Biodiversity in the Pettaquamscutt River Estuary

Sunshine Menezes, PhD candidate, Graduate School of Oceanography

The Pettaquamscutt River Estuary, or the Narrow River, is a small estuary in southern Rhode Island, fed at its headwaters by Gilbert Stuart Stream and at its mouth by Rhode Island Sound. For the past year, I have been conducting a biodiversity survey of a particularly small component of the phytoplankton (microscopic, single-celled, floating protists) in the Pettaquamscutt. This is an exciting location for my research because of the peculiarities of the environment.

The Pettaquamscutt River was carved by retreating glaciers approximately 10,000 years ago. During this process, two kettle lakes were formed in the northern portion of the river. These lakes, the Lower Pond and Upper Pond, have depths of approximately 19 and 15 meters, respectively. Relatively deep, these lakes are a sharp contrast to the rest of the river, which is only 2 meters deep, on average. This morphology creates what is called a fjord-type estuary: incoming tidal waters sink to intermediate depths in the lakes, resulting in a zonation of the water. There is a well-oxygenated surface layer, an intermediate layer of low oxygen (a hypoxic zone), and a bottom layer that is very rarely mixed, and therefore, remains anoxic (without oxygen) for long periods of time.
The Pettaquamscutt River is also subject to nutrient inputs, such as nitrogen and phosphorous, from the activities of people along the river. These nutrients fuel the growth of phytoplankton, and given the right conditions, blooms (a period of very high growth by a single species) may occur.

For all of these reasons, the Pettaquamscutt estuary is an interesting place to study biodiversity. My research focuses on nanoplankton, which have been studied very little, primarily due to their small size (Figure 1). It is useful to survey the diversity of species in this group for several reasons. First, previously undiscovered species will most likely be identified by this work. Second, it is useful for scientists and managers to understand the composition of the species for a particular ecosystem so that we have a means of gauging changes in biodiversity. If we don’t know what lives in the river, we can’t possibly know if it has been invaded by exotic species. This is especially important with phytoplankton, since certain species are often used as indicator species to give a rough assessment of the status of an ecosystem (an indication of human-mediated nutrient inputs, for example). Without a survey that has recorded which species are present, we might not be able to detect change. Of course we can record the invasions by larger organisms such as the green crab (Carcinus maenas) and periwinkles (Littorina littorea) rather easily, but that doesn’t tell us anything about the changes that we cannot see with the naked eye. Thus the phytoplankton can yield valuable information about changes in an ecosystem; biodiversity surveys are the most thorough means of recording these changes.

The Pettaquamscutt River is home to several novel phytoplankton species and interactions. One of these species, Hermesinum adriaticum, is a heterotrophic protist (a non-photosynthetic protist that gets energy by consuming other small protists or bacteria), which has only been found in a handful of locations around the world, including the Adriatic Sea, the Mediterranean Sea, Chesapeake Bay, and the Pettaquamscutt River Estuary. Considering that many phytoplankton species are cosmopolitan (found throughout the world), the scarcity of Hermesinum, and its presence in the Pettaquamscutt River, is noteworthy. (For more details on the interesting life history of Hermesinum in the Pettaquamscutt, see Maritimes, Vol. 35, No. 2.)

Another way of determining (very) long-term changes in phytoplankton diversity is to look at sediment cores. In anoxic basins, such as the Lower Pond, the sediments are well preserved due to the absence of burrowing benthic animals such as worms and clams. This enables us to look at different layers of the sediments (varves) that represent several time periods, many hundreds to thousands of years old. Since some types of phytoplankton are encased in silicon walls, and others have scales that resist decomposition, we can look through the sediment cores for these mementos of past phytoplankton communities. Thus, we can get an idea of the species (among those groups that have these preservable structures) that were present hundreds of years ago. Hargraves and Miller looked for Hermesinum adriaticum in cores from the Lower Pond and found that this species has been present for approximately the last 300 years. This roughly corresponds to the time of European settlement in this region. While this is not proof of an invasion by Hermesinum, it does make one wonder how many phytoplankton species may have hitched a ride with the Europeans.

Although previous surveys have found low species diversity in the Narrow River among the larger (net) phytoplankton, this is only now being resolved for the smaller nanoplankton. Some ecologists argue that communities with low diversity may be more susceptible to biological invasion, so the significance of this diversity issue is clear. Through the intensive field surveys of the nanoplankton in the Pettaquamscutt River Estuary and an analysis of sediment cores from the Lower Pond, I will provide a database against which future changes in species composition can be compared. By measuring how species diversity changes in response to changes in nutrient concentrations, we may get a better picture of possible future scenarios for the phytoplankton in the Narrow River.

Sunshine Menezes, PhD candidate, Graduate School of Oceanography

Sunshine Menezes earned a BS in zoology from Michigan State University. The Great Lakes were not big enough, however, and she made the jump to the East Coast in 1996. Her major professor is Paul Hargraves.