tests for water clarity by measuring Secchi depth in the rivers.
Generally, the tributary rivers are not as clear as the harbor
because they have higher concentrations of particulates and algae.
The water clarity in the rivers has not changed significantly
since monitoring began in 1992.
Charles River has an interesting pattern of water clarity: the
water is more clear in the urbanized basin than it is upstream.
As the river slows and widens through Cambridge and Boston, the
rate of flow decreases, allowing solids to settle out. As a result,
Secchi depths increase downstream. (See Spatial
Trend in Charles River Water Clarity.)
ALGAE AND CHLOROPHYLL
concentrations are typically higher in the rivers than in the
harbor, because nutrient levels are higher in rivers. Also, the
residence time of water is greater in the dammed portions of the
rivers than in the harbor. Both these factors allow more algae
to build up. Algae levels are at their highest in the rivers and
the harbor in the summertime. The Mystic and Charles rivers have
the highest amounts of algae; the Neponset the least. Since river
chlorophyll monitoring began in 1992, there have been no significant
trends in algal blooms over time.
OF CHLOROPHYLL IN THE RIVERS AND THE HARBOR.
|Rivers have higher chlorophyll
levels than the harbor, but both ecosystems are subject to
dramatic seasonal fluctuations. Chlorophyll levels are high
in the summer when the light is intense, and low in the dimmer
winter months. River chlorophyll is measured downstream near
the river mouth but before mixing with saltwater. Both the
Mystic and Charles are very eutrophied, with high average
chlorophyll due to poor flushing and high nutrient inputs.
The Neponset has the lowest chlorophyll of all the rivers,
probably due to a relatively natural river flow and lower
nutrient inputs. (See larger