We report the effects of small-scale turbulence on the feeding rates of the marine copepod Oithona davisae. Laboratory experiments were conducted under a range of turbulence dissipation rates between 10 Ϫ4 and 10 1 cm 2 s Ϫ3 . Net enhancements of feeding were observed only at the lowest, whereas negative net effects appeared only at the highest, turbulence intensities. These results contrast with expectations from an encounter-based model for this copepod species that predicted positive feeding enhancements at all turbulence intensities. This disagreement suggests the presence of detrimental effects at moderate and high turbulence intensities, very likely driven by either a lower mechanosensor perception capability or lower capture success. In comparison to other ambush copepods, O. davisae appears much more sensitive to the presence of turbulence, which might be the result of its strict ambush behavior, whereas copepods like Acartia tonsa or Centropages typicus, which can switch into different feeding modes, appear to benefit more from turbulence. The response of O. davisae feeding to turbulence in our experiments agrees with recent field observations on changes in the vertical distribution of Oithona as a function of wind-driven turbulence events. Hence, O. davisae seems to choose those depths where small-scale turbulence favors feeding.

We determined the egg production efficiency (EPE: egg production/ingestion) and egg viability of the copepod Acartia tonsa Dana under different heterotrophic and autotrophic diets. EPE was estimated in adult females either as the slope of the linear relationship between specific egg production (EPR) and ingestion rates, or as the quotient: EPR/ingestion rate. The diets, offered in monoculture, were the heterotrophic ciliates Strombidium sulcatum or Mesodinium pulex, the heterotrophic dinoflagellate Gymnodinium dominans, the autotrophic cryptophyte Rhodomonas salina and the autotrophic dinoflagellate Gymnodinium sanguineum. The diets were also analyzed for fatty acid contents and composition, relationships with EPE and reproductive success were determined. Clear differences were found in the fatty acid contents and the composition of the different diets offered, but these differences did not correspond with variability in EPE. However, egg viability was correlated with ingestion of certain prey essential fatty acids; interestingly, our data do not show that ciliates and heterotrophic dinoflagellates are nutritionally superior prey for marine copepods, contrary to general expectations.

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...

The transfer of matter and energy in planktonic food webs is determined by the encounter rate between predators and prey and subsequent attack and capture success. We tested if differences in swimming behavior and escape ability of 2 planktonic ciliates could explain differences in their susceptibility to predation from the calanoid copepod Acartia clausi. We chose 2 similarly sized ciliates, which exhibited very different swimming behavior: Strobilidium spiralis (Strobilidiina) and Metacylis sp. (Tintinnina). Behavioral analyses were combined with feeding incubations. The behavior of S. spiralis was characterized by slow upward and downward swimming interrupted by sudden jumps. Metacylis sp. showed a swimming pattern typical for many tintinnids, with steep helical trajectories and without jumps. Feeding experiments while A. clausi was incubated in a 1:1 mixture of both ciliate species showed a higher predation rate on Metacylis sp. According to behavioral observations, the copepod exhibited a 2-fold difference in ingestion rate between the ciliate species. Furthermore, film analyses revealed that S. spiralis escaped the attacking predator with rapid jumps while Metacylis sp. lacked any effective behavioral response to the approaching predator. We discuss the relative importance of velocities and dispersal rates in encounter processes and conclude that the escape response shown by S. spiralis significantly reduces the risk of predation. This study emphasizes the importance of mechanistic knowledge at the level of behavior and individual sensory abilities to improve present models of biogeochemical fluxes in the water column.KEY WORDS: Predator-prey interaction · Swimming behavior · Ciliate · Copepod · Acartia clausi Resale or republication not permitted without written consent of the publisher

To minimize the dispersal of aquatic nuisance species through shipping, ballast water can be treated to kill, remove, or inactivate organisms. Ultraviolet (UV) radiation is used in some ballast water management systems to address this goal. Because UV treatment renders cells nonviable (by sterilizing them, preventing reproduction) and does not necessarily kill organisms instantaneously, the efficacy of UV treatment has been verified historically by regrowth assays in which microorganisms are cultured (and thus, reproduce) under optimal growth conditions. Although regrowth assays are definitive, they are time consuming-lasting for days or weeks-and, importantly, are applicable only to organisms amenable to culturing. Furthermore, these estimates of cell concentrations are often accompanied by large error estimates. In this paper, several rapid alternatives to regrowth assays are described and evaluated. An ideal approach would shorten or simplify the analysis burden and, potentially, could be used for shipboard testing to determine compliance with national and international ballast water standards. Complicating this task is the requirement that compliance with the ballast water standards will be determined by quantifying the number of living organisms in ships' ballast water, and while organisms may be living following UV treatment, they may not be viable (i.e., they may not reproduce). To address this dichotomy , alternatives to regrowth assays were categorized based upon the complexity of the analysis and the means used to determine the status of microorganisms (either as viable or living): 1. Instantaneous growth and cell replication, 2. Cell activity and metabolic rates, 3. Cell structural integrity, and 4. Biomolecule presence and status. With the suite of approaches currently available, it is not possible to determine the viability of organisms rapidly, that is, within minutes of collecting a ballast water sample. Measurements of the photosystem integrity via variable fluorescence and the presence of adenosine triphosphate (ATP) are currently the most promising for rapidly estimating concentrations of living cells in compliance testing of ballast water discharges; however, extensive validation is required to verify the applicability of these approaches for the complexity of real-world samples.

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