Microzooplankton, the missing link in Finnish plankton monitoring programs

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 µg POC l–1, R2 = 0.66; for phytoplankton to Chl a: 0.037 g Chl a (g MDC)–1, R2 = 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.