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JPR Advance Access originally published online on February 9, 2007
Journal of Plankton Research 2007 29(4):377-389; doi:10.1093/plankt/fbm023
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© The Author 2007. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

The paradox between the long-term decrease of egg mass size of the calanoid copepod Eurytemora affinis and its long-term constant abundance in a highly turbid estuary (Gironde estuary, France)

Valérie David1,2,*, Benoît Sautour1 and Pierre Chardy1

1 Université Bordeaux 1—CNRS UMR 5805, 2 rue du Professeur Jolyet, 33120 Arcachon, France 2 Virginia Commonwealth University, Department of Life Science, 1000 West Cary Street, Richmond, Virginia 23284, USA

* Corresponding author: vdavid{at}vcu.edu

Received on September 22, 2006; accepted on January 30, 2007


  Abstract

Eurytemora affinis egg mass size—a commonly used demographic parameter—was surveyed in the Gironde estuary. Its decrease between 1978 and 2003 contrasted with the stability of population density during this long-term period. Different hypotheses were tested to explain this paradox. (i) Upstream shift of the population: Even though the population of E. affinis shifted upstream due to a greater penetration of marine water, no relationship between the population shift and demographic parameters was observed: the egg mass size exhibited the same evolution upstream. (ii) Water warming: No early egg production and no change of female size that could metabolically limit the egg mass size was observed. (iii) Food limitation: The change in food quality observed did not result in any change in total copepod abundance. (iv) Faster renewal of egg masses: The paradox cannot be explained by this factor and the number of adults observed nowadays is higher than the number of eggs produced by females, whereas fecundity rates strongly increased. The paradox is probably explained by the expulsion of viable subitaneous eggs. This may represent a recent behaviour adaptation of the copepod to environmental changes.

Communicating editor: R.P. Harris


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