Zooplankton monitoring data
The zooplankton monitoring data from the Gulf of Riga is the most extensive dataset of zooplankton species composition and abundance in the Baltic Sea. Data were collected by the Estonian Marine Institute (UT-EMI) and Institute of Food Safety, Animal Health and Environment in Latvia (BIOR). The coastal data originate mostly from Pärnu Bay, covering areas up to 10 m deep. Samples were collected with a Juday 90 µm net as a single haul from the bottom (1m above the sea floor) to surface. Open Gulf data were collected with Juday 160 (mesh size 160 µm, upper ring ⌀ 37 cm). Sample preparation for counting was performed according to HELCOM standardized methods (HELCOM 2017). During analysis, zooplankton were identified to species level or to the lowest possible taxonomic level and their number in subsample determined. The number of Copepoda was counted separately for each stage, i.e., nauplii NI-6, copepodites CI-V and adult females and males CVI. The counting procedure in the sub-samples was repeated until the number of three dominant species reached 100 individuals. The individuals of large-sized zooplankton species (Cercopagis pengoi and Limnocalanus macrurus ) were counted in the whole sample. Finally, the biomass of each species was determined using individual wet weight factors (Hernroth 1985). Cercopagis pengoi biomass (BIOR data) was calculated using individual weight factor 0.4 according to Ojaveer (1997). Data covered the time period from 1957 to 2014. Seasonal data for May and August were consistently sampled both in the coastal and open gulf areas; therefore the data from these two months were used, consisting in total of 2118 profiles (970 coastal, 1148 open, 1108 in May, 1010 in August).