Discovering Silica Cycling

By Joanna Carey

Rivers draining more forested watersheds contain significantly less silica than those draining more developed watersheds.

I am standing, engrossed in quiet, on a wooden bridge in Northern Massachusetts, with a perfect view of the Ipswich River.  I can see it meander once before it eventually opens up to form a babbling riffle. This river is alive, performing complicated metabolic processes as the water moves downstream.

Thanks to my EPA Science To Achieve Results (STAR) Graduate Research Fellowship, I went to this bridge (among others) weekly for a year, sampling the river for nutrients. While filtering my water samples here, people walking by would often ask, ‘how is the river doing?’

Before answering, I would hesitate; it turns out this is a complicated question!

From a human health perspective, most of the rivers I studied were in fine shape (thanks to the Clean Water Act and EPA), meaning that people could swim in the river without getting sick. However, other aspects of the river condition could use improvement.

Human activities, such as wastewater discharge, use of fertilizers, and fossil fuel combustion, are increasing the amount of nutrients flowing into rivers, which can spark excess algal growth and other negative repercussions on the entire ecosystem.

As an EPA STAR Fellow, I had the opportunity to be one of the first in the world to examine how watershed land use impacts the amount of silica in the rivers. Silica, or SiO2, is a required nutrient for diatoms, a common type of phytoplankton (tiny photosynthetic organisms) in temperate waters.

Why is the amount of silica in the rivers important?

Well, it all goes back to the fact that rivers supply over 80% of the silica that’s found in marine waters. And the amount of silica directly controls the amount and type of phytoplankton that grow in the ocean. Because phytoplankton makes up the base of the marine food chain, their type and abundance directly impacts organisms higher up on the food chain, such as commercial fisheries.

My research resulted in the discovery that land use type is indeed an important driver of the amount of silica in rivers.

I found that rivers draining more forested watersheds contain significantly less silica than those draining more developed watershed, which may be because of the large amount of silica taken up by land plants. It appears that lack of vegetation in urbanized landscapes results in more silica entering river systems. While more silica in rivers is not a bad thing, the research highlights a previously unrecognized way in which human actions are altering the environment.

For the last three years, I have been honored to be an EPA STAR Fellow. The award not only allowed me to perform the research of my dreams, but highlighted for me the importance of these fellowships for training the next generation of scientists. Thanks to the EPA, I can now count myself among the experts in aquatic biogeochemistry!

About the Author: Joanna Carey, a former STAR Fellow, is currently an ORISE post-doctoral fellow with the EPA Atlantic Ecology Division in Narragansett, RI studying the impact of oysters on nitrogen cycling in Southern New England.