Macrocosm: Mangroves are nature's filters and transformers, but they may be due for a cleaning
Marie Hickman
The ecosystems of southern Florida contain multitudes of mysteries. Giant reptiles prowl the waterways, seeking out fish and wayward small mammals that traipse into their path. Roseate spoonbills - large, pink-plumed birds with yellowish-green heads and strange flat bills - walk along the shallow channels, dipping their ladle-like bills within to gather small fish and invertebrates before flying back to their nests.
And, like strange architecture from an Alien movie, mangroves protrude from the surface of the swamps, held aloft by stilt-like roots poked full of holes. Though they may look like they belong on another planet, mangroves are one of the earth's evolutionary wonders, and they serve a vital purpose within Florida's waters - one that, as many are, is threatened by climate change.
Mangrove swamps are wetlands found in tropical and subtropical regions, such as Central America, the southern tip of Florida, and Texas's Gulf Coast. They are characterized by salt-tolerant trees, shrubs, and other plants that can grow in brackish or saline tidal waters. Generally found bordering the equator, a crucial factor in their survival is temperature - a fluctuation of ten degrees over a short period of time can kill mangroves, and therefore collapse the entire ecosystem of the mangrove swamp. Florida's southwest coast supports one of the most extensive mangrove swamps in the world, characterized by its large quantity of mangrove trees and near-impenetrable density of vegetation.
There are, in fact, five different types of mangrove swamps. Mangrove forests along bays and lagoons that experience full sun are called 'mangrove fringe,' dependent on normal tides that flush out detritus from the swamp, keeping the area clear for the roots to pull in nutrients continually. An overwash forest is similar to this, but the entire forest becomes flooded at high tide. This is a popular place for birds to nest due to the amount of fish regularly washed in from these tides.
Riverine mangrove forests, predictably, are within river floodplains by coastal areas and are sometimes inundated with fresh river water. However, these areas are highly variable so that the soil can become highly salty in summer droughts. Basin mangrove forests extend far inland and usually occur in coves. Dwarf, or scrub, mangrove forests have shorter vegetation than others, only attaining canopy heights of less than 5 feet, although they contain the same species as other forest types. This is attributed to a lack of nutrients, high salinity, and rocky soils.
Why are mangroves so vital?
Coastal tides feed mangrove swamps but require shelter from wave action within. Wave action can be a potential danger to the short shrubbery that lives along these coastal shores, uprooting the roots placed in very tenuous locations within the soil. In their impenetrable placement, the thickets of mangroves are a shelter for the land from waves and wind as well.
These tightly clustered trees provide an essential service to the surrounding area: the process of denitrification. Nitrogen comes in many forms: the harmless N2 gas that swirls around in our atmosphere, the ammonia expelled from animals through waste, and the nitrate plants take up from nearby synthesizing bacteria. Each of these has a purpose in an ecosystem, but by interfering with the mechanism's gears, the whole process starts to overbalance and teeter. Human-made fertilizers introduce a glut of nitrate into soil systems to feed large swaths of crops, most of which aren't even taken up by plants. The remainder threatens to fall into nearby waterways, if not for the actions of a mangrove swamp, behaving like a filter to keep hazardous nutrients out of the ecosystems where they don't belong.
Denitrification occurs only through the existence of two cooperating types of soil. The topsoil in mangrove swamps is loosely formed as a sandy residual matter of alluvium washed in from the river or oceanic tides. This topsoil is porous and facilitates aeration (the introduction of air and, hence, oxygen) during low tide. The subsoils are a dark brown or black color. This dark soil is full of organic matter that is poorly aerated due to being stuck underwater and therefore produces a strong odor due to anaerobic sulfur-reducing bacteria. These two types of soil are significant for various factors, chief among them being the cycling of nitrogen. The denitrification of nitrate is only possible with these specific factors operating in tandem.
The nitrogen must first be introduced and transformed into a suitable form through aerobic reactions, then denitrified and put back into the air as gaseous nitrogen gas, harmless to the atmosphere and the surrounding ecosystem. This is an essential step for removing NO3- from bodies of water that cannot be taken up by plants and nitrogen-consuming bacteria and is, therefore, able to result in algal blooms that drain oxygen from coastal areas.
These algal blooms create eutrophication, a phenomenon in which oxygen is no longer available to creatures beneath the water's surface due to the dense growth and subsequent death of algae. This prevents oxygen from being bubbled into the waterways, choking off animals beneath it. Animal life remaining present on our coasts is essential for financial, commercial, recreational, aesthetic, and environmental reasons, so this process of denitrification done by wetlands is critical.
What's happening to the mangroves?
Mangrove trees and swamps are not helpless victims in the face of climate change. One of those fascinating things about these organisms and ecosystems is their adaptability. Mangroves have developed many complex anatomical and morphological adaptations to succeed in the high-stress, high-saline waters of saltwater tidal marshes. For example, along with their aerial roots, Mangrove trees have specially developed aerenchyma with interconnected air channels throughout the plant. The bases of leaves develop this aerenchyma with increasing pore space at the stem junctions, allowing for gas exchange to the leaves more directly than through the roots.
Submerged mangroves have specialized root adaptations to conserve atmospheric oxygen by diffusion into the underground sections of the roots used in the event of anoxic conditions. To prevent O2 from releasing from the aerenchyma from simple diffusion, an impermeable layer within the exodermis developed to hold that oxygen in. Mangrove trees can even overcome salt stress on a molecular level, stopping themselves from producing toxic ROS, which disrupts metabolism in the tree. They can overcome this stress through various defensive enzymes and antioxidants, preventing this toxic breakdown of lipids, proteins, and nucleic acids within the cells.
However, mangroves also face substantial threats from climate change, significantly rising sea levels. The adaptations that allow mangroves to keep taking in oxygen - tiny holes poking through their roots - are at risk of becoming useless as the high tide gets higher. Without these methods of gathering oxygen, mangroves drown in their own homes, their stalwart corpses unable to stabilize the soil or provide nutrients and shelter to the species that depend on them. In some areas like Louisiana, land sinking due to water removal and erosion can accelerate that process.
Mangrove swamps provide invaluable resources to coastlines, prevent damage during natural disaster events, and filter nutrients out that could otherwise leave a bay looking like a graveyard. Their presence is highly adaptable but not immune to damage. Only through fast action and a careful examination of the ecological and economic incentives to keep these forests do we have a chance of allowing them to remain standing above our coastal waters.