Limnetica 34

The grazing characteristics of the natural zooplankton community in the shallow hypertrophic lagoon Albufera de Valencia (Spain) were compared in two periods of the annual cycle: (1) at the onset of spring, after a period of increased water flow through the lagoon that reduces phytoplankton density and cyanobacterial prevalence, and (2) in mid-spring, after the spring phytoplankton bloom and under conditions of cyanobacterial dominance. Clearance, ingestion and assimilation rates were measured by labelling natural seston with ¹⁴C-bicarbonate and ³H-thymidine/leucine. The clearance rates (CRs) of Daphnia magna, the dominant species at the onset of spring, were low, very likely as a consequence of the high abundance of filamentous cyanobacteria in the lagoon. The CRs were nearly equal for phytoplankton and bacteria, corroborating the unselective feeding of Daphnia. Bacteria supplied ≈ 40 % of the planktonic carbon ingested by D. magna, and this carbon was assimilated with the same efficiency as that provided by phytoplankton. This suggests a strong coupling between the microbial loop and the classic grazer trophic web at the onset of spring in the lagoon and, consequently, an important contribution of the microbial loop to energy transfer to higher trophic levels (fish). The CRs of Bosmina longirostris, the dominant species in mid-spring, were also low probably because the food concentrations were well above the incipient limiting level. The CRs were higher for 1 – 15 µm algae than for < 1 µm bacteria, indicating positive selective feeding towards larger cells. Algae sized 3 – 15 µm provided most of the planktonic carbon ingested by B. longirostris. Similarly, regarding the total mid-spring zooplankton community, phytoplankton contributed 90 % of the planktonic carbon ingested. This suggests that during the spring phytoplankton bloom when cyanobacteria dominate, the energy transferred to zooplankton via bacteria constitutes a smaller fraction than during periods of lower phytoplankton density. Finally, our results indicate that zooplankton consumed a very small proportion of phytoplankton and bacterioplankton production during the two studied periods, which highlights the reduced ability of zooplankton to control phytoplankton growth in hypertrophic systems.