Polluted protected areas

Artificial light can have large consequences for stream communities – aquatic insects, for example, are less likely to drift downstream in the presence of streetlights. Because some aquatic organisms use natural variations in sunlight as cues telling them when and where to migrate, or eat, or gather up into a group, artificially altered light patterns can disrupt those behaviors.

Marine protected areas, despite their status as ocean zones that have been set aside and (to some extent) shielded from human impacts, are affected by light pollution, too.

A group of researchers from the University of Exeter, in the United Kingdom, recently published an analysis of just how much artificial light is reaching the global network of marine protected areas. By analyzing satellite images taken at night, the scientists were able to quantify the extent of light pollution within marine protected areas, and how that number changed over 20 years, between 1992 and 2012.

The researchers found that in 2012, 35 percent of the protected areas they analyzed were exposed to some degree of artificial light. Of those areas, more than half experienced “widespread” light pollution, indicated by visible illumination present in all of the image pixels within the protected area.

Furthermore, although the majority of protected areas did not experience any change in artificial light levels over the 20 years the scientists analyzed, light pollution increased in 14.7 percent of the protected areas. (The authors note, however, that a lack of change doesn’t indicate a lack of light pollution – it simply means that light levels were constant between 1992 and 2012.)

“Given the importance of light in guiding the behaviors of many marine species,” the authors write, “these results suggest that nighttime lighting may influence the ecology of many of the most valued regions of the ocean.”

The first step in mitigating the negative consequences of artificially illuminating marine protected areas is to determine the extent of the problem. Now that researchers are aware of which areas are particularly impacted by light pollution, resource managers can begin addressing the problem by “[s]witching off, dimming or shielding lights, preserving naturally dark landscapes, and limiting use of spectra known to cause ecological impacts,” among other possible solutions.

Marine protected areas provide shelter from human activities for marine organisms like the sea lion pictured here. 

(Image by NOAA's National Ocean Service via Flickr/Creative Commons license)

Stuck sediment

The flowing course of a river carries with it more than just water – insects and other living things move downstream, too, and so do rocks, trees and branches, and sediment. When a dam is built, the fast-moving water of a river transforms into the slow-moving water of a reservoir, and stuff that might have moved downstream in the past can get stuck behind the dam.

River restoration and dam removal projects have proliferated in recent years, leaving scientists and resource managers wondering what happens to the multiple decades-worth of accumulated sand and mud that can built up at the bottom of a reservoir when the dam that formed it is suddenly gone.

When the two dams on Washington State’s Elwha River were demolished, more than ten million cubic meters of sediment were released into the river and allowed to flow downstream to the river’s estuary where it meets the Strait of Juan de Fuca. You’d need to rent more than two hundred and twenty-seven thousand 26-foot U-Haul moving trucks – the largest size they offer – to move that much sediment by truck.

A team of scientists, curious about how that much sediment would affect the Elwha River and its estuary, monitored two ‘pocket estuaries,’ small areas protected by barrier beaches but influenced by the tide, both before and after the dams were destroyed.

The researchers recently published the results of their study: the sediment rode downstream to the lower river, where it settled in the river channel and pushed the river delta more than 100 yards further into the sea, cutting the estuaries off from the influence of salt-water tides and filling them instead with water from the river, “changing the estuary from a brackish and tidally influences system to a perpetually freshwater system.”

When the river was dammed, water quality measurements (including salinity, depth, and temperature) varied according to the tides; after the dams were dismantled, they fluctuated in response to the amount of water flowing down the river. These physical changes to the estuary habitat have already altered the biology of the place – different insects live there now, and fish communities and diets are shifting as well.

The authors write that “[t]he removal of the [two] dams and the subsequent delivery and deposition of sediment to the river delta has caused the Elwha River system to lose its small, but important estuary habitat.” They also note, however, that “the potential for new estuary habitat to develop is high.”

Though the dams have come down, the Elwha River is still changing – and scientists plan to monitor the evolution of the river for years to come.

This photo was taken in 2012, during the dam removal project on the Elwha River. Sediment released by the demolition was deposited in and around the estuaries at the river mouth; some sediment also flowed into the Strait of Juan de Fuca as a coastal plume.

(Image by John Felis via USGS/Public domain)

Crab claws

Sand fiddler crabs are a common sight on the beaches of the eastern U.S. The small creatures (typically about an inch wide, not including their legs) are distinguished by the unusual asymmetry exhibited by the claws of the males – one claw grows to almost grotesque proportions, longer than the body of the crab itself.

The males use their claws to defend their breeding burrows from other males, and as an advertisement to females looking for mates – they stand at the opening to their burrows, waving their giant claws in circles, trying to entice female crabs to join them.

In areas with a semidiurnal tide regime, sand fiddler crab mating tends to peak during spring tides, which occur when the moon is full or new – when the gravitational forces between the sun, the moon, and the earth line up to create higher high tides and lower low tides, and the ‘tidal flux’ (the difference between the high tide and the low tide) is especially large. The sand fiddler crab larvae that result from mating during a spring tide are released during the next spring tide, when that greater tidal flux can help the larvae survive.

A group of researchers working on the western coast of Florida recently investigated whether or not male sand fiddler crabs’ claw-pinching strength follows the lunar cycle to peak during spring tides, too.

Their results, published in the Journal of Experimental Marine Biology and Ecology, suggest that male sand fiddler crab strength and behavior does correspond to the lunar cycle. The scientists found that crabs who were courting females and defending burrows (which were located in relatively high and dry areas without much food nearby) had more force behind their claw-pinches than crabs roaming around in large groups, or droves, looking for something to eat, and that both the number of courting males and the males’ claw strength peaked during new and full moons.

Courting females and defending burrows appear to be energetically costly activities, and male crabs periodically took a break from mating activities to restore their strength by eating; as the scientists note, “claw power declined during courtship and increased while droving.” In general, the strongest male sand fiddler crabs were able to synchronize their eating-and-mating cycle with the lunar and tidal cycles, allowing them to stand outside their burrows waving their claws during the times when they were most likely to successfully mate.

A male sand fiddler crab will try to attract females to its breeding burrow by waving its large claw in circles above its body.

(Image by Rachid H via Flickr/Creative Commons license)

Dynamic adaptation

Elegant terns roam far and wide during the course of a year: the seabirds, dapper despite their raggedy black mohawks, migrate as far south as northern Chile during the (North American) winter, then range up the U.S. Pacific coast after breeding during the early summer.

During the breeding season, however, almost the entire worldwide population of elegant terns congregates in one place – Isla Rasa, a tiny island (just one quarter of a square mile in size) in the middle of Mexico’s Gulf of California. 

At least, that’s what used to happen.

In recent decades, as the population of elegant terns has grown, scientists have noticed a shift – during some years, elegant terns have abandoned Isla Rasa in favor of breeding colonies in southern California. In a study recently reported in the journal Scientific Advances, a group of researchers details their investigation into a few possible explanations for the move.

The researchers monitored elegant tern nests on Isla Rasa as well as three breeding colonies in southern California, starting in 1980 at Isla Rasa and 1991 at the California sites. After the year 2000, there were four years during which the sea surface temperature was anomalously high around Isla Rasa – and during each of those years, more than 70 percent of the nests the scientists found were in California. During yeas without particularly high sea surface temperatures around Isla Rasa, less than 20 percent of the elegant tern nests were located in California.

Higher sea surface temperatures may make fishing more difficult for the terns, driving them away from Isla Rasa to the north; the researchers also found some evidence that more intense human sardine fishing may push the birds away from Isla Rasa, too. (Sardines and other small fish are elegant terns’ main source of food.)

The shift in where elegant terns are nesting is not necessarily a bad thing for the species – in fact, it’s probably at least partially due to a recent population boom, which, as the scientists note, “seems to be pushing reproductive pairs onto new nesting grounds in California.”

The ability of elegant terns to shift where they nest from year to year suggests that they “can make fast decisions and dynamically adapt to rapid changes in the global environment,” a useful skill for any species.

A group of elegant terns on a fishing boat in Chile.

(Original image by tk-link via Flickr/Creative Commons license)