WATER CLARITY MONITORING
Secchi disk is used to measure water clarity in Boston Harbor
Water clarity is one way people decide whether to
swim in Boston Harbor; it also determines how much light reaches
the plants in the water. These plants, which include algae and
seagrasses, are important to the ecology of the harbor.
Measurements of clarity in the harbor (and anecdotal
observations by the public) indicate that water clarity has improved
since the early '90s. One of the measurements that has shown this
improvement is Secchi depth. This is the depth at which a white
disk, a Secchi disk, lowered into the water is no longer visible.
Water clarity shows a west-to-east increasing gradient
across the harbor. Water is more turbid toward the rivers and
shallow margins of the harbor, and clearer (deeper Secchi depths)
toward the mouth of the harbor and bay. Water in Dorchester Bay
is naturally turbid, because it is a shallow depositional area
for sediments carried down by the Neponset River.
Map of water clarity
changes after South System flows were transferred to the Deer
Island Treatment Plant for secondary treatment.