Chlorophyll

As part of Snapshot Day 2005 each sampling group collected and filtered water samples to collect the solids which were collected in the field and sent back to Lamont-Doherty Earth Observatory for laboratory analysis of chlorophyll a levels (protocol below) by Dr. Ajit Subramaniam.

What is chlorophyll a and why do we care about it? 

Chlorophyll a is the green pigment found in all photosynthetic organisms that allows plants (including algae - phytoplankton) to conduct photosynthesis. Remember photosynthesis is the process by which plants create sugar (food) and oxygen using light (from the sun), water, and carbon dioxide.  Phytoplankton are primary producers - the base of the aquatic food chain - and thus are important to sustain life in the river.  However, too much phytoplankton could cause degradation in water quality and so quantifying phytoplankton biomass is important.  Scientists have found that there is a strong correlation between phytoplankton  biomass  and chlorophyll a concentration in the water.  Therefore we are measuring chlorophyll a as a proxy for how much phytoplankton there is in the Hudson River. 

Protocol:

Water samples were collected at each site. 120cc of river water was forced through a glass fiber filter. This method screens and collects the phytoplankton in the water (as well as other small particles) on the filter.  The filters were placed in vials and frozen at the collection sites and returned to Lamont-Doherty Earth Observatory for analysis. Lamont scientists measured the samples by extracting the sample in acetone and then using a spectrophotometer.  The spectrophotometer calculates the amount of light absorbed at specific wavelengths of the extract and that is used to calculate the chlorophyll a concentration. 

Discussion:

Note that this data was collected as part of an educational activity and provides some interesting items for discussion and further analysis. As part of a first time collection process there are refinements that would be suggested and incorporated into future collection and analysis,  which we hope to continue next year. Several of the samples did not completely extract and thus were not included in the data analysis. Data collected this first year is an excellent opportunity for study refinement, and for establishing some baseline numbers.

The results of the analysis are listed here and graphed below. A more representative sampling of chlorophyll levels would be collected in the main channel of the river where there is more complete mixing of the water column. Our sampling groups are located along the sides of the river thus the sampling might represent small micro systems in the river ( i.e. more representative of the specific areas where the water is collected).

Overall the chlorophyll readings were more elevated than when the test samples were collected in August.  This could be the result of recent rains bringing an increased nutrient load into the river system which had suffered a month long dry spell. 

Trends in the data suggest extremely low phytoplankton biomass in the Gowanus Canal samples, where growth could be light limited as a result of high turbidity in that area (secchi readings of 4.5 inches and 6 inches visibility).

Readings in the harbor area are relatively high which could be due to the increased light availability and high nutrient loads in this area where the river water mixes with the ocean.  Chlorophyll levels made a noticeable drop at the Harlem River sampling site and remained somewhat level until a small increase at the Piermont Pier. This small reduction in the chlorophyll levels in the area between RM 6 and RM 25 could be the result of the increased turbidity caused by the mixing from the saltwater intrusion.   The area in the Hudson between RM 18 and RM 23 is often referred to as the area of turbidity maximum because of this active mixing.

In the upper estuary there were small spikes in several locations, with a large spike at Cohotate.  Productivity was so high in this area that samplers at Cohotate had to filter a reduced amount of water because of plugging the filter. Both the samples from RM 2 in the harbor and RM 118 at Cohotate were green with chlorophyll.  It will be interesting to continue collections at this site to see if this high level of biomass continues.

Visual Evaluations:

As part of the Chlorophyll collection the samplers visually evaluated the color of the filter after collecting the solids.  A color scale with numeric equivalents was provided to the samplers to match against.  The numeric measures are provided here with the Chlorophyll readings.  It should be noted that sometimes the score from the color card match was more representative of silt and sediments being collected in the filtration than in the chlorophyll readings.  This appears to be true for samples in the 5.0 range which is almost a light chocolate color on the chart.

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