Incorporating Vegetated Wetlands into Watershed Nitrogen Loads

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No matter how watershed loading rates are modeled, all land use types, including vegetated wetlands, must be assigned a loading or loss term. In the 1990s, the Buzzards Bay National Estuary Program and other agencies and investigators would add precipitation to estuary surfaces into their watershed loading calculations. Precipitation to fresh surface waters were typically assigned the value of natural landscapes (0.46 lbs per acre in the MEP model), or some value intermediate between natural landscapes and estuary surface waters (acknowledging some attenuation within the pond). Vegetated wetlands on the other hand were lumped in with upland natural landscapes, and assigned that much lower loading rate, or sometimes assigned a loss term.

In their watershed nutrient loading models, the Massachusetts Estuary Project assigns ponds and coastal waters a high “water body surface area” nitrogen loading coefficient, but vegetated wetlands are assigned the same low loading coefficient as other “natural landscapes.” By default, vegetated wetlands are lumped with the Natural Landscape” category which has a very low watershed loading rate shown in the table below.

Selected MEP Landscape Loading Coefficients

The MEP’s loading coefficients, as expressed in various reports and loading tables submitted to the state and towns, are summarized below.

Table 1. Effective loading rates for selected land use types incorporated into MEP loading models.

Slocums River watershed exception

The one exception to these loading rules occurs in the Slocums River watershed loading report (posted on the www.oceanscience.net/estuaries/Slocums-Little.htm). In this study, the MEP found unexpectedly high nitrogen concentrations in river sampling locations. There is considerable discussion of this issue in the report, and based on a number of observations the MEP ultimately concluded:

“In the Slocums River watershed exist extensive wetland and swamp lands surrounding most of the streams and rivers feeding into the estuary (Figure IV-5). Based on nitrogen species and loading information collected at the gauges to the Paskamanset River, Destruction Brook, BJR North and BJR South, it is clear that these wetlands are nearly saturated with nitrogen and are exporting nearly the same nitrogen loads as are being added to them by precipitation. This is equivalent to how surface waters act and how they are incorporated into the watershed nitrogen loading model. Because of this, MEP staff digitized these wetland areas using GIS techniques and treated these areas as surface waters for the purposes of developing their nitrogen loads in the Slocum River watershed nitrogen loading model. A total of 5,511 acres of this type of wetland exists in the Slocum River watershed; no wetlands of this type were assigned in the Little River watershed.”

Consequently, the Slocums River, which has only 639 acres of water body surface area (two-thirds of which is estuary surface area), had all wetlands assigned to the water body surface area in the loading model, and this combined acreage totaled 6,178 acres. The areas of vegetate wetland and water body surface area in this MEP analysis nearly match the “open water” and vegetated wetland categories in the Mass DEP’s wetlands GIS coverage.

This approach was a significant deviation from their standard model because there are 639 acres of water body surface area in this watershed (2/3 of which are estuary) but there 6,178 acres of wetlands plus open water. The adoption of this approach in this wetland dominated watershed increased unattenuated watershed nitrogen loading by roughly 40%.

Implications of Wetland Saturation with Nitrogen

From a management point of view, the fact that vegetated wetlands, including vast areas of forested wetlands (which do not have standing water in the growing season), could be saturated with nitrogen and have negligible nitrogen removal capacity has important implications. In the Slocums River watershed, the assumption resulted in a more than 50% increase in watershed loading (see our own calculations for the Slocums River Watershed. For example, this would result in wetlands having higher nitrogen loading contributions per unit area that from roofs and lawns, and twenty times higher than natural landscape loading as illustrated by Table 1. The lack of nitrogen removal in these watersheds is a testable hypothesis that merits further scientific study and evaluation.