Toxicity Evaluation of Waters from a Tributary of the River Po Using the 7-DayCeriodaphnia dubiaTest

Seven-day toxicity tests withCeriodaphnia dubiawere used to assess ambient water toxicity in the River Lambro, a tributary of the River Po, and in the corresponding stretch of the main river located downstream from their confluence. About once a month, toxicity tests were conducted on water samples of the River Lambro, using as dilution water the water of the main river collected upstream from the confluence. With a lower frequency, the downstream stretch of the River Po was tested for ambient toxicity at four sites located at 6, 11, 16, and 21 km from the emission of the tributary. The River Lambro demonstrated variable toxicity in different time periods, although the most frequent effects were sublethal, on both reproduction and growth. Ammonia, nickel, and zinc can be indicated as possible toxicants. Acute toxic effects toC. dubiawere observed with spring samples only, when the action of some pesticides, likely insecticides, have to be taken into account. Tests conducted on downstream water gave limited results, seemingly because the dilution capacity of the River Po reduced Lambro toxicity to a level close to and often below the detection limit of the 7-day test. Accordingly, toxic effects at downstream sites could only be observed on the growth ofC. dubia,which was found to be the most sensitive endpoint. Fairly good agreement was found between predictions based on toxicity tests of the River Lambro and the effects observed for downstream samples.