Nutrient Loading

Nutrient over-enrichment is a threat to the bays.

Increased nutrient (nitrogen and phosphorus) loading to the Coastal Bays leads to degraded water quality and ecosystem health. For the latest assessment, three-year (2001-2003) annual nutrient concentration medians were calculated for each monitoring station and compared to established thresholds. 

Nitrogen Status:  The upper tributaries in the northern Coastal Bays and Newport Bay are severely enriched in nitrogen.  The southern Coastal Bays, Sinepuxent and Chincoteague, have the lowest total nitrogen concentrations. View total nitrogen status map.

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Phosphorus Status:
  Phosphorus enrichment appears to be more widespread than nitrogen enrichment with few sites meeting the seagrass threshold.  View total phosphorus status map.

Where do nutrients come from?

Nutrient inputs to the Coastal Bays are dominated by non-point sources (e.g., surface runoff, groundwater, atmospheric deposition and shoreline erosion).  The amount of nutrients coming from an area is largely dependent on the predominant land use - agriculture and developed land versus wetlands and forests.

Nutrient budgets are a tool managers use to organize diverse nutrient data sets.  The essential parts of simple budget include inputs (point, diffuse, atmospheric) and exports (denitrification, burial in sediment, oceanic exchange, fisheries harvest).  Additional elements that should be included are nitrogen storages in the sediments, eroding marshes (a form of inputs) and nitrogen fixation (another potential input).

November 22, 2004 The Maryland Coastal Bays Program Scientific and Technical Advisory Committee, STAC, held a one day workshop to refine the nutrient loading budget for the MD Coastal Bays.  In the past few years there has been an abundance of data collected related to nutrient inputs to the bays that can be used to verify (or challenge) the modeling estimates that have been calculated. The goal of the workshop was "to develop an updated nutrient budget for the Coastal Bays (including VA and DEL) based on locally collected data."  Minutes from the workshop are available online and a synthesis of the results will be completed by June 2005.

How Excess Nutrients Harm the Coastal Bays
Excess amounts of nutrients are the most extensive pollution problem affecting the Maryland Coastal Bays. Just as nutrient fertilizers are used to promote plant growth on our lawns and farm fields, nutrients in streams and the bays encourage the growth of aquatic plants. Although some aquatic plants are beneficial and provide food, oxygen, and habitat, excessive nutrients may result in thick growths of aquatic plants (especially algae) that contribute to an unhealthy environment. These algae blooms block light needed by seagrasses (submerged aquatic vegetation, or SAV), either by clouding the water column or coating the grasses themselves. In addition, when algae die, they settle to the bottom and are consumed by bacteria during the decomposition process. This process consumes oxygen, depleting it from bottom waters. The resulting low dissolved oxygen concentrations drive fish and blue crabs from their preferred habitat and can kill clams, worms, and other small bottom organisms on which crabs feed.

The most important nutrients affecting aquatic plant growth are nitrogen and phosphorus. Plant and animal matter (including animal and human waste), fertilizer, and even car and power plant exhaust, all contain nutrients. When these nutrient sources are not controlled, excess nutrients find their way into the groundwater, creeks, rivers and, eventually, the open bays. Nutrients can have detrimental effects on these tributary systems as well.

Definition of nutrient loads versus nutrient concentrations
The nutrient load refers to the total amount of nitrogen or phosphorus entering the water during a given time, such as "tons of nitrogen per year." Nutrients may enter the water from runoff, groundwater, or the air (in the form of wet deposition such as rain or snow as well as dry deposition). The nutrient concentration refers to the amount of nitrogen or phosphorus in a defined volume of water (such as milligrams of nitrogen per liter of water). Total nitrogen concentration is the total amount of nitrogen in one liter of water; total nitrogen includes both dissolved nitrogen in the water column and particulate nitrogen contained in algal cells and in organic detritus such as degrading leaves from trees. Like nutrients, concentrations of oxygen, algal abundance (measured as chlorophyll a), and total suspended solids are a measure of how much oxygen, chlorophyll a, or total suspended solids are in a defined volume of water. The relationship between nutrient concentration and nutrient load can vary and depends on the flow, the volume of water in the river, and watershed characteristics.

Although water quality goals refer to decreasing nitrogen and phosphorus loads, concentrations are generally used as thresholds because laboratory analyses of water samples are reported in terms of concentration with a known level of precision and accuracy. The Maryland Coastal Bays Scientific and Technical Advisory Committee (STAC) developed criteria for threshold categories based on living resources indicators, most notably seagrasses.

Living resource objective

TN thresholds

TP thresholds

Better than seagrass objective <0.55 mg/L <0.025 mg/L
Meets seagrass objective <0.64 mg/L <0.037 mg/L
Does not meet seagrass objective <1 mg/L <0.043 mg/L
Does not meet STAC objectives <2 mg/L <0.1 mg/L
Does not meet any objectives >2 mg/L >0.1 mg/L

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