Distribution of macrozooplankton at time series station MIGZOO-4

A large population of the colonial pelagic tunicate Pyrosoma atlanticum occurred in April 1991 in offshore waters of the Ligurian Sea (Northwestern Mediterranean). The high numbers of colonies caught allowed their vertical distribution and diel migration in the 0-965 m water column to be described as a function of their size. Daytime depths and amplitudes of the migration were correlated with colony size. The amplitude of the migration ranged from 90 m for 3-mm-length colonies to 760 m for 51-mm-length colonies, with a mean amplitude of 410 m for the whole population, all sizes pooled. The results of horizontal hauls at a given depth around sunrise and sunset showed a marked diurnal symmetry of the migratory cycle relative to noon, and that migration of the population was not cohesive. For example, the larger the colonies, the later after sunset they reached the upper layers during their upward migration.

1991年4月，地中海西北部的利古里亚海近海水域出现了大规模群栖浮游被囊动物大西洋海樽（Pyrosoma atlanticum）种群。本次研究捕获了大量该物种的群体，得以基于群体体长，系统描述其在0~965米水层中的垂直分布与昼夜迁移行为。研究结果显示，日间栖息深度与迁移幅度均与群体大小呈显著相关：迁移幅度范围为90米（对应体长3毫米的群体）至760米（对应体长51毫米的群体），合并统计所有体长个体的种群平均迁移幅度为410米。在日出、日落前后于特定水层开展的水平拖网结果表明，该种群的迁移周期相对于正午呈现显著的昼夜对称性，且种群整体的迁移并非同步集群式的。例如，群体体型越大，其向上迁移过程中于日落之后抵达上层水域的时间越晚。