Trophic impact and prey selection by crustacean zooplankton on the microbial communities of an oligotrophic coastal area (NW Mediterranean Sea)

Abstract

Selection for ciliates in the presence of phytoplankton has been previously shown for some species of copepods. However, the factors determining preference for this heterotrophic prey and how crustacean zooplankton predation can affect the ciliate community are not yet fully understood. In this study, we investigated predation rates on phytoplankton and ciliates by the most abundant copepod and cladoceran species in a coastal area of the oligotrophic NW Mediterranean Sea monthly over an annual cycle. Three major results were apparent. Firstly, ciliates were important contributors to zooplankton diet, representing a median of 37 and 17% of the carbon intake, and 51 and 34% of the nitrogen intake for copepods and cladocerans, respectively. Secondly, ciliates were positively selected in most cases, this selection was species specific and apparently independent of phytoplankton concentration. And finally, in spite of the high clearance rates on ciliates, the impact of the crustacean community on the ciliate standing stock was low (median 2%), suggesting a bottom-up control of the ciliate community. KEY WORDS: Copepods · Cladocerans · Ciliates · Food web · NW MediterraneanResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 35: 65-78, 2004 al. 2002a) is commonly used to argue that ciliates may be important to zooplankton diet. Yet, relatively few studies have paid attention to the actual nutritional contribution of ciliates as carbon and nitrogen sources for zooplankton metabolism, and compared them to the one supplied by phytoplankton ingestion (e.g. Froneman et al. 1996, Batten et al. 2001. Furthermore, it is scarcely known how ciliate contribution to crustacean zooplankton diet may vary by season, in which different zooplankton species and microbial communities occur.A second aspect of interest in the trophic interactions between ciliates and zooplankton deals with feeding selection mechanisms. Copepods frequently exhibit higher clearance rates on ciliates than on phytoplankton (e.g. Gifford & Dagg 1991, Fessenden & Cowles 1994, Levinsen et al. 2000, a fact that has been attributed to positive selection patterns. What exactly determines this positive selection is not clear, and factors like food quality and encounter rates have been proposed (Jonsson & Tiselius 1990, Stoecker & Capuzzo 1990. However, the preference for a given food item could also be dependent on the availability of an alternative suitable prey, and switching responses may appear as well (Kiørboe et al. 1996, Gismervik & Andersen 1997. Unfortunately, few field studies have considered these aspects.Finally, little is known concerning the strength of the trophic control that zooplankton may exert on ciliate communities. Because of their nexus position, as both prey for zooplankton and top predators for other microbes, ciliates represent a crucial link within marine planktonic food webs. Empirical evidence up to now, however, indicates that copepods, the major component of mesozooplankton, in g...

Trophic role and carbon budget of metazoan microplankton in northwest Mediterranean coastal waters

“…Some studies suggest that the effects of micrometazoans grazing on phytoplankton may be mediated mostly by their control of microzooplankton populations (Fessenden and Cowles 1994). However, our results indicate that their grazing pressure on protozoans appeared to be modest in agreement with other previous studies for mesozooplankton (Broglio et al 2004). The low effect of micrometazoans as compared to protozoans may be due to both their lower biomass in the field (one or two orders of magnitude in this study) and the lower specific ingestion rates (Jacobson and Anderson 1993).…”

“…For instance, bottle effects and crowding (Peters and Downing 1984) and the lack of turbulence during the incubations may result in lower feeding rates than under natural conditions (Saiz et al 2003). Nevertheless, predator biomass used in the feeding experiments was in the range commonly used in incubation experiments with larger zooplankton (Nejstgaard et al 2001;Broglio et al 2004) Another source of error could be inclusion of all nauplii stages as predators for the calculation of feeding rates, because some calanoid nauplii start feeding at naupliar stage II or III (Landry 1975).…”

“…Expts, experiments. metazoans (Stoecker and Capuzzo 1990; but see Broglio et al 2004). Hence, although the biomass contribution of protozoans to the diet may be very variable (Saiz and Calbet in press), protozoans may be qualitatively important in the metazoan microzooplankton diet because they are able to fill the nutritional deficit created by feeding solely on phytoplankton (Kleppel 1993).…”

“…Some studies had reported that phytoplankton can be controlled by the metazooplankton grazing (Roman and Gauzens 1997), whereas others indicate metazoans exert little grazing pressure on phytoplankton (, 5%; Broglio et al 2004;Atienza et al 2006b). However, most of these studies excluded the micrometazoans and consequently underestimated the trophic effect by metazooplankton.…”

Seasonal variability of the dynamics of dimethylated sulfur compounds in a coastal northwest Mediterranean site

“…Thus, there is no indication that microzooplankton impact on DMSP producers should be higher in summer. Besides, mesozooplankton never accounted for more than 6% of the grazing effect of the whole community in a seasonal study conducted 45 km south of our sampling site (Broglio et al 2004). It seems thus plausible that a large proportion of the summer increase in DMS production comes from the DMSP producers themselves.…”