In their seminal paper, Goldman et al. suggested that phytoplankton close to maximum growth rate attains a restricted optimal N : P ratio close to the Redfield ratio of molar N : P = 16. Recently, the presence of such a global attractor for optimal phytoplankton stoichiometry has been questioned in models and empirical analyses. As the chemical composition of phytoplankton is of major importance for our understanding of global elemental cycles and biogeochemical transformations, we assembled 55 data sets of phytoplankton growth rate and biomass N : P ratios in a meta‐analysis testing (1) whether phytoplankton N : P converges at high growth rates, (2) whether N : P ratios scale with growth rate, and (3) whether the optimal N : P ratios achieved at highest growth rates reflect organism traits or environmental conditions. Across systems and species, phytoplankton N : P decreased with increasing growth rate and at the same time showed decreasing variance, i.e., fast‐growing phytoplankton is more P rich and has a more confined elemental composition. Optimal N : P increased with increasing N : P of available nutrients, i.e., with increasing P limitation. Other differences were rare, except cyanobacteria showed higher optimal N : P than diatoms. Understanding the role of phytoplankton in biogeochemical transformation requires modeling approaches that are stoichiometrically flexible to reflect the dynamics of growth and nutrient supply in primary producers.

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Cite this article as: Christoph Plum, Florence Pradillon, Yoshihiro Fujiwara and Jozée Sarrazin, Copepod colonization of organic and inorganic substrata at a deep-sea hydrothermal vent site on the Mid-Atlantic Ridge, Deep-Sea Research Part II, http://dx. AbstractThe few existing studies on deep-sea hydrothermal vent copepods indicate low connectivity with surrounding environments and reveal high endemism among vents. However, the finding of non-endemic copepod species in association with engineer species at different reduced ecosystems poses questions about the dispersal of copepods and the colonization of hydrothermal vents as well as their ecological connectivity.The objective of this study is to understand copepod colonization patterns at a hydrothermal vent site in response to environmental factors such as temperature and fluid flow as well as the presence of different types of substrata. To address this objective, an in situ experiment was deployed using both organic (woods, pig bones) and inorganic (slates) substrata along a gradient of hydrothermal activity at the Lucky Strike vent field (Eiffel Tower, Mid-Atlantic Ridge). The substrata were deployed in 2011 during the MoMARSAT cruise and were recovered after two years in 2013.Overall, copepod density showed significant differences between substrata types, but was similar among different hydrothermal activity regimes. Highest densities were observed on woods at sites with moderate or low fluid input, whereas bones were the most densely 2 colonized substrata at the 2 sites with higher hydrothermal influence. Although differences in copepod diversity were not significant, the observed trends revealed overall increasing diversity with decreasing temperature and fluid input. Slates showed highest diversity compared to the organic substrata. Temperature and fluid input had a significant influence on copepod community composition, resulting in higher similarity among stations with relatively high and low fluid inputs, respectively. While vent-specialists such as dirivultids and the tegastid Smacigastes micheli dominated substrata at high vent activity, the experiment demonstrated increasing abundance and dominance of non-vent taxa with decreasing temperature and fluid input. Effects of the substratum type on community composition were not significant, although at sites with moderate or low fluid input, woods exhibited distinctive communities with high densities and relative abundance of the taxon Nitocrella sp.. In conclusion, copepod colonization and species composition were mainly influenced by hydrothermal fluid input and temperature rather than the type of substratum. The outcome of this study provides fundamental knowledge to better understand copepod colonization at hydrothermal vents.

Despite the progress made in explaining trophic interactions through the stoichiometric interplay between consumers and resources, it remains unclear how the number of species in a trophic group influences the effects of elemental imbalances in food webs. Therefore, we conducted a laboratory experiment to test the hypothesis that multispecies producer assemblages alter the nutrient dynamics in a pelagic community. Four algal species were reared in mono- and polycultures under a 2 x 2 factorial combination of light and nutrient supply, thereby contrasting the stoichiometry of trophic interactions involving single vs. multiple producer species. After 9 d, these cultures were fed to the calanoid copepod Acartia tonsa, and we monitored biomass, resource use, and C:N:P stoichiometry in both phyto- and zooplankton. According to our expectations, light and N supply resulted in gradients of phytoplankton biomass and nutrient composition (C:N:P). Significant net diversity effects for algal biomass and C:N:P ratios reflected the greater responsiveness of the phytoplankton polyculture to altered resource supply compared to monocultures. These alterations of elemental ratios were common, and were partly triggered by changes in species frequency in the mixtures and partly by diversity-related changes in resource use. Copepod individual biomass increased under high light (HL) and N-reduced (-N) conditions, when food was high in C:N but low in C:P and N:P, whereas copepod growth was obviously P limited, and copepod stoichiometry was not affected by phytoplankton elemental composition. Correspondingly, copepod individual biomass reflected significant net diversity effects: compared to expectations- derived from monocultures, copepod individuals feeding on algal polycultures remained smaller than predicted under HL and N-sufficient (+N) conditions but grew larger than predicted under HL, -N and low light +N conditions. In conclusion, multiple producer species altered the stoichiometry of trophic interactions between phyto- and zooplankton, with divergent effects under high and low resource supply.