The size of 30 small (2-60 pm) phytoplankton species was examined with a microscope and a Coulter Counter before and after fixation. Acid Lugol's iodine caused cells to shrink immediately. The shrinkage effect was constant for concentrations of l-10% Lugol's iodine (in seawater). For optically measured cells fixed in 2% Lugol's iodine, volume of live cells = 1.33 x (volume of fixed cells). Coulter Counter and optically measured volumes did not agree. For live cells, optical cell volume = 1.24-2.04 x (Coulter Counter determined volume); this difference is likely due to inaccurate volume measurements of nonspherical cells by the Coulter Counter and by inaccurate microscopy resulting from optical distortions (errors of ~0.5 pm in cell dimensions). Cell quota estimates were presented following the relation y = a.x?, where x = optically measured cell volume (pm3), y = any cell constituent (pg cell-'), and a and b are constants. The constants a and b were 0.109 and 0.99 1 for carbon, 0.0172 and 1.023 for nitrogen, 0.043 and 1.058 for protein, and 0.00428 and 0.9 17 for Chl a. Our relation of carbon to volume differs from other literature values, in which there is no consensus. Our data can be used to determine carbon, nitrogen, protein, and Chl a estimates from field material that has been fixed with Lugol's iodine, observed live, optically measured, or Coulter Counter measured; however, the variability in published data suggests that any of these estimates will have a large potential error.

An inverse relationship between organism size and rearing temperature is widely observed in ectotherms ('the temperature-size rule', TSR). This has rarely been quantified for related taxa, and its applicability to protists also required testing. Here, we quantify the relationship between temperature and mean cell volume within the protists by a meta-analysis of published data covering marine, brackish water and freshwater autotrophs and heterotrophs. In each of 44 datasets, a linear relationship between temperature and size could not be rejected, and a negative trend was found in 32 cases (20 gave significant negative regressions, p < 0.05). By combining 65 datasets, we revealed, for each 1 degrees C increase, a cell-size reduction of 2.5% (95% CI of 1.7-3.3%) of the volume observed at 15 degrees C. The value did not differ across taxa (amoebae, ciliates, diatoms, dinoflagellates, flagellates), habitats, modes of nutrition or combinations of these. The data are consistent with two hypotheses that are capable of explaining the TSR in ectotherms generally: (i) resource, especially respiratory gas, limitation; and (ii) fitness gains from dividing earlier as population growth increases. Using the above relationship we show how changes in cell numbers with temperature can be estimated from changes in biomass and vice versa; ignoring this relationship would produce a systematic error.

The Eppley curve describes an exponential function that defines the maximum attainable daily growth rate of marine phytoplankton as a function of temperature. The curve was originally fitted by eye as the upper envelope of a data set, and despite its wide use, the reliability of this function has not been statistically tested. Our analysis of the data using quantile regression indicates that while the curve appears to be a good estimate of the edge of the data, it may not be reliable because the data set is small (n 5 162). We construct a contemporary, comprehensive data set (n 5 1,501) and apply an objective approach, quantile regression, to estimate its upper edge (99 th quantile). This analysis yields a new predictive equation, m max 5 0.81e 0.0631T , that describes the maximum specific growth rates (m max , d 21 ) of marine phytoplankton as a function of temperature (T, uC). The Liverpool phytoplankton database (LPD) curve is higher than the Eppley curve across all temperatures, and at temperatures below 19uC, the Eppley curve falls below the lower 95% confidence interval of the LPD curve. However, the LPD Q 10 value (1.88) is identical to that of the Eppley curve and thus supports the use of models that incorporate this as an estimate of phytoplankton growth-rate response to temperature change. To assess the potential effect of the LPD curve on primary production, we embedded the LPD function into a one-dimensional numerical model of a temperate, pelagic ecosystem. This analysis suggests that models using the Eppley function will underestimate primary production by as much as 30%.

We examined the response of diatoms to naturally experienced temperatures and tested these hypotheses: (1) diatoms follow the rule that organism size decreases with increasing temperature; (2) diatom growth rate follows a Q 10 -like response; (3) diatom carbon (C) and nitrogen (N) content per unit volume (V) decrease with increasing size, and changes in temperature affect this relationship; and (4) diatom C : V is the same as that of other phytoplankton. We also present, as predictive equations, relationships between (1) growth rate, temperature, and size; (2) C content and V; and (3) N content and V. Eight diatoms and two flagellates were acclimated for approximately five generations and grown for approximately five more generations at five temperatures (9-25ЊC) on a 14 : 10 light : dark cycle at ϳ50 mol photons m Ϫ2 s Ϫ1. Growth rate, cell V, and C and N content per cell were measured; relationships between these parameters and temperature were determined. For five diatoms and both flagellates, cell V decreased with increasing temperature; cells decrease by ϳ4% of their mean V per ЊC. Growth rate appeared to increase linearly with temperature in all cases. The literature suggests that a linear response is the rule, not the exception. Temperature did not significantly affect C or N per V of diatom species. When all diatoms were considered, both C and N per V decreased with increasing cell size; our data support the argument that diatoms differ from other protists in this respect, but the difference is less pronounced than stated in previous reports.As diatoms are indisputably a major component of many food webs, estimating their abundance, biomass, and growth rate has been, and will be, an essential component of marine studies. Like all organisms, diatoms are influenced by ambient temperatures, a point that has long been accepted (e.g., Eppley 1972; Goldman and Carpenter 1974). There is now an increasing awareness of global-warming impacts and other anthropogenic and natural changes in marine systems. Concomitantly, there is a need to better understand the influence of temperature on phytoplankton in general and on diatoms specifically.This study improves our ability to assess the effect of temperature change on diatoms by making estimates of how their size, biomass, and growth rate vary over naturally occurring ranges. Furthermore, three biological paradigms are examined: the rule of diminishing size with increasing temperature (Atkinson 1994); the Arrhenius/Q 10 relationship (e.g., Cossins and Bowler 1987); and the difference in carbon : volume (C : V) ratio between diatoms and other phytoplankton (Strathmann 1967). Atkinson (1994Atkinson ( , 1995 indicated that for ectotherms, size decreases with increasing temperature. One of the few exceptions to this rule was the diatom Phaeodactylum tricornutum (Atkinson 1994), but there are 1 Corresponding author (dmontag@liv.ac.uk). Paradigm i-Reviews by

Phagotrophic protists are diverse and abundant in aquatic and terrestrial environments, making them fundamental to the transfer of matter/energy within their respective food webs. Recognising their grazing impact is essential to evaluate the role of protists in ecosystems, and this includes appreciating prey selectivity. Efforts have been made by groups and individuals to understand selective grazing behaviour by protists: many approaches and perspectives have been pursued, not all of which are compatible. This article, which is not a review, is the product of our discourse on this subject at the SAME 10 meeting. It is the work of individuals, assembled for their breadth of backgrounds, approaches, views, and expertise. Firstly, to communicate ideas and approaches, we develop a framework for selective feeding processes and suggest 6 steps: searching, contact, capture, processing, ingestion, digestion. We then separate study approaches into 2 categories: (1) those examining whole organisms at the community, population, and individual levels, and (2) those examining physiology and molecular attributes. Finally, we explore general problems associated with the field of protistan selective feeding (e.g. linking food selection into food webs and modeling). We do not present all views on any one topic, nor do we cover all topics; instead, we offer opinions and suggest avenues for continued study. Overall, this paper should stimulate further discourse on the subject and provide a roadmap for the future.

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Functional ecology is a subdiscipline that aims to enable a mechanistic understanding of patterns and processes from the organismic to the ecosystem level. This paper addresses some main aspects of the process-oriented current knowledge on phagotrophic, i.e. heterotrophic and mixotrophic, protists in aquatic food webs. This is not an exhaustive review; rather, we focus on conceptual issues, in particular on the numerical and functional response of these organisms. We discuss the evolution of concepts and define parameters to evaluate predator-prey dynamics ranging from Lotka-Volterra to the Independent Response Model. Since protists have extremely versatile feeding modes, we explore if there are systematic differences related to their taxonomic affiliation and life strategies. We differentiate between intrinsic factors (nutritional history, acclimatisation) and extrinsic factors (temperature, food, turbulence) affecting feeding, growth, and survival of protist populations. We briefly consider intraspecific variability of some key parameters and constraints inherent in laboratory microcosm experiments. We then upscale the significance of phagotrophic protists in food webs to the ocean level. Finally, we discuss limitations of the mechanistic understanding of protist functional ecology resulting from principal unpredictability of nonlinear dynamics. We conclude by defining open questions and identifying perspectives for future research on functional ecology of aquatic phagotrophic protists.

The growth, grazing, and cell volume of Strombidinopsis multjaurls, a large (-100 pm) coastal planktonic cil~ate, IS affected by food concentration and temperature. Using growth and grazlng data, we modelled small-scale bloom dynam~cs between the clliate and ~t s prey. Growth expenments were conducted at 13°C on S. niultiauris fed the 10 pm d~noflagellate Gymnodiniurn simplex; changes in cell numbers and cell volume were monitored. Ingestion rate was measured by 3 methods (uptake of fluorescently labelled latex beads, heat-killed, fluorescently labelled G nmplex; and I4C-labelled G. simplex). Growth rate vprsus food concentration followed a rectangular hyperbolic response, with a maxlmum of p = 0 6 d ' above 104 prey ml-l (480 ng C ml-l), below 1.3 X 10"l-' (62 ng C ml-l), mortality occurred. Cell volume followed a rectangular hyperbolic response to food concentration, and showed a doubling in size between zero and maxmum prey levels. Grazing rate initially ~ncreased with food concentration and was then inhib~ted at levels >10"rey ml-l. The cil~ate ~ngested 14C-labelled live prey at higher rates than either dead or artificial prey at subsaturating concentrations; above saturating concentrations, ingestion rates were similar for the 3 prey types. The maximum observed grazlng rate was 35 prey cihate ' h-' Growth rate and cell volume were measured under steady-state conditions at 9 temperatures between 3 5 and 22°C: clliates died at 3.5 and 5"C, growth rate increased hnearly to a maximum of p = 0.9 d-l at 15"C, did not change between 15 and 20°C, and decreased at 22°C. Cell volume ~ncreased between 5 and 10°C and decreased between 10 and 22°C. The population dynamlcs model revealed that the ciliate was able to control the dinoflagellate population. Over the 20 d model simulation, virtually no predator-prey cycle occurred when prey growth rates were p < 0.2 d-' As prey growth rate was increased bloom dynamics became apparent, with a minunum duration of-10 d for a bloom to begin and end at a prey growth rate of p = 0.65 d-l. During these simulated blooms ciliates reached maxlmum levels of 35 cells n~l-' , and prey reached levels of 1.7 X 104 cells ml-', slmilar to numbers found in a typical coastal bloom. Our data and model suggest that ciliates and their prey produce episodic, short-term blooms, and we recommend that these events be evaluated more carefully in the field and be incorporated into models. KEY WORDS. Blooms. Cell volume. Grazing rate. Growth rate. Microzooplankton. Mortality rate. Ohgotnch ciliate. Plankton. Temperature response