Responses of calanoid copepods to changes in phytoplankton dominance in the diatom/Phaeocystis globosa dominated Belgium coastal waters
|University||Vrije Universiteit van Brussel (Belgique)|
|Thesis supervisor||Marie-Hermande Daro|
|Thesis co-supervisor||Tackx Michèle|
|Thesis co-supervisor||Gasparini Stéphane|
|Financement||Vrije Universiteit van Brussel|
|Abstract||Copepods feeding behaviour according to changes in composition and availability of their potential preys in Belgian coastal waters were appraised through the combination of different field experiment approaches.
The gut fluorescence method was combined with egg production measurements to estimate the contribution of herbivory to the total ingestion required to sustain Temora longicornis' egg production. The results showed that during fall and winter herbivory was often sufficient to cover the nutritional needs of T. longicornis, while during spring and summer they needed heterotrophic food to meet their energetic demands for egg production. When T. longicornis carbon requirements for egg production increase the contribution of non-phytoplankton food sources to total carbon ingestion becomes more important. The phytoplankton spring bloom, either during diatom dominance or during P. globosa dominance, did not enhance the contribution of herbivory to the copepods' diet. HPLC gut pigment analysis showed that diatoms were the main phytoplankton group ingested, whereas no evidence for ingestion of P. globosa and nanoflagellates was found.
In situ feeding incubation experiments were also conducted with two dominant copepod species in Belgian coastal zone, T. longicornis and Centropages hamatus, designed to estimate diet composition and prey selectivity upon natural plankton assemblage. The results showed that copepods were not only omnivorous, but highly selective as well, consuming the diverse categories of prey in a different proportion than expected from prey availability in situ. Although diatoms were an important contributor to the copepod’s diet in this study, they were only ingested according to their abundance or even slightly rejected. There was an overall positive selection for ciliates by both copepod species, with ciliates < 18 µm being the preferred prey category, whereas P. globosa and nanoflagellates were selected against. Dinoflagellates were diversely appreciated according to the copepod species considered. Temora longicornis showed a clear preference for small dinoflagellates and a more neutral selection for dinoflagellates > 18 µm. Prey-selectivity appeared to be not size-dependant, and other factors such as food quality and prey motility are assumed to be the driving factors of selective feeding in Belgian waters.
Total carbon ingestion estimated for T. longicornis from the field incubation experiments (phytoplankton + microzooplankton) appeared to be not sufficient to cover egg production carbon requirements estimated from the egg production measurements. This supports the hypothesis that another possible source of carbon (e.g. detritus, copepod eggs) was not taken into account in the incubation experiments.
This study also critically examined the use of HPLC pigment analyses to trace the fate of phytoplankton in the food chain. In particular we demonstrated that 19’-hexanoyloxyfucoxanthin is not the appropriate pigment to estimate Phaeocystis abundance and trace its trophic fate in BCZ, and that the use of alloxanthin as biomarkers for copepod grazing on Cryptophyceae is not suitable both for quantitative and qualitative estimates.