The role of ciliates, heterotrophic dinoflagellates and copepods in structuring spring plankton communities at Helgoland Roads, North Sea

Abstract

Mesocosm experiments coupled with dilution grazing experiments were carried out during the phytoplankton spring bloom 2009. The interactions between phytoplankton, microzooplankton and copepods were investigated using natural plankton communities obtained from Helgoland Roads (54°11.3′N; 7°54.0′E), North Sea. In the absence of mesozooplankton grazers, the microzooplankton rapidly responded to different prey availabilities; this was most pronounced for ciliates such as strombidiids and strobilids. The occurrence of ciliates was strongly dependent on specific prey and abrupt losses in their relative importance with the disappearance of their prey were observed. Thecate and athecate dinoflagellates had a broader food spectrum and slower reaction times compared with ciliates. In general, high microzooplankton potential grazing impacts with an average consumption of 120% of the phytoplankton production (P_p) were measured. Thus, the decline in phytoplankton biomass could be mainly attributed to an intense grazing by microzooplankton. Copepods were less important phytoplankton grazers consuming on average only 47% of P_p. Microzooplankton in turn contributed a substantial part to the copepods’ diets especially with decreasing quality of phytoplankton food due to nutrient limitation over the course of the bloom. Copepod grazing rates exceeded microzooplankton growth, suggesting their strong top-down control potential on microzooplankton in the field. Selective grazing by microzooplankton was an important factor for stabilising a bloom of less-preferred diatom species in our mesocosms with specific species (Thalassiosira spp., Rhizosolenia spp. and Chaetoceros spp.) dominating the bloom. This study demonstrates the importance of microzooplankton grazers for structuring and controlling phytoplankton spring blooms in temperate waters and the important role of copepods as top-down regulators of microzooplankton.

Appendix

See Tables 5, 6, 7, and 8.

Table 5 Microzooplankton grazing g (day⁻¹), phytoplankton growth rates k (day⁻¹) and instantaneous growth rate values μ_o (day⁻¹) from bottles without added nutrients, percentage of initial stock P_i (%) and potential production P_p (%) grazed as determined in four dilution experiments for each registered prey taxon

Table 6 Microzooplankton carbon-specific filtration rates F_c (mL μgC predator⁻¹ day⁻¹) and carbon-specific ingestion rates I_c (μgC prey μgC predator⁻¹ day⁻¹), total ingestion rates of the microzooplankton community I_total (μgC prey L⁻¹ day⁻¹) and electivity E* (−) for each registered prey taxon

Table 7 Temora longicornis grazing g (day⁻¹), phytoplankton and microzooplankton growth rates k (day⁻¹) and phytoplankton instantaneous growth rate values μ_o (day⁻¹) from bottles without added nutrients, percentage of initial stock P_i (%) and potential production P_p (%) grazed as determined in four grazing experiments for each registered prey taxon

Table 8 Temoralongicornis individual filtration F_i (mL Ind.⁻¹ day⁻¹) and ingestion rates I_i (ngC Ind.⁻¹ day⁻¹) as well as carbon-specific filtration rates F_c (mL μgC predator⁻¹ day⁻¹) and carbon-specific ingestion rates I_c (μgC prey μgC predator⁻¹ day⁻¹), total ingestion rates I_total (μgC prey L⁻¹ day⁻¹) and electivity E* (−) for each registered prey taxon