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Ben Thornes

Phytoplankton abundance and Manila clam condition in Poole Harbour

Tuesday 15th July 2014

In 1988 government scientists and local shellfish farmers introduced the non-native Manila clam (Ruditapes philippinarum) to Poole Harbour in the belief that this high-value species would provide good possibilities for commercial cultivation (Humphreys, 2010). Although the clam subsequently naturalised (Jensen et al. 2004) recurrent bouts of natural mortality (Humphreys et al. 2007) have reduced the viability and attractiveness of the species to local shellfish farmers. It is unclear what causes the observed non-fishing mortality but disease and food supply are two possibilities. In order to better understand the role of food supply and physical factors on clam biology, we measured a suite of biological and physical parameters in Poole Harbour using in situ monitoring whilst simultaneously assessing clam condition over an annual cycle (June 2011 to August 2012). We recorded a period of consistently low autumn and winter phytoplankton abundance between October and March. In contrast, phytoplankton abundance showed a highly variable pattern in spring and summer. This variability was driven by both prolific summer diatom blooms and a more gradual seasonal change in the abundance of the smaller phytoplankton, i.e. the picoeukaryotes. The phytoplankton abundances observed in this study represent some of the highest levels ever recorded in Poole Harbour but, and in contrast with some previous observations, are consistent with the expected seasonal cycle. The condition of the Manila clam population closest to our monitoring site was at its lowest in March, approximately coinciding with the annual minimum in phytoplankton abundance. There were significant differences in clam condition at different sites within the Harbour although it was difficult to relate these differences to food supply. We conclude that changes in the composition and abundance of phytoplankton in Poole Harbour correlates closely with clam physiological status, as indicated by condition index measurements, and that the low winter food availability likely increases clam vulnerability to disease and other external factors. Together, these environmental factors contribute to a variable clam survivorship between years.

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