THE INFLUENCE OF COPEPOD AND KRILL GRAZING ON THE SPECIES COMPOSITION OF PHYTOPLANKTON COMMUNITIES FROM THE SCOTIA-WEDDELL-SEA - AN EXPERIMENTAL APPROACH

• Published in: Polar Biology
• Main Authors: Fransz, G, Cuzinroudy, J, Alder, Va, Graneli, E, Graneli, W, Rabbani, Mm, Daugbjerg, N
• Other Authors: Station Zoologique de Villefranche, Université Pierre et Marie Curie - Paris 6 (UPMC)-Centre National de la Recherche Scientifique (CNRS), Department of Biology
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 1993
• Subjects: [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Antarctic; The Antarctic; Weddell Sea; Weddell; Antarctic Ocean; Antarc*; Euphausia superba; Polar Biology; Copepods
• Online Access: https://hal.science/hal-03476497; https://doi.org/10.1007/BF00238930

The influence of copepods (mainly Oithona similis) and krill (Euphausia superba) grazing on the species composition of plankton communities in ship-board containers was investigated during the spring and post-spring period in the Scotia-Weddell Sea in the Antarctic ocean. Numbers of grazers were experimentally manipulated in containers with natural phytoplankton assemblages. With natural levels of copepods but no krill a high (700 950 mug C . l-1, ca 30 mug chl a . l-1) phytoplankton biomass developed. In these cultures large diatoms, e.g. Corethron criophilum and chains of Thalassiosira sp., made up 80% of total phytoplankton cell carbon at the end of the experiment. In cultures with elevated numbers of copepods (5X or 10X the natural level) phytoplankton biomass was somewhat reduced (ca 23 mug chl a . l-1) compared to cultures with natural copepod abundance, but still high. Phytoplankton species composition was on the other hand greatly influenced. Instead of large diatoms these cultures were dominated by Phaeocystis pouchetii (70%) together with small Nitszchia sp. and Chaetoceros neogracile (20%). In containers with krill (both juveniles and adults), but without elevated numbers of copepods, phytoplankton biomass rapidly approached zero. With 10X the in situ level of copepods, krill first preyed on these before Corethron criophilum and Thalassiosira sp. were grazed. When krill were removed a plankton community dominated by flagellates (60-90%), e.g. Pyramimonas sp. and a Cryptophycean species, grazed by an unidentified droplet-shaped heterothrophic flagellate, developed. These flagellates were the same as those which dominated the plankton community in the Weddell Sea after the `spring bloom'. A similar succession was observed in situ when a krill swarm grazed down a phytoplankton `bloom' in a few hours. Our experiments show that copepods cannot control phytoplankton biomass in shipboard cultures even at artificially elevated numbers. Krill at concentrations similar to those in natural swarms have a great ...