Algal Blooms and Plankton Ecology

Primary Productivity in Florida Bay Waters

Florida Bay Project Profile - 33

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Questions: What areas in Florida Bay have the most actively growing phytoplankton populations? What factors limit growth in these drifting microscopic plants?

Background/Project: Dense populations or "blooms" of phytoplankton (microscopic algae drifting in the water column) have been observed in some areas of Florida Bay since the early 1990’s. Persistent blooms may be detrimental because they reduce water clarity affecting the quantity of sunlight reaching seagrasses on the bay bottom. Since phytoplankton is a major food source for zooplankton which are fed upon by small fish and invertebrates, blooms may also change the structure of the food web in the bay. In the early 1990’s, thousands of filter-feeding sponges in southeastern Florida Bay, died after being exposed to blooms which originated in northern Florida Bay. Since 1994, monthly water samples containing living phytoplankton populations were taken from four permanent sites located in different regions of the bay. A portion of each live sample was placed in a glass bottle and held in a plexiglass incubator having constantly circulating bay water. This incubator kept the phytoplankton at natural temperature and sunlight conditions. Primary productivity, the rate at which new plant material is produced in each sample, was measured by exposing the sample to different intensities of sunlight with the tracer element, Carbon 14. This measurement indicated how fast the populations from the different regions of the bay grew. Another portion of the sample from each site was also exposed to different nutrient treatments. The growth of phytoplankton, measured by a fluorometer, indicated how much of the green plant pigment, chlorophyll, was produced with each treatment. In this manner, each nutrient added could be evaluated in its role in stimulating growth. These data, in addition to those obtained in the field, showed how fast and what helped to make the bloom species grow.

Findings To Date: The greatest primary production was measured in the north central bay where the blooms are dominated by a primitive blue-green species. Under some conditions, individual phytoplankton reproduced as much as two times per day. The western bay, which was dominated by diatoms and flagellates, also showed very high production rates. The southwestern bay had the next highest rates. The lowest production, the lowest organic loads, and the lowest phytoplankton populations were found in eastern Florida Bay indicating that blooms in this area were rare. Nutrient incubations showed that low levels of phosphorus in the eastern bay was limiting the growth of phytoplankton populations while nitrogen and silica stimulated growth in the western regions.

Status: Funded through 1999.

Restoration Impacts: Understanding what controls phytoplankton blooms helps ecosystem modelers and managers to better predict water quality and the energy flow through the food web in Florida Bay.

Funding Sources: National Oceanic Atmospheric Administration, Atlantic Oceanographic Meterological Laboratory; Florida Marine Research Institute

Related Profiles: • FBPP 32 • FBPP 39 • FBPP 58