Benthic Flux

The Benthic Zone

By Michael Heller Water LIFE editor

Sewage and pollutants, all the phosphate mining seeps and spills, all the agchemicals .... everything liquid we put on top of the ground makes it into the porous underground of Florida. So I have to believe some of it makes its way into the Gulf, coming up through the Benthic Zone, the lowest level of the Gulf’s water.

I like to think of Florida as one of those green-on-one-side, scrubby yellow kitchen sponges you buy at the supermarket. The thin green side is Florida’s lush vegetation, the big fat yellow spongy part is Florida’s porous underground, the part where the aquifers flow.

Years ago there were bubbling freshwater springs out in the Gulf - actual upswelling of water you could see on the surface, on a calm day. Those springs were fed from aquifers beneath the Gulf floor.

Today the freshwater springs in Florida have a diminished flow, but fishermen are still fishing the offshore springs in the Gulf, so there must still be something happening down there, but what?

A 2007 report titled: Submarine Groundwater Discharge Along the West Florida Shelf: subtitled: Is Groundwater an Important Nutrient Source for Florida's Red Tides? doccumented scientists from the US Geological Service (USGS) and the University of South Florida as they looked at the precipitation associated with hurricanes Charley, Frances, Ivan, and Jeanne. They found had a two-fold effect

on the movement of nutrients on land to the submerged west Florida shelf in the Gulf: (1) an immediate effect by enhancing surface runoff and (2) a delayed effect by recharging the coastal aquifer and increasing submarine groundwater discharge along the coast, a year later. The same thing happened last Summer after 2017s heavy rains and then Hurricane Irma’s rain on top of that.

In descriptions of Florida's groundwater system, the most commonly discussed features are freshwater springs, sinkholes and the highly transmissive Floridan aquifer.

Sinkholes north of Lake Okeechobee have been found to be connected to the Gulf 70-miles away. Scientists also know there is a connection between Warm Mineral Springs at Northport and a cluster of underwater springs off Fort Myers Beach. All our underground pollution, all the deep injected sewage we are pumping underground.... is that in those connections?

Ten years ago, to test for a link between submarine groundwater discharge and nutrients on the west Florida shelf, USGS scientist Christopher Smith measured and tracked radon (222Rn) and radium (223, 224, 226 and 228Ra) isotopes.

He recorded: “Movement of nitrogen from the land to the ocean through submarine groundwater discharge—the flow of water from underground aquifers into the ocean, either through discrete submarine springs or by more diffuse flow from benthic sediment into overlying seawater.”

Smith added: “In coastal areas it is these shallow aquifers that are directly

Nitrogen and Heat: This graphic from another 2009 report, shows heating and exchange of underground salt and fresh water flows into the sub-surface ocean by Miami. The same thing is happening on Floridaʼs SWW coast, but to date very few studies have been done.

recharged by precipitation and they are also the most sensitive to human activity— such as application of pesticides, herbicides, and fertilizers.”

One aquifer he studied was at Tampa where a relatively thick section of mixed sedimentary deposits form a prominent ridge inland from the coast, referred to as the "Pinellas Ridge." This local ‘high area’ is a recharge zone for the surface aquifer and the underlying Floridan aquifer.

Smith found that recharge along the Pinellas Ridge created a ‘steep hydraulic gradient’ between the sub-marine groundwater and adjacent surface-water bodies which was ‘subsequently driving nutrients derived from human activities, such as fertilizer application and septic-system seepage,

to the coastal ocean.’

Judging from estimated spring-discharge rates and measured nutrient concentrations, Smith found the total dissolved inorganic nitrogen being discharged by one Gulf spring near Tampa to be: “The same as the nitrogen level in the major rivers feeding Tampa Bay.”

Along with the nitrogen are dinoflagellates, some of which feed red tide; dinoflagellates are the most prominent microbes found in the benthic layer.

There is still uncertainty as to the overall role of submarine groundwater discharge to the Gulf and red tide. Very few studies, other than Smith’s, of benthic flux in the Gulf, have ever been conducted.

I think we just don’t want to know.