Deployment of the Video Plankton Recorder. Frontphoto: Salps form colonies of barrel-like organisms that filter the water (individual length 20 mm; Loppahavet , October). E. Larvaceans build mucus houses, that are really food-collecting sieves, and sit inside beating their “tail” to create a water current (Æ = ca 11 mm; Kongsfjorden, January). Click on photo for full frame. Photos: F. Norrbin

 

By Fredrika Norrbin, UiT – Arctic University of Norway

 

Read the rich illustrated article in Fram Forum 2014

 

Zooplankton are tiny creatures that populate the seas, much as insects swarm in the air. They feed on even smaller organisms: phytoplankton, which convert sunlight and nutrients to energy, and microplankton that eat particles. Zooplankton are adapted to the seasonal cycle, and are sensitive to temperature. Because they have little motility of their own, they depend on water currents to move from place to place. 

What makes zooplankton so important to study is that they provide the major pathway of energy to fish and higher organisms. Environmental changes may alter the composition of plankton so that the ecosystem becomes unfavourable to species that humans rely on. Clearly we need detailed knowledge about plankton, for instance if we want to know whether there is enough of the right kind of plankton at the time of year when fish larvae need abundant food.

 

The video plankton recorder

We use an instrument called the Video Plankton Recorder (VPR) to study the abundance and distribution of common types of zooplankton. The VPR consists of a digital video camera, a strobe light, and sensors for salinity, temperature, chlorophyll fluorescence and turbidity. This device is deployed from a winch and either raised and lowered repeatedly at a single station or towed behind a ship moving through the water.

 

The VPR at the surface, showing the illumination from the Xenon ring strobe. The focus of the 1.4 megapixel B/W camera is midway between the two arms, providing an almost undisturbed sampling area. Twenty images (22 × 32 mm – sampling volume 26 ml) are taken every second. Photos: F. Norrbin.

 

What is so fascinating about this instrument is that it provides images of undisturbed plankton and particles in the water column, with exact information on the depth for each observation. Some of these animals are too fragile to be caught with plankton nets, so marine biologists rarely see them in a natural state.

We use the VPR to investigate zooplankton in North Norwegian fjord areas, partly within the Fram Centre Flagship program Effects of climate change on sea and coastal ecology in the north. The VPR work package is concerned with zooplankton composition and vertical structure. Many models assume that plankton are homogeneously distributed in the water column, but high-resolution studies have shown a large degree of aggregation, and especially layering of the plankton. Although zooplankton are weak swimmers, they can easily adjust their vertical position in the water column. Thus they can choose optimal environmental conditions, and approach or avoid other plankton. Even in rather turbulent coastal waters, plankton form distinct horizontal layers a few metres to tens of metres thick.

 

Early findings—insight from images

 

From VPR stations taken at various times of the year, a pattern emerges of outer fjords and coastal areas dominated by the copepod Calanus finmarchicus, while inner fjord arms are populated by smaller species of copepods and hydromedusas. Although these facts could also have been determined using plankton nets, the VPR provides additional information about depth segregation of species (termed “habitat selection”) and the position of invertebrate predators in relation to their prey.

We also observed that the tiny copepod Microsetella was almost always associated with detritus in the water column. Detritus, or “marine snow”, is formed from decaying plankton, and eventually sinks to the bottom, providing food for benthic organisms. Small bacteria and plankton colonize and start to break down the detritus even as it falls through the water.

The VPR lets us visualize many aspects of the lives of plankton, such as environmental preferences and behaviour. The individual observations build up to thousands of data points to be used in statistical analyses of the data. This is valuable information for modellers, and can contribute greatly to predictions related to environmental change.

The author on deck with the VPR. Photo: F. Norrbin.

For further reading

Gallager SM, Yamazaki H, Davis CS. (2004) Contribution of fine-scale vertical structure and swimming behavior to formation of plankton layers on Georges Bank. Mar Ecol Prog Ser 267:27-43

Jacobsen HP, Norrbin MF. (2009) A fine-scale layer of hydromedusae is revealed by the Video Plankton Recorder (VPR) in a semi-enclosed bay in northern Norway. Mar Ecol Progr Ser 380:129-135

Norrbin F, Eilertsen HC, Degerlund M. (2009) Vertical distribution of primary producers and zooplankton grazers during different phases of the Barents Sea spring bloom. Deep-Sea Research II 56:1945-1958

Primicerio R. (2003) Habitat choice and community dynamics of zooplankton in the subarctic lakes Takvatn and Lombola (northern Norway). Dissertation, University of Tromsø, Norway