Experimental phosphorus enrichment in Everglades National Park: I. Approach
and Methods
Evelyn E. Gaiser, Adrienne Edwards, Krish Jayachandran, Ronald Jones, David
Lee, Thomas Philippi, Jennifer Richards, Leonard Scinto, Joel Trexler
Florida International University, Miami, FL
This presentation describes the approach and methods of phosphorus (P) dosing
research at the experimental flume facilities established by in Everglades National
Park by the Southeast Environmental Research Center at Florida International
Univerisity. This research was originally designed to provide critical baseline
information needed to establish Class III water quality standards for the
Everglades Protection Area. Research began in 1998 when we completed the
installation of three, four-channel 100-m long flumes in previously unimpacted
wet prairie marsh of Shark River Slough in Everglades National Park. Low-level
P-addition treatments of 0, 5, 15 and 30 ppb above ambient concentrations are
imposed at the upstream end of the channels and delivered downstream by natural
flow and spiraling through biota. This study differs from other P enrichment
experiments by delivering measured concentrations of P continuously into water
flowing into long (100 m), replicated, experimental channels established in
natural marsh; this mimics the way P enters the natural system from canal inputs.
Responses in all relevant abiotic and biotic parameters have been measured
downstream from inputs for a period of 4 years.
The central hypotheses guiding our dosing research are that:
1) Low-level additions of P to the water column will induce an ecosystem
state change in the Everglades that will eventually lead to the types of
disturbed ecosystems that occur in enriched areas elsewhere in the system.
2) Responses measured first in microbial components will cascade through
the food web to induce imbalances throughout other components of the
ecosystem.
3) Ecosystem state change will occur more rapidly when the concentration of
experimentally added P is greater, but the endpoint of this ecosystem state
change will not be affected by the concentration of experimentally added
P (i.e., there is no P threshold; time, not P concentration, is the
independent variable of importance).
4) During ecosystem state change, the system may become N limited and
may actually be a source of P to downstream wetlands.
These hypotheses are illustrated in Figure 1.
Figure 1. Schematic representation of the hypotheses driving experimental dosing
research at FIU.
High Dose
upstream
downstream
Degree of
Imbalance
Fast-response
Parameter
Low Dose
upstream
downstream
Same
endpoint
regardless
of dose
Slow-response
level
Parameter
Duration of P enrichment
Briefly, our experimental design consists of three, four-channel 100-m long
flumes that were established in pristine Eleocharis "wet prairie" marshes. Each
flume has a control channel and 3 experimental channels representing a range of P
concentrations bracketing currently proposed water quality standards: a low dose,
treated with P to increase ambient total P (TP) concentrations to 5 µg l-1 (5 ppb, or
≈0.17 µM), medium dose, treated with P to increase ambient total P (TP)
concentrations to 15 µg l-1 (15 ppb, or ≈0.5 µM), and high dose, treated with P to
increase ambient total P (TP) concentrations to 30 µg l-1 (30 ppb, or ≈1.0 µM)
above ambient. Reference plots have been designated in an untreated area of
marsh adjacent to each flume to facilitate the detection of “edge” effects in the
flumes. We attempted to minimize “edge” effects in the experimental design by
restricting our sampling to the center of broad (3-m wide) channels and by using
floating docks with plastic retaining walls, rather than elevated platforms, which
are known to produce shading effects.
Phosphorus additions to our flume channels are electronically controlled and
based on instantaneous water volume to maintain consistent experimental
concentrations. The first 10 m of each flume channel is the nutrient mixing area
and the zone where we measure water flux. This mixing area is devoid of
vegetation and has a solid fiberglass “floor” fixed to the soil surface. Highly
sensitive acoustic Doppler flow sensors, which measure flow rates down to 1-2
mm sec-1 and integrate flow across the width and depth of the channel, are set up
in these mixing areas. They transmit channel-specific flow rates in real time to an
on-site computer that also receives real-time water level data from a pressure
transducer. The computer calculates channel-specific flux and determines the rate
at which P must be pumped into that channel, based on its programmed treatment
level. Phosphorus is added from a reservoir filled with NaH2PO4 + Na2HPO4 (to
make a pH of 7.0). We are thus adding P as soluble reactive P (SRP) which is
quickly taken up into particulate fractions (measured as TP) in the mixing areas.
The on-site computers are interfaced to cellular communications, allowing us to
access water flux data, met station data, pump status, and system diagnostics at
any time. All on-site electronics are powered by solar panels and a bank of deepcycle batteries.
Our research team has been sorted by discipline into 8 Key Element Groups,
including: environmental monitoring, biogeochemistry, microbial ecology, soils,
periphyton, macrophytes, fauna, and data integration. All key parameters are
sampled on a regular schedule when water is flowing and P is being added.
To analyze ecological response to P dosing in our experimental flumes, we
acknowledged that flow rates into the channels differed among the 3 sites. As in
the natural system, low-dose, low-flow channels received a lower load of P than
high-flow channels receiving the same P concentration. Therefore, we first
determined the cumulative load of P received at the head of each channel
throughout the 4 year dosing period. We then related each response parameter to
cumulative load at the head of the channel, with distance from that load entered as
a covariate. The following series of presentations will address results pertaining to
the 4 hypotheses, the application of these results to interpreting large-scale
enrichment patters in Everglades marshes, and the implications of our design in
setting water quality compliance standards.
Evelyn Gaiser, Department of Biology and the Southeast Environmental Research
Center, Florida International University, Miami, FL, 33199,
Phone: 305-348-6145, Fax: 305-348-4096, gaisere@fiu.edu, Ecology