dike<\/a> was constructed on the refuge to concentrate standing water for mosquito control and was later used to manipulate the water level to create habitat for migratory waterfowl. despite these advantages, the disruption to the refuge\u2019s natural hydrology has detrimentally impacted mangroves in \u201cimpounded\u201d areas that are disconnected from the surrounding waterways. <\/p>\nwater exchange from incoming and outgoing tides is critical for healthy water quality \u2014 and mangroves. when bacteria break down algae and other organic matter in the water, they consume oxygen. if the oxygen level of the water drops too low, other bacteria use sulfur compounds instead of oxygen, producing hydrogen sulfide in the process. mangroves in impounded areas don\u2019t experience sufficient tidal flushing, which can lead to a buildup of hydrogen sulfide that stresses the trees. <\/p>\n
\u201cwe\u2019ve seen this all over southwest florida, where even small changes in the tidal hydrology that was original to the system can cause long-term stress,\u201d said ken krauss, an ecologist at usgs. <\/p>\n
due to this stress, mangroves in impounded areas of the refuge struggle to produce seedlings. they also receive fewer seedlings from trees outside the impoundment, which are delivered through tidal flushing. collectively, these factors can impede the regeneration of tidally restricted mangroves after a hurricane. <\/p>\n
tunnel-like culverts have been constructed to reconnect some of the water flow between the two sides of the dike, but they don\u2019t fully restore the refuge\u2019s natural hydrology. \u201cit\u2019s like breathing through a straw,\u201d said jeremy conrad, an ecologist at the united states fish and wildlife service.<\/p>\n
the dike is not the only example of altered hydrology on sanibel island. in a waterfront residential area, mangroves were cut off from tidal flushing by the construction of a road, which requires regular maintenance as the rising sea laps at its edges. debris from the roadwork combined with organic material to form a hard deposit along the coastal side of the canopy, further restricting water flow. <\/p>\n
the forest die-off was a case of \u201cmangrove heart attack\u201d \u2014 a seemingly overnight manifestation of underlying issues caused by reduced tidal flushing. the death of the mangroves has exacerbated shoreline erosion on this vulnerable part of the island, said bob gerwig, a wildlife refuge specialist at “ding” darling. \u201cwhen these mangroves die off, all that root structure decays, and the soil subsides and it collapses. so then we’re losing that resiliency to sea level rise and climate change,\u201d he said.<\/p>\nmangrove heart attack leaves this forest in rough shape. \u201cthis is an awesome example of what\u2019s not supposed to be happening,\u201d gerwig said. (sarah anderson\/medill)<\/figcaption><\/figure>\nfor mangroves growing in smaller bodies of water, simple concrete culverts have made a big difference in restoring hydrological connection. since a culvert linking a mosquito control ditch to the island\u2019s tarpon bay was repaired three years ago, its mangroves have begun to show signs of recovery. \u201cyou may be breathing through a straw, but you\u2019re breathing,\u201d gerwig said. <\/p>\n
gerwig hopes to help larger swaths of impounded mangrove in the refuge breathe through bigger \u201cstraws\u201d by breaking up the dike with raised boardwalks and bridges. \u201cmangroves are a very slow grower and they respond to stressors and reliefs in a very slow manner. so these restorative actions are going to take time,\u201d he said. \u201cit’s so easy to break something, and it takes decades to put it back together.\u201d<\/p>\n
unfortunately, mangroves might be impacted by another anthropogenic factor: eutrophication, or the enrichment of nutrients such as nitrogen and phosphorous in the water. the water bodies surrounding sanibel island receive large amounts of agricultural and wastewater nutrients from discharges from lake okeechobee and the caloosahatchee river. \u201csanibel is right at the dumping point where everything flows right toward us, and it very much influences what happens on the \u2018ding\u2019 darling wildlife refuge,\u201d said avery renshaw, a biological science technician at the refuge. <\/p>\n
just as nutrients in fertilizer help plants in a garden grow, nitrogen and phosphorous supplied by tidal flushing benefit mangroves in low-nutrient environments. \u201cbut what we’re finding is that when the system is under a high level of fertilization or eutrophication for long periods of time, it can actually become a detriment to the system,\u201d conrad said.<\/p>\nthis power pole \u201cused to be on dry ground,\u201d gerwig said. \u201cso that\u2019s just an idea of the loss of mangroves and then the subsequent erosion.\u201d (sarah anderson\/medill)<\/figcaption><\/figure>\nto adapt to sea level rise, mangroves gain vertical elevation by accumulating leaf litter and sediment and growing their roots. when they are exposed to low levels of nutrients, mangroves develop extended root systems to forage for nitrogen and phosphorous, helping them build vertical elevation. but mangroves that are bombarded with nutrients may shift their energy to growing the aboveground portion of the tree at the expense of the roots, which could hinder the elevation process.<\/p>\n
to evaluate the impact of nutrient loading on the mangroves at \u201cding\u201d darling, conrad fertilized the soil with nitrogen or phosphorous and measured the growth of the tree above and below ground and any changes in the elevation of the soil over four years. while he observed some growth and metabolic shifts that hinted at adaptations to eutrophication, he did not find a significant difference in the soil elevation change in the phosphorous- or nitrogen-treated mangroves compared to the control plots. the minimal effect might be because \u201cwhat we put in the ground was a drop in the bucket compared to what was already in the system,\u201d conrad said. it\u2019s also possible that the influence of the additional nitrogen and phosphorous could take more time to appear in a high-nutrient environment, krauss said.<\/p>\nbob gerwig checks on mangrove recovery in a tidally restricted area after its culvert was repaired. (sarah anderson\/medill)<\/figcaption><\/figure>\nwith some experimental optimization, the team aims to incorporate the results into a model to forecast the effect of sea level rise on mangrove cover at the refuge over time. \u201cin perpetuity, as we continue to get more data and a better understanding, we can begin to model and project what elevations look like in the forest versus what the elevations of the oceans are doing,\u201d conrad said. \u201cand then that gives us a model to predict what our lands will look like 30, 40, 50 years from now under these accelerated rates of sea level rise.\u201d <\/p>\n
krauss hopes the project will help mangrove managers at the refuge and beyond prepare for and respond to climate-related disturbances. \u201cthis is a changing world, and we have to figure out how to adapt properly to it,\u201d he said. <\/p>\n","protected":false},"excerpt":{"rendered":"
volunteers helping in recovery at benedict key aim to build up an oyster reef to shield the fledgling mangroves from destructive waves, sarah anderson reports.<\/p>\n","protected":false},"author":9417,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[4904],"tags":[4324,3976,4762,4625],"storyfest_categories":[],"class_list":["post-11064","post","type-post","status-publish","format-standard","hentry","category-water","tag-eutrophication","tag-hydrology","tag-mangroves","tag-sea-level"],"acf":[],"yoast_head":"\n
restricted water flow and nutrient pollution threaten southwest florida\u2019s mangrove forests - planet forward<\/title>\n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n\t \n\t \n\t \n