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Phytoplankton Responses to Experimental Nitrogen and Phosphorus Loading in the Eutrophic and Colored Caloosahatchee River, Florida

Scientific Investigations Report 2026-5141
Environmental Health Program
Prepared in cooperation with the U.S. Army Corps of Engineers, Nova Southeastern University, and Florida Gulf Coast University
By: , , , , , , and 
https://doi.org/10.3133/sir20265141

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Abstract

The Caloosahatchee River, located in southwest Florida, is a eutrophic and colored river that flows from Lake Okeechobee westward into its estuary and the Gulf of America. Cyanobacterial harmful algal blooms (HABs) are a documented problem along this freshwater-to-marine waterway where nutrient enrichment has been identified as a key factor in bloom occurrence but has not been experimentally tested in the river. This study is the first to test the effects of inorganic nutrient loading on phytoplankton assemblages in the Caloosahatchee River and the effects of different nutrient sources on phytoplankton dynamics at different times of the year. Three independent, in situ experiments were conducted to test the effects of daily, incrementally increased ammonium, nitrate, and phosphate loading on phytoplankton at different times of the year (summer, fall, winter). Over the 72-hour enclosure period, phytoplankton abundance metrics (cell concentration, chlorophyll-a, and phycocyanin), dissolved oxygen, and pH increased, and fluorescent dissolved organic matter and turbidity decreased in all treatments and controls. Increased phytoplankton abundance metrics relative to controls were observed after 72 hours of exposure to elevated ammonium and nitrate in summer and only ammonium in winter, suggesting periodic nitrogen limitation; however, no treatment effects on phytoplankton assemblage structure in terms of resemblance and diversity metrics were found. Increases in total cell concentrations were driven by elevated growth rates of already dominant taxa but not sufficiently to form a visible bloom. Cyanobacteria consistently dominated the phytoplankton, particularly Aphanocapsa and Merismopedia, whereas the common HAB-forming Microcystis maintained consistently low abundance. This study provides new information on the ecology of phytoplankton assemblages in the Caloosahatchee River and could be used by water resources managers to evaluate strategies for controlling cyanobacterial HABs in the river.

Suggested Citation

Mazzei, V., Loftin, K.A., Karwacki, E., Lopez, J.V., Krausfeldt, L.E., Rosen, B.H., and Urakawa, H., 2026, Phytoplankton responses to experimental nitrogen and phosphorus loading in the eutrophic and colored Caloosahatchee River, Florida: U.S. Geological Survey Scientific Investigations Report 2026–5141, 32 p., https://doi.org/10.3133/sir20265141.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Study Area
  • Methods
  • Results
  • Discussion and Conclusions
  • References Cited
  • Appendix 1. Supplemental Tables and Figures
Publication type Report
Publication Subtype USGS Numbered Series
Title Phytoplankton responses to experimental nitrogen and phosphorus loading in the eutrophic and colored Caloosahatchee River, Florida
Series title Scientific Investigations Report
Series number 2026-5141
DOI 10.3133/sir20265141
Publication Date April 10, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Caribbean-Florida Water Science Center
Description Report: x, 32 p.; 3 Data Releases
Country United States
State Florida
Other Geospatial Caloosahatchee River
Online Only (Y/N) Y
Additional Online Files (Y/N) N
Additional publication details