Chlorophyll: Chlorophyll is the green pigment that plants use for photosynthesis. Just like plants on land, microscopic algae in the water also contain chlorophyll. Thus, the chlorophyll concentration is a measure of the amount of microscopic algae present in the water. These algae are food for larger organisms and also produce oxygen that many other organisms need to survive.

During our sampling trips, we measure chlorophyll as relative units. Water with no suspended chlorophyll will read zero. Much of the open waters of the river read about 1- 2 units, and in areas with intense algal blooms the reading could exceed 20.

Enterococcus: Enterococcus is a genus of Gram positive, facultative anaerobic, a lactic acid bacterium that is commonly used as an indicator of water quality and sewage contamination. Enterococcus counts are useful as a water quality indicator due to their abundance in human sewage, correlation with many human pathogens and low abundance in sewage free environments (Jin et al. 2004). The EPA reports Enterococcus counts as colonies (or cells) per 100 ml of water.

Oxygen: Oxygen concentrations in the water are closely connected to biological activities. When aquatic plants photosynthesize, they release oxygen into the water. Meanwhile, bacteria and other organisms use up oxygen in their metabolic activities. Higher animals, such as fish and shellfish, need well-oxygenated water to survive. The absolute amount of oxygen that can be dissolved in water is dependent on the temperature and salinity, we therefore record oxygen concentration at different locations as the % concentration relative to full saturation. In aquatic systems in general, surface waters are typically in equilibrium with the atmosphere such that the oxygen concentration near the surface is close to fully saturated (100%).

Because of intense microbial activity, oxygen concentrations in the surface waters of the Hudson are highly variable. We sometimes find areas of the River where the surface oxygen concentrations are too low to support fish or other higher animals (much less than 100%). We also find other locations where the surface waters are supersaturated (more than 100%) because of high photosynthesis from algal blooms.

Salinity: Estuaries like the Hudson are where freshwaters in rivers meet the sea. Salinity tells us about the relative contribution of fresh and salt water within a particular region of the estuary. Salinity generally increases towards the mouth of the estuary, although salinity at any location can change with the tide. Heavy rains will push freshwater further down the estuary, while dry weather will allow salty water to penetrate further north. Many aquatic organisms have a preferred range of salinities where they can best survive. During our sampling trips, salinity is measured in parts per thousand (ppt). Ocean salinity is about 34 ppt and the freshwaters of the upper Hudson River are close to 0 ppt.

Temperature: Water temperature is one of the most basic and most important things we measure during our sampling trips. For example, the activities and metabolic rates of many organisms in the River, from bacteria to fish, are closely connected to water temperature. In general, water temperature in the River changes seasonally, warming through the summer and then cooling as winter approaches. However, surface water temperature can change on small scales in the River. For example, because deeper waters are generally cooler than surface waters, surface temperature can fall if deeper waters are mixed up to the surface. In addition, effluents from industrial facilities, power plants, and sewage treatment plants often are often a different temperature than the rest of the River.

Turbidity: Turbidity is a measure of the amount of fine particles suspended in the water. As turbidity rises, light penetration, a key factor for aquatic plants and algae, diminishes. Particles can be the sites of intense microbial activity and can play a role in the transport of contaminants. In addition, areas of heavy runoff and erosion often have high particle concentration. In general, turbidity in the Hudson River increases in the lower estuary, reaches a maximum around upper Manhattan, and then falls as one goes further south.

During our sampling trips, turbidity is measured as nephelometric turbidity units (NTU). Typical values range from close to zero (not turbid) to several hundred (very turbid).

For more detailed information on our testing protocol view our report: Swimmable River