Fine-scale layer of hydromedusae is revealed by video plankton recorder (VPR) in a semi-enclosed bay in northern Norway

Abstract

High resolution images of zooplankton organisms were recorded in situ using a digital autonomous video plankton recorder (VPR) in a semi-enclosed bay in northern Norway. Data from vertical VPR profiles was used to estimate the depth distribution and abundance of zooplankton along a bay-wide grid. Images were recorded simultaneously with hydrographical data, and the spatial distribution of different organisms was related to several water column characteristics, such as chlorophyll a, density, and buoyancy frequencies. In June, unusually high abundances of hydromedusae (Rathkea octopunctata and Obelia sp.) were located in a fine-scale layer of a few metres thickness. This layer could be associated with relatively high buoyancy frequencies, reflecting a density gradient present throughout the entire sampling area. Aggregation of hydromedusae around density discontinuities may have enabled retention of medusae in the bay. Ctenophores and appendicularians were distributed above the medusae layer, while peaks of copepod abundance were found outside the layer boundaries. We suggest that the bloom of R. octopunctata was initially caused by liberation of new medusae from successful benthic hydroid colonies, and that proliferation of this species was sustained by asexual, manubrial budding in the medusae stage. KEY WORDS: Zooplankton · Hydromedusae · Video plankton recorder · Vertical distributionResale or republication not permitted without written consent of the publisher

Fine-scale vertical distributions of Mnemiopsis leidyi ctenophores: predation on copepods relative to stratification and hypoxia

“…Graham et al 2001, Jacobsen & Norrbin 2009), the possible importance of density discontinuities and stratification per se must be considered. Both Arai (1992) and Frost et al (2010) concluded that hydromedusae behaviorally aggregate at salinity discontinuities.…”

“…Ctenophores, medusae, and copepods are known to accumulate at density discontinuities in the water column (Arai 1992, Graham et al 2001, Malej et al 2007, Jacobsen & Norrbin 2009, Frost et al 2010. Arai (1992) concluded that Sarsia tubulosa hydromedusae behaviorally aggregate at salinity discontinuities.…”

Marine snow, zooplankton and thin layers: indications of a trophic link from small-scale sampling with the Video Plankton Recorder

“…Generally, the existence of thin vertical layers has been rarely investigated, which is due to the low sampling efficiency of traditional net sampling for marine snow and the generally low ability of this method to resolve small-scale distribution patterns (Widder et al 1999, Alldredge et al 2002, Jacobsen & Norrbin 2009). Studies using optical and acoustical instruments have only recently started to quantitatively describe thin layers of marine snow at density gradients (Alldredge et al 2002, McManus et al 2003.…”

“…Recent advances in optical sampling methods, however, have allowed observations of plankton patches from microscale (<1 m) to small scale (1 to 10 m) (e.g. Davis et al 1992, Ashjian et al 2001, Jacobsen & Norrbin 2009). …”

Optics-based surveys of large unicellular zooplankton: a case study on radiolarians and phaeodarians

“…Such methods allow us to investigate marine multicellular plankton (e.g., copepods, pteropods, larvaceans and jellyfish) at ecologically meaningful scales (Benfield et al 1996, Remsen et al 2004, Jacobsen & Norrbin 2009). Optics-based surveys are also optimal for the observation of unicellular plankton possessing fragile siliceous skeletons, which are often fragmented and broken by sampling with a plankton net, with the result that their ecology (e.g.…”