Director’s Note: Mangroves, mangrove trimming and water quality
Recently, there has been a quite a bit of concern raised about the topic of mangrove trimming, and potential impacts to the bay’s health from what some refer to as “excessive” trimming. To be clear, the SBEP is not a regulatory agency – we issue no permits and have no formal role in the determination of what is and what is not allowed to occur with mangrove trimming. However, the recent attention to mangrove trimming is a good way for us to highlight the topic of mangroves and their importance, including benefits to water quality.
We all know that mangroves are important for the seabirds and wading birds in our region. I live on the shoreline of Terra Ceia Bay, and a mangrove island in the southeast part of that bay is magical place to see pelicans and egrets and roseate spoonbills in the wild. To reproduce, these birds need protection from predators such as outdoor cats (bad idea) and racoons, and mangrove islands provide the refuges from predation that allow them to continue to exist in our region. Simply put, lose the mangrove islands and you can lose our seabirds.
Mangroves are also incredibly important features for fish as well – as any angler knows. If you’re not aware of the importance of mangroves, fish along the mangrove fringe on a high tide. Spotted seatrout and red fish and mangrove/grey snapper are common, and early life stages of snook and tarpon use mangrove fringes as well.
Mangroves protect our shorelines from erosion and damage from storm surges – a topic well documented across the globe. Natural shorelines are much less impacted by storm surge than hardened shorelines – a fact obvious to anyone who has worked in hurricane relief efforts or who spends enough time on the water.
Mangroves are also important for water quality. The prop roots of red mangroves are attachment sites for oysters and barnacles and sponges and other filter feeders. Mangroves forests also filter out pollutants from upland sources. In the summer of 2020, more than 10 million gallons of raw sewage gushed out of a hole in the sewer line that takes wastewater from Longboat Key to Manatee County’s Southwest Treatment Plant. Perhaps because the spill occurred several hundred feet into a mangrove forest, no elevated levels of fecal indicator bacteria were found in the bay after the leak, even right next to the shoreline.
Mangroves also help to keep nuisance and harmful algal blooms under control. The tannins released from leaves that fall off of mangroves leach out “tannins” which give the water coming off of mangrove fringes a tea-like color (see photo). The word “tannins” is a catch all phrase that includes compounds that have been shown to reduce the growth of algae. You may have read of “barley straw” or similar compounds being tested by Mote and others to mitigate the impacts of red tide – well, decomposing mangrove leaves release similar compounds for free. If you don’t like “gumbo” algae and aren’t fond of red tide, keep in mind that not only are we loading our bays with more nutrients than what they naturally received, we have also removed a lot of the mangroves that help moderate the transformation of nutrients into algae.
Tannnins coming off mangrove fringe
Without doubt, mangroves are better protected now than in past decades. In the post-WW2 years, we pulled them out by their roots or smothered them with fill material as we replaced our mangrove shoreline with seawalls. That doesn’t happen anymore. But what still does happen is that we trim mangroves, to preserve or create unobstructed views of the water from our homes and parks and businesses. Trimming comes with many, many restrictions – see the guidance from FDEP here - Mangrove Trimming Guidelines for Homeowners (floridadep.gov).
Mangrove trimming guidance is meant to ensure that mangroves continue to exist along our shorelines, which is good for all the reasons noted above. Recently, however, a topic has arisen that may require a bit more attention – what happens to the trimmed material that is cut down from the trees themselves. As noted above, the SBEP is not a regulatory agency, and we have no formal role in permitting – nor do we wish to have one. However, there appears to be a lack of attention in current guidance as to the potential impacts to water quality of material left to decompose in the water after mangrove trimming. The guidance in one permit we have examined – if consistent with other permits – is a bit troubling. Language in the reviewed permit simply states that “trimmings” over 3 feet long or more than 3 inches in diameter are to be removed for disposal elsewhere. It appears that such guidance would allow someone to take a five-foot section of material and cut it in half and leave both pieces in place. Or, more likely, anyone trimming a tree would make sure that cuts would be no longer than three feet in length. A brief visit to one site where mangrove trimming took place shows quite a bit of material left to decompose in the water.
So should the leaving behind of trimmed mangrove material be expected to impact water quality? The short answer is yes. In my prior career in the private sector, I used a mangrove nutrient loading model developed by Louisiana State University (LSU) researchers to look into this very topic, in response to a 30 square mile algal bloom in Southeast Florida. Earlier today, I ran a hypothetical scenario through this same model, to find out how concerned we might want to be about mangrove trimmings left to decompose in place in Sarasota Bay. Under this hypothetical scenario, if you trimmed 10 acres of mangroves from 24 feet down to 12 feet in height, you would be expected to create “trimmed material” that would contain nearly 2 tons of nitrogen. If 90% of that material was removed, leaving just 10% to decompose in place, that 10% alone would equal 377 pounds of nitrogen – equal to about 75 bags of typical lawn fertilizer.
Some might wonder – I thought tannins from leaves were good – why be concerned about mangrove trimmings left to decompose in the water? Well, the difference is the same that occurs between the brown leaves that fall from maple trees in the fall, and the bright green leaves of maple trees in the spring to summer. Fallen leaves have lots of tannins, but not much nitrogen. Leaves from trees cut while the leaves are still growing are much more nutrient dense – whether from maple trees or mangroves. Leaving masses of trimmed mangroves to decompose in the water is counter productive and is inconsistent with our existing guidance to homeowners. We tell people to not blow grass clippings into their canals or into storm drains that flow to the bay, but you can drop mangrove branches into the water, as long as the branches are less than 3 inches thick and the cut material is less than three feet in length? That’s illogical.
There appears to be a problem with mangrove trimming guidance. Even with the scenario described above – 10 acres trimmed from 24 to 12 feet, with 90% of the material removed – the potential load of nutrients to the water is likely equal to multiple dozens of bags of fertilizer added to our bay. Is this moving in the right direction, in terms of getting our water quality under control? Not really. Can we do better – continue to allow mangrove trimming without creating the potential for worsening our water quality problems? Probably, if we acknowledge the topic is worthy of addressing in our permitting of mangrove trimming (and the monitoring of compliance of existing guidance).
Our expected rates of sea level rise are fairly modest over the next 30 years – perhaps 8 to 9 inches or so. But for some mangroves, trapped on the outside of seawalls, this will be more than they can overcome, if we do not figure out a way for them to migrate upslope. If we start to lose mangroves from sea level rise, and we continue to trim remaining mangrove fringes without sufficient concern over the nutrient problems associated with such practices, then we will be working against our own self-interests from two directions – reducing the assimilative capacity of our bay to absorb nutrients, while simultaneously adding more nutrients. We can’t be successful in restoring and protecting our water quality by doing the same things we’ve been doing for the last few years – we must do better, or we’ll reap the consequences of not doing enough.