Jellyfish abundance and Chlorophyll a data from a shallow semi-enclosed cove, Kertinge Nor, Denmark

Carnivorous gelatinous zooplankton (GZ) can be very abundant in marine ecosystems around the globe and exert considerable predation pressures on micro- to macrozooplankton as well as larval and juvenile fish. As these species are in many cases intermediate and top predators, their biomass can be easily evaluated, which could indicate overall food web stability or alteration. There is growing concern worldwide about increasing abundances of GZ species and consequently negative impacts on food webs and human coastal zones activities. In this paper, I present a case study from a shallow Danish cove and a long-term record spanning 29 years (1991-2019) obtained by unconventional means. Jellyfish were collected using horizontal sub-surface net tows in August and September during a summer student class. Sea surface temperature and chlorophyll a concentration were measured in parallel. No clear long-term trend in jellyfish biomass could be seen, whereas the values were highly variable from year to year. This is in contrast with other published Scandinavian long-term GZ time series. Sea surface temperature and chlorophyll a concentration had significant (and interactive) effects on the jellyfish biomass in Kertinge Nor. The scarcity and shortness of GZ long-term series do not allow solid conclusions in most marine ecosystems, so it becomes clear that publishing time series (even with small spatial extent) can contribute to improve the perception of interannual GZ population developments. Thus, I strongly recommend extending monitoring activities, explicitly including GZ taxa, in as many marine ecosystems as feasible.