Volume 3, Number 2
Table of Contents
Inside the Bay's "Green Box"
From 500 feet in the air, scientist Larry Harding can measure the total amount of chlorophyll in the Bay and how it varies in space and time. From chlorophyll abundance, he has developed a model to calculate primary productivity, or how much carbon the Bay's floating plants fix by photosynthesis in a specific area in one day. But it takes some wet shipboard sleuthing to identify the cast of characters that are driving the Bay's complex food web. A postdoctoral scientist in Harding's group, Jason Adolf is trying to figure out which groups of phytoplankton are in the Bay and to link this information back to data collected by aircraft and satellite surveys in order to tease apart the environmental factors that are driving their distribution.
"The goal of this work was to open up the 'green box' of chlorophyll in the Bay and to decipher what groups of phytoplankton are both present and active at a given location or point in time," says Adolf, taking care to credit ecologist Hans Paerl at the University of North Carolina in Chapel Hill for coining the phrase.
All photosynthetic algae and certain bacteria (cyanobacteria) contain chlorophyll a (one subtype), but the techniques used to quantify it from an aircraft or satellite cannot resolve the differences among groups of photosynthesizers. The major groups that contain chlorophyll a also have other distinctive photopigments (such as fucoxanthin, peridinin, zeaxanthin, and others), so the unique chemical structures of these pigments can be used to identify what types are in a sample.
From 1995 to 2000, Adolf worked with Harding's group to collect and analyze water samples taken seasonally across regional scales that match Harding's aircraft surveys. Using a method called high performance liquid chromatography (HPLC), which separates compounds and quantifies them based on their chemical structure and light absorption properties, he identified which groups of phytoplankton were present at different times of the year.
Adolf found that 93 percent of all the chlorophyll a in the Bay's "green box" came from four major groups: diatoms, dinoflagellates, cryptophytes, and cyanobacteria. But he found that the relative abundances of these groups in different seasons changed dramatically from year to year. "We didn't expect to see such a variable pattern," he says.
Adolf's findings did support the current paradigm of a spring to summer transition in species composition that is driven by early high freshwater flow in the winter and low flow in the summer. Diatoms tend to be very abundant in spring and less abundant in summer. Adolf found that they make up roughly an average of 70 percent of the biomass in spring but only 28 percent in summer.
Although only single-celled, diatoms have distinctive cell walls made of silica and a characteristic structure called a frustule, which consists of two valves that fit within each other. They vary widely in size, ranging from two microns to several millimeters, and in shape, from spheres to cylinders to pancake-like discs.
Most diatoms are not capable of active movement. Conventional thinking holds that in the Chesapeake Bay diatoms dominate in the spring because the water is turbulent enough to keep them in suspension, explains Adolf. When tributary inflow to the Bay subsides in the summer, diatoms sink to the Bay floor forming a thick carpet of organic matter that contributes to the low oxygen conditions typical of late spring and summer. During high flow years, diatoms are more abundant and persist longer into the summer.
Diatoms are generally good food sources for filter-feeding zooplankton, bottom-dwelling invertebrates and larval fish. Cyclotella is one genus that is exceptionally plentiful in Chesapeake Bay. Some species in this genus have long, thin spines that are thought to help with flotation.
Dinoflagellates comprise an average of 20 percent of the total phytoplankton biomass of the Bay at their highest seasonal abundance, which according to Adolf's analysis occurs in the summer, dropping to as little as 4 percent in the fall. Single-celled organisms, dinoflagellates are often photosynthetic - but not always. Their name is derived from the two whip-like flagella that allow them to move actively through the water column. Unlike the silica-laden diatoms, dinoflagellates have a cell wall made of cellulose, which, in some species, is divided into a distinctive armor known as theca. These plates form unique geometries that can be used for classification.
Dinoflagellates are important sources of food for higher trophic levels in the Bay, but they are also somewhat infamous for releasing harmful toxins and for changing the color of the water when they proliferate and reach high densities. The dinoflagellates Prorocentrum minimum and Karlodinium micrum are the chief culprits in the Chesapeake behind a phenomenon known as mahogany tide. When these species bloom, the high biomass may severely limit the amount of oxygen available to other organisms, sometimes resulting in local fish kills - K. micrum is known to have caused several fish kills in aquaculture facilities in the Bay. Although no cases of shellfish poisoning resulting from toxins produced by these dinoflagellates have been reported in Maryland waters, according to the Department of Natural Resources, scientists and managers suspect that they do have the potential to be toxic to shellfish. Prorocentrum micans is a cousin of P. minimum.
Cryptophytes comprise an average of 35 percent of the plankton biomass in the fall, according to Adolf's analysis, but are not as well known as other groups of algae in the Bay. Like dinoflagellates, they are single-celled, have flagella, and are capable of movement. They have a distinctive carotenoid pigment called alloxanthin, in addition to chlorophyll, making them easy to distinguish from the other groups. Cryptophytes are also notable for an additional compartment in their nucleus that contains nucleic acid. This structure is thought to be a relic of the nucleus of a formerly free-living red algal cell that had been taken up through a process called "endosymbiosis." The genus Cryptomonas
The final major group of photosynthesizers in the Bay are the cyanobacteria. Cyanobacteria are not algae, but true bacteria that also contain chlorophyll and perform photosynthesis. In the summer, Adolf found that cyanobacteria account for an impressive average 26 percent of the chlorophyll biomass in the Bay. Cyanobacteria are quite small and usually unicellular, though they may grow in colonies large enough to see, particularly in fresh water, arranged in long, filamentous formations. They contain an accessory pigment called phycocyanin, which gives the group its name and, combined with a unique carotenoid called zeaxanthin, allows for ready identification.
Although in some ecosystems cyanobacteria are a valuable food source, in the Bay they tend to be harbingers of poor water quality. Recent cyanobacteria blooms in Colonial Beach, Virginia, for example, have caused repeated beach closures. There are many cyanobacteria that are either toxic or inedible, and they tend to thrive under nutrient-enriched, eutrophic conditions. Some cyanobacteria can actually metabolize nitrogen from the atmosphere (N2), converting it to ammonium. In some bodies of water, nitrogen fixation caused problems with nutrient overenrichment.
Ultimately, Adolf hopes that information about species composition at the base of the food chain will help to untangle the web of culinary preferences that drive the consumers in the Chesapeake Bay ecosystem. "We know from other research in the field that a varied diet of different species of phytoplankton tends to promote more successful growth and reproduction by grazers higher up the food chain," says Adolf. "And we know that there will be differences in the food web when the Bay is filled with one type of phytoplankton versus another," he says.
Making the link between photosynthetic algae and species dynamics at higher trophic levels still remains a challenge. "People would love to be able to use floral composition data to address questions about fisheries abundance," says Adolf. But the "green box" of the Bay still has plenty of secrets yet to share.
- Erica Goldman