Algal Growth: The Role of Metals

All algae are not alike. There are thousands of species – while most are beneficial to the ecosystem, some can have toxic impacts. Particular algal species shift in dominance from one season to another depending on a number of factors. Primary among them are the relative proportions of nitrogen, phosphorus and silica that are available in the water and their interaction with a host of environmental factors, for instance, temperature, salinity and light.

As Pat Glibert has pointed out (see "Uncommon Blooms"), the form of available nitrogen can influence which algal species will succeed; also important to success, says James Sanders, are the concentrations of trace metals. Metals like mercury, arsenic, copper, selenium – while natural constituents of estuarine waters – can become contaminants if human activities lead to higher than normal concentrations.

For some years, Sanders and his colleagues at the Academy of Natural Sciences Estuarine Research Center (ANSERC) in St. Leonard, Maryland, have been studying the complex effects of elevated levels of contaminants. While contaminant levels of arsenic, for example, may be toxic to one species of algae, another may be able to tolerate those concentrations and grow well.

In early experiments, algae in mesocosms (large experimental tanks) immersed in the Patuxent River were dosed with metals and compared with algae in mesocosms that were left alone. In general, says Sanders, while growth and biomass in the dosed and undosed mesocosms did not change, species that were normally dominant were often replaced by algal species that were resistant to the higher loads. And those algal species, he says, tend to be smaller. (Whether coincidental with Glibert’s hypothesis that organic nitrogen may favor smaller, potentially harmful, algae is an open question.)

While these studies added to an appreciation of how contaminants may influence the kind of algae that succeed or fail, they did not necessarily simulate the real world of aquatic ecosystems where neither nutrients nor contaminants are constant. To do this, ANSERC scientists took on an ambitious long-term project (supported by NOAA’s Coastal Ocean Program) that aims to assess the interactive effects of multiple stressors simultaneously – nutrients and contaminants – and explore the implications of such effects on species further up the food chain.

So far, says ANSERC researcher Gerhardt Riedel, experiments have shown that nutrient loading is the dominant effect on algal growth and species. "Plants need nutrients, and all other things being equal, they will use up all available nutrients." The effects of elevated levels of contaminants are much more subtle, he says. "If you add a toxic element, you inhibit growth of a few species, so that what you do at low concentrations is knock out a few sensitive species. If the species is not dominant, you won’t see it at all. If it is important, then you will."

One such dominant species in the Patuxent River is Rhizosolenia fragilissima, a large diatom that is sensitive to copper and arsenic. When you add these metals, says Riedel, "you selectively inhibit its growth." But the explanations of just why may depend, once more, on the availability of particular nutrients. The chemical form of arsenic is arsenate, a chemical analogue of phosphate – if phosphate is low, the diatom will take up arsenate which is toxic to it. Toxicity, then, may depend on phosphate concentrations. In general, says Riedel, experimental results so far indicate that the effects of contaminants on algae may vary considerably, depending on the concentrations of metals but especially on the availability of nitrogen and phosphorus.

The bottom line is that the Bay is overenriched with nutrients – if the influence of metal concentrations on the growth of potentially harmful algae is to be minimized, it will depend on major cutbacks of the nitrogen and phosphorus loads that flow into the Chesapeake system.

Home • Search • Our Program • Chesapeake Bay
Research  Policy • Extension Program • Marine Education • Book Store & Videos • Hot Topics

This page was last modified May 23, 2002

http://www.mdsg.umd.edu/MarineNotes/May-June98/side1.html

Accessibility Statement