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Isotopes of Zooplankton to measure climate and human impacts on pelagic food webs

Final Report Summary - ISOZOO (Isotopes of Zooplankton to measure climate and human impacts on pelagic food webs)

In all pelagic marine food-webs, zooplankton are the critical link between the phytoplankton primary producers and higher trophic levels, including fish. On-going changes observed for zooplankton communities in the Mediterranean and other oceans around the world therefore
have serious implications for food-web productivity. However, gaps in our knowledge of how the zooplankton component of the food-web functions, limits our understanding of their response to change and their continued provision of ecosystem services (e.g. food security).

The ISOZOO program was designed to make a significant contribution to our understanding of zooplankton food-web processes, and how climate change and anthropogenic impacts affect these. Our particular aim was to understand how zooplankton communities, and particularly the food chain length which determines energy transfer efficiencies, will be impacted by changes in nutrient supply and phytoplankton composition in our future oceans.

ISOZOO adopted an approach of space for time substitution to tackle this problem. By sampling and characterising food web processes in the present day ocean, across a broad spectrum from nutrient rich to nutrient poor environments, we aimed to develop an understanding of how food webs in any one region will respond to change over time. Our method to characterise food web linkages and transfer efficiencies comprised reducing food-webs to size spectra component size classes and using stable isotope tracers to identify their feeding relationships. This approach effectively reduced the species complexity of ocean food webs to more manageable component size classes. It is well established that size is one of the primary determinant of trophic interactions, i.e. big eats small. Additional data requirements of ocean biochemistry and physical properties necessitated a multidisciplinary approach.
Facilitated by IIF mobility, ISOZOO established a multinational network of collaborators from the Mediterranean Institute of Oceanography (France), Institute for Research and Development (France), Secratariat for the Pacific Community (New Caledonia), National Institute for Water and Atmospheric Research (New Zealand), Alfred Wagner Institute (Germany) and the University of British Columbia (Canada). This network enabled us to the successfully collect data from the Mediterranean, South Pacific (New Caledonia, Marquesas Islands, Chatham Rise), North Pacific and South Atlantic oceans, spanning low nutrient to high nutrient realms.

Systematic analysis of the vast data set generated over the two year program remains ongoing. The initial analyses focused on the tropical South Pacific providing the first detailed stable isotope based characterisation of whole food web processes in the New Caledonia region. The hypothesis of a long food chain in the nutrient poor New Caledonia region was upheld by the study. Comparison with published data for the nutrient rich California Upwelling System found there to be at least one extra step in the food chain in the low nutrient region. This corresponds to a significant loss of energy to higher trophic levels, with the implication of lower productivity under the future reduced nutrient conditions predicted for a number of ocean regions. This initial analysis provided proof of the ISOZOO concept, and at the same time provided new insights into stable isotope methodologies which have been broadcast through peer reviewed publications.

The coupled size spectrum-stable isotope approach has provided further insights into food web processes across a range of spatial and temporal scales. Over Chatham Rise, New Zealand, a north to south decrease in food chain length was observed, associated with high, diatom dominated primary production in the south. In the Gulf of Lion, Mediterranean Sea, size structured dynamics showed a seasonal shift from bottom up to top down trophic control over the period of the spring bloom. The utility of our method for food web studies in pelagic ecosystems has expanded both through presentations and a recently published review paper.

Ongoing analysis of the data generated during the 2 year IIF stage of ISOZOO will continue to disseminate the data collected for each ocean region sampled. The longevity of this ambitious project is ensured by the long term partnerships established with a number of international organisations, and through formalization of the ISOZOO approach across a number of international research programs. Ultimately we aim to incorporate the improved knowledge of zooplankton food web processes into ecosystem models, enhancing our predictive capability for ocean food web management.