Dr. Bradley Peterson’s Research on Threats and Stressors to Seagrasses in New York’s Marine Waters

Eelgrass Meadow

Seagrasses are a type of submerged aquatic vegetation – plants which spend their entire lives under the waves.  There are two species of seagrass in New York coastal waters – widgeon grass, Ruppia maritima, with short blades and lots of branching, and eelgrass, Zostera marina, a much larger plant with strap-like blades.  Eelgrass is the dominant, northern temperate seagrass species in the world, occurring on both coasts of the US and Canada, as well as throughout northern Europe.  Our local Long Island waters are no exception – eelgrass is the dominant species.

Eelgrass once formed extensive, luxuriant undersea meadows throughout all of Long Island’s estuaries.  These meadows provided a variety of ecosystem services.  Eelgrass helps stabilize the sediments both by an extensive root and rhizome network, but also by the blades baffling water currents.  Seagrasses help maintain water quality through absorption of nutrients and carbon dioxide, and the release of oxygen.  They provide food to numerous species – locally, eelgrass is a part of the diet of many native and introduced water fowl.  But perhaps one of the most important services provided by eelgrass is as a shelter for many resident marine species, a nursery ground for migrant species and a foraging ground for larger species.  In fact, many of New York’s commercially and recreationally important marine fish species spend at least a portion of their lives utilizing eelgrass as a habitat.

Scallop on Seagrass

In 2007, prompted by substantial declines in the distribution and abundance of seagrass in the Peconic-Gardiners Bays Complex and other Long Island estuaries, the New York State Legislature constituted the New York Seagrass Task Force and charged it with developing recommendations on how to best conserve and protect healthy seagrass meadows and, where meadows have been degraded or destroyed, how these areas can restored.  The Task Force submitted its final report to the Governor and Legislature in December 2009.

Eelgrass is a vitally important species and Assistant Professor Bradley J. Peterson of SoMAS leads the charge in trying to understand what is happening with this seagrass around Long Island and what might have produced the considerable declines that have been documented.  A major focus of Dr. Peterson’s seagrass lab is examining the interplay between reduced light, increasing nutrients and increasing temperature, among other stressors – these are all now becoming common characteristics of many of our local estuaries.  His students have carried out a series of lab experiments investigating these multiple stressors in both large and small mesocosms – essentially just a fancy scientific term for a bucket – but all projects include field components.  “Sometimes, there is a disconnect between what happens in a controlled lab setting and what occurs in the field,” says Peterson.  “It is important that field experiments are conducted to corroborate the lab results.”

Shrimp on Seagrass

The hope is that by gaining insight into these stressors and their interactions, impacted seagrass meadows can be more effectively restored and conserved.  Like all plants, seagrasses require sufficient light and nutrients to grow.  Nutrients are not typically considered to be limiting in New York’s coastal waters because of the typically high levels of anthropogenic nutrient loadings these waters experience.  However, nutrient loading leads to algal blooms, which can effectively shade out eelgrass lying further below.  Low light resulting from poor water quality is typically considered the major contributing factor to seagrass loss.  But Dr. Peterson warned, “It is not as simple as a low light issue.  When we conducted extensive surveys in Great South Bay, we never encountered eelgrass in sediments consisting of more than 2% organic matter, indicating that there are other factors at play in controlling the extent of eelgrass in local waterways.”  This observation led to a New York Sea Grant-funded study of the impacts of multiple stressors on eelgrass in Long Island.  An important aspect of this study is to examine the impacts of not just the stressors individually, but also in combination with each other.

Some of the research findings to date from the Peterson group include:

• Injecting sulfide* into the seagrass sediments in the lab reduces growth on its own
• A combination of sulfide injection with other stressors, such as reduced light and increased water temperature, leads to more dramatic growth reduction and loss
• In field experiments, the sediment sulfide is increased by boosting the organic content in the sediments
• Shade and sulfide impact seagrass growth in the field, although the extent is not quite so dramatic as the laboratory
• While nutrients are a vital building block for seagrasses, extremely high nitrogen content groundwater has a negative impact on seagrass growth
• The herbicide Diuron, formerly used on farms for weed control, and introduced to the bays through groundwater seepage, has a negative impact on eelgrass growth
• Hard clams are capable of alleviating light and nutrient stresses to eelgrass, enhancing growth of grass in both lab and field settings
• The clams burrowing activities have not been demonstrated to alleviate sulfide stress
• Despite the multiple stressors, eelgrass populations in Great South Bay are genetically diverse, which might indicate either some level of ecological resilience or the last reproductive gasp of a clonal organism

*sulfides – Hydrogen sulfides are produced in the sediments when bacteria breakdown organic matter (sugars and carbohydratess) in the absence of oxygen.  This occurs in many estuarine sediments.  Sulfides are also toxic to seagrasses, so having high sulfide concentrations in the sediments can harm the root-rhizome complex of seagrass, affecting its growth.

Planting Seagrass

Brad Peterson is planning additional research to further investigate some of these issues.  “There is some eelgrass that grows in a high stress environment known as the deep edge,” he explained.  The deep edge occurs where the eelgrass extends to its depth limit as controlled by a combination of light penetration, water temperature and sediment biogeochemistry.  Understanding how this plant persists by integrating these stressors, and the impact this has on the genetics of the meadow, is critical information for managers trying to restore and conserve eelgrass.  In addition, the Peterson lab hopes to enlist the assistance of Dr. Jackie Collier, a physiological ecologist at SoMAS, to investigate the impact of the sediment microbial community on seagrasses via changing nutrient dynamics.  This work will aim to characterize the sediment microbes, quantify how they change with the different seagrass stressors and determine whether they can help to alleviate these stressors.