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).