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June 24, 2003
"Black Tide" on St. Leonards Creek, Calvert County.

A dense bloom of Karlodinium micrum was identified on St. Leonard’s Creek, Wednesday June 18th. The beginning of the bloom condition was documented one week earlier by Stella Sellner of the Academy of Natural Sciences Estuarine Research Center

High density bloom of Karlodinium micrum (1.5 million cells/ml) concentrated at the water surface and producing a "black tide" appearance in St. Leonard Creek, MD.
Figure 1. High density bloom of Karlodinium micrum (1.5 million cells/ml) concentrated at the water surface and producing a "black tide" appearance in St. Leonard Creek, MD. Photo by Stella Sellner, ANSERC.
(ANSERC) in St. Leonard, MD. The bloom density of 1.5 million cells/ml on Wednesday was so concentrated that the water appeared black on the surface (Figure 1); by comparison, bloom densities that have been associated with toxic algal events of K. micrum linked to fish kills have occurred at 10,000 to 30,000 cells/ml based on research by Dr. Alan Place and Jon Deeds (Center of Marine Biotechnology, Baltimore, MD). Water samples were collected by Richard Lacouture (ANSERC) and showed the bloom was concentrated in the upper 0.3 meter (1 foot) of the water columnn (Figure 2). At 1 meter below the surface down to the bottom of the creek the water was essentially clear of the algal bloom. Figure 3 illustrates the boundary of the bloom with nearshore water in the creek.

On Friday June 20th, a fish kill of silversides was identified in association with the St. Leonard’s Creek mouth related to ponded water that was cutoff from the main channel of the creek during low tide. The fish kill prompted additional investigation of St. Leonard Creek, however, there was no evidence of a broader scale fish health event. Algal samples were again collected and the cell concentrations declined but remained significant at 50,000 cells/ml in the surface water.
Water samples collected from a vertical profile of the water column in St. Leonard Creek, MD, on June 20, 2003. From left to right, samples were collected at the surface, then below the surface at 0.3 meters (1 ft), 1 meter and 2 meter depths.
Figure 2. Water samples collected from a vertical profile of the water column in St. Leonard Creek, MD, on June 20, 2003. From left to right, samples were collected at the surface, then below the surface at 0.3 meters (1 ft), 1 meter and 2 meter depths. Photo by Richard Lacouture, ANSERC.

Water quality conditions were measured throughout St. Leonard’s Creek during the rainstorm on Friday morning by ANSERC. Dissolved oxygen in the surface water measured 8.1 to 10.5 mg O2/L except at the head of the creek which measured 3.5 mg O2/L. Cloudy conditions Friday likely limited the levels of dissolved oxygen observed which will be expected to increase with sunny weather and may cause severe hypoxia at night with such extreme densities of algae. Bottom waters were severely hypoxic and ranged from 0.1 to 0.25 mg O2/L in the creek and 4.4 mg O2/L at the mouth.
A photo of "black tide" in St. Leonard's Creek.
Figure 3. Illustration of the boundary between clear water nearshore and the dense offshore bloom of K. micrum on June 18th, 2003. Photo by Stella Sellner, ANSERC.