Post-Hurricane Impacts on the Middle St. Johns River

By Earl Lundy

Hurricane Irma had a definite impact on Florida, with over 2/3 of its counties feeling some of its effects. Biologists from the DeLeon Springs Freshwater Fisheries Lab have been monitoring conditions on the lakes and selected stretches of the middle St. Johns River since the storm subsided.

To start with, U.S. Geological Society (USGS) gages recorded river levels rose 3 to 6 ½ feet above normal water levels, depending on location. What was unique about Hurricane Irma was that it took so long for water levels to recede: three months out, water levels in some areas are still above their normal levels. While it took three months for waters to recede when hurricanes hit the state in 2004, that was with three hurricanes hitting the St. Johns River basin. Hurricane Irma was just one hurricane.

The high waters brought several issues. First, it flushed vegetation and debris out of area swamps and low-lying areas and set up a Biological Oxygen Demand (BOD) that contributed to lowered dissolved oxygen levels. Fish generally need a minimum of 2.0 mg/L of dissolved oxygen (DO) in the water, else a fish kill is imminent. FWC biologists, along with the Army Corps of Engineers, measured oxygen levels at several points along the St. Johns River and DO levels ranged between 0.09 and 1.5 mg/l due to the associated

impacts from Hurricane Irma. As was to be expected, dead fish of several species were observed. Recent community sampling activities have recorded dissolved oxygen levels more in line with historical levels, indicating that oxygen levels have rebounded.

Dead fish of various species at a St. Johns River boat ramp.

Further, runoff from terrestrial sources brought a lot of particulate matter into the river. This increased the turbidity and lowered light penetration, minimizing plant growth and oxygen production by submerged plants. Dark, tannin-stained waters that were flushed out of the local swamps also added to the shading effect and prolonged the time it took for regular oxygen production to resume. Secchi readings of lakes during community sampling have been one-half to one-third of historical levels. Discussions with local anglers indicated most of the fish caught have been higher up in the water column than they are normally found. While fish are being marked at deeper depths, it seems they are feeding higher, possibly due to an inability to see bait or lures.

Also, the winds Hurricane Irma brought caused A LOT of wave action, wave action that uprooted submerged and emergent plants. Biologists have observed large rafts uprooted plants in the lakes and river sections they are monitoring, mainly American Eelgrass Vallisneria americana, a native submerged freshwater tape-grass that provides valuable nursery habitat for sportfish, and refugia for various freshwater forage fishes. An additional stressor on what submerged aquatic vegetation is left will be the more turbid waters. High turbidity can prevent light from penetrating to submerged aquatic vegetation, causing the plants to cease photosynthesis and rely on dissolved oxygen in the water for respiration – dissolved oxygen that is initially at low levels after a storm. This can result in further loss of submerged aquatic vegetation due to oxygen deprivation. We’ve observed a loss of vegetation in most of the lakes we’ve surveyed so far, which was expected. We won’t begin to know the true extent of the loss of vegetation until water levels recede and the waters begin to clear. Further information will be gathered when the Long-Term Monitoring crew performs its vegetative surveys this summer. We’ll be able to compare the before and after surveys to better understand how much vegetation was lost. However, if past hurricanes are any indication, the lakes and river will bounce back, possibly taking a few years, but they will bounce back.