Copepod and microzooplankton grazing in mesocosms fertilised with different Si:N ratios: no overlap between food spectra and Si:N influence on zooplankton trophic level

Abstract

We hypothesized that the trophic level of marine copepods should depend on the composition of the protist community. To test this hypothesis, we manipulated the phytoplankton composition in mesocosms and measured grazing rates of copepods and mesozooplankton in those mesocosms. Twelve mesocosms with Northeast Atlantic phytoplankton were fertilised with different Si:N ratios from 0:1 to 1:1. After 1 week, ten of the mesocosms were filled with natural densities of mesozooplankton, mainly calanoid copepods, while two remained as mesozooplankton-free controls. Both before and after the addition of copepods there was a positive correlation of diatom dominance with Si:N ratios. During the second phase of the experiment, copepod and microzooplankton grazing rates on different phytoplankton species were assessed by a modification of the Landry-Hassett dilution technique, where the bottles containing the different dilution treatments were replaced by dialysis bags incubated in situ. The results indicated no overlap in the food spectrum of microzooplankton (mainly ciliates) and copepods. Ciliates fed on nanoplankton, while copepods fed on large or chain-forming diatoms, naked dinoflagellates, and ciliates. The calculated trophic level of copepods showed a significantly negative but weak correlation with Si:N ratios. The strength of this response was strongly dependent on the trophic levels assumed for ciliates and mixotrophic dinoflagellates.

An indoor mesocosm system to study the effect of climate change on the late winter and spring succession of Baltic Sea phyto- and zooplankton

“…It has been shown in summer experiments that copepod grazing primarily reduces large phytoplankton, while small phytoplankton is at times even favoured, because they are released from ciliate grazing pressure (Feuchtmayr 2004;Grané li and Turner 2002;Sommer et al 2003aSommer et al , b, 2005a. Contrary to summer conditions, when ciliates preferentially feed on small phytoplankton (Sommer et al 2005b), the ciliates in our experiment had also fed on large algae (Aberle et al 2006), which is quite typical for the start of the seasonal growth cycle (Montagnes et al 1988). Thus, ciliates could maintain the grazing pressure on large phytoplankton even after copepods had declined to population levels ineffective for top-down control.…”

Spring bloom succession, grazing impact and herbivore selectivity of ciliate communities in response to winter warming

“…The present study was geared to determining morphospecies and changes in ciliate assemblages, and thus such complexity was not required. This is an accepted way to proceed in the literature (see: Setaelae and Kivi 2003;Gaedke and Wickham 2004;Johansson et al 2004;Sommer et al 2005;Berninger and Wickham 2005).…”

“…A modified version of Landry and Hassett's (1982) dilution technique was used by replacing glass bottles by dialysis tubes of 300-ml volume (Medicell International Ltd., London) and a molecular weight cut-off of 12-14,000 Da (Wiltshire and Lampert 1999;Sommer et al 2005). The dialysis membrane enabled the free exchange of nutrients between the interior of the bags and the surrounding water.…”

“…Traditionally, ciliates were considered as major grazers of microalgae, preying mainly upon nanoplankton size fractions (Jonsson 1986;Kivi and Setaelae 1995;Grané li and Turner 2002;Sommer et al 2005;Berninger and Wickham 2005), whereas large phytoplankton size fractions like, e.g., diatoms are usually considered as the preferred food item of copepods (Kivi et al 1996;Grané li and Turner 2002;Sommer et al 2005). In addition, recent studies also point to the ciliates' importance as bacterivores and to their role as a trophic link between the microbial components in the food web and higher trophic levels (Posch et al 1999;Simek et al 2000).…”

“…The re-structuring of planktonic systems in response to global warming is one of the major topics in aquatic ecology at present, and there is much debate on the degree of climatic sensitivity of different trophic components (Walther et al 2002;Hays et al 2005;Winder and Schindler 2004). In this context, the microzooplankton is an important player as it is considered as the trophic link between the microbial components and higher trophic levels (Johansson et al 2004;Sommer et al 2005). Microzooplankton communities are generally regarded as being major consumers of planktonic primary producers, consuming as much as 60-70% depending on the species composition and the nutrient status of the environment (Landry and Calbet 2004).…”