Comparative analysis of diets and diel vertical migration patterns of dominant zooplankton in the Gullmar Fjord, Sweden

Sammanfattning: This study aimed to quantify and compare zooplankton feeding interactions in terms of diet composition, ingestion rate, and food size selectivity of a copepod (Acartia tonsa) and a cladoceran (Evadne nordmanni). This was achieved by surveying the grazing impact of A. tonsa and E. nordmanni on the natural phytoplankton, microzooplankton and microbial community occurring in autumn in coastal Skagerak. A. tonsa cleared a higher proportion of the silicoflagellate Dictyocha speculum (p<0.001) compared to E. nordmanni. Prey selection analysis confirmed a strong preference towards D. speculum by A. tonsa (α = 0.084). Results give indication towards an active prey selection with adjustable feeding methodology from filter feeding to ambush feeding in A. tonsa. Furthermore it suggests feeding upon prey taxa independently of prey abundance in the environment. However, E. nordmanni ingested significantly more Ceratium lineatum (p<0.001) compared to A. tonsa. Density dependent ingestion and stable clearance rates by E. nordmanni suggest a passive filter feeding method. Moreover, both species preferred prey of different size ranges. A. tonsa preyed predominantly on medium sized prey (e.g. D. speculum, Dinophysis norvegica and Lohmaniella oviformis). In contrast, E. nordmanni preferred prey > 100 μm (e.g. Nitzshia longissima, Ceratium furca, Ceratium fusus). Under the influence of interspecific competition (E. nordmanni + A. tonsa treatment), both grazers continued to graze upon their preferred prey and size range. The zooplankton community was observed on their diurnal vertical migration behaviour over two consecutive days in Skagerrack. Here, we tested for differences in mean abundance at two depth intervals (0-10 m and 10-20 m) over a time series of 48 hours, with sampling every 6 h. Significant migration behaviours were reported for adult (p< 0.0001) and juvenile (p< 0.0001) copepods of the entire community on both days (2-way repeated measure ANOVA). At low light levels, adult and juvenile abundance at surface was significantly higher compared to deeper waters. Similarly, at high light levels, abundance was significantly higher in deeper waters compared to surface layers. Juveniles of all 4 investigated species underwent significant migration, whereas reduced migration by adults of A. tonsa and E. nordmanni were reported. Reasons for ontogenetic differences in migration behaviour may include higher predation pressure towards juveniles or species specific predation pressure.