Although most phycologists use natural seawater for culturing marine species, artificial media continue to play important roles in overcoming problems of supply and seasonal variability in the quality of natural seawater and also for experiments involving manipulation of micro‐ and macronutrients. Several artificial media have been developed over the last 90 years; enriched seawater, artificial water (ESAW) is among the more popular recipes. ESAW has the advantage of an ionic balance that is somewhat closer to that of normal seawater. The original paper compared the growth of 83 strains of microalgae in natural seawater (ESNW) versus ESAW and determined that 23% grew more poorly in the artificial water. Since 1980, however, the composition of ESAW, as used by the original authors, has changed considerably. In particular, the added forms of phosphate, iron, and silicate have been changed and the trace metal mixture has been altered to include nickel, molybdenum, and selenium. We tested whether these changes improved the ability of the artificial medium to grow previously difficult to grow phytoplankton species. To test this, we selected eight species that had been shown to grow better in ESNW than in ESAW and compared their growth again, using the currently used recipe with all the above modifications. For all but one species (Apedinella spinifera), growth rate and final yield was no different between the media but in one case (Emiliania huxleyi) was slightly higher in ESAW. No differences in cell morphology or volume were found in any case. We conclude that changes to the enrichment portion of the recipe have significantly improved this artificial seawater medium and that it can be used to grow an even wider range of coastal and open ocean species.

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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

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Abstract. Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs): six types of phytoplankton, three types of zooplankton, and heterotrophic procaryotes. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing macrozooplankton (e.g. krill), and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean high-nutrient low-chlorophyll (HNLC) region during summer. When model simulations do not include macrozooplankton grazing explicitly, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there is no iron deposition from dust. When model simulations include a slow-growing macrozooplankton and trophic cascades among three zooplankton types, the high-chlorophyll summer bias in the Southern Ocean HNLC region largely disappears. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community, despite iron limitation of phytoplankton community growth rates. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.