Evaluation of a Periphyton-based Stormwater Treatment Area (PSTA) in the
margin of the C-111 canal and the Everglades National Park (ENP): Results of a
two-year investigation (2001-2002)
Serge Thomas, Evelyn E. Gaiser, Miroslav Gantar, Aga Pinowska, Leonard J.
Scinto and Ronald D. Jones.
SERC/FIU, Periphyton Group, Miami, FL, USA.
Introduction
The Everglades is a large subtropical peat-based wetland located in a limestone
depression of the South Florida peninsula, USA. Originally, this oligotrophic
hydrosystem naturally depleted in phosphorus (typically <0.32 µM TP)
encompassed a large surface area (10,000 km2) and was characterized by shallow,
slow-moving water originating from Lake Okeechobee and flowing to the Gulf of
Mexico. Specific hydropatterns of seasonal drying and inundation (rainfall-driven
hydrology) linked to low rates of nutrients loads allowed paradoxically the
development of tremendous biomass of emergent macrophytes interspaced with
tree islands and open water sloughs containing abundant benthic/floating mats of
periphyton, thus sustaining a flourishing secondary production. However, for
more than a century, this fragile hydrosystem has gradually been modified by the
construction of dikes and canals aimed to drain some portions of the Everglades
for the development of agricultural and urban areas. The result is a profound
change in hydropatterns and nutrient loads of this region: the canals channelizing
water and concentrating diffuse agricultural/urban runoffs containing high rates of
pollutants, which pour into the remaining natural marsh with negative impacts on
the ecosystem. In particular, increasing amounts of TP pouring into the
Everglades profoundly affects the periphyton community which switches from a
thick, cohesive diatom-cyanobacteria dominated mat in the pristine parts of the
hydrosystem to a scattered non-cohesive filamentous green algae community in
the impacted hydrosystem with a lower biomass per unit of surface area. Thus, it
appears important to limit TP loads into the natural ecosystem and to restore
natural hydropatterns.
The high growth rates of Everglades periphyton linked to high TP affinity led to
the development of Periphyton-based Stormwater Treatment Areas (PSTA).
These are hydrology-controlled shallow constructed wetlands positioned in the
margin of polluted and natural systems and aim to remove TP from the water
before it enters into the natural marsh. One obvious advantage of the PSTA over
other ecotechnologies involving algae such as phytoplankton is that TP is
sequestered in the periphyton matrix and can be easily harvested with the
periphyton or permanently sequestered in the soil (accretion).
At the Southeast Environmental Research Center, we are studying how
periphyton reacts to various TP loads (flume experiments, transects studies along
TP gradients and PSTA), but also how PSTA can be managed for better TP
removal. In the study presented, we evaluate a PSTA resulting from the southern
levee removal of the C-111 canal, which once bordered the northeast side of the
Everglades National Park (ENP).
Problematic
The removal in 1997 of the Southern levee bordering the C-111 canal and the
ENP allowed not only more freshwater intrusion into the ENP, but also provided a
relatively flat limestone area promoting, when flooded 6 months a year from June
to December, the growth of thick short-hydroperiod calcareous periphytic mats
(epilithon). Thus, this site is potentially an opened PSTA, which hydrology can be
moderately controlled through the C-111 water gates and pumps. The study aims
to evaluate/manage this PSTA through different harvesting periodicity to promote
the best TP removal per unit of surface area. In that time, hydrology was not
considered or managed.
Methods
In March 2001, the southern edge of the C-111 canal was dried and covered by
thick, calcareous periphytic crusts about three years old (3 kg m-2), interspaced
with scarce rooted macrophytes. Three sites interspaced about 1 km were chosen
for their visual similarity and elevation. A grid with one side parallel and the other
one perpendicular to the canal (20 by 40 m2, square cells of 3 m of side) was
selected for each site in an area containing mats of similar TP content (110 µg TP
g-1). Within each grid, 5 different harvesting treatments (three replicates for each
treatment) were randomly selected among the cells:
* Treatment A- harvesting 0, 2, 3, 4, 6 months after inundation of the site
* Treatment B- harvesting 0, 2, 6 months after inundation of the site
* Treatment C- harvesting 0, 3, 6 months after inundation of the site
* Treatment D- harvesting 0, 6 months after inundation of the site
* Treatment E- never harvested
Harvesting is defined as removing through scraping the whole periphytic mat
from the treatment cell and discarding the harvested periphyton, downstream in
the C-111 canal. Every month and if applicable, before harvesting, a digital
picture of each treatment cell was taken and cohesive periphyton was randomly
cored within the cell and analyzed afterwards for TP content in the laboratory. TP
sequestered per unit of surface area was then computed by taking into account of
the surface area of the core and of the periphyton mat coverage in the cell as
deducted from the digital picture. Number of stems, periphyton
cohesiveness/thickness, the percentage of floating periphyton in the cell and
water-level were also recorded.
Results and discussion
The southern edge of the C-111 canal was flooded for 6 months with about 10 cm
of water from the end of July 2001 to the end of January 2002. Two months after
inundation, two-month old mats found in treatments A to D were cohesive and
sequestered the same amount of phosphorus (111 mg TP m-2). These treatments
contained significantly less TP (P<0.05) than treatment E with 3 years and 2
month old mat (157 mg TP m-2). After three months of flooding, there was no
difference among the treatments (A to E) in TP sequestration per unit of surface
area. Three months after inundation was also the time when sloughing (floating
mats appearing) just occurred in all treatments. This suggests that mats older than
three-years sequester significantly less TP per surface area than newly established
mats on a bare substratum flooded 3-months or less. After four months of
inundation, a decrease in the cohesiveness of the mat, especially in the youngest
mats was observed denoting a decline of the productive capacity of the mat. This
phenomenon occurring at the end of the flooding season is observed elsewhere in
the Everglades and is still poorly understood. The drying down of the sites
occurring during the month of January led to an increase in both mat cohesiveness
and TP sequestration per unit of surface area suggesting that drying contributes to
increase TP retention.
From the harvesting treatments described above, 6 harvesting frequencies (HF),
which can be used as PSTA management practices, can be deducted:
* HF1= harvesting every two months (335 mg TP m-2 y-1)
* HF2= harvesting 0, 2 and 6 months after inundation (262 mg TP m-2 y-1)
* HF3= harvesting every 3 months (354 mg TP m-2 y-1)
* HF4= harvesting 0, 4 and 6 months after inundation (222 mg TP m-2 y-1)
* HF5= harvesting every 6 months (208 mg TP m-2 y-1)
* HF6= no harvesting (78 mg TP m-2 y-1)
It appears that HF6 has the lower TP retention per year. Therefore periphyton
harvesting as a PSTA management practice should be envisaged. HF3 mimicking
the best the periphytic cycle is the best HF with the most TP sequestered per
surface area per year.
Serge, Thomas, SERC/FIU, UP OE-148, Miami, FL, 33199, Phone:
305 348 6167, Fax: 305 348 4096, thomasse@fiu.edu, GEER program, question 3
(Water quality and water treatment technologies).
ORAL