As she crouches precariously on the uneven shoreline of the Sian Ka'an Biosphere Reserve, a protected area just outside of Tulum in Mexico's Quintana Roo state, Cristina Mittermeier shakes her head. "Some days, it's hard to be optimistic," says the 52-year-old marine biologist, photographer and co-founder of ocean conservation nonprofit SeaLegacy.

Instead of the pristine white beaches you see in Tulum tourist brochures -- Quintana Roo is home to the resort city of Cancún -- the ground below us is a tangle of plastic debris and crunchy, stinking seaweed, piled high over the packed sand. The brown mass undulates down the coast like rotting sand dunes; the persistent wind carries its rotten egg stench far beyond the boundaries of the beach. As Mittermeier rifles through the plastic graveyard taking photos, her feet sink into the mass with each step.

Mittermeier isn't here as a tourist. Her mission is to protect the oceans by sharing arresting images of climate change and the local communities most affected by it. Her hope is that the images -- her Instagram account has 1.2 million followers -- will act as a catalyst for a movement, inspiring more people to advocate for the environment.

Oceans are on the front line of climate change. They cover 70 percent of the world's surface and contribute more than half of the oxygen we breathe every day. They regulate the climate and provide food and ingredients used in medicine for cancer, heart disease and Alzheimer's disease. But the oceans are in danger. They work overtime to absorb the heat generated by carbon dioxide and other greenhouse gases. The gases change the pH of the water, damaging coral reefs and harming marine animals, which more than 3 billion people rely on for their main source of protein.

"The oceans are so big and so remote that people don't understand how our food is caught, and just how fragile [the oceans] are," Mittermeier says. "Don't you want to join in this vision of a future where our children can aspire to a clean beach -- and maybe some breathing air?"

Not only does sargassum seaweed smell awful, it poses health risks to humans and sea life when it reaches the coast. Sarah Tew/CNET

Stinking seaweed

The seaweed is called sargassum -- "it sounds like a sassy orgasm," Mittermeier had joked earlier, sounding out the oddly named algae. Sargassum began to take over beaches like Sian Ka'an and other areas of the Caribbean Sea and Gulf of Mexico in 2011. Studies link its coastal invasion to deforestation in the Amazon. As forests in Brazil are cut down to make way for farmland, fertilizer used for the crops flows directly into the Amazon River when it rains. Eventually, the fertilizer reaches the ocean, changing the nutrients in the water and causing sargassum to bloom out of control.

As we walk on the beach, each wave brings in more sargassum, piling it higher and higher over sand once frequented by turtles to lay their eggs. There are bottle caps, toothbrushes, lighters -- pieces of people's lives, long ago discarded -- that surfed the ocean currents to get here.

I need to get up tomorrow and try even harder than today, because I cannot imagine a planet in which my children have to live in this postapocalyptic world. Cristina Mittermeier

Candace Crespi, campaigns manager for the Blue Sphere Foundation, is here, too. The Blue Sphere Foundation is an ocean conservation organization that acts as SeaLegacy's fiscal sponsor, making it possible for SeaLegacy to ask for grants and tax-deductible donations under Blue Sphere Foundation's nonprofit status. Crespi is Mittermeier's assistant in the field, but like Mittermeier, she's also a biologist, a conservationist and an experienced diver.

The two form a strong partnership. Mittermeier calls Crespi her Swiss Army knife, because of her varied skills; Crespi considers Mittermeier a role model. "Cristina is the epitome of selfless commitment without ego ... always humble and willing to go the extra mile to make this world a better place for all beings," Crespi says. "Watching Cristina in her element makes it impossible not to be inspired and want to help in some way."

Crespi (left) and Mittermeier (right) work together to get videos of plastic waste and sargassum for Instagram. Sarah Tew/CNET

Mittermeier has dedicated her life to protecting oceans and documenting climate change in far-flung places, from Antarctica and the Galápagos Islands to French Polynesia and beyond, but she began walking these very beaches 30 years ago. She now lives in British Columbia with her partner and SeaLegacy co-founder, Paul Nicklen. But she returns to her native Mexico regularly. Each time she visits, she notices drastic changes.

On her last trip to Sian Ka'an five years ago, plastic waste was the main problem. Now the sargassum overshadows the plastic. It's the new "normal," it seems. But even when she's surrounded by mountains of garbage and an invasive seaweed, she's far from resignation. "I need to get up tomorrow and try even harder than today, because I cannot imagine a planet in which my children have to live in this postapocalyptic world."

Over email, Nicklen says Mittermeier is extraordinarily compassionate, but feisty when necessary. "She's not scared of diving with sharks or jumping into Arctic waters, and she doesn't back down when presented with an opportunity to stand up against environmental or social injustices."

Her advice to herself, and to anyone else overwhelmed by environmental issues, is to take action today. "I want a better future for them and for your children, too," she says. "I get up [every day and do this work] because I have to. And if I can do it with a smile, even better."

Dreaming of dolphins

Mittermeier was born in Mexico City in 1966 and grew up in Cuernavaca, a town of about 350,000, roughly two hours south of the capital. She's the second of five siblings; she has an older brother and three younger sisters.

She fell in love with the ocean at a young age, even though her childhood in landlocked Cuernavaca didn't exactly set her up for this life. "As a young teenager I imagined myself swimming with dolphins, but I didn't know how to make it happen," she says.

Mittermeier snorkels in Casa Cenote, a sinkhole near Tulum. Candace Crespi

Her dad was an accountant, her mom a psychologist. Though her parents didn't have a "special affinity to nature," as Mittermeier puts it, they encouraged her early love for it. She attended summer camp in the US and in Canada, where she learned English, swam in icy lakes and learned how to canoe and kayak. At home, she'd sneak into her brother's room and read his pirate books, imagining far-off places.

Science also drove her education. She earned an undergraduate degree in biochemical engineering in marine sciences from the Monterrey Institute of Technology and Higher Education in 1989 and at her friend's urging, moved the same year to Akumal, a coastal town 30 minutes north of Sian Ka'an.

She got a job cataloging the wildlife in the area through her friend's uncle and helped establish protections for turtle nesting sites, which led to the development of an ecological center that's still there today. Mittermeier also earned her dive certification in Akumal in 1989. Akumal is just 30 minutes north of Tulum, so we make a brief stop there on our way to Sian Ka'an, heading south from Cancún. A lot has changed since her last visit five years ago.

Now teams employed by hotels and restaurants, armed with pitchforks, gather sargassum and fling it into trailers pulled by ATVs, stinking and heavy from the salt water. It's a daily chore for employees in Akumal to remove the seaweed, a man tells us, pitchfork in hand. The sargassum gets dumped behind the hotels where it waits in piles until a larger truck, and another team of people, haul it to a landfill.

In Akumal, teams of people employed by area hotels and restaurants use pitchforks to load sargassum onto trailers. Sarah Tew/CNET

While the seaweed is a "critical marine habitat" out in the ocean, it's problematic on the coast, says Mengqiu Wang, a postdoctoral researcher at the University of South Florida College of Marine Science. "It has an unpleasant smell, it covers up the beach, it's detrimental to local tourism and it has been reported to harm people's health."

Research ties sargassum on the beach to respiratory issues like asthma, headaches and even memory loss in humans. Even as it dies in coastal waters, it uses up more oxygen, creating a low-oxygen environment that's unhealthy for fish and other sea life.

"People are not coming here [because of the sargassum]," one of the people removing it says. Akumai isn't crowded, especially for a weekend in late July, but it isn't empty either. I see a young couple sitting together on a beach chair, taking selfies. A family with young children walks on the beach. Some braver souls traverse the sargassum to swim in the ocean.

Strangely, the sargassum and the plastic waste seem normal, both to the tourists and the locals we encounter. At the very least, they've adapted to it. Mittermeier doesn't accept this. "I know what the problem is and I can actually do something, so I have to," she says defiantly, as she snaps photos.

We find the ecological center -- the location has moved since Mittermeier lived here -- but it's closed because it's Sunday. A simple sign marks the nondescript building.

Mittermeier stands next to the entrance of the Akumal Ecological Center. Sarah Tew/CNET

Behind the camera

Mittermeier can't remember a time when she didn't care about the planet, but living in Akumal decades ago opened her eyes even more to the fragility of the oceans. "I wanted to say something about it and I didn't know how," she says. "It takes 30 years sometimes to find a way."

Mittermeier started on her way in 1990 when employees from the environmental protection nonprofit Conservation International visited Akumal, saw what she was doing there and asked if she'd like to work for them.

She said yes and began co-authoring scientific papers about biodiversity hotspots -- areas with high concentrations of endemic species under the greatest threat of losing their intact ecosystems. Very few people read the academic papers, she says, limiting her ability to effect real change, despite months of painstaking collaborative work.

This photograph helped launch Mittermeier's career, even though it was originally attributed to her ex-husband. Cristina Mittermeier

"It became very clear to me early on that [scientific papers] don't connect with general audiences and that if we really wanted to build a constituency of people who care about reform we need a different vehicle," she says. Though she didn't know it at the time, the "different vehicle" would end up being photography and social media.

She met her now ex-husband, Russ Mittermeier, in 1991. He was the president of Conservation International at the time and they moved to the Washington, DC, area, where the organization is headquartered. They have three children: John, Michael and Juliana, all adults now. Mittermeier joined Russ on expeditions where she'd carry his gear, including his camera. One day, she saw a man in the Amazon and instinctively took his picture.

"He was beautifully framed by the black door and I just snapped a shot. I really didn't know anything about exposure or whatever," she explains. The photo ended up being used as the centerpiece of a 1992 exhibit in the Houston Museum of Natural History, credited to her ex-husband, because she had used his camera to take the picture.

This was just the beginning. "I became a photographer because I found out that I was good at it," she says casually. Outside of some formal photography at Washington's Corcoran College for the Arts, she's mostly self-taught.

But the path wasn't all easy. She was raising her children, traveling the world with her husband and working as a portrait photographer in the Washington suburbs. Mittermeier took family photos of people she refers to as "Stepford wives," who were singularly focused on having the best Christmas card, she remembers with a laugh.

In 2005, she joined National Geographic as a photographer. She has visited every continent and around 120 countries, capturing images ranging from sled dogs in Greenland to dry riverbeds in Madagascar and cowboys traveling by horse in Brazil. Four years later, she met Nicklen in the cafeteria at National Geographic's Washington headquarters. They started dating and working together on assignments.

We have nowhere else to go. This is the only planet we have and it needs the fabric of life to operate. Cristina Mittermeier

They founded SeaLegacy in 2014. "I said to Paul, 'You know what? We need to start our own nonprofit and we need to just start shooting for ourselves,'" Mittermeier says. She still works as a National Geographic contributing photographer, with work appearing in the National Geographic Image Collection, but now she has more freedom to focus on the causes closest to her.

Nicklen's interest in social media algorithms has contributed to their 1.7 million followers on SeaLegacy's Instagram page and their goal of using social media as a platform to spark conversations about climate change. He didn't study or work in social media before co-founding SeaLegacy. He simply wanted to grow an audience to share SeaLegacy's message, and became proficient at it by spending hours pouring over the analytics.

"I found that not all types of posts, stories, pictures, posting times and days got an equal response," Nicklen says. "The wisdom and interest of the audience determines what content is most popular."

Mittermeier photographs a young man fishing off the coast of Sian Ka'an. Sarah Tew/CNET

Working from the heart

"The strongest personality traits that Cristina brings to both assignments and daily life are integrity and focus," Nicklen says. "While she is adept in business, she lives and works from the heart."

Everywhere we go, Mittermeier talks to people and asks them questions. She also takes their picture. Crespi records short videos of Mittermeier describing where they are and what they're doing, for Instagram Stories.

Mittermeier uses a Sony A7 III and a Sony A9, two mirrorless cameras, but is otherwise a minimalist when it comes to equipment. But she always brings noise-canceling headphones. "I travel a lot and they allow me to be in my own head," she says. But when she's visiting a place, she's totally present.

Her mission is much larger than the sargassum or the plastic on these beaches, but it's all interconnected. Wherever she is, Mittermeier's goal is to bring people along with her through her images shared on social media.

"Creating a sense of community, of tribe, of belonging, of a movement is the most important thing that my photography attempts to do," she says. "It's an invitation, really, an open door that says, 'Come with me.'"

Mittermeier bikes to a Maya pyramid in Cobá. Sarah Tew/CNET

She says that getting rid of single-use plastics like straws is a great first step if you're concerned about the environment. Becoming an advocate for an organization you support is even better. "We have nowhere else to go," she says. "This is the only planet we have and it needs the fabric of life to operate, so just pick one [issue you support] and start doing something today."

Mittermeier is tireless as she works. I'm exhausted watching her, but also heartened. "It's difficult to separate what working with Cristina is like from what it's like living with her," Nicklen says. "There is very little separation. She's the hardest working and most dedicated and compassionate person I know."

Nicklen adds that he has to occasionally remind her to take a break and tell her that it's OK to recharge. I have no trouble believing this.

Our trip is almost over and we pile back in our dirt-coated maroon van and drive through a short, intense rainstorm down a quiet road past small Maya villages. We head through the interior of Quintana Roo back to Cancún and, eventually, the airport home.

It's been a fast-paced, eye-opening trip and we're talking about everything we've seen.

"You probably have this feeling [of excitement and satisfaction at work] every day," CNET Senior Video Producer Mark Licea says to Mittermeier at one point.

"A little bit, yeah," Mittermeier replies, with a small, proud smile.