment within the Southern DPS, meaning that fewer individuals are surviving to reproduce and maintain the size of the population. With this obstacle to the population’s survival so deeply rooted in the ecology of the San Francisco Bay and Delta, the actions that result in the take of an individual animal seem relatively easy to avoid. The proposed take prohibitions are a necessary step in slowing the decline of the Southern DPS, but they are by no means the solution. The elimination of selenium discharges to the San Francisco Bay and Delta may be the only way to ensure the population’s long-term survival.

(Photo credit: David Gotschall)

After 150 Years - Clarity and Consequences

Earlier this spring during a spate of unusually hot weather in the Bay Area, rays of sunlight stretched below the surface of Richardson Bay to trigger an intense algal bloom. Like all blooms, this rapid proliferation of algae was encouraged by the warm temperatures and an adequate supply of nutrients. About a month later another bloom occurred. Both events resulted in floating clumps of innocuous red algae and calls to the Baykeeper pollution incident hotline from concerned shoreline residents. My response: Don’t panic. If it doesn’t smell, it’s not a sewage spill.

According to an article by James Cloern, et al in the 2006 Pulse of the Estuary, Bay Area residents should become familiar with this sight. Algal blooms have been occurring with increasing frequency in the San Francisco Bay since the late 1990s, and the trend is likely to continue. The cause has been uncertain, however, because a host of factors promote the growth of algae. These include predators, nutrients supply, temperature, and metals. In every ecosystem one of these variables must be the limiting factor that controls algae growth and prevents bloom events. In many aquatic systems, such as the Chesapeake Bay, nutrients are the limiting factor. Given the excessive agricultural runoff in the Chesapeake Bay watershed, it is no surprise that algal blooms have been a serious problem. Despite the always reliable winter sewage spills, however, nutrient levels in the San Francisco Bay have been consistently low. So what is the variable that allows this unusual and unseasonable growth of algae? The upcoming 2009 issue of the Pulse of the Estuary will shed more light onto this question. The answer, in fact, is light.

The San Francisco Bay is becoming clearer! The concentration of suspended sediment in the Bay has been steadily decreasing since 1999, allowing sunlight to reach further below the surface of the water, stimulating algae growth and causing blooms. Incredibly, the reason for our water clarity today stems from human activities during the Gold Rush Era. In the late 1800s hydraulic gold mining sent tons of sediment, waste from the search for gold in the Sierra foothills and the Coast Range, down the Sacramento River and other Central Valley rivers. At the same time, development in the Bay Area caused the erosion of stream banks. Shoreline tidal marshes that were diked off to increase buildable and farmable land area could no longer capture this eroded sediment at the shore before reaching open water. As a result, the sediment settled on the floor of the Bay – so much sediment in fact, that the Bay became shallower. Bay Area residents are very familiar with the dredging platforms that regularly remove sediment, carving navigation channels into the floor of the Bay. In addition to dredging, natural wave patterns and burrowing wildlife can stir up sediment and re-suspend it in the water column. High concentrations of suspended sediment reduce the depth to which sunlight can penetrate the water, thus controlling algae growth and preventing most blooms.

Recent USGS data suggest that the Bay experienced a dramatic increase in clarity when this erodible supply of sediment was depleted in the late 1990s. In this year’s Pulse of the Estuary, David Schoellhammer of USGS offers an explanation as to how this may have happened. As long as the San Francisco Bay received sediment from upstream sources and held suspended sediment at capacity, erosion from the floor of the Bay was minimal. Although the Sacramento River delivered sediment, it also gently flushes the Bay and gradually pushed sediment through the Golden Gate. River banks in the Central Valley were protected during the 1900s to prevent erosion, and other sources of sediment are trapped behind dams. The remainder of the hydraulic mining supply slowly moved downstream until it reached the Bay. As a result the Sacramento River delivered clear water, which increased the erosion of sediment on the Bay floor. In 1998, a wet year during which the strong, clear flows from the Sacramento River persisted well into the summer, most of the remaining sediment supply was likely eroded pushed out of the Bay. The following year saw the suspended sediment concentration of the Bay waters decrease by 50%.

This great sweep of sediment through the Golden Gate did not unearth an ecological time capsule to the Bay’s pre-Gold Rush condition. The subsequent increase in clarity in the Bay is a major shift in water quality, which is causing a cascade of ecological and economic consequences in light of modern environmental stressors. As Bay Area residents have recently witnessed, the low concentration of suspended sediment in the Bay makes more light available to stimulate the growth of photosynthetic organisms – aquatic plants, algae, and other phytoplankton. As these organisms thrive they feed higher trophic levels, and the Bay food web becomes more robust. As Schoellhamer points out, the San Francisco Bay has crossed a threshold and become an estuary with a level of primary production that is more typical of temperate latitudes. This increased productivity has implications of its own. With a greater availability of light, nutrient inputs have a greater impact in the growth of algae. While the San Francisco Bay regularly receives nutrients from agricultural runoff or sewage spills, the low light has always prevented excessive growth of phytoplankton. Under current conditions, however, these inputs may trigger more intense bloom events and their associated problems.

The loss of sediments may also hinder coastal wetland restoration efforts. Wetland restoration usually involves opening up a previously diked area to the tides, so that suspended sediments in the water will naturally settle out along the shore, gradually building up until the land is high enough for plants to colonize. The lower the concentration of suspended sediments in the water, the longer it will take for the wetland to develop. Now with rising sea levels threatening to inundate our shorelines, the growth of new wetlands will likely be outpaced. To speed up the process, wetlands restoration projects may also utilize dredge spoils. With the loss of sediment from the Bay bottom, however, there is less of a need for dredging and a limit to sediment available for these restoration projects. Incredibly, the natural expulsion of sediments from the Bay, which caused supplies to shrink while demand has recently grown, has changed hidden Gold Rush waste into a valuable natural resource.

Learn more about Bay sediment in the 2009 Pulse of the Estuary, from the San Francisco Estuary Institute. The Pulse is the annual report for water quality in San Francisco Bay. You can find it at www.sfei.org

(Photo Credit: Michael Slater 2006)