Ecological Effects of Hurricane Irma
On Sept. 10, 2017, Hurricane Irma became the first major hurricane (Category 3 or higher) to make landfall in Florida since Wilma in 2005. The storm affected every part of the South Florida Water Management District's 16-county region, from Orlando to the Florida Keys. In addition to high winds and storm surge, Irma brought a District-wide average of 8 inches of rain over a 48-hour period, exacerbating already high water levels in the midst of what would be a record-setting wet season.
In the immediate aftermath of Irma and in the weeks and months that have followed, SFWMD scientists have been assessing the hurricane's short- and long-term impacts on South Florida's diverse ecosystems. The tabs below feature some of their findings as of April 2018. Each section includes presentation slides with more details, graphs and photos.
Kissimmee River Basin
Inundation of the Kissimmee River floodplain is dependent on higher levels of discharge from water control structures to the north. After a large rain event like Hurricane Irma, flow from the north may need to be increased dramatically to provide flood control in the Kissimmee Chain of Lakes upstream. In the days after Irma, water overtopped the river's banks and inundated the floodplain. Ecologically, annual flood pulses are desirable – although ideally in less extreme events than Irma – and are essential to maintain floodplain wetlands in low-gradient river ecosystems.
In 2017, flow as measured by discharge at the S-65A structure had a negative correlation with dissolved oxygen in the water, which is typical of this relationship in the Kissimmee River. Rising flow causes dissolved oxygen to decline primarily because it causes water depth to rise, reducing the amount of sunlight reaching photosynthetic organisms. When discharge must be increased quickly for flood control, the result is often deep sags in dissolved oxygen, especially in the wet season when temperatures are high. Such declines can impact fish populations. Hypoxic conditions (dissolved oxygen < 2 mg/L) causes physiological stress in many fish. Anoxic conditions (dissolved oxygen < 1 mg/L) can be lethal to sensitive species. In the Kissimmee, largemouth bass, bluegill, and other sunfish are very sensitive to hypoxic conditions and may be killed, while other fish species (e.g., Florida gar, catfish and many exotic fish) are relatively tolerant.
SFWMD scientists collected survey data on both intolerant species (primarily sunfish) and tolerant species in 2017. With these samples, they were able to document likely effects on populations of sunfish. In particular, largemouth bass experienced substantial setbacks in the wet season, with a severe dissolved oxygen crash and fish kill in June followed by another crash in September after Hurricane Irma. Bass (especially larger, reproductive-sized bass) are highly sensitive to low dissolved oxygen. Populations have been impacted for years by hypoxic events, resulting in low abundance of bass in recent years compared to data collected from a semi-restored channel in 1988. The hypothesis of SFWMD scientists is that this decline is due to almost annual hypoxic or anoxic events since restoration of flow in these channels. Almost no bass were recorded in the 2017 summer and fall surveys, and that none were large suggests it will take years for the population to recover even to pre-2017 levels. The District continues to work to reduce the severity and duration of Kissimmee River hypoxic events to the extent possible.
Irma did have some positive effects on the Kissimmee River. Numbers of small, first-year individuals of one sunfish, bluegill, were very high in the post-Irma survey, suggesting that this species, which reproduces in the summer, may have benefitted from refugia available on the floodplain during the long period of floodplain inundation. Field staff also reported that areas formerly dominated by exotic grasses (e.g., West Indian marsh grass) were cleared out by the prolonged deep water and high flow during the hurricane, providing newly open substrate for native plant recruitment and wading bird foraging.
Hurricane Irma caused dramatic differences in water levels from one side of the lake to the other during peak wind events, with more than 10 feet of difference in lake stage between the east and west shorelines (Figure 1). These intense winds and associated wave energy uprooted approximately 5,200 acres of vegetation along the outer edges of the marshes (Figure 2), with species like cattail (Typha spp.) especially affected. Cattail is not well suited for high-energy environments and plants that were uprooted during the storm formed extensive wrack lines where they were pushed farther into surviving emergent vegetation (Figure 3). These mounds of decaying organic material can exist for many years, as evidenced from prior hurricanes, and can eventually become floating islands capable of supporting woody species. They may also hinder fish and wildlife movement between the marsh and open water areas and reduce recreational access.
Irma also stirred up significant amounts of nutrient-laden organic sediment from deeper portions of the lake, causing extremely high turbidity and total phosphorus (TP) levels following the storm. Although conditions appeared to improve slightly in the month after, strong winds from passing cold fronts in January caused conditions to deteriorate again, reaching even higher levels of turbidity and TP than what was recorded even after Irma (Figure 4). After back-to-back hurricanes in 2004-2005, it was documented that disturbed sediments may take years to fully settle out of the water column and will likely be resuspended many times from strong wind events before they do. These resuspension events often correlate with algal blooms and poor growth of submersed and emergent plants in deeper areas of the lake. For example, submersed aquatic vegetation was rebounding in the summer of 2017 after 3-4 years of declining coverage, but Irma reduced that acreage to the lowest total since the previous hurricanes in 2004 and 2005.
St. Lucie River Estuary
In preparation for Hurricane Irma between Sept. 6-8, ~5,222 cubic feet per second (cfs) of water was released from Lake Okeechobee into the St. Lucie River Estuary and another ~4,993 cfs from the C-23 and C-24 basins and Ten Mile Creek. Following Irma's passage, the St. Lucie experienced maximum freshwater inflows of ~17,707 cfs on Sept. 11. Most of the water on that day came from the tidal basin (~5,443 cfs, or 30.7 percent), followed by inflows from C-23, C-24 and Ten Mile Creek (~8,341 cfs, or 47.1 percent) and the C-44 basin (3,923 cfs, or 22.2 percent). No water was released from Lake Okeechobee on that day. The inflow of water slowly decreased within the next two weeks.
Increased freshwater inflows and a record amount of precipitation during and after Irma lowered salinity in the North Fork and South Fork to nearly 0 practical salinity units (PSU) and in the middle part of the estuary to < 2 PSU near the surface and < 8 PSU near the bottom, based on data from Florida Atlantic University's (FAU) Land/Ocean Biogeochemical Observatory (LOBO) stations and the U.S. Geological Survey (USGS). The water column was strongly stratified in the lower part of the estuary, with fresher, less dense water laying on top of saltier, denser water for an average salinity difference of 5-6 PSU, based on USGS data.
FAU LOBO data showed a significant increase in oxygen levels after the hurricane as increased freshwater inflows mixed surface and bottom water layers and oxygenated the entire water column. Water temperature decreased significantly during the first week following the post-Irma freshwater inflows to the estuary and record amount of precipitation during that time.
Turbidity significantly increased across the estuary following the hurricane's passage. Winds and increased rates of water flow easily disturb unconsolidated surface sediment layer that are not stabilized by submerged aquatic vegetation (SAV), which is absent in most parts of the St. Lucie River Estuary. Turbidity slowly decreased over time in the first week after the hurricane and then increased again as a result of increasing freshwater inflows.
Based on FAU LOBO data, chlorophyll a – a proxy for algal biomass – decreased after Irma. This was most likely due to the increased flushing time/lower residence time of water related to increased freshwater inflows to the estuary. Algae did not have enough time for nutrient uptake and accumulation of biomass for growth, and increased turbidity levels decreased light availability within the water column. Monthly SFWMD water quality monitoring since September indicates low algal biomass and slowly increasing salinity across the bay, especially in the lower part of the bay. Additionally, no dissolved oxygen deficiencies have been recorded since Irma. High flow in the first months following the hurricane mixed the surface and bottom water layers and oxygenated the entire water column. Irma, the record 2017 wet season and high freshwater inflows to St. Lucie Estuary prevented a build-up of algal biomass within the estuary.
Caloosahatchee River Estuary
In preparation for Hurricane Irma between Sept. 5-8, ~36,451 cubic feet per second (cfs) of water was released into the Caloosahatchee River Estuary. A total of ~16,206 cfs of that water came from the C-43 basin, ~9,001 cfs from the tidal basin and ~11,244 cfs from Lake Okeechobee. Following Irma's passage, the Caloosahatchee experienced maximum freshwater inflows of ~27,335 cfs on Sept. 11. A total of ~25,300 cfs came on that day from the C-43 basin, and ~2,035 cfs came from the tidal basin. No water was released from the lake. Inflows gradually decreased over the next two weeks following the hurricane and then increased again with fresh water from the basins and the lake.
Surface and bottom salinity significantly decreased across the estuary after the passage of Irma as a result of the increased freshwater inflows and a record amount of precipitation. Turbidity significantly increased after the hurricane due to storm surge as winds pushed water from the Gulf of Mexico into the estuary, according to data from the Sanibel-Captiva Conservation Foundation. Increased freshwater discharges also increased the concentration of colored dissolved organic matter (CDOM) in the water column, which decreased light penetration within the water column and light availability to SAV.
Chlorophyll a data recorded by the Sanibel-Captiva Conservation Foundation at Fort Myers show a significant decrease in algal biomass following the hurricane, which was most likely due to increased freshwater flows that increased flushing time/decreased water residence time within the estuary and high turbidity that decreased light penetration down the water column. High freshwater inflows into the estuary in the following months and the pulsed releases of fresh water over the last few months have kept the chlorophyll a concentrations low throughout the estuary.
No algal blooms or low oxygen levels were recorded in the estuary after the hurricane or in the following months. High discharges most likely prevented algal bloom development within the estuary as high flushing time, low water residence time and low light within the water column decreased algal growth potential. However, red tides have developed in the following months along the Lee County coast.
Stormwater Treatment Areas (STAs)
Stormwater Treatment Areas (STAs) are constructed wetlands that remove and store nutrients through plant growth and the accumulation of dead plant material that is slowly converted to a layer of peat soil. Five STAs south of Lake Okeechobee with an effective treatment area of 57,000 acres remove excess nutrients from water before sending it to the Everglades and other natural areas. SFWMD manages water levels in the STAs as best as possible to optimize their performance and achieve water quality improvement targets.
Following Hurricane Irma, the STAs experienced deep water conditions and high turbidity. The STAs received high inflow volumes, phosphorus concentrations and phosphorus loads, impacting treatment performance with high outflow phosphorus concentrations. In recent months, most STAs are showing improved performance, but some cells continue to have impacted performance and poor vegetation conditions.
Irma's high winds also resulted in vegetation damage that included shredded and flattened cattail, floating vegetation and mats and submerged aquatic vegetation (SAV) damage and loss. Vegetation strips added in SAV cells after previous hurricanes minimized damage. Based on experience with prior storms, the cattail should recover.
It was already a hydrologically stressful environment in central Water Conservation Area 3A (WCA-3A) when Hurricane Irma traveled north and then northeast across the Florida peninsula. The upper depth tolerance of tree islands around one gauge was reached almost two months before Irma came ashore. Irma made things significantly worse. Post-Irma water depths in the marsh reached a maximum of about 4.5 feet. Some areas of the Everglades exceeded the upper depth tolerance of tree islands for more than 220 days. Depending on the species, water depths that exceed the upper tolerance of tree islands for 60-120 days can kill saplings, prevent growth or severely stress adult tree populations. However, these forested trees do not respond to stressful hydrology immediately. It could take 1-2 years before the impacts of Irma and the extreme high water of 2017-2018 are fully realized.
Irma increased water levels across the WCAs and Everglades National Park. However, levees, roads, canals and wetland elevations distributed the hurricane's impacts unevenly across the landscape. Normal environmental conditions were found in Northern WCA-3A and WCA-1. Deep water was found in WCA-2 and WCA-3. Irma greatly enhanced the pile up of water along the L-67A canal (seen here as deep blue). These deep conditions along the L-67A might prevent tree islands woody species from recruiting and cause sawgrass ridges to diminish. However, fish populations can thrive under these water conditions and can provide excellent foraging habitats as water levels recede during an extreme dry season.
Wind damage to tree islands was variable. Even though most tree islands were not directly impacted, some tree islands suffered minor damage including loss of leaves (e.g., coastal plain willow and pond apple), leaves wind burned (e.g., cypress spp.), or branches bent or broke. Some islands had about 90 percent of their trees knocked over or severely broken and understory flattened. Most of damage was to the most numerous tree species (e.g., coastal plain willow), and the largest tree species (e.g., strangler fig or cypress spp.). Less common species (sweetbay, redbay, cocoplum and dahoon holly) were noted to have less damage. The scientific community is of the opinion that all these species will recover. However, the recovery rates from wind damage will need to be monitored.
Many large cypress trees were knocked down at the Jetport South wood stork colony in southwestern WCA-3A. Given that few trees were damaged in nearby cypress domes, it is possible that the colony was hit by a tornado that was known to pass through this area. This wood stork colony is the largest in the WCAs (857 nests in 2017). It was found that the damaged trees did limit stork nesting locally, but the storks moved to adjacent islands (850 nests in 2018). On the other hand, great egrets were attracted to the damaged and downed trees and nested there in greater numbers than in previous years (63 nests in 2017 versus 200+ nests in 2018). No nesting occurred during Hurricane Irma.
Water quality and algal dynamics were assessed at 81 locations across Biscayne Bay after the passage of Hurricane Irma. The surveys were conducted between September 2017 and January 2018. The data used for the post-hurricane assessment were collected by multiple agencies including SFWMD, the National Oceanic and Atmospheric Administration (NOAA), Miami-Dade County Department of Environmental Resources Management (DERM) and the National Park Service (NPS).
The September survey data indicate that salinity and temperature dropped across the Bay, especially near shore, during the hurricane and the first week following the hurricane. Salinity and temperature decreases were most likely related to increased freshwater inflows from the mainland via the drainage canals, overland flow and submarine groundwater discharges, and to the record amount of precipitation during the hurricane. Additional cooling of the bay was caused by the hurricane itself. Hurricanes act like "heat engines" that transfer heat from the water surface to the atmosphere through evaporation. Also, upwelling of colder water from below due to the suction effect of the low-pressure center of the storm, and cloud cover most likely also played a role in cooling of the Bay. The highest turbidity levels were recorded in north-central Biscayne Bay, Card Sound and Barnes Sound – the areas that experienced the highest storm surge. The post-hurricane nutrient enrichment of the Bay stimulated algal growth. The highest algal biomass was recorded in north and north-central parts of the Bay and in Card Sound and Barnes Sound. Biomass at near shore sites consisted mostly of cyanobacteria and green algae. Ammonia levels were elevated across the Bay, with highest values recorded in the south-central part of the Bay. Nitrate and nitrite levels were the highest in the south-central part of the Bay.
In the months following the hurricane, nutrient and algal biomass levels and salinity decreased to pre-hurricane levels, and cyanobacteria and green algae were outcompeted by diatoms (diatoms are usually the dominant algae across the Bay, while green algae are often more abundant in some (not all) canals). Based on data collected during the surveys, there is no evidence of a long-term water quality decline or hurricane-induced algal blooms in Biscayne Bay.
Hurricane Irma passed across Florida Bay as a Category 4 storm, producing strong and immediate impacts on the environmental conditions and the ecology of the bay. These effects continue to be felt in many ways. The primary impacts during storm onslaught were the physical action of winds on bay waters, the bay bottom and vegetation, primarily affecting seagrasses and mangroves, which were mechanically sheared and uprooted. Also, direct precipitation on the bay and in the Everglades watershed discharged a pulse of freshwater into the bay. These inputs reduced salinity, notably along the north shore closely connected with the creeks and sheetflow from the Everglades. One month prior to the hurricane, a high-resolution mapping survey by SFWMD in August, showed that bay waters were unusually hypersaline for that point in the rainy season (Figure 1 in the presentation), owing to a deficit of rain and creek flow. Salinity ranged in the high 30s to low 40s practical salinity units (PSU); that is above full-strength seawater salinity of 35-37 and far above average Florida Bay salinity of about 25-30 for late August. By October, one month after the hurricane, mapping showed salinities had declined drastically, ranging in the coastal bays from near-freshwater to about 10 and in the main bay from 20-30. The hurricane had reduced salinity levels to about average for September, which helped avoid stressful hypersaline conditions later in the dry season. Five months after the hurricane in February 2018, there was no evidence of hypersalinity in the bay; levels ranged from 15-30, which is optimal and likely to help reduce development of excessive salinity conditions in 2018.
Chlorophyll a is a common measure of the amount of microscopic algae (phytoplankton) in the water and is regularly monitored by SFWMD. In August 2017 prior to the hurricane, bay waters were mostly clear and chlorophyll a was low, ranging from below 1 in the east to below 5 µg/L in the central bay, which is about normal for those areas (Figure 2). Following Irma’s passage, runoff from land along with decaying vegetation biomass, released excess nutrients into bay waters, inducing an algal bloom over a large region of the bay with chlorophyll ranging as high as 50 µg/L. After an increase in chlorophyll of as much as eight times the typical (median) value (Figure 3), algal biomass slowly declined through the winter months. By February 2018, the algal blooms had subsided to background levels in the eastern bay although a bloom persisted in the central bay. SFWMD scientists expect bloom conditions to continue to decline in spring 2018 if there are no additional perturbations to the system. The after-effects of the storm, including elevated algae levels and the resuspension of fine sediments from the bay bottom caused a persistent turbid condition of reduced water clarity in the bay for months (Figure 4). As the bloom subsides and particles settle back to the bottom, SFWMD scientists expect water clarity to continue improving.