Eelgrass in Narragansett Bay: A Case Study An activity developed by Kristin Van Wagner, Education Coordinator for the Narragansett Bay Research Reserve based on Prudence Island, Rhode Island (www.nbnerr.org) Updated 8/19/09 Overview: The following information and activities can be used as they are, or adapted to your location and local native aquatic vegetation. These materials are appropriate for middle and high school science students. The activity is made up of the following components: Eelgrass Introduction (PowerPoint Presentation) Introduce your students to eelgrass and eelgrass mapping with aerial photography by running through this presentation with your class. There are notes for some of the slides that provide additional background information on the presentation s contents. Eelgrass Morphology Have students use your eelgrass morphology handout and answer key to identify and become familiar with eelgrass. You can collect washed up eelgrass plants from the shoreline of most northeastern estuaries during the summer months and keep the plants frozen until you need them for your classroom morphology lesson, or simply use the diagrams. Eelgrass Test You can have your students take the test before you begin this activity as a pretest and use it as a jumping off point for a discussion about eelgrass, its ecological importance and some of the threats to its survival. Or, you can use it as a knowledge assessment at the end of your studies. The provided answer key will help to guide your discussion, it is not meant to be used as a guide for scoring your students answers to the test questions. Eelgrass Background and Activity Read the background section and interpret the contents to your students, or have older students read it themselves. Using the aerial mapping photographs included in the background and in your PowerPoint presentation have your students answer the Drawing Conclusions questions either as a group to foster discussion, or as a way to stimulate critical thinking skills individually.
Eelgrass Background and Activity Introduction Eelgrass (Zostera marina) is a type of seagrass that grows in quiet bays along the northeast and northwest coasts of the United States. Eelgrass plays a crucial role in the health of coastal systems because it provides critical habitat for juvenile marine life, helps stabilize sediments, and aids in filtering particles from the water column. Eelgrass has been deemed a critical marine resource and is currently protected by both Federal and Rhode Island legislation. Eelgrass is also a valuable indicator of the health of an estuary because it is sensitive to environmental change. Excess nutrients, sewage, and algal blooms can reduce water clarity and increase algal growth that limits the sunlight available to eelgrass, often causing it to die off. Stormwater runoff loaded with herbicides, pesticides and polluting chemicals directly harms eelgrass. Dredging can stir up sediment and prevent light from penetrating into deeper water. Boat wakes, propellers, and shade producing docks can disturb the growth of eelgrass beds. Seawalls and other erosion control structures can increase wave energy and alter the sediments where eelgrass grows. Mapping where eelgrass is growing and how much eelgrass there is a critical first step in understanding, managing, and protecting these shallow, submerged estuarine habitats. Map data provides essential baseline information for government agencies, town planners, and the scientific community. The only time eelgrass has been mapped throughout Narragansett Bay, RI was in 1996 when 99.5 acres of eelgrass were identified. During the past10 years, results from some site-specific mapping efforts and improvements in geographic information systems (GIS) and mapping technology have illustrated a need to update the 1996 Bay-wide mapping project. True-color aerial photography is a common tool for mapping eelgrass in shallow estuarine habitats. It is especially useful when scientists are mapping eelgrass in relatively large estuaries. This technique was used to map eelgrass in Narragansett Bay (118,550 acres of water and 630 miles of shoreline, shown in Fig. 1) in 1996. Figure 1. The study area for Narragansett Bay and Block Island.
However, aerial photography does have limitations including issues with water clarity, photographic quality, and challenges in interpreting where eelgrass is growing based on its photo-signatures. The overall goals of this project were to: 1) Conduct a complete and comprehensive survey of eelgrass throughout Narragansett Bay and Block Island; 2) Analyze and compare eelgrass mapping techniques (photo-interpretation of true-color aerial photography vs. field-mapping methods); and 3) Examine status and trends of eelgrass from 1996 to 2006. Methods Aerial Photography Acquisition True color aerial photographs of Narragansett Bay and Block Island (Fig. 1) were taken by James W. Sewall Co. on August 5th 2006. The photographs were taken following research protocols at a low sun angle, two hours within low tide, when wind and atmospheric haze where minimal, and when water clarity was high. Volunteers using secchi disks measured water clarity around the Bay as the target dates for taking pictures approached. Photo-interpretation The resulting images were sent to the Environmental Data Center at the University of Rhode Island for interpretation. These researchers had the tough job of identifying eelgrass beds in the Bay based on the aerial photographs. They used GIS software to outlined areas they though were eelgrass on the photographs using clues like the presence of eelgrass wrack on shore, historical eelgrass data, and the shape of the shoreline. This process is known as delineation. Field Work and Ground-Truthing Because it is difficult to identify eelgrass beds on true-color aerial photographs, efforts were made to quickly and thoroughly identify areas thought to be eelgrass so they could be groundtruthed by underwater video, view scopes, and in some cases volunteer divers. Ground-truthing began two months after the photography was taken and continued during the following summer. URI s digital eelgrass delineations were taken into the field on a laptop computer and viewed simultaneously with global positioning systems (GPS) positions. Viewing the GIS delineations and GPS locations in realtime eliminated the need for using hard-copy maps. During ground-truthing, crews started in the center of each delineated area and if eelgrass was present, they determined the deepwater edge and the extent of the bed and mapped them with the GPS unit. Field delineations and observations were incorporated into the final GIS database.
Trend analysis A major goal of this study was to compare the outcomes of the 1996 mapping effort and the 2006 project. After reviewing the mapping methods and technology used for the two studies, it became clear that a direct comparison was not possible between the 1996 and 2006 datasets for the whole Bay. However, three sites had similar aerial photography and ground-truthing efforts between 1996 and 2006 and all three support some of the largest eelgrass beds mapped in Narragansett Bay in 2006. Figure 2. The three areas within Narragansett Bay that can be used to make a direct comparison from 1996 to 2006 eelgrass coverage data. Results Eelgrass Extent and Distribution During the fall of 2006 and summer of 2007, 465.5 acres of eelgrass beds were mapped in Narragansett Bay and around Block Island. The most eelgrass (208.9 acres) was found in the east passage of Narragansett Bay and the largest eelgrass bed (63 acres) in the study area was found at the Sakonnet Lighthouse. No eelgrass was found in most of the Sakonnet River or north of Prudence Island.
Trend Analysis At two of the three sites that could be directly compared from 1996 to 2006 the geographic area covered by eelgrass appears to have at least doubled in size over the past 10 years (Fig. 3A & 3B). The eelgrass bed at Potters Cove (Fig. 3A) represented one of the largest beds identified in 2006. The T-wharf site also appears to have expanded tremendously around the wharf and along the southern tip of Prudence Island (Fig. 3B). Figures 3A & 3B. GIS delineated eelgrass coverage maps showing 1996 and 2006 coverages for Potters Cove, Jamestown and T-wharf, Prudence Island within Narragansett Bay.
Finally, at the Fort Getty site (Fig. 3C), the geographic area covered by eelgrass also seems to have increased slightly with the largest gain along the southern end of the site, although it is unclear how much ground-truthing was done here in 1996. Figure 3C. GIS delineated eelgrass coverage map showing 1996 and 2006 coverages for Fort Getty, Newport within Narragansett Bay. Drawing Conclusions It is important to keep in mind that we cannot draw strong conclusions from this study because we only have two years worth of data. This is not a strong data set as yet, but our hope is to continue this monitoring project over time and continue to collect data regarding eelgrass coverage in the Bay. In addition, we cannot show direct cause and effect with this data, so there it is not appropriate to say emphatically that there is more eelgrass due to decreasing nitrogen loads in the Bay. We can only speculate and perhaps formulate future research questions. Below are some possible discussion topics you might use with your students. Discussion topics: - Knowing that eelgrass is an indicator of the overall health of Narragansett Bay, what conclusions can you draw about the health of the Bay based on the eelgrass mapping results above? - Are these strong conclusions based on the data? Why or why not? - What could we infer from the results of this study regarding the levels of nutrients like nitrogen that have been entering the Bay over the past 10 years?
- What might these results lead us to conclude about water quality trends in Narragansett Bay over the past 10 years? Is this a strong conclusion? Why or why not? - If the water quality in the Bay is improving, what implications could these results have on wildlife, fisheries, and tourism in Narragansett Bay? The images and text in this activity are from: Report on the Analysis of True Color Aerial Photography to Map and Inventory Zostera marina L. in Narragansett Bay and Block Island, Rhode Island by Mike Bradley, Environmental Data Center, University of Rhode Island, Kenneth Raposa Narragansett Bay National Estuarine Research Reserve and Sue Tuxbury, Save the Bay. The full report is available on www.nbnerr.org
Name: Date: 1) What is eelgrass? Eelgrass Pretest 2) Where does it grow? 3) Are there eelgrass beds in Narragansett Bay? Yes No If yes, is there more or less eelgrass growing in Narragansett Bay today than there was in the 1700s? 4) Describe how eelgrass is part of a food web 5) Name some important ecological functions provided by eelgrass 6) List three important coastal habitat types in Rhode Island 7) What does habitat restoration mean?
8) Does the health of eelgrass beds impact the Rhode Island economy? 9) What are some of the threats facing eelgrass in Narragansett Bay? 10) Who owns the eelgrass beds in Narragansett Bay? 11) Who is responsible for protecting and restoring eelgrass habitat in Narragansett Bay?
Name: Date: 1) What is eelgrass? Eelgrass Pretest Eelgrass is a rooted, flowering, vascular seagrass. Rhode Island s primary seagrass species is eelgrass (Zostera marina). 2) Where does it grow? It grows underwater in meadows or small beds 3) Are there eelgrass beds in Narragansett Bay? X Yes No If yes, is there more or less eelgrass growing in Narragansett Bay today than there was in the 1700s? There is less eelgrass growing in the Bay today. It is estimated that the majority of historic eelgrass beds have been lost. 4) Describe how eelgrass is part of a food web The vast primary productivity of eelgrass provides food and shelter for epiphytes (plants that grow on other plants), clams, mussels, oysters, quahogs, lobster, scallops, shrimp, eels, crabs, and juvenile fish. In turn, these animals support larger fish, shorebirds, water fowl and marine mammals like harbor seals. 5) Name some important ecological functions provided by eelgrass Eelgrass produces organic material that becomes part of the food cycle, provides a settling substrate for scallops, helps cycle nutrients, prevents shoaling and erosion by binding sediments, and provides nursery habitat for many finfish and shellfish. 6) List three important coastal habitat types in Rhode Island Sandy beach, pebble or cobble beach, rocky shore, salt marsh, coastal forest, mud flats, and salt ponds.
7) What does habitat restoration mean? Habitat restoration is the return of a habitat to its original community structure, natural complement of species, and natural functions. 8) Does the health of eelgrass beds impact the Rhode Island economy? Eelgrass is the primary source of food and shelter for economically important finfish and shellfish like the bay scallop. 9) What are some of the threats facing eelgrass in Narragansett Bay? The major causes of eelgrass loss in Rhode Island include nutrient loading from development and sewage outfalls, physical damage from dredging and boating activities, disease, and shading from fixed piers and docks. The most serious threat to Rhode Island s eelgrass is increased nutrient levels (usually nitrogen) from polluted runoff, septic systems, and sewage treatment plants. The resulting enrichment leads to excessive blooms of algae -- both microscopic plankton and large nuisance drift algae such as sea lettuce (Ulva lactuca) -- that limit the available light required by eelgrass to grow. 10) Who owns the eelgrass beds in Narragansett Bay? Submerged lands beneath the mean high tide are publicly owned, so the short answer is that everyone owns the eelgrass beds in Narragansett Bay. 11) Who is responsible for protecting and restoring eelgrass habitat in Narragansett Bay? Eelgrass habit is protected and managed by regulations established by state agencies such as the Coastal Resources Management Council (CRMC), the Rhode Island Department of Environmental Management, and the Narragansett Bay National Estuarine Research Reserve.