Mid-Po river zooplankton : a seven year study

The temporal crustacean-zooplankton variation in the mid-Po River (Italy) was analyzed from November 1974 through November 1982. Samples were collected at three stations at monthly intervals. Separate taxon-by-taxon multiple regression analyses (MLTRG) were carried out with zooplankton taxa as the criterion variable and river depth, river discharge and water temperature as the predictor variables. Moina brachiata and Eucyclops serrulatus were correlated with water temperature, Cyclops vicinus inversely correlated with temperature, Eudiaptomus padanus, C. vicinus and Daphniidae with river discharge; Copepodites inversely correlated with river depth. MLTRG showed that water temperature and river discharge account for about 15% of total zooplankton variation. Factorial analysis of variance (ANOVA) was carried out on residuals calculated from MLTRG. Among-years and among-months variation were significant for all the examined taxa. Significant years × months interactions indicated that seasonal variations were also different in different years. ANOVA stressed that there were still unexplained significant residuals in years and months variations. Weekly and daily samples collected in Summer 1979 were submitted to MLTRG and ANOVA as above. Among-week variations were significant for all the examined taxa, whereas among-days were significant for Daphiidae and M. brachiata (Cladocera), and not significant for any of the other taxa. Caution is recommended in interpreting the results of river zooplankton studies. Differences between species responses warn us not to pool species into larger taxa. Temporal variation changes from year to year, so long-time series are necessary to clarify the most important trends of river zooplankton.