Research

People, Microbes and their Shared River

Compared to the ocean, rivers are awash in nutrients and organic material. This is largely due to drainage from the surrounding watershed and marshes, which carries both particulate and dissolved minerals into rivers, along with particulate and dissolved organic material.

This material supports a variety of microbes that grow at a frenetic pace during the warm months of the year. Bacteria levels in an estuary in summer, for example, can exceed those in adjoining areas of the ocean by more than one thousand times. These bacteria, in turn, are eaten by other microorganisms, creating a miniature food web that fuels the growth of larger river inhabitants and also degrades a significant amount of organic matter before it reaches the sea.

This microbial food web has served as the biological hub of river ecosystems since long before people arrived on the scene. However, the growth of large human populations in the watersheds of rivers and estuaries has changed the nature of the microbial community in a variety of ways that are only now beginning to be understood in any detail.

Some of the most worrisome changes include the introductions of pathogenic bacteria and viruses into the estuarine community. Such water-borne diseases kill more than 5 million people each year, mainly children in tropical counties. In developed countries, these risks have been greatly reduced, but pathogen levels remain high enough to restrict beach and other recreational uses of waterways, particularly in the summer.

Human settlements also increase the input of nutrients and organic matter to rivers and estuaries in a process called eutrophication, which stimulates growth in the microbial food web and also enhances the growth of phytoplankton at river mouths. When this organic matter decomposes, it can lead to greatly reduced oxygen levels that make sections of the estuary uninhabitable for aquatic invertebrates and fish.

Untangling the microbial web

In order to examine the link between the Hudson River's microbial food web and the flow nutrients into the river, four sites in the lower part of the estuary were repeatedly sampled during the summer of 2005 (Piermont, N.Y.; Pier 26, Manhattan; Newtown Creek; and the Gowanus Canal). The nutrients that were measured, including both dissolved organic nitrogen and phosphorus (DON and DOP), reflect the system's ability to support the microbial loop, an important component of the total biological production in the estuary.

Piermont consistently had the lowest phosphate levels and nitrate levels were higher along the main trunk of the river. At Newtown Creek, nearby sewage outfalls may be associated with the very high bacteria levels, elevated phosphate and ammonium levels, and relatively low DOP and DON levels that were found.

In general, the abundance and diversity of microbes were lower at Newtown Creek than at the other sites. Moreover, patches of diatoms, which accounted for most of the elevated chlorophyll, appeared as small patches (10s to 100s of meters across) moving through the lower estuary with the prevailing currents rather than in sustained, large-scale blooms.

Diurnal sampling at Gowanus has indicated that run-off is high in nitrogenous nutrients, while phosphorus was mainly associated with the water in the Upper Harbor. This has important implications to engineering work that will be needed to mitigate the effect of combined sewer outflows in the region in order to reduce nitrogen in the Long Island Sound.