We analyze the relationship between species richness, salinity and resource use efficiency from 10712 summer (June to September) surface phytoplankton samples from the Chesapeake Bay (n=3967) and the Baltic Sea (n=6745). As sample species richness (alpha diversity) has a U-shape distribution along an estuarine salinity gradient and species richness is known to scale with resource use efficiency – an important ecosystem function, we hypothesized that the ecosystem function can be predicted from salinity.

Samples from Chesapeake Bay were taken from fixed statutory monitoring station. From The Baltic Sea samples represented a combination of fixed and spatially scattered locations.

Sampling and sample processing details are given in respective program manuals (Michaels and Kahrr 2010; HELCOM 2014). Aliquots of surface layer samples were preserved with acid Lugol’s solution or formalin, and cells were enumerated from aliquots of 10–50 mL using inverted microscopes. The counting effort varied but in general was better than 100 cells of the most numerous taxon.

Michaels, B., and R. Kahrr. 2010. Maryland Chesapeake Bay water quality monitoring program: phytoplankton component. https:// www.chesapeakebay.net/documents/3684/3_mdphdoc.pdf.

HELCOM. 2014. Manual for marine monitoring in the COMBINE pro- gramme. http://www.helcom.fi/Lists/Publications/Manual%20for%20 Marine%20Monitoring%20in%20the%20COMBINE%20Programme %20of%20HELCOM.pdf. HELCOM, Helsinki.

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Missing values denoted with NA

Geographic range of data: Chesapeake Bay (USA); Baltic Sea. The sampling station coordinates are given in the data table in columns lat and lon

Temporal coverage: 1984-06-06 to 2008-09-30