The Marine Insight web page mentions the following as being the effects of dredging.
* The soil deposits in any water-body have a certain pre-disposed composition. Through dredging this composition is altered
* Because of the said alteration, the existing habitat of creatures and organisms that depends on the original composition of the soil dies out due to the unfavourableness of the changes caused
* The turbidness of the soil under the water also changes because of this alteration in the underwater soil composition. This poses problems by way of creation of newer and harmful organisms, transferring of unwanted organisms to other parts in the water-body leading to a wider spread of contamination and organic processes by way of release of extra and unwanted nutrients. The turbidness also causes the already existing contaminations to spread further into the water-body which also affects the marine environment adversely
The dredging of contaminated materials will cause the harmful particles to regroup and spread to a larger area in the water body
Since dredging loosens up the soil, those substances which were previously held fast to the contaminated deposit will find their way into the water and the un-dredged soil. If these substances are harmful organisms then they will cause a substantial degradation to the environment even after dredging the area
The water could get polluted because of the soil particles mixing with the water. And while this does not have huge biodiversity impacts, it is indeed an unwanted side-effect of dredging
After watching a video of dredging on our local beaches it is hard to imagine that there isn't some sort of environmental effect after 4 months or more of this.
Here in Oceanside during the 2017 dredging project they have had to look into displacing the eel grass.
According to the Smithsonian Ocean Portal site the following marine animals utilize the eel grass. "As a result, seagrasses can be home to many types of fish, sharks, turtles, marine mammals (dugongs and manatees), mollusks (octopus, squid, cuttlefish, snails, bivalves), sponges, crustaceans (shrimp, crabs, copepods, isopods and amphipods) polychaete worms, sea urchins and sea anemones—and the list goes on."