Phosphorus and Nitrogen Limited Phytoplankton Productivity in Northeastern Gulf of Mexico Coastal Estuaries

Abstract

An understanding of nutrient limitation of estuarine phytoplankton growth is important in making environmentally sound decisions concerning watershed development and the use of aquatic environments for waste disposal. Experiments to determine nutrient limitation of phytoplankton productivity were conducted monthly during the summers of 1975 and 1976 in several shallow north Florida coastal and estuarine systems by inorganic carbon-14 uptake and phosphorus-32 bioassays. The results of these nutrient enrichment experiments suggest that phosphorus is frequently more important than nitrogen in limiting phytoplankton productivity in nearshore northeastern Gulf of Mexico. A multiple regression model was constructed to determine which combinations of environmental and nutrient variables could explain the most variation of phytoplankton productivity in these coastal systems. The final regression model was:

$${\text{P}}{\text{.P}}{\text{. = 32}}{\text{.1 + 48}}{\text{.4 S}}{\text{.R}}{\text{.P}}{\text{. = 0}}{\text{.54 Salinity}}$$

where: P.P. is phytoplankton productivity in μgC 1^-1hr^-1, S.R.P. is soluble reactive phosphate in μg-atom PO₄-P 1^-1, and salinity in part-per thousand. This model explained 64 percent of the variation in phytoplankton productivity.

Nitrogen has been identified as the primary limiting nutrient for phytoplankton in coastal waters and it has been proposed that the removal of phosphorus from marine waste discharges will have little impact on the control of eutrophication. The observation that phosphorus is important in limiting phytoplankton productivity in these coastal and estuarine areas suggests that water quality planning for the coastal zone is best done on a regional basis, with consideration given to local nutrient cycling processes.