Theme 2 - Spatial Arrangements

How do the spatial arrangements and connectivity between habitats in coastal watersheds and estuaries influence ecological processes?

Complexities of water flow affect capacity of wetlands “filter” to remove nitrogen pollution.

In the Plum Island ecosystem, urbanization (shown at right as degree of impervious surfaces) has been most intense in the portion of the watershed furthest from the coast (nearest Boston).  Urbanization increases nitrogen (N) inputs to streams and rivers in the watershed through sewage and runoff. Runoff increases with the amount of impervious surfaces, and channelizing water flow through culverts and outflow pipes bypasses wetlands that help mitigate the nitrogen loads the water is carrying.

                                                Percent of impervious surfaces in and                                                Beaver Pond on a stream of the
                                                     adjacent to PIE watersheds                                                                             Upper Parker River
Curiously, however, we have not seen an increase in N export, even though loading has increased.  Why?

One explanation arose from our observation that beaver ponds have increased significantly in the watersheds over the last 15 years. Beaver ponds, where water flow is slowed, should significantly increase N retention.  Inadequate culverts also lead to ponding, connecting wetlands and increasing potential for mitigation of nitrogen loads in the water. These finding led us to improve our simple river network model to take into account heterogeneity within the network created by beaver ponds, lakes, and floodplains as well as by other transient storage features (hyporheic zones, surface pools).  The model suggests that nitrogen retention in the watersheds varies with the complexity of the flow.  See Wolheim et al……[link]


Will the tidal marshes survive sea level rise?

Saltmarsh cordgrass, Spartina alterniflora, grows within a range of elevations with respect to tide height, but there is an elevation that is optimal for growth.  Rising sea level coupled with disruption of sediment runoff from land compromises the ability of marshes to maintain suitable elevation for Spartina.  In the absence of significant new sediment inputs to the estuarine system, will the marsh be able to keep up with sea level rise? Observations are being made to determine changes in marsh elevation and experimental manipulations are being conducted to determine what controls marsh accretion or erosion (see Surface Elevation Table measuring right).

We see evidence for substantial geomorphic change in some areas of the marsh.  Ponds shown on the1953 map,(top panel left) had expanded greatly in area by 1985 (aerial photo, top panel center).  By 2001 (top panel right) several of these ponds had been captured by creeks and drained, and are now vegetated by Spartina alterniflora.  Recognizing this history helps us predict how PIE marshes may look in the future (cartoon, bottom panel).

The conceptual diagram shows ponds present in the marsh "Now", expanding and connecting to
drainage creeks in "Short Term Future", then draining and being revegetated in the "Long Term Future"