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Ph.D. student fights for mangroves — not concrete seawalls — on Miami's shorelines

Ph.D. student fights for mangroves — not concrete seawalls — on Miami's shorelines

November 10, 2021 at 11:47am

Miami’s future could be one of enormous concrete seawalls stretching along the coast. Or something a little different — where mangroves become the type of infrastructure that really protects the city.

Marbelys Garriga is fighting for a future where coastal urban landscape and design is dominated by living shorelines that benefit precious water resources, protect wildlife and help ensure humans can continue to live in coastal communities.

The FIU Institute of Environment Ph.D. student is spearheading a project to gather quantifiable evidence on how traditional, more well-known shoreline infrastructure, such as seawalls or rip rap retaining walls, compare to natural forms, like mangroves and oysters. She’s focused on how these different types of infrastructures could impact an issue Miami continues to grapple with — water quality.

“Miami Dade has been focused on rehabilitating and restoring natural coastal areas where possible, but living shoreline projects have not been significantly considered in the heavily urbanized shorelines of Miami,” Garriga said “If we quantify the water quality regulation capacity of each infrastructure type, and provide a way for policymakers to use measurable outcomes of living shorelines for water quality regulation, it becomes more difficult to still favor seawalls that provide no ecological function.”

Garriga knows that when most people think infrastructure, they probably envision manmade structures and not mangroves with their gnarled, twisted, long leggy roots stretching down into water.

But, in many ways, mangroves are infrastructure — a very complex natural type that does more for our shorelines than what we’re currently capable of building to protect ourselves.

The benefits mangroves provide to people are well-documented. They protect coastal communities from sea level rise. They defend South Florida from powerful hurricanes. And there’s another benefit that Garriga wants to explore — how they impact water quality.

Garriga — who is a part of the NSF-funded CREST Center for Aquatic Chemistry and the Environment and is also funded by the Urban Resilience to the Extremes Sustainability Research Network — would use flow-through flumes placed in artificial, hybrid, and natural shorelines to test for nutrient fluxes. Once, before the water comes into contact with the infrastructure. Then again at the point it intercepts with it. At a few of the sites, the flumes will also target stormwater outfall flow, which will allow Garriga to understand how living shorelines can intercept stormwater runoff before entering coastal waters.

These before and after comparisons will show what’s happening to certain nutrients, like nitrogen and phosphorous — caused by fertilizer, sewage and other human sources.

Concrete can’t do anything to remove these nutrients from the water. Trees, though, rely on nitrogen and phosphorous to live and can filter them from the water, improving water quality.

Garriga already knows seawalls won’t be shown to do any good in terms of water quality. But, the problem is seawalls get built because they have quantifiable ways to physically stop water. Even though she knows the mangroves and other living shorelines do a lot more, the first step is to gather evidence to accurately prove these benefits.

Currently, Garriga’s speaking with city officials throughout Miami to get permission to conduct her research at sites in Biscayne Bay and Lakeworth Lagoon. These areas were selected because past water quality research has been conducted there — like the Coral Gables Water quality assessment, which was led by FIU Director of Science for Sea Level Solutions Tiffany Troxler, as well as Miami Waterkeeper water quality sites.

Lakeworth was also selected because there’s hybrid types of living shorelines. In one example, mangroves were planted near a crumbling seawall. From a distance, the mangroves look nothing like their historic counterparts in the Everglades. They’re trimmed, maintained and resemble squat hedges. In addition to bringing some greenery to the landscape, they also have helped stabilize the seawall, so that it doesn’t need to be repaired or replaced.

“Even little mangroves provide so much good,” Garriga said. “They don’t have to be tall enough to block our coastal views, because it’s that bottom half, the root system, that’s needed for water quality regulation — and that also serves as a habitat for fish and other wildlife.”

One issue, though, that presents a challenge is that current policy in Miami controls when and where mangroves are planted, and also states they cannot be trimmed. Garriga hopes that it can be changed to reflect an important distinction — that the critically endangered habitats in the Everglades would remain protected, but the ones planted as ecological engineering infrastructure would fall under a different set of rules. Essentially, it requires a different viewpoint, to see the mangroves in a new way. It’s a subtle difference, but an important one.

Garriga looks forward to getting out into the field. To begin this research will bring her one step closer to her dream.

When it comes down to it, a living shoreline is about choosing to allow nature to work to our human advantage. It’s about designing places that are pleasant to visit — a quiet, peaceful spot that looks out over the ocean without a wall blocking the view or perhaps a bench among some mangroves.

In the middle of a city, it can be a pocket of nature, of life, of beauty.