Vertical distribution and seasonal abundance of Aglantha digitale (O.F. Müller) (Coelenterata: Trachymedusae) and other planktonic coelenterates in the northeast Atlantic Ocean

Abstract

The vertical distribution and seasonal variability in abundance of the hydromedusa Aglantha digitale (O.F. Müller) and other coelenterates from the north-east Atlantic Ocean are described. The coelenterates were abundant from May to September and reached peak numbers in June, July and September when concentration of 40 individuals m ⁻¹ were recorded. A. digitale numerically dominated the codenterate population and accounted for over 80% of the dry weight biomass in hauls taken from 1971 to 1974. The data suggests a 2½ month development period for this medusa from egg to mature adult (8 mm) which supports previous published data. During the growth of the medusa there were considerable changes in the ratios of wet to dry weights. At a bell-height of 1 mm the mean dry weight was 22% of mean wet weight and at 8 mm the mean dry weight was 5% of mean wet weight; the percentage decreased further in the largest specimens to 2% at 25 mm. The majority of A. digitale occurred in the upper 100 m but in late July the larger specimens extended their distribution to the limit of sampling (500 m). The two most abundant siphonophores , Dimophyes arctica (Chun) and Nanomia cara A. Agassiz, were found deeper than the main population of A. digitale . It is suggested that coelenterates could be an important source of nitrogen for the development of the late bloom of phytoplankton in the north-east Atlantic Ocean.