Long-term trends in the main organism groups
We compared the mean proportions of each groups’ abundance (rotifers, cladocerans, copepods, others) to the total abundance of organisms in order to detect patterns in the long-term dynamics of the main groups. In the coastal area in May (Fig. 4a), the proportion of rotifers increased from 1960s onwards from ca. 40% to ca. 70% of the whole community abundance; however, the proportion varied strongly over time. In the 1960s, copepods were dominating the community. Since then a large decrease in the abundance of copepods has been recorded in connection to an increase of rotifers, and more recently, an increase in the abundance of e.g., bivalves (‘others’). Later in the summer (August) in the coastal area, the most notable change was the decrease in the proportion of cladocerans of their peak in the mid-1980s to very low abundances in the mid-1990s and beyond (Fig. 4b). After the mid-1990s, rotifers accounted for >50% of the organisms recorded. In the open sea area in May (Fig. 4c), the community was mostly dominated by copepods until the 1980s, whereafter the proportion of rotifers increased and dominance relations started to shift from copepods to rotifers. Towards the end of the investigated time period, from ca. 2005, the proportion of copepods decreased by ca. 50 %, related to the increased proportion of ‘others’. In August in the open sea area (Fig. 4d), the large decrease of rotifers from 1990s to 2010 gave rise to increasing dominance of cladocerans and copepods.