Phytoplankton and bacterial response to inorganic and organic nutrient enrichment and
alteration in Florida Bay: Results from bioassay enrichment experiments
Cynthia A. Heil
University of South Florida, St. Petersburg, FL
Patricia M. Glibert, Marta Revilla, and Jeffrey Alexander
University of Maryland Center for Environmental Science, Horn Point Laboratory,
Cambridge, MD
Susan Murasko, David Hollander, and Ana Hoare
University of South Florida, St. Petersburg, FL
The proposed restoration of freshwater, surface-flow through the Florida Everglades is
expected to alter the forms, sources, and ratios of nutrient inputs to Florida Bay. Of
particular concern is the expected change in the availability and ratio of dissolved
inorganic to dissolved organic nitrogen, as previous research in coastal oceans and
estuaries has recognized that alteration in nitrogen cycling has the potential to promote
selection and succession of phytoplankton species, change the distribution and magnitude
of primary production and associated trophic levels, cause degradation of benthic habitats
leading to mass mortality, and destroy recreational and commercial interests. As part of a
NOAA-funded program, we addressed the nutrient composition and the potential for
alternation in nutrient availability to impact plankton dynamics using short-term (48
hour) bioassay enrichment experiments in November 2002.
Water samples were collected from 6 geographically and ecologically unique sites within
Florida Bay: Little Maderia Bay, Sunset Cove, Rankin Bight, Barnes Key, Rabbit Key,
and a Gulf of Mexico influenced western Florida Bay site. Surface water samples from
each site were immediately returned to the laboratory, and analyzed for a full suite of
dissolved and particulate nutrients as well as phytoplankton (as chlorophyll a) and
bacterial biomass. Water from each site was dispensed into duplicate 2 L carboys and
enriched with ammonium (2 g at N l-1), organic nitrogen (a mixture of urea and amino
acids at 2 g at N l-1), inorganic phosphate (2.0 g at P l-1), organic phosphate (2.0 g at
P l-1), and humic acids (2 mg l-1). A control with no additions was also maintained. All
treatments were incubated under natural light and water temperature conditions and
monitored over 48 hours.
Although ambient inorganic and organic phosphorus concentrations were low at all
stations (<0.17 and <0.73 μM respectively), after 48 hours chlorophyll a in bioassays
increased significantly (P<0.05) upon inorganic phosphorus enrichment only in the
Sunset Cove and Little Madeira incubations, and increased with organic phosphorus
enrichment only in the Little Madeira incubation. Although a phosphorus stimulus
response was only evident only at these two stations, an increase in particulate P:Chl a
ratios with both inorganic and organic phosphorus enrichment was observed within 24 hr
at all stations, suggesting either luxury consumption and storage of both inorganic and
organic phosphorus additions, or that a stimulation of the heterotrophic rather than the
autotrophic community, occurred. In contrast, with the Rankin Bight bioassays, a
significant increase in chlorophyll a was observed only upon the addition of dissolved
organic nitrogen and at both Barnes Key and the Gulf stations with the ammonium
addition. No significant stimulation in chlorophyll a was noted by any nutrient addition
at Rabbit Key, indicating either nutrient sufficiency or simultaneous limitation by both
nutrients.
Although increases in chlorophyll a varied with nitrogen and phosphorus additions in
different locations in the Bay, physiological indices suggest that cells were poised to
utilize any source of nitrogen or phosphorus that was supplied. Potential utilization of
organic nitrogen and phosphorus were assessed in the bioassay experiments by measuring
alkaline phosphatase and urease activity after 48 hrs. Both are inducible enzymes that
allow cells to exploit organic sources of nutrient. Although enzyme levels varied from
station to station, in general alkaline phosphatase activity increased upon additions of
ammonium, organic nitrogen, and humic acids, whereas urease activity increased upon
additions of phosphorus. Thus, these cells demonstrated the ability to utilize organic
nitrogen and phosphorus when other cellular nutritional needs were met.
The response of the bacterial community to nutrient enrichment differed from that of the
phytoplankton community at each station. With Sunset Cove and Rankin Bight
incubations, there were no significant changes in bacterial abundance after 48 hour
exposure to any of the added substrates. At Little Madeira, additions of ammonium,
dissolved organic nitrogen and humic acids resulted in a significant (P<0.05) decrease in
bacterial abundance, suggesting that a stimulation of bacterial grazers by nutrient
additions occurred within 48 hrs. At Rabbit and Barnes Keys, inorganic phosphorus
stimulated bacterial abundance, as did dissolved organic phosphorus at Barnes Key.
Thus, the nutrients that stimulated production of phytoplankton biomass at each site were
not the same as those that either stimulated or depressed bacterial levels, suggesting that
different nutrients and processes regulate bacterial and phytoplankton biomass within
Florida Bay, and that altered nutrient inputs may potentially act differentially upon
bacteria and phytoplankton communities.
Cynthia, Heil, University of South Florida, College of Marine Science, 140 7th Ave S., St.
Petersburg, FL, 33701,
Phone: 727-355-1667, Fax: 727-553-1189, cheil@seas.marine.usf.edu, Question 3