Citation:

Schmidt, C., M. Bradley, AND G. Cicchetti. Historical context of the Tier 1 Submerged Aquatic Vegetation mapping efforts in Narragansett Bay RI. 9th National Summit on Coastal and Estuarine Restoration and Management, Long Beach, CA, December 08 - 13, 2018.

Impact/Purpose:

Seagrass (a rooted plant growing in the shallow areas of estuaries and coasts) serves as a nursery for fish and shellfish, filters water, and enhances the natural beauty of our waters. Seagrass, also known as Submerged Aquatic Vegetation (SAV), is very susceptible to nutrient pollution, which favors the growth of nuisance algae. Excess nutrients can result from road runoff, septic systems, fertilizers, and sewage treatment plants. To better understand changes in seagrass abundance we compared the current extent of seagrass in Narragansett Bay RI to the historical extent of seagrass in the Bay back to the 1900s and described the significant events over time that led to seagrass losses and gains over the last century. The study shows a broad negative relationship between the amount of seagrass and the amount of nutrients entering the Bay, with a significant seagrass decline in the 1930s due to a disease outbreak. These results lead to a deeper understanding of how our activities affect seagrass and thus the health of the Bay. This leads to a better ability to communicate changes in seagrass and in overall Bay health to the public, thus supporting management of this estuary.

Description:

Rooted submerged aquatic vegetation (SAV) is an ecologically important habitat, and SAV loss is considered an indicator of nutrient pollution in estuaries. Therefore, the Narragansett Bay Estuary Program (responsible for producing technical reports on the state of the Narragansett Bay watershed) uses the extent of SAV as a critical status and trends indicator. In order to help quantify and discern trends (if any) of SAV, the R.I. Eelgrass Mapping Taskforce has led a series of aerial photographic surveys of SAV since 2006. These surveys have utilized orthophotography, GPS, and underwater video cameras to quantify the presence or absence of SAV (Tier 1). In order to place these Tier 1 surveys in a historical context, we reviewed and created GIS layers of historical (1900 - 1995) SAV maps and data. These maps were geo-referenced, and SAV beds were digitized to derive historical acreages of SAV. From this analysis, we estimated that about 2700 acres of SAV were present in the Bay during the early 1900s. Later, the wasting disease event of the 1930s and the hurricane of 1938 decimated SAV. After the 1930s, we believe SAV increased steadily as SAV recovered to about 1100 acres during 1970s, until a precipitous drop to around 100 acres during the mid-1990s. The more recent aerial surveys of SAV beginning in 2006 point to an increase of SAV until 2016 when an 8% decrease was observed from the previous years' survey (2012). We note that SAV trends correlated broadly to nutrient input over the span of data, with the significant interruption in the 1930s due to wasting disease and the hurricane. Our analyses give us a baseline of SAV coverage for management decisions and will further our understanding of SAV dynamics.

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