Phytoplankton ecology of the Indian River near Vero Beach, Florida

The average, annual total phytoplankton cell density in the Indian River near
Vero Beach was 1.38 X 10¹⁰ cells m⁻³ and the average net (>20 µm) phytoplankton cell density
was 9.88 X 1(!1 cells m-3• Average total chlorophyll a for the year was 10.69 mg m⁻³ and 3.42 mg
m⁻³ for the total and net fractions respectively. This portion of the river was characterized by
relatively high nutrient concentrations. Yearly mean values for total nitrogen, orthophosphate
and silicate were 9.24 µM, 1. 79 µM and 53.67 µM, respectively. Most biological standing stock
belonged to the phytoplankton size fraction passing through a 20 µm mesh net. These
nanophytoplankton accounted for 99.3% of the cell density, 90.7% of the biomass and 78.0% of
the active chlorophyll a observed throughout the year study. Variations in diatom cell densities
accounted for most total phytoplankton cell density fluctuations. Skeletonema costatum (Grev.)
Cl. was the dominant diatom in terms of numbers and biomass. The initial spring bloom was
comprised mainly of relatively small chain-forming diatom species of the genera Chaetoceros,
Nitzschia and Thalassiosira. This pulse was followed by a period of increased phytoflagellate and
dinoflagellate numbers. The diatom genera during this period were Thalassiosira, Nitzschia and
Skeletonema. The late summer-fall phytoplankton community was primarily composed of
Chaetoceros, phytoflagellates, Thalassiosira, Skeletonema and Paralia. The late fall-winter
phytoplankton community was dominated by phytoflagellates and a diatom assemblage comprised
of Thalassiosira, Nitzschia and Skeletonema.

Florida Atlantic University. Harbor Branch Oceanographic Institute contribution 279

This manuscript is an author version with the final
publication available and may be cited as: Mahoney, R. K. & Gibson, R. A. (1983). Phytoplankton
ecology of the Indian River near Vero Beach, Florida. Florida Scientist 46(3-4), 212-232.