Growth responses of planktonic ciliates in the genera Strobilidium and Strombidium

Abstract

Intrinsic growth and mortality rates need to be estimated to assess the potential for oligotrich ciliate populations to exploit prey populations, and since species specific differences exist, data for a variety of ciliates grazing on a variety of prey are also needed. In this paper, data are presented on the growth and mortality rates of 4 planktonic oligotrichs (2 Strobilidium species and 2 Strombidium species), at a number of prey concentrations. Growth and mortality data for these and other species in these 2 genera are reviewed. In general, these oligotrichs have maximum growth rates (p, d-l) ranging from -0.3 to 2.2 (at temperatures ranging from 6 to 31°C); they have similar threshold concentrations (the food concentration where growth rate equals zero) of 10 to 100 ng C ml-'; they have rapid mortality rates at sub-threshold concentrations (cells only survive 1 to 3 d), and the range of food concentrations where the growth rate increases from zero to near its maximum is 20 to 300 ng C ml-l Although generalities can be made about ciliates in these 2 genera, the data suggest that in terms of growth and mortality rates, planktonic oligotrichs cannot be considered a single functional group. There is, however, a need to predict 'typical' oligotrich growth rates. To this end, several formulae have been established to predict maximum growth rates from ciliate cell volume and ambient temperature. In this paper, the growth data for Strobilidii~m and Strombidium are reviewed and compared to the predicted values from formulae In the literature; the formula presented by Muller & Geller (1993, Arch. Hydrobiol. 126: 315-327) appears to be the best pred~ctor of maxlmum growth rate from cell volume and ambient temperature. Finally, based on the review of growth data for these 2 genera, some speculations are made as to how these ciliates may exploit prey populations in a planktonic environment composed of small-scale patches.

Experimental and modeling evidence of appendicularian‐ciliate interactions

“…In the ciliate growth model, the difference between F max and L can be compared with the maximal growth rate (m max ) estimated by Montagnes (1996) from several studies. Our estimation of m max is within the higher range of growth rates observed and corresponds to those estimated for several Strombidium species (1.28-2.71; Rivier et al 1985;Fenchel and Jonsson 1988;Ohman and Snyder 1991).…”

“…where L represents a term of mass loss (e.g., respiration and mortality), F max is the maximum net feeding rate, and k is the half-saturation constant of the feeding rate in relation to the concentration of food (Montagnes 1996 The model parameters were calibrated according to a least square minimization method (Nelder-Mead simplex method) in absence of appendicularians (C).…”

Determining Parameters of the Numerical Response

“…For instance, planktonic ciliates suffer high mortality rates below threshold levels of their phytoplankton prey, and these threshold levels are likely to be encountered in situ [11]. Furthermore, the threshold parameter (p 0 ) may be used to compare different planktonic species, indicating that similar taxa have distinctly different responses [11], and to compare single species under different ecological conditions, indicating, for instance, the response to temperature (e.g., [16]). Accurate estimates of these values could thus substantially alter food web models that include protozoa (see [4]), and if the threshold level is used as a key measurement to compare taxa, it needs to be precisely and accurately estimated.…”

“…Typically this relationship is modeled following a rectangular hyperbolic function with a nonzero intercept; this model is known to be a good predictor of the numerical response (e.g., [1,11,15]):…”

“…Such numerical responses are often incorporated into microbial food web models [4]. Furthermore, the parameters of the response are often used to compare taxa (e.g., [11]) and are used to assess the autecology of species [5,16]. Consequently, for these comparisons to be made it is essential to obtain accurate and precise estimates of the parameters for this response.…”