Plants hitching a ride

Growing up, I spent my summers exploring the lakes of New York’s Adirondack Mountains (I recently wrote about one Adirondack lake in particular here). I loved to swim, but I did not love swimming through patches of plants growing up from a lakebed – the feeling of their tendrils swaying in the wake of my passing, clinging to my skin as if they wanted to grab my body and pull me down into the depths of the water, was enough to send me thrashing back to the lakeshore.

Eurasian water-milfoil, an aquatic plant native to Eurasia and northern Africa that has spread across much of North America, grew in such abundance in Upper Saranac Lake (a lake that I swam in many times as a child) that a local foundation raised $1.5 million to begin removing the plant from the lake in the early 2000s. Invasive aquatic plants often thrive and proliferate in their new environments so much that they crowd out native plants, degrade fish habitat, and clog waterways, preventing them from being used for boating or swimming.

The control effort in Upper Saranac Lake – which involves sending divers into the water to pluck milfoil by hand – has been largely successful at reducing the amount of the plant in the lake (in recent years, divers have collected roughly one-fortieth the amount of milfoil harvested at the beginning of the project), but if the removal stops, Eurasian water-milfoil could quickly rebound – meaning maintenance dives will have to continue, and someone will have to keep paying for them, indefinitely.

Dealing with invasive species in aquatic environments is expensive and time-consuming, and, as is the case with Eurasian water-milfoil in Upper Saranac Lake, they often cannot be completely eliminated. Perhaps the best offense, then, is a good defense – if invasive aquatic plants aren’t allowed into lakes to begin with, then no one has to dive down to the lakebed to remove them later.

One way for aquatic plants to spread among lakes is to hitch a ride on boats or trailers. If the bits of plant matter that get wrapped around a boat propeller or caught in the wheel wells of a trailer can survive out of water long enough to reach the next lake a hapless boat-owner visits, the plant might be able to spread to that lake. (This is why many states require boaters to wash their boats between lake visits.)

Research recently reported in the journal Hydrobiologia shows that some plant-parts are particularly adept at surviving dry spells. A team of scientists working in northern Wisconsin collected stems from two aquatic plants, Eurasian water-milfoil and curly-leaf pondweed, as well as buds from the pondweed, allowed them to dry outdoors (in order to simulate the conditions plants caught on a boat or trailer might experience), then placed them back in tubs of water to see if they were still capable of growth and, presumably, establishing themselves in a new lake.

Single plant stems were able to grow after up to 12 to 18 hours of drying, and stems that were coiled, as if twirled around a boat propeller, grew after up to 48 hours out of the water. The curly-leaf pondweed buds were able to survive for the longest – some sprouted after 28 days on land.

“The high cost and difficulty of eradicating introduced invasive species makes preventing secondary spread a management priority,” the authors write. Knowledge of how aquatic plants are able to spread between lakes, and how long they can survive out of the water, can help lake managers develop guidelines for cleaning boats and, hopefully, eliminate the need to send divers down to weed invasive plants from the beds of any more lakes.

Native aquatic plants are an important part of lake ecosystems, but invasive species, like Eurasian water-milfoil in North American lakes, can grow so much that they take over, killing other plants and preventing boating and swimming. 

(Image by dhobern via Flickr)

Manatee stressors

Last month, a herd of 19 manatees briefly made a splash on the national news circuit when they got stuck in (and were subsequently rescued from) a storm drain in Florida. The group swam into the drainpipe in an apparent attempt to find warmer water during a cold snap – manatees are sensitive to cold water, and temperatures below 68 degrees Fahrenheit can hurt or even kill them.

Florida manatees – marine mammals that are about 10 feet long and weigh 800 to 1,200 pounds – also face threats from other sources, including boat collisions, habitat loss, and red tides, or harmful algal blooms. Concentrations of the dinoflagellate Karenia brevis are the most common cause of red tides in the Gulf of Mexico; these microorganisms produce a suite of toxins, called brevetoxins, which are harmful to humans and other vertebrates, and can kill hundreds of manatees at a time. (The overall population of Florida manatees is estimated at 5,000 to 6,500 individuals, so hundreds of deaths during a single event is a big deal.).

Although many Florida manatees that encounter red tides succumb to the toxins the algae produce, some manage to survive (sometimes with the help of humans). A team of scientists working in Florida reported the results of a study they conducted on the adverse effects of red tide exposure on surviving manatees in a recent issue of the journal Aquatic Toxicology.

The researchers collected blood samples from 12 manatees that were rescued from a red tide, and from 11 free-ranging manatees from a different, red tide-free location. They measured several parameters that indicate immune response, as well as the concentration of brevetoxin in each manatee’s plasma.

Some immune system indicators were the same regardless of whether or not the manatees had been exposed to the red tide, but others, particularly lymphocyte proliferation (a measurement of the body’s ability to defend itself against pathogens), showed that manatees that endured a red tide displayed reduced immune system functioning. The scientists also found that the manatees with the highest concentrations of brevetoxin in their plasma tended to have the lowest lymphocyte proliferation scores. As the authors point out, the effect of red tides on manatee lymphocyte proliferation “has the potential to result in an immunosuppressed animal that could likely exhibit greater susceptibility to other stressors.”

Florida manatees encounter many hazards in their environment; according to this research, even threats that don’t prove immediately lethal may have lasting negative consequences.

Florida manatees at Crystal River National Wildlife Refuge, located on central Florida's gulf coast. 

(Image by U.S. Fish & Wildlife Service/David Hinkel via Flickr)

Native or not?

“I think it’s important for people to rethink how we see lakes and the things that live in ’em,” says Dr. Curt Stager, professor of biology at Paul Smith’s College, nestled in the heart of New York’s Adirondack Mountains. Stager says we’re used to thinking of lakes as playgrounds and resources, places where we go to catch fish or cool down on a hot summer day, but we should also recognize that lakes, and the organisms that live in them, might have something to teach us. “They’re marvels of evolution,” Stager says. “They’ve got amazing stories.”

One lake in particular has a story to tell, a story that has been collecting in the sediment at the bottom of the lake for more than 2,000 years. It’s a story that the humans living on the shores of the lake haven’t been able to hear, until now.

Yellow perch have long been considered non-native to the Adirondacks – the range map for the species shows a big blank spot in the northeastern corner of New York State, where the Adirondacks are located. Because of its non-native status, the New York State Department of Environmental Conservation has eradicated yellow perch (and other non-native fish) from several ponds to make way for brook trout, a highly prized native species which has declined in recent decades.

But what if yellow perch aren’t non-native to the lakes and ponds of the Adirondacks?

“I was skeptical that perch were not native to the Adirondacks,” Stager says. “It just doesn’t make sense that they’re in mountainous areas all over eastern North America . . . and then there’s just one little spot here where they’re not supposed to be.”

So Stager and a team of researchers from Paul Smith’s College decided to investigate the claim that yellow perch were introduced to the region, rather than a native species. They recently published the results of their study in the journal PLoS ONE.

As aquatic plants and animals living in a lake go about their lives, shedding cells and scales and waste products, that material falls through the water and builds up on the bottom as sediment – the top layer of sediment contains the most recently shed cells and scales, and deeper layers hold progressively older records of what used to live in the lake.

On a bright and cold winter day, Stager and his team drilled through the snow and ice covering Lower St. Regis Lake, on the edge of Paul Smith’s College’s campus, and used a long, skinny tube to extract a core of sediment, just under four and a half feet tall, from the bottom of the lake. The deepest part of the sediment core they lifted out of the lake was between 2,131 and 2,315 years old – well before the relatively recent era of non-native fish introductions in the Adirondacks.

The researchers took samples from the middle of the core, where the edges of the tube couldn’t have smudged and mixed the sediment, and analyzed them for yellow perch DNA.

Study co-author Dr. Lee Ann Sporn extracting samples from a sediment core in the lab. 

(Image by Dr. Curt Stager)

They found evidence of yellow perch throughout the entire sediment core – meaning that yellow perch have likely been living in Lower St. Regis Lake for over 2,000 years. (They also analyzed samples from cores taken from lakes without any yellow perch, to make sure that they hadn’t contaminated their tools with perch DNA, and those samples all came back negative.)

Based on the evidence contained within its sediment, Lower St. Regis Lake is telling us that yellow perch are native to the Adirondacks after all.

Still, something has been changing in the lakes of the Adirondacks – as brook trout declined, yellow perch became more numerous than they were in the past. In the 1800s and early 1900s, there were so few perch in the Adirondacks that naturalists usually didn’t find them at all during net surveys – which is why they were thought to be non-native in the first place – but as Stager says, “a net survey is not a reliable way to tell if something’s not there. If you catch it, you know you have it, but if you don’t catch it, it doesn’t mean it’s not there.”

So why have yellow perch done so well in recent decades? Stager says there are lots of possible explanations – climate change and warming lake temperatures, overfishing of brook trout, human activity fueling ecosystem production, and probably others, too, all of which created conditions that were detrimental to brook trout but advantageous to perch.

Stager and his team only looked for yellow perch DNA in the core from Lower St. Regis Lake, but they have big plans for the future. Brook trout are cherished partly because there are unique strains of the fish that exist only in the Adirondacks; now that they know that yellow perch have been there for thousands of years, the researchers wonder if they could have evolved unique strains, too. Stager believes he can use the DNA stored in sediment cores to reconstruct the history of entire lake communities.

And he hopes that people will start paying closer attention to the stories that lakes can tell us. “I hope this kind of thing really helps us see there’s a much richer heritage we’ve got here than just something to snag on a hook.”

The researchers drilled through snow and ice to extract a sediment core from Lower St. Regis Lake in the Adirondack Mountains of New York State.

(Image by Dr. Curt Stager)

Turtle bycatch

When you’re standing knee-deep in a stream, branches growing together into a leafy green tunnel above your head, slippery cobbles under your feet, expecting to find a steelhead or two in the net you just jerked out of the water, it can be a little startling to see a five-inch crayfish staring back at you, waving its claws.

Sometimes at work, when I’m lucky, I get to step outside my office for a little while and help the graduate students in my lab as they collect samples and data in the field. The project I work on the most involves scooping up fish in hand-held nets, but occasionally we capture other things, too – organisms we weren’t targeting or trying to catch, like that five-inch crayfish. In the fisheries world, this accidental take is called bycatch. Usually when scientists and managers are studying or trying to regulate bycatch, they’re dealing with marine life, but it happens in freshwater fisheries, too – and not just with crayfish.

When I catch a crayfish in my net, I pull it out and set it back in the stream where it came from (carefully, to preserve my fingers), but that’s not always possible in other situations. Some fishing nets are set underwater, and left to soak and collect fish for several hours before the catch is retrieved; that can be a problem if air-breathing aquatic animals – like turtles – also get caught.

A team of Canadian scientists recently published a case-study of freshwater turtle bycatch in a small lake in the journal Aquatic Conservation: Marine and Freshwater Ecosystems. They estimated the effectiveness of two methods for reducing turtle bycatch in Lake Opinicon, in southeast Ontario, Canada, which supports a small commercial fishery: adding “exclusion devices” to fish net openings that would keep most turtles out but let fish in, and shortening the fishing season.

The researchers measured the shells of four species of female turtles, and found that the smaller exclusion device they evaluated (which had an opening about two inches wide) would keep most turtles out of the fishnets – between 92 and 100 percent of three turtle species in the lake had shells too large to fit through the device. For the fourth and smallest turtle species the scientists studied, however, the exclusion device was not as successful – only 27 percent of those turtles were bigger than the opening.

If managers close the fishery during the time when turtles are most active, that can reduce turtle bycatch; to evaluate the effect of closing the fishery a month earlier in the spring, the scientists measured how active the turtles were during May and June. Only one turtle species was most active right before the current closing date of June 20, but, as the authors note, “[w]hile seasonal activity rates vary among species, a shortened fishing season would still decrease the total number of turtles captured, provided that there is no compensatory increase in fishing effort.”

Based on demographic traits like the current number of turtles in the lake, how old female turtles are when they first reproduce, and the proportion of female turtles that reproduce each year, the researchers estimated how the population of each turtle species might increase or decrease over the next 500 years under different bycatch scenarios. They determined that for three of the turtle species, just one or two female turtle deaths due to bycatch each year could lead to a complete loss of the population from the lake within that timeframe.

When I catch a crayfish in my hand-held net I can let it go right away, but turtles caught in underwater fishnets don’t have that option. The unintended capture of turtles during fishing can have disastrous consequences – as the authors write, “it is imperative that appropriate bycatch mitigation measures . . . are put in place to ensure the long-term persistence of freshwater turtles.”

Researchers studied four turtle species in Lake Opinicon, in southeastern Ontario, Canada, including the painted turtle. 

(Image by Micheal Jewel via Flickr)