| Summary: | Finnish plankton monitoring is divided into phytoplankton and mesozooplankton sampling. Using the phytoplankton protocol, we included all organisms identified in samples from the Baltic Sea during spring (n = 125). The plankton was converted to carbon, and including all microscopy derived carbon (MDC), increased the carbon content by 22%, on average, compared with only phytoplankton. Particulate organic carbon (POC) and chlorophyll a (Chl a) were also measured, and the general relationship between MDC and POC was: slope = 1.04, intercept = 240 mu g POC l(-1), R-2 = 0.66; for phytoplankton to Chl a: 0.037 g Chl a (g MDC)(-1), R-2 = 0.68. Our results demonstrate that a variable fraction of the plankton biomass is not recorded in the monitoring programs. Most of the unaccounted biomass was ciliates, which constituted 14.1% +/- 3.7% (mean +/- maximum error) of the plankton biomass. Based on the results we recommend including microzooplankton in the existing phytoplankton monitoring program. Peer reviewed
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