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.