Aquatic sediments, both freshwater and marine, are home to a beautiful and diverse community of microscopic algae. Many people are familiar with seaweeds such as kelp and popweed, which are easily visible. However, microalgae are single-celled organisms and can be seen only through a microscope.

In Kachemak Bay, microalgae on tidal flats are mostly diatoms. Their presence on the sediment (mud or sand) can be seen from the golden brown color that appears on the tidal flat surface as the tide ebbs. This color is due to fucoxanthin, an accessory pigment characteristic of diatoms. Diatoms also have the chlorophyll pigment, which they use to capture light for photosynthesis as all green plants do. Algal species have a wide variety of accessory pigments in addition to chlorophyll, which result in a range of colors from red to brown to yellow to green.

A unique feature of diatoms is their silica frustule, which encloses the cell in a wide array of shapes and designs. These frustule designs and shapes are species-specific and help in the identification of diatoms species (see illustration). The silica frustule has several functions, which vary according to where the diatom lives. For example, diatoms that prefer to stay floating in the water column may have large, thin frustules with many projections or spikes, which help them to stay afloat, and may make them more difficult to eat. Conversely, diatoms that prefer to stay on the sediment surface have thicker frustules that include an opening called a raphe. By exuding protoplasm through the raphe, these bottom-dwelling, or benthic, diatoms can move quite rapidly, and many exhibit a vertical migration up and down through the surface sediments, following the tides and sun.

We have examined the diatom population in the tidal flats around Kachemak Bay and collected data on their abundance and diversity. We estimate abundance by extracting chlorophyll from surface sediments and measuring the chlorophyll with a spectrophotometer. Benthic diatom abundance on the tidal flats offshore Homer, at China Poot and around McDonald Spit is very high — as high or higher than that reported from other temperate intertidal flats. The diversity of the benthic diatom population also is very great, with more than 300 species occurring in a single sample of approximately half-inch diameter of mud surface. Many of these benthic diatoms get suspended into the water column as the tide rises, and may be transported to other locations in the bay.

Next: We’ll describe results of a study examining the role of benthic diatoms in the Kachemak Bay food web.

Carolyn Currin is a research microbiologist located at the National Oceanic and Atmospheric Administration’s Center for Coastal Fisheries and Habitat Research in Beaufort, N.C. There she is the leader of the Coastal and Estuarine Ecology Team. Funding for the research in Kachemak Bay came from NOAA’s National Ocean Service. If you have questions about Kachemak Bay, contact Kachemak Bay Research Reserve staff at 235-4799 or visit the Web site at www.kbayrr.org.