COOL Classroom 2.1

Discover Comparing Seagrass Data Over Time


While we are fortunate in having a series of seagrass surveys dating back over thirty years, the great difference in mapping methods and technologies make a rigorous comparison of the surveys problematic. The earliest 1970’s survey relied on aerial photography complemented by float plane-assisted field checking. The 1980’s survey relied on a boat-based systematic sampling of a grid at every one-quarter mile interval. Whereas, the 1990’s survey was a boat-based survey that traced the outer boundaries of individual seagrass meadows using a Global Positioning System (GPS) unit. The most recent surveys in 2003 and 2009 (not shown above) were accomplished using aerial photography.

As a scientist one must always question the data and try to make sure that you are comparing “apples to apples.” In this case, some of the differences we see between the surveys represent a “real” change in the amount and distribution of seagrass meadows and some represent an “artifact” of the technique used. While we cannot directly compare the amount or distribution of seagrass meadows in a specific place due to the different techniques used(as we are comparing apples to oranges), these surveys do suggest a general decline in seagrass distribution across the entirety of Barnegat Bay.

In 2003 and again in 2009, scientists used the same techniques (high spatial resolution digital camera imagery and advanced computer techniques) to map seagrass distribution. Therefore, these two surveys are more comparable across time. The map and the table below indicate that while the overall area of seagrass did not change much between 2003 and 2009, there does appear to be a significant decline in the amount of the dense cover class (from over 2,000 hectares or 7.7 square miles to under 500 hectares or 1.9 square miles).