Water Quality Info

The Narragansett Bay watershed covers a land area of 1,657 square miles, more than ten times the area of the Bay itself. Only 40% of the Bay's watershed is in Rhode Island and the remaining 60% is in Massachusetts. Due to the sheer size of the watershed and the fact that it includes over 100 cities and towns in two states, it is very difficult to control all of the pollutants entering the Bay.

When regulators and scientists discuss "pollutants" in Narragansett Bay, they generally mean substances in the water that can cause harm to people and the environment. The Clean Water Act of 1987 defines pollution as man-made or man-induced alteration of the chemical, physical, biological and radiological integrity of water. Some of these contaminants may include metals, bacteria, nutrients, organic waste, and organic compounds. Not only are there different types of pollutants, but there are different ways for these substances to get into the Bay:

In Narragansett Bay and Mt. Hope Bay, some of the most common water quality problems include heavy metals, fecal coliform bacteria, and low dissolved oxygen.

The main part of the EMPACT project involves deploying and maintaining monitoring buoys located in both Narragansett and Mt Hope Bays. On each of these buoys is a series of sensors that tests for the following parameters: dissolved oxygen, temperature, conductivity (salinity), pH, and chlorophyll. More information is provided on the different water quality results that this EMPACT project will be presenting as well as on the research buoys and equipment.

Why is Dissolved Oxygen Important?

When monitoring water quality, one of the most frequent measures we wish to take is dissolved oxygen. Just as with land plants and animals, most creatures in the water need a constant adequate supply of oxygen to survive. But life in the water provides additional challenges that we do not experience on land. The air we breathe contains about 21% oxygen, and this number changes very little unless access to fresh air is completely cut off. Water can only contain relatively low concentrations of oxygen. A cubic foot of air contains about 8˝ grams of oxygen. A cubic foot of water at room temperature contains only about Ľ gram of oxygen if it is saturated. Colder water can contain somewhat more oxygen than this, but the salinity in seawater reduces the amount of oxygen the water can contain. But it gets worse. Because the oxygen supply in water is so low to begin with, it is easily used up. And because water is so much denser than air, it is more difficult to replace the oxygen that gets used. Imagine for a moment that you are a fish or lobster trying to get enough oxygen by "breathing" water instead of air, and you can understand the importance of dissolved oxygen in maintaining a healthy habitat for sea creatures.

Where does it come from and where does it go?

There are two principal sources of dissolved oxygen to the water column. The first is the air above the water. In the process of reaeration, gasses in the air dissolved in the surface water and are then mixed down into deeper water. The other source of oxygen is the plants in the water. In most cases, this will be the phytoplankton, the simple microscopic plants that live in the water column, but in shallower water near shore, larger plants, seaweeds and seagrass, may also be important. The process of plant growth, photosynthesis, absorbs carbon dioxide and water to create new plant organic matter, releasing oxygen in the process. During the day, when there is light for plants to grow, photosynthesis is usually the most important source of oxygen to the water column.

Every living thing in the water or on the bottom consumes oxygen (directly or indirectly). Respiration is the process whereby organisms take up oxygen to "burn" organic matter, releasing carbon dioxide and producing energy for survival. The animals respire constantly. Plants are a source of oxygen during the day (given enough light), but in fact they too respire constantly, and during the nighttime, they take up oxygen. Even the bacteria, living both in the water column as well as on the bottom, respire, and are a significant drain on the oxygen supply in many aquatic systems.

So if we have an abundant source of oxygen in the air and more being produced in the water by plants during the day, why is there a problem? There are two issues, which we have already referred to above: water is dense, and water can only contain a limited amount of oxygen. Reaeration takes place only at the surface. Moreover, photosynthesis, depending as it does on light, is also primarily a surface phenomenon. For creatures living at the surface there is rarely a problem, but for the deeper-dwelling life, and especially for bottom-dwellers, oxygen has to get down to them somehow and may be limited. The small amount of oxygen that can dissolve at the surface will first be used up in part by the creatures living there. Mixing processes in the water column can then stir what is left down into the deeper waters. In shallow areas, and in high-energy environments where waters are well mixed, this process may occur readily, providing a good supply of oxygen (providing the system has not become eutrophic). The water column is not always uniform, however, and when waters become stratified, as frequently occurs in estuaries, for instance, mixing is inhibited, and the transfer of oxygen to the deeper waters is correspondingly slowed. In many cases, and there are places in Narragansett Bay where this is common in the summer time, the mixing of oxygen to the bottom is slowed so much that much of the available oxygen is used up and the bottom waters cannot support many of the creatures that would otherwise live there. This low-oxygen condition is referred to as hypoxia. In extreme cases, all of the available oxygen is used up and a condition of anoxia exists. Few organisms can survive this condition, and residents will either leave, if they can, or die. Only some bacteria will survive these conditions if they persist.

Summer time conditions put extra stresses on dissolved oxygen in marine systems. There are a number of reasons for this, but the important factors are, first, that, as noted above, warm saturated water contains less dissolved oxygen than cold water does, and, second, sea creatures use oxygen at a greater rate at warmer temperatures. As a result, we commonly think of hypoxic or anoxic bottom waters as being conditions that take place primarily in the summer.

Hypoxia in our bay is most pronounced in the summer. More sunlight, warmer temperatures and abundant nutrients cause algal blooms to occur. The waters of the bay become stratified, or layered, since warmer, less salty water is lighter or less dense. This warmer layer floats on top of the cooler salty water. When these conditions exist, the bottom waters can rapidly become hypoxia or anoxic. Healthy levels of dissolved oxygen for Narragansett Bay, Mt. Hope Bay and the urban rivers range from 5mg/L to 12 mg/L.

Properties of Seawater

Salinity is the measure that oceanographers use to indicate how salty the ocean is. Pure seawater, from the open ocean, is about 3˝% salt by weight, and the unit used to indicate this is parts per thousand: 3˝% is 35 parts per thousand or 35 0/00. Actually, today oceanographers may refer to the units somewhat differently, but the value is essentially the same. Coastal waters are measurably more diluted than ocean water, so that the waters off the mouth of Narragansett Bay may have a salinity of only 32 ppt. The Bay is an estuary, so by definition, the waters there are further diluted, and there is a gradient of dilution from the mouth up to the head at each of the various rivers and streams that enter the Bay. Even at a single place in the estuary, salinity will fluctuate with movement of the tides, dilution by rain or snow, and mixing of the water by wind.

Oceanographers use salinity as one way of determining where water masses in the ocean originate and their flow directions. In an estuary, salinity tells us how much of the water at any given place originated from offshore and how much is from freshwater sources within the estuary. This can be an important measure, for example, when trying to understand sources of pollution in an estuary. Salinity values are calculated from conductivity measurements.

The salinity of freshwater ranges from 0 to 1 part per thousand (ppt). Brackish water is considered slightly salty and ranges from 1 to 10 ppt. The salinity of seawater defined as anything over 10 ppt. Salinity levels for Narragansett Bay generally range from 20 ppt to 31 ppt. Ocean salinity is usually near to 32 ppt.

Animals, plants and even bacteria that live in water react in different ways to changes in water temperature. Extreme changes in temperature can restrict how big and where plants, animals and microbial organisms can grow in the bays. Some fish or plants will die if the water gets too warm, and some cannot survive if it gets too cold. Sunlight and changes in air temperature control water temperature. In colder winters, parts of the bay's surface water freezes. In general though, warmer, fresher, or less salty water makes up more of the surface water. Near the bottom of the water column, cooler salty water is drawn in from Rhode Island Sound. The temperature range for bay waters is large and can vary from freezing (0^oC or 32^oF) to a much warmer 25^oC (78^oF).

Salinity and temperature together have an important influence on the density of seawater, and this in turn may affect how well the water column can mix vertically. Full salinity seawater is about 2˝% more dense than freshwater (depending on temperature). While this does not sound like a large difference, in fact it can be a strongly stabilizing force in the water. Because water flows freely to the lowest possible state, denser waters will stay at the bottom, or sink through the lighter waters, which stay at the surface. It may take considerable energy (wind or tidal currents) to overturn this condition and mix the lighter surface waters into the deeper layers. This stable condition of lighter water overlying denser water is called stratification. In freshwater, such as a lake, temperature alone can lead to stratification. In such a system, typically there will be a region of the water column in which most of the temperature change occurs called the thermocline, which separates the surface waters from the bottom waters. In an estuary or the ocean, a similar layer may exist, but it will be the result of both salinity and temperature, or of salinity alone. This type of density change layer is called a pycnocline. The formation of a pycnocline, and the resulting stratification may be an important factor in controlling dissolved oxygen concentrations in the deeper layers of the water column.

pH is a measure of how acidic or basic (alkaline) a solution is. A pH measurement indicates the hydrogen ion (H+) activity in a solution, and is expressed as a negative logarithm. Testing water for pH measures the capacity of the water to neutralize acids associated with pollution.

pH is one of the primary indicators used for evaluation of surface-water quality. Most marine plants and animals are sensitive to pH variations. Water's pH is affected by the minerals dissolved in the water, aerosols and dust from the air, and human-made wastes as well as by plants and animals through photosynthesis and respiration. Several factors affect the pH of the water, including:

The pH of water is critical to the survival of most aquatic plants and animals. Many species have trouble surviving if pH levels drop under 5.0 or rise above 9.0.

Chlorophyll is found in all plants and algae. Chlorophyll measurements help us estimate the amount -- or biomass -- of microscopic plants (also called phytoplankton) in the water.

Phytoplankton are closely linked to · the level of nutrients in the water, · temperature, · sunlight, and · the degree to which the water column is mixed.

Phytoplankton grow in surface waters where sunlight penetrates (also called the photic zone). If the phytoplankton are mixed deeper into waters with little light, their population, and chlorophyll, will decline.

Fecal coliform

Fecal coliform bacteria indicate the presence of human and animal waste in the water. Fecal coliform levels in the bays and rivers rise after heavy rainstorms for several reasons, including storm runoff and combined sewer overflows (CSOs). Stormwater running off yards, parking lots and streets also affects fecal coliform levels because it carries pollution, such as animal or bird waste into the water. CSOs happen when rainwater overwhelms the sewer pipes causing a mixture of rainwater and sewage to overflow into a river or bay. The NBC is currently working on a project to end CSO discharges.

People who swim in or eat fish or shellfish from waters with high levels of bacteria may become ill. In seawater, acceptable levels of fecal coliform bacteria are less than a geometric mean of 14 MPN/100ml. In freshwater, acceptable concentrations of fecal coliform is less than the geometric mean of 20 MPN/100ml.

To see the fecal coliform levels in Narragansett Bay click here.

An estuary is an enclosed place where the river meets the sea. More specifically, an estuary is defined as an enclosed arm of the ocean in which the waters are measurably diluted by freshwater sources. There are a number of types of estuaries, and they cover a large range of sizes, from the smaller coastal ponds of Rhode Island and Massachusetts to the extensive and complex estuary that is Chesapeake Bay. Categories or types of estuaries are generally defined according to the nature and extent of the salinity gradient that forms between the freshwater riverine head to the coastal marine waters at the mouth. This results in different levels of stratification in the various types of estuaries and over the length of each type.

A salt wedge estuary is the most highly stratified type in which there is a strong river flow compared to the mixing that is generated by tidal currents. A distinct freshwater layer flows out over a distinct saltwater layer (or "wedge") and the two are distinct over some length of the estuary.

The partially mixed estuary, of which Narragansett Bay is an example, displays a consistent gradient of salinity from the mouth to the head with a corresponding gradient of salinity from the surface to the bottom over a significant length of the estuary.

The well-mixed estuary is dominated by tidal currents over river flow and is thus well mixed over most of its length. While there is a gradient of salinity from the freshwater sources to the mouth, there is little vertical salinity variation, and thus little stratification in this type of estuary.

The fjord is a distinct estuarine type with a different geometry. The typical fjord is relatively long and narrow and is deep along much of its length. A defining feature of a fjord is a sill of shallow water at the mouth, which serves to isolate the deeper waters of the fjord from the ocean. As a result, the deeper waters may not be well mixed, and a fjord can be a highly stratified type of estuary if there is sufficient freshwater.

Many of the organisms that live in the estuary are the same as those that might be found along the coastal ocean. But, as one travels up the estuary into increasingly fresher and more protected areas, there is a shift in the species found there. Some mobile species may move from offshore into the more protected areas to reproduce, so that estuaries are important nursery areas for many species. And, for a number of reasons, estuaries may be highly productive, yielding a high biomass of fish and shellfish per unit area. For these reasons, estuaries are generally considered to be valuable biological resources requiring protection against abuse.

Waterfront residential property has become very popular and commerce has always developed near the water (both for rivers and the ocean). Because estuaries allow access to the water while at the same time providing a protected environment, they are the focus of a great deal of human activity. This results in heavy usage of the waters for shipping, fishing, and recreation, as well as providing an appealing aesthetic environment and a useful place to discharge wastes. Obviously, such a wide range of uses will result in conflicts that may prove difficult to resolve. As population pressures increase, and as we come to better understand and appreciate the values that estuaries have at every level, there will be a need for ever more conscientious, persistent and costly custodianship of these resources.

Nutrients and Eutrophication

Plants require nutrients to grow. This applies to plants in the water just as it does to plants on land, and the nutrients required are much the same. There are three principal sources of nutrients to a marine system (the ocean or an estuary): the atmosphere, runoff from the land (whether it be rivers, groundwater or sewers), and internal cycling of nutrients already in the system. These sources have existed as long as the oceans, but human influences have accelerated the delivery of nutrients to the oceans, most obviously in the nearshore environment, and this is especially apparent in enclosed bodies of water such as estuaries. Nutrients, and life itself, are normally very diluted in seawater. Typically, their concentrations are measured in parts per million. Thus, it is easy to see how it is possible to change the nutrient concentrations in seawater with the addition of a sewer outfall to an estuary or hundreds of houses on septic systems contributing to the groundwater that flows to an embayment.

Estuaries tend to experience eutrophic conditions more severely than open water areas for two main reasons. They are frequently more heavily developed, and their enclosed nature reduces tidal flushing, or the rate at which polluted water in the estuary may exchange with cleaner offshore water. Thus, there is less dilution of the nutrients, and their concentrations tend to build up.

The most important nutrients of concern when we discuss human impacts to an estuary are nitrogen and phosphorus, and in most marine systems, there is already excess phosphorus for plant growth, so it is nitrogen that we think of as the "limiting nutrient." It is, then, this limiting >nutrient, nitrogen, that will increase plant growth directly as the result of being added to such a marine system. In freshwater, it is often phosphorus that behaves as the limiting nutrient.

When excess nutrients are added to an estuary, a series of changes take place in the biology and ecology. The term eutrophication is applied to this process. Sometimes the word eutrophication is used to refer to the process of excess nutrient addition, and sometimes it is used to refer to the effects of this nutrient addition, so it may mean either or both. The added nutrients result in increased plant growth. More plant growth results in more organic matter in the water column, and this organic matter is eaten by animals in the water column and in or on the bottom (thus more animal production) and settles to the bottom. There is an overall buildup of organic matter in the water and on the bottom, and this organic matter breaks down over time either as living things undergoing respiration, or dead organic matter being decomposed by bacteria. These processes consume oxygen. Near the surface, there may be plenty of oxygen for this to occur, but in deeper, poorly mixed water, oxygen may be reduced to low levels, or even used up completely. This chain of events results in a whole range of changes in the ecosystem. Different species of planktonic plants preferentially grow in high nutrient environments, and to some extent, this change in plants may result in a change the in animal community to one that prefers (or better tolerates) the change in diet. Increase phytoplankton growth reduces the transparency of the water, and bottom dwelling plants such as eelgrass may lack sufficient light for growth and die off. And finally, and most dramatically, where eutrophication is severe enough and all the oxygen is used up, there can be a wholesale die off or emigration of the bottom community, leaving only bacteria to thrive in the anoxic conditions.