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

Abstract

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.

Popular Abstract

The diet and vertical migration pattern of zooplankton in Gullmar Fjord, Sweden

Zooplankton are small crustaceans which display the link between photosynthesizing phytoplankton and higher trophic levels such as planktivorous fish. Understanding their dietary needs and feeding behaviour is important in order to understand how the entire marine food web is structured and that future prediction about the health of the marine ecosystem can be made. Furthermore, zooplankton undergo a diel vertical migration pattern, a predator induced defence behaviour, during which they sink into low-light depths to avoid predation during the day. During night time they rise to the surface to feed and restore energy levels.

The current study investigated prey types and feeding behaviour by two dominant zooplankton species: A. tonsa (a copepod) and E. nordmanni (a cladoceran). Both species are morphological different and employ different feeding techniques. A. tonsa is more motile and uses an ambush feeding technique to capture certain prey. Alternatively, they can also use a current feeding technique during which they create water currents around their mouth and can ingest particles. E. nordanni on the other hand is less motile and only employs the water current feeding technique. In addition, diel vertical migration was also investigated in order to see if food abundance and prey types have an impact on the vertical migration pattern. Diel vertical migration is a balance between energy spent during migration and energy restored during feeding, it was hypothesized that smaller copepods will not migrate as far as adults, since they are less detectable by predators, but also since the energy spent during migration is not proportional to the amount of energy taken in during feeding.

The grazing experiment has shown that E. nordmanni mainly fed on highly abundant foods (e.g. dinoflagellate Creatium lineatum). This was expected since they employ a water current feeding technique. Results have also shown feeding on large, non-motile diatoms, which are large in size. Even though E. nordmanni is a non-motile species, they are capable of holding onto prey types and ingest them. The copepod species A. tonsa was observed to ingest a wide range of prey but mainly motile prey such as silicoflagellates (Dictyocha speculum) and ciliates (Loahmaniella oviformis). This gave an indication that A. tonsa and E nordmanni employ different feeding techniques and that preferred prey types do not overlap. Hence, competition for food is unlikely.

Results from the migration study has shown that smaller and juvenile zooplankton engage in a stronger diel vertical migration pattern compared to adults and larger sized zooplankton. There are several possible reasons for this. Juvenile zooplankton species are a desired food source for larger zooplankton and juvenile fish and it is desired to reach a large body size as quickly as possible in order to reach maturity and reproduce. Hence, juveniles need to make sure to remain undetected. Larger adults showed high variations in migration patterns. Perhaps desired prey types or prey abundances were not sufficient enough for them to undergo extensive migration.

This research study confirms results from previous grazing studies which confirmed that copepods and cladocerans employ different feeding strategies and have thus different prey preferences. Previous results from the migration study did not overlap with the current study, however, it give us an indication migration patterns differ seasonally depending on reproductive cycle, prey types and prey abundances.

Advisor: Per Carlsson
Master´s Degree Project in Marine Biology 60 credits 2015
Lund University, Department of Biology

• author: Eimer, Friederike
• supervisor: Per Carlsson
• organization: Degree Projects in Biology
• course: BIOP21 20112
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 5463823
• date added to LUP: 2015-06-03 14:37:39
• date last changed: 2015-11-10 09:48:18