reactietermijn vacature verstreken - Promovendus voortplantingsproces schaaldieren in kustwateren
NIOZ Koninklijk Nederlands Instituut voor Zeeonderzoek

PhD position (NIOZ)

For more information on how to apply follow link to the Mares website


Host institute 1: P6 – Royal Netherlands Institute for Sea Research (NIOZ)
Host institute 2: P13 - University of Aveiro

Subject description

The bivalve community of marine soft sediments is an important component of coastal food webs transmitting primary produced energy to higher trophic levels (Herman et al. 1999). Understanding the interactions between bivalves, their food and their (human) predators is almost synonymous with understanding the carrying capacity for the upper trophic levels of a coastal system.

One of the main conclusions drawn from a now four decades operative benthos monitoring program in the Dutch Wadden Sea is that bivalve population size and secondary production is mainly a matter of recruitment success (Van der Meer et al. 2001, Beukema & Dekker 2005). However, recruitment success is still poorly understood. Fecundity is a poor predictor of the number of recruits (Honkoop et al. 1998) not just because of stochastic effects of the abiotic environment (see, for example, the review by Queiroga et al. (2007) for an example on the importance of physical forcing), but also due to density dependent mortality in the first life stages, in particular the early post-settlement phase (Dekker & Beukema 2007). The main aim of our project is to unravel the bottom-up and top-down processes that are responsible for this density dependent mortality. We hypothesize that density dependent mortality during this early life phase and the simultaneous absence of density dependent mortality in the adult phase, is a combination of three mechanisms: (1) A particular type of reserve dynamics during starvation resulting in an almost linear increase of the time until death by starvation with size (adults can survive longer periods of food shortage than juveniles); (2) A limited diet overlap between juveniles and adults. The two main food sources for bivalves are phytoplankton and microphytobenthos. Especially microphytobenthos is of major importance in the first post-settlement life stages (Kang 1999). So we expect strong feedbacks between spat density and (local) microphytobenthos biomass and productivity. Such feedback mechanism is essential for density-dependent regulation. Once the surviving spat has matured, the relatively few surviving adults merely feed on phytoplankton showing no or little feedback. (3) With increasing spat density food levels go down and growth is retarded. Consequently, the period that the bivalve spat is within the vulnerable size range will increase, leading to increased predation

The importance of these mechanisms can only be appreciated by combining a model study with experiments and field observations. A tri-trophic community model, containing two or more algal resources, size-structured populations of various bivalve species and one or more of their main crustacean predators will be developed. Processes at the level of the individual will be described according to the theory of Dynamic Energy Budgets (DEB), developed by Kooijman (Kooijman 2000, Van der Veer et al. 2006, Van der Meer 2006). The modeling work will expand a recently developed physiologically-structured bivalve population model (Kooi & van der Meer 2010). The community model will be used to explore a long standing theoretical problem on competition between adults and juveniles. Surface/volume ratio considerations imply that juveniles are competitively superior to adults, i.e. that critical food levels that are just sufficient for maintenance are lower for juveniles compared to those for adults. Yet, empirical evidence from the Wadden Sea points to a different outcome: density-dependent regulation occurs during the first post-recruitment phase, but adults do not respond in terms of reduced growth or increased mortality to high influxes of juveniles. The model will examine whether the hypothesis of the absence of diet overlap between juveniles and adults and the idea of a synergistic effect of food competition among settled spat and predation could be reliable and sufficient to solve this puzzle. If not, additional mechanisms such as interference competition, which may work through microgradients in food density will be incorporated.

Laboratory- and field-experiments will provide additional information on the most relevant physiological processes at the individual level. The dynamics of the reserves in relation to maintenance requirements will be studied in the laboratory under different food conditions including severe starvation, both for juveniles and adults. In a second type of laboratory experiments newly settled laboratory-reared bivalves will be offered either benthic or pelagic food and growth rates will be measured. Experiments will be done under flow conditions in replicated flume systems to prevent sedimentation of pelagic algae. Further experiments with both benthic and pelagic algae together will be carried out, applying isotopic tracers in order to distinguish between the two food sources and estimate intake rates. In a field experiment microphytobenthos will be manipulated by shading (Huxham et al. 2006) and nutrient additions. The effect magnitude of these treatments will first be quantified in a pilot experiment. After successful pilots the actual experiments will be done during the settlement and post settlement period, between May and August. The expectation is that shading will reduce biomass and productivity of microphytobenthos resulting in slower growth and reduced numbers of spat due to increased predation. The results of our recent (and partly still unpublished) work on bivalve spat size dependent functional response relationships for the predatory brown shrimp Crangon crangon will become available for the project (see also Andresen & van der Meer 2010).

Finally, model results will be compared to available long-term data on the abundance and size- and age structure of bivalve and crustacean populations of the Wadden Sea.

Further information on this project can be obtained from Prof. Dr. Jaap van der Meer, e-mail: or by phone; +31-222-369357
vacature geplaatst op 13 Dec 2010 door NIOZ Koninklijk Nederlands Instituut voor Zeeonderzoek



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