Nitrogen Isotope Analysis Project

 

 

 

Funded by:

 

Massachusetts Coastal Zone Management

Non-Point Source Pollution Grants Program

2005

 

Project Partners:

 

Town of Oak Bluffs Shellfish Department

United States Environmental Protection Agency’s

 Atlantic Ecology Division

Martha’s Vineyard Commission

Martha's Vineyard Shellfish Group Inc.

 

Also Supported by:

 

Tisbury Shellfish Department

Edgartown Shellfish Department

Lagoon Pond Association Inc.

Tisbury Waterways Inc.

Friends of Sengekontacket Inc.

 

Prepared by David W. Grunden, Oak Bluffs Shellfish Department

Richard McKinney, US EPA Atlantic Ecology Division

William Wilcox, Martha’s Vineyard Commission

 

Table of Contents

 

 

                                                                                                                                    Page

           

Introduction                                                                                                                     4

 

Methods                                                                                                                          5

 

Land use                                                                                                                         6

 

Sample site selection                                                                                                       12

 

Results                                                                                                                           13

 

Discussion                                                                                                                      15

 

Public outreach                                                                                                               16

 

Report distribution list                                                                                                     17

 

References cited                                                                                                             18

 

 

 

Tables

 

1          Land use summary statistics by watershed                                                             7

 

2          Nitrogen loading breakdown form all sources                                                        8

 

3          Breakdown of nitrogen sources – present day                                                        9

 

4          Nitrogen loading from on-site wastewater                                                             11

 

5          Summary statistics for 2005 – tissue analyses for nitrogen isotope ratios                14

 

6          Results of stable isotope analyses of ribbed mussels previously sampled

            Form 1999-2000                                                                                                15

 

 

 

 

 

 

 

Figures

 

                                                                                Page

 

1                    Lake Tashmoo – nitrogen sources – present day                                                  9

 

2                    Lagoon Pond  - nitrogen sources – present day                                                  10

 

3                    Sengekontacket Pond – nitrogen sources – present day                                      10

 

4                    Quahog nitrogen sources                                                                                   14

 

 

 

 

Maps

 

 

Sample site locations                                                                                                       19

 

Watershed map of all three ponds                                                                                   20

 

 

 

 

Appendices (Not Included)

 

Appendix 1      Media coverage of this project

 

Appendix 2      Financial report

 

Appendix 3      Interim report to Massachusetts coastal Zone Management  

 

Appendix 4      Raw data collected

 

Appendix 5      Letters in support of this grant project

 

 

Introduction:

 

            On the island of Martha’s Vineyard there are several coastal ponds and embayments that currently support both recreational and commercial shellfisheries. The value of this fishery is in the millions of dollars to the local economy. The island including the watersheds of these ponds has also seen a tremendous amount of development, mostly single family homes both year round and seasonal almost all with on site septic systems. The real estate value of homes has increased dramatically over the past several years. This is especially true of homes on the shores of the coastal ponds or with a water view. 

It wasn’t until the late 80’s and early 90’s that people began to notice changes in the coastal ponds that were indicative of the excessive nutrient loading. An example of this is between 1989 and 1991 almost all the eelgrass (Zostera marina) died off in Sengekontacket Pond. In 1995 the Martha’s Vineyard Commission (a regional planning agency) received a grant to collect and analyze water quality samples from eight coastal ponds across the island. Since that first monitoring program, there has been at least some water quality monitoring in many of the island’s salt ponds. Many of these ponds are indeed monitored on an annual basis now. The result of this effort is the realization that there are no longer any truly pristine waters left on Martha’s Vineyard Island.

All water bodies on the island have been detrimentally impacted to some degree by excessive nutrient loading. Nitrogen has been found to be the nutrient that is limiting the growth of algae and phytoplankton and, when it is added, system productivity increases.  As the watershed develops, more nitrogen is added and reaches the pond stimulating an increase in the growth of aquatic plants. The consequences of adding ever-greater amounts of nitrogen include reduced water clarity, growth of macro-algae, loss of eelgrass, increase in phytoplankton populations, increase in organic matter smothering bottom dwelling shellfish and a shift away from desirable species like the bay scallop to less desired species such as snails and other detritus feeders.   It is believed that the largest source of the excessive nitrogen entering the ponds is from the wastewater from septic systems.  We have looked at land use maps, counted the number of homes within the watershed and multiplied the average nitrogen output from well-maintained Commonwealth-approved systems to get a good estimate of the level of nitrogen entering the ponds.  These loads have been compared to calculated allowable loads (TMDLs) and found to be excessive or close to the loading limit for these systems.   The local boards of health have been reluctant to encourage the installation of nitrogen reducing septic systems. Some members of the local boards have stated that they did not think the science was there to support either the encouragement or requirement of the installation of nitrogen reducing septic systems and that looking at land use maps to estimate the contribution of nitrogen from septic systems was little more than a guess.

            One method to assess the sources of nitrogen to a pond is an analysis of nitrogen-stable isotopes.  The ratio of nitrogen-stable isotopes varies with the source from which the nitrogen is derived.  By analyzing the ratio of these isotopes from aquatic animal and plant material collected from coastal ponds we can determine the source of the nitrogen they are utilizing in their growth. The results allow us to be able to differentiate the source of the nitrogen between septic system/animal, agricultural (fertilizers) and precipitation. Wastewater derived nitrogen in the groundwater is enriched with nitrogen 15 compared with nitrogen 14 by 10 to 20 per thousand (1 to 2%) when compared to atmospheric nitrogen gas.  Groundwater nitrogen derived from precipitation is typically enriched by 2 to 8 per thousand (0.2 to 0.8%) over nitrogen gas.  Fertilizer derived nitrogen 15 ratios range from 3 per thousand depleted to 3 per thousand enriched (Kreitler, 1978).   The ratios found in primary producers (phytoplankton and algae) reliably reflect inputs of nitrogen from the land and air (Cole et al, 2004). The values of the isotope ratios we found in shellfish in our ponds can be compared with that found in shellfish from pristine locations to provide us with an indication of the source of the nitrogen.  Knowing the probable sources will allow us to make a more convincing argument to the local Boards of Health and ask them to begin to require de-nitrifying systems or at the very least encourage the homeowner to consider it and to do their part to protect, preserve and improve the water quality of our coastal ponds. We need to start to treat our own waste differently to maintain and/or improve the water quality of our coastal ponds. 

 

Methods:

 

               Sample sites were selected in three coastal Ponds: Sengekontacket Pond, Lagoon Pond and Tashmoo Pond. These sites were not random. Land use in the immediate watershed was considered.  The sites were chosen with the intention of seeing different influences related to the use of the land up gradient from the sample sites. We chose three coastal ponds so that if the results came back similar to each other we could more easily extrapolate to watersheds with similar conditions on Martha’s Vineyard.

            The sampling rounds were scheduled to be every other week beginning in late May and continuing until early September. We decided that the quahog, (Mercenaria mercenaria) would be the main targeted species sampled, as it was a common species that could be sampled at each station. Every other sample round would collect quahogs for analysis. On the alternate sample rounds we would collect other species so that we could see if the other species took up the same percentages of source nitrogen as the quahog. We were sure to include some ribbed mussels (Guekensia dismises) in these rounds so that we could directly compare the results of ribbed mussels collected by the US EPA several years ago from the same ponds in this study. Other species sampled in these rounds include; eelgrass (Zostera marina), bay scallop (Argopecten irradians), stout razor clam (Tegulus sp), Blue mussel (Mytillus edulis) and the salt marsh grass Spartina alterniflora.  

            The samples were always collected on a Friday morning and the shell was marked with the sample site identification. The samples were then brought to the Martha’s Vineyard Shellfish Group. The shellfish samples were shucked open and the meats were kept on the shell marked with the site identification and placed into a drying oven set at 65 degrees Celsius. The plant material samples were placed onto aluminum foil labeled with the site identification and also placed into the drying oven. The EPA protocol required drying the samples for at least 48 hours however with the first round of sampling we found that the sample meats were not dry enough to be ground into a fine powder, probably due to the ambient humidity due to a lack of air conditioning in the room with the drying oven. We therefore increased the time in the drying oven to 72 hours and that allowed pulverizing the tissue into a dry fine powder using a mortar and pestle. Once ground, the sample was put into small glass bottles marked with the date the sample was processed and the sample site. Two small samples were removed from each bottle and weighed using a Mettler Toledo analytical balance. Each sub sample was extracted to weigh between 1 and 2 milligrams (recorded to 0.00 mg accuracy) and placed into a small pressed tin cup. The cup was then squeezed closed and balled up and placed into a tray and the sample’s position in the tray was recorded along with the sample weight, species and site identification. When a tray was filled it was mailed to Rick McKinney at the US EPA’s Atlantic Ecology Division in Narragansett, RI where they were analyzed.

            We made several attempts to find quahogs in Nantucket Sound to use as controls. We were unsuccessful in those attempts. We collected sample quahogs from just inside Cape Pogue Pond as we thought that was perhaps as close as we could get to a pristine embayment, however we expected there might be some impact from nitrogen isotope enrichment even there. The results of the analysis showed more of an impact than we believed when choosing this alternative control site. Instead of using these results as the control we decided to use the same control group as the EPA lab has used before from quahogs collected from within Narragansett Bay off Prudence Island. This also allowed a better comparison of the ribbed mussel samples to the samples that the EPA collected several years ago.

 

Land Use:

Land use in the watershed is the primary determinant of nitrogen loading.  The MV Commission evaluated land use in the watersheds of the three ponds under a grant from the Coastal Zone Management Coastal Pollution Remediation grant program (MV Commission, 2004).  This study defined the watershed boundaries based on the application of a groundwater model that was constructed using the US Geological Survey’s Modular Three-Dimensional Finite Difference Groundwater Flow Model (MODFLOW).

 

Watershed area and land use components were identified by the MV Commission using GIS to provide the statistics described in Table 1.

 

 

Table 1: Land Use Summary Statistics by Watershed

Watershed	Town	Total acres	Open space- Acres	Existing residences- number	Existing commercial- number	Projected residences- number	Projected commercial- number
Tashmoo	Tisbury	1687		699	69	1219	86
	Oak Bluffs	44		9	2	24	2
	West Tisbury	885		187	1	282	1
Lagoon	Oak Bluffs	2265		1156	11	1910	11
	Tisbury	592		638	23	1023	24
	Edgartown	138		0	0	0	0
	W. Tisbury	906		179	0	245	0
Sengekontacket	Oak Bluffs	1167		381	10	530	13
	Edgartown	2915		937	5	1533	5
	W. Tis.	390		77	4	101	4

 

The figures in Table 1 and the resulting nitrogen loads in Table 2 are present day estimates for the watersheds that do not reflect the present day nitrogen inputs to the ponds due to the lag time between land use changes, nitrogen generation and the travel time to discharge in a pond.  It is reasonable to expect that the proportions of the different nitrogen sources are similar when extrapolated back in time.  Nitrogen loading figures were calculated based on precipitation (46.9 inches per year), known household population data from the US Census for each Town and an assumed wastewater generation rate of 48 gallons per capita per day and estimated discharges from commercial uses where wastewater flows were calculated at 60 percent of the Title 5 design flow.  All wastewater discharges except treatment plant sources were assumed to carry nitrogen at a concentration of 35 milligrams per liter.  The nitrogen loading figures are presented in Table 2.

 

 

TABLE 2: Nitrogen Loading Breakdown From All Sources  Loads in Kilograms/year
	Residential	Guest house	Large volume	Misc. higher		Direct fall	Direct	Impervious		Impervious		Groundwater		Lawns		Farms		Large	Total
Watershed	Septic	septic	Wastewater	flow septic*		precipitation	Discharge	road loading		roof runoff		background		& land scapes				Turf	N
				Commercial			runoff	at 0.75 mg/l		at 0.25 mg/l		at 0.072 mg/l						Sources	Load
EXISTING
Tashmoo	3997.4		443	480.4		1419	10	435		44.6		430		273		793		122	8447.4
	2500		510	0		1510	0	560		0		360		240		630			6310
Lagoon	8304.8		897	248.6		3012	80	653		98.2		641		606		792			15332.6
	7597		1436	0		3267	0	2801		0		345		732		1062			17240
Sengekontacket	5525.9		124.5	48.8		3953	10	672		69.5		735		368		51		1010	12567.7
	6000					2692	0	0		0		467		601				1492	11252