Observing the River

The data you'll be using has been collected by Cary Institute scientists, who've been sampling the tidal freshwater portion of the Hudson River at specific stations for many years. Launching a small motorboat called a Boston Whaler from various points along the river, they systematically draw water samples at precise locations repeatedly over time.

The scientists collect samples according to two different plans:

Transects

This approach consists of collecting water samples at closely-spaced intervals (every 2-4 km (1.2-2.5 miles)) along a 170 km (105 miles) stretch of the river over 1-3 days. Scientists launch a boat at the northern end of the river and sample in the main channel while traveling downriver, usually between Castleton and Haverstraw Bay.

Cardinal stations

These six key stations — Castleton, Hudson, Kingston, Poughkeepsie, Fort Montgomery and Haverstraw Bay — are spread along some 150 km (93 miles) of the tidal freshwater Hudson River. The scientists measure the concentrations of more characteristics at the Cardinal stations than they do during the transects, but cannot sample them all on the same day. These include oxygen, nutrients like nitrogen and phosphorus, and different types of plankton like rotifers. This makes it possible to compare many variables across time.

The scientists then test the samples for a number of factors:

Zebra mussels

Twice in the summer scuba divers collect ten rocks from the hard or rocky areas of the river bottom from each of the seven sampling sites. They bring these rocks back to the lab in, count the number of mussels attached to each, and measure shell length. Samples are archived in ethyl alcohol and stored in the freezer. In “soft-bottom” areas, scientists use a device called a benthic grab to collect material at 48 random sites throughout the freshwater portion of the river. Back in the lab, the material is sieved, and all bivalves counted and identified. A subset is measured for shell length. Since scientists knew approximately how much of the river bottom is rocky and how much is soft, they combine these averages for an annual estimate of the total number of mussels in the freshwater portion of the river, as well as the average per unit of river bottom.

Total Suspended Solids (TSS)

Researchers determine the amount of Total Suspended Solids (TSS) in the river water by pouring a precise amount through a pre-weighed filter. Material too large to pass through is considered “particulate” (a suspended solid), while the material that passes through the filter is considered “dissolved”. After the filter dries, it’s weighed again. The difference is a dry weight measure of the particulates in the water sample. TSS is measured in milligrams per litre (mg/l).

Chlorophyll

Chlorophyll correlates with the amount of phytoplankton in the river. To measure it, researchers pass water samples through filters. The chlorophyll is extracted from the particles that collect on the filters and measured with a device called a fluorometer, which measures the fluorescence of chlorophyll.

Temperature and Dissolved Oxygen

Researchers lower probes into the river that measure temperature, dissolved oxygen, as well as other variables such as pH and conductivity

Zooplankton (bacteria, copepods, rotifers, cladocera)

Small zooplankton (micro-zooplankton) are sampled by passing two liters of river water through a fine mesh net. Large zooplankton (macro-zooplankton) are sampled by pumping 100 liters of river water through a net with a larger mesh. The samples are preserved in formaldehyde aboard the boat, carried to the lab, and counted using different microscopes. The microscope depends on the size of the specimens and the volume of the sample.