2022 Estuarine Nutrient Enrichment Status Report: Assateague Island NS

2022 Status : over-enriched explore our interactive data viewer

Quick dive

Many of our coastal parks are home to dynamic estuaries, where saltwater meets freshwater. These waters also experience the detrimental effects of estuarine nutrient enrichment, largely from human activities, that impact water quality. The Northeast Coastal and Barrier Network (NCBN) parks depend on high-quality aquatic resources to sustain complex estuarine and near- shore ecosystems.

Condition of majority of sampled area.
Note: Conditions of chlorophyll-a and light attenuation at ASIS in 2022 indicate that the estuarine system remains nutrient over-enriched.

	Park-wide
Dissolved oxygen	Good
Chlorophyll-a	Fair
Light attenuation	Fair

Additional Resources

The data referenced in this brief is available here. Our water quality dashboard provides an interactive visualizer for this data and that of our other parks.

Fig. 1: Map of the Assateague Island National Seashore estuarine area that is monitored for nutrient enrichment and located in Maryland and Virginia.

Site Context

Assateague Island National Seashore (ASIS) is a 60km long barrier island off the eastern coast of the Delmarva Peninsula, extending into the Atlantic Ocean (Fig. 1). The northern two-thirds of the island is in Maryland, while the southern third is in Virginia. ASIS one of two NER coastal parks with a highly developed water-quality monitoring program, through a joint effort among many federal and state partners. The park boundary encircles the NPS holdings, the U.S. Fish and Wildlife Service’s Chincoteague National Wildlife Refuge, and Maryland’s Assateague State Park. Further, Sinepuxent Bay and the MD portion of Chincoteague Bay are managed under the Maryland Coastal Bays Program as a National Estuary Program estuary. Consequently, the park has monitoring interests throughout the coastal bay, and is not confined in its interests to the area within the park boundary. Approximately 37,319 ha of estuary is monitored for this protocol. About 6,867 ha of that area falls inside park boundaries, representing 100% of NPS estuarine resources within ASIS.

The threat of nutrient enrichment is a primary management concern. Water quality monitoring follows Northeast Coastal and Barrier Network’s Protocol for Monitoring Estuarine Nutrient Enrichment (Kopp and Neckles, 2009). The protocol’s primary goal is to assess how estuarine natural resources are affected by changing nutrient inputs by determining the status and trends of response indicators within the monitored estuarine areas of each park.

Presented here are condition status summaries of three key response indicators:
1) bottom water dissolved oxygen (DO)
2) surface water chlorophyll-a
3) water column light attenuation (K_d)

Threshold values are used to determine “good”, “fair”, and “poor” condition categories (Table 1), which are consistent with those used by the US Environmental Protection Agency (USEPA) for national coastal condition reporting (USEPA 2001b, 2004) and thresholds used by National Oceanic and Atmospheric Association for the National Estuarine Eutrophication Assessment (Bricker et al. 1999).

Water clarity is expressed as K_d (m^-1). Cutoff values of K_d have been calculated from the USEPA criteria for turbidity, using the thresholds for areas with significant submerged aquatic vegetation (SAV).

Table 1: Threshold values associated with each measured nutrient enrichment condition category.

Measure	Good	Fair	Poor
Chlorophyll-a	< 5 µg/L	5-20 µg/L	> 20 µg/L
Dissolved oxygen	> 5 mg/L	2-5 mg/L	< 2 mg/L
Light attenuation	< 0.92 m-1	0.92-1.61 m-1	> 1.61 m-1

Bottom Water Dissolved Oxygen

Bottom water DO concentration is an indirect indicator of nutrient load and direct indicator of estuarine water chemistry and organism health. Oxygen is consumed when microbial decomposers mineralize organic matter on and within estuarine sediments. When sediment organic matter increases due to nutrient over-enrichment it can cause an overgrowth of nuisance algae. The resulting heightened microbial decomposition can lower DO concentrations in bottom waters. It can also lead to increased extent and duration of bottom water anoxia and toxic sulfide concentrations. Nutrient enrichment can, ultimately, impact fauna diversity and abundance and shift species dominance.

Fig. 2: Bottom water dissolved oxygen concentration (mg/L) condition categories by percent of total estuarine sample area of Assateague National Seashore.

During the 2022 summer index period, ASIS park-wide average near bottom water dissolved oxygen (DO) concentration was good within 98.8% of the sampled estuarine area, and poor within 1.2% (Fig. 2). 2022 conditions were consistent with previous index periods. Since the study began, bottom water DO concentrations have been observed as good in a majority of the ASIS sampled area.

Surface Water Chlorophyll-a

Nutrient enrichment often stimulates phytoplankton production, leading to population growth. Phytoplankton growth is often nutrient-limited. Trends of increasing biomass can suggest increasing nutrient load. Surface water chlorophyll-a concentration is an indicator of phytoplankton population growth, nutrient enrichment, and overall water quality.

Fig. 3: Surface water chlorophyll-a (μg/L) concentration condition categories by percent of total estuarine sample area of Assateague National Seashore.

During the 2022 summer index period, ASIS park- wide surface water chlorophyll-a average concentration was good within 33.3% of the sampled estuarine area, fair within 65.0%, and poor within 1.7% (Fig. 3). The 2022 conditions were consistent with previous years, with a majority of the estuarine area in fair condition, and some portion of both poor and good condition area. 2022 showed an increase in good condition area. There has always been a small percentage observed as good, besides 2016, 2012, and 2005.

Water Column Light Attenuation

Submerged aquatic vegetation (SAV) provide important ecosystem services. SAV productivity and distribution depend on light. Light attenuation through the water column is dependent on concentrations of phytoplankton, which can correlate with nutrient enrichment, and suspended inorganic and dissolved organic matter. Regardless of current SAV presence, NCBN continues to monitor this critical SAV parameter, as it provides information relevant to the overall estuarine eutrophication status.

Fig. 4: Water column light attenuation, represented by coefficient Kd (m-1), condition categories by percent of total estuarine area of Assateague National Seashore.

During the 2022 summer index period, ASIS average water column light attenuation (K_d) was good within 16.2% of the sampled estuarine area, fair within 43.4%, and poor within 40.4% (Fig. 4). The 2022 index period was improved from previous years, with a lesser majority of the sampled area in poor condition, and a larger portion in good condition. 2022, 2020, 2014, and 2010 have been the only years where there was some area observed in good condition.

Conclusion

In 2022, Assateague Island National Seashore saw varying conditions of bottom water dissolved oxygen, surface water chlorophyll-a, and water column light attenuation (K_d).The majority of estuarine area sampled for bottom water dissolved oxygen concentration was in good condition. Historically, bottom water dissolved oxygen levels have been good for most of the estuary. In terms of surface water chlorophyll-a, the majority of the estuary was in fair condition. ASIS has seen similar surface water chlorophyll-a conditions in previous index periods. Water column light attenuation was improved compared to previous years, with a greater percentage in fair and good condition, breaking the trend of a majority poor water clarity condition between 2005 and 2020.

Conditions of surface water chlorophyll-a and water column light attenuation at Assateague National Seashore in 2022 indicate that the estuarine system remains nutrient over-enriched.

Fig. 5: Park staff collects dissolved oxygen and other water quality data in one of the monitored salt ponds.

NPS Photo / Michaela Compo

References

Click to expand

Bricker, S.B., C.G. Clement, D.E. Pirhalla, S.P. Orlando, and D.R.G. Farrow. 1999. National estuarine eutrophication assessment: effects of nutrient enrichment in the nation’s estuaries. NOAA, National Ocean Service, Special projects Office and the National Centers for Coastal Ocean Science, Silver Spring, MD. 71pp.

Kopp, B. S. and H.A. Neckles. 2009. A Protocol for Monitoring Estuarine Nutrient Enrichment in Coastal Parks of the National Park Service Northeast Region. Natural Resource Report NPS/NCBN/NRR—2009/110. National Park Service, Fort Collins, Colorado.

U.S. EPA. 2001. National coastal assessment: field operations manual. U.S. Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Gulf Ecology Division, Gulf Breeze, FL. EPA 620/R-01/003.

U.S. EPA: 2003. Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries, EPA-903/R-03/002, United States Environmental Protection Agency, Region III Chesapeake Bay Program Office, Annapolis, MD; Region III Water Protection Div., Philadelphia, PA, and Office Water, Office of Science and Technology, Washington, D.C.