Biophysical and ionic controls of buoyancy in diapausing calanoid copepods

Non-technical summary

Calanoid copepods are key players in World's oceans. They are the largest constituent of oceanic zooplankton biomass and are a major link within global carbon cycles. In the North Atlantic and Arctic, calanoid copepods are a vital food for commercially important fish species such as cod, mackerel and herring. A key feature of many calanoid copepod life-cycles is a phase of overwintering at great depth, in a state analogous to hibernation. This increases their chances of surviving to the next season through avoiding predation at times when there is little else to be gained by remaining within the surface layers.

A notable feature of calanoid copepods is that they contain exceptionally high amounts of fat (or lipid). The large lipid store is both a valuable energy reserve and a major determinant of buoyancy. The attainment of neutral buoyancy is important to copepods over winter since they must minimise swimming effort in order to save energy. A balance must be sought between provisioning for the winter without disturbing the ability of the copepod to achieve neutral buoyancy. The best scientific efforts at trying to simulate this balance have so far proved to be unsatisfactory.

Recently, two potential additional mechanisms of buoyancy control have been identified. In one study, Sartoris and colleagues found that diapausing copepods contained a different balance of ions in their bodily fluids (haemolymph) compared to active, surface dwelling copepods. In a second study, scientists involved in the present proposal showed that lipids rich in omega-3 polyunsaturated fatty acids (PUFAs) changed from liquid to solid state when under pressures typical of the deep sea. The effect only happened when PUFAs comprised more than 50% of the lipid store which, coincidentally, was commonly found in deep diapausing copepods, but not in those still active at the surface.

At present, both of the mechanisms have only been identified in Southern Ocean copepods, although previously 'misinterpreted' evidence in the scientific literature also suggests that northern hemisphere species employ similar techniques. We will carry out surveys across a number of locations in the North Atlantic, Arctic and adjacent sea-lochs to determine lipid composition and haemolymph-ion concentrations in three calanoid copepod species. The surveys take into account environmental influences, particularly the type and availability of the microplanktonic food of copepods. This will determine whether there is any active regulation of the levels of omega-3 fatty acids in the lipid stores. Such active regulation may be of particular importance towards the end of winter as a means of controlling the timing and rate of ascent back into the surface layers. Our sampling strategy, application of novel analytical techniques and datasets generated during the research will allow these questions to be addressed.

Secondly, using statistical techniques we will reconsider efforts made so far to simulate overwintering depth and seek improvements through including additional data and mechanisms. For instance, in changing from a liquid to solid state, the volume occupied by a lipid will be decreased and its response to increasing pressure will change. The effects of ionic balance will also be considered, mainly in how it may assist copepods maintain their theoretical neutral buoyancy depth in the face of any physical disturbance.

This research proposal is based on our recent discovery, that the biophysical properties of lipids are a major factor controlling the distribution of life in the oceans. This finding gives an exciting new perspective on the role of lipids in marine organisms, opening up a fundamentally new direction for research, with profound implications for our understanding of the entire ocean food web.

Impact summary

Biophysical and ionic controls of buoyancy in diapausing calanoid copepods

1. Who will benefit from the research and how?

Scientists and policy makers need to develop rigorous predictive capabilities of zooplankton populations in the North Atlantic and Arctic sea to be able to advise on fisheries policy and sustainable management of these key natural resources.

2. What techniques, methods activities will be used to promote the research?

Workshop at BAS Cambridge with the project participants and UK oceanographic modellers/fishery scientists.

Engaging with industry, i.e. Setaram have application notes on their website relating to previous work completed by the research grant proposers. A LinkedIn users group for UK, Ireland and Scandinavia scientists has been set up by Seteram to promote the application of thermal analyses by environmental scientists. The group has been set up to allow users to network, discuss applications and keep up to date with system developments etc.

The media. The finding of, Pond and Tarling (2011) Limnol. Oceanogr. 56: 1310-1318, have already attracted extensive media coverage. e.g.

http://news.sciencemag.org/sciencenow/2011/06/tiny-marine-crustaceans-construc.html?rss=1
http://planetearth.nerc.ac.uk/news/story.aspx?id=997

3. Expected milestones and measures of success

A workshop will be held at BAS, Cambridge during October 2013 with the aim of disseminating the findings of the research to a wider group of users and in particular scientists engaged in modelling zooplankton population dynamics in the North Atlantic and Arctic.

We will publish the results of the findings in high impact factor journals and will issue joint BAS-SAMS press releases.