Global patterns of planktonic diversity are mainly determined by the dispersal of propagules with ocean currents. However, the role that abundance and body size play in determining spatial patterns of diversity remains unclear. Here we analyse spatial community structure - β-diversity - for several planktonic and nektonic organisms from prokaryotes to small mesopelagic fishes collected during the Malaspina 2010 Expedition. β-diversity was compared to surface ocean transit times derived from a global circulation model, revealing a significant negative relationship that is stronger than environmental differences. Estimated dispersal scales for different groups show a negative correlation with body size, where less abundant large-bodied communities have significantly shorter dispersal scales and larger species spatial turnover rates than more abundant small-bodied plankton. Our results confirm that the dispersal scale of planktonic and micro-nektonic organisms is determined by local abundance, which scales with body size, ultimately setting global spatial patterns of diversity.

The ratio of tissue RNA to DNA (R/D) is a widely used index of recent growth and nutritional condition in larval and juvenile fish. To date, however, no standard technique for measuring nucleic acids has been adopted. Because methodological details can affect the estimate of R/D, researchers using different analytical protocols have been unable to compare ratios directly. Here, we report on the results of an international interlaboratory calibration of 4 spectrofluorometric protocols to quantify nucleic acids. Replicate sets of 5 tissue samples and 2 standards (common standards) were supplied to each of 5 researchers for analysis with their own methods and standards. Two approaches were evaluated for mitigating the observed differences in values: 1) the use of common nucleic acid standards and 2) standardizing to a common slope ratio (slope of DNA standard curve/slope of RNA standard curve or m_DNA/m_RNA). Adopting common standards slightly reduced the variability among protocols but did not overcome the problem. When tissue R/Ds were standardized based on a common m_DNA/m_RNA slope ratio, the variance attributed to analytical protocol decreased dramatically from 57.1% to 3.4%. We recommend that the ratio of the slopes of the standard curves be provided to facilitate intercomparability of R/D results among laboratories using different spectrofluorometric methods for the analysis of nucleic acids in fish.