The Plum Island Ecosystems LTER Scientific Merit: The Plum Island Ecosystems (PIE) LTER is an integrated research, education and outreach program whose goal is to understand the long-term response of watershed and estuarine ecosystems to changes in climate, sea level and human activities. Some key findings are: 1) Over the last 20 years, climate change has led to a 20% increase in rain and snow falling on watersheds in the PIE region. However, this increase in precipitation has not lead to an increase in water delivery to the coast because over the same period, the amount of water diverted from rivers for human use has increased an equivalent amount. So, on an annual basis, the average amount of water reaching the coast has not changed. However, during summer, when river flow is lowest and human use is highest, water withdrawals have a significant impact on water levels in the river, at times causing the rivers to "run dry". 2) Much of the nitrogen that gets into streams from land sources (lawns, septic, sewage, runoff) may be removed by biological processes that occur within streams. By naturally removing nitrogen from the water, streams help protect estuaries from excess nitrogen. At PIE, we have found that more nitrogen is prevented from reaching the estuary than is the case for most watersheds in the northeastern United States. This may be because most of our nitrogen sources are far from the estuary, unlike many other watersheds. 3) Nitrogen that does reach the estuary may be further removed in sediments and in the marsh through processes that are controlled by a variety of factors. In the upper estuary, sediment removal of N varies most with changes in salinity. In the lower estuary, bivalves influence nitrogen cycling. In the marsh, loss of nitrogen varies with vegetation type, and can be very high in some types of salt marsh vegetation. Nitrogen removal also happens in the root zone of marsh plants, a finding we were able to demonstrate by developing a new method. 4) We have found that yearly variations in sea level have a dramatic impact on how much marsh grass is produced, with production being higher in years when sea level is higher. Using this and other information we developed a model to predict how marshes are responding to sea-level rise. Both the rate of sea-level rise and the availability of sediments in the water column determine what range of sea-level rise rates marshes can tolerate before being lost through submergence. 5) We have found no evidence that herbivory plays a significant role in controlling marsh grass production at PIE, in contrast to findings for some other, more southern marshes. 6) The periodic harvesting of salt marsh hay appears to be sustainable. It has no long-term effect on the abundances of fish, or breeding birds and only minor effects on the food web overall. 7) A great deal of organic material enters the water columns of PIE estuaries from land but we have been able to determine that much of this material simply passes through the estuary without being used by bacteria or higher organisms. In contrast, organic matter derived from the marsh is important in fueling estuarine food webs. 8) Our work has shown the importance of small fish in marsh food webs. Mummichogs, small bait fish, are omnivores that eat small invertebrates but will also consume algae growing on the marsh surface. They switch their diet depending upon food availability. The population of these fish has fluctuated markedly at PIE, and data suggest their abundance changes in response to numbers of a top predator, striped bass. 9) Using radio receivers we have been able to track the movement of striped bass and herring within PIE estuaries, and beyond. This work has helped us understand where these fish feed, whether or not they return to PIE each year, and what their diet and growth rates are in PIE estuaries. 10) Beaver populations...