Mass occurrence of gelatinous organisms in the sea

Abstract

Posljedice velike brojnosti meduza su dobro znane: opasnost kupačima; začepljenje ribljih mreža; kompetitori u ishrani s ribama, osobito juvenilnim stadijima; predacija jaja i ličinki riba. Mogući uzroci porasta brojnosti meduza su kontroverzni. Vrlo vjerojatno da je prelov ribe jedan od mogućih uzroka prekomjerne pojave karnivornih želatinoznih organizama. Riba koja jede zooplankton je u kompeticiji s meduzama i što je ribe manje, veća je količina hrane za meduze. Također, dobro je poznato da su meduze otpornije od mnogih vrsta riba, osobito u smislu znatno veće tolerancije na smanjenje kisika. Nedostatak kisika u oceanima je sve češće primijećen, i proporcionalan porastu eutrofikacije, odnosno porastu organske tvari u morima. Rezultat je sve veće tonjenje organske tvari prema dnu gdje ga razgrađuju mikroorganizmi uz veliki utrošak kisika što često dovodi do hipoksičnih uvjeta. Klimatske promjene i porast površinske temperature mogu dodatno smanjiti koncentracije otopljenog kisika i samo manji dio kisika potrošenog u većim dubinama za razgradnju organske tvari može biti nadoknađen. Stoga, povećanje štetnog djelovanja meduza predstavlja kombinaciju stresnih promjena morskih ekosustav uzrokovanim klimatskim promjenama koji za posljedicu donose promjene u klasičnoj hranidbenoj mreži: fitoplankton-zooplankton-riba. Iz novo-nastalih uvjeta utjecaja sinergetskog djelovanja prekomjernog ribarstva i povećanja eutrofikacije, korist su izvukle meduze.The impacts of explosive jellyfish proliferations are quite well known: injured swimmers; clogged fish nets; feeding competition for fishes, especially for juvenile stages; predation of fish eggs and larvae. The possible causes of these increasing growths are somewhat controversial. One significant problem, presumably, is overfishing. Fish that feed on zooplankton are in feeding competition with the jellyfish. If the fish are absent, then the jellyfish have an abundance of available food. It is also known that jellyfish are more robust than many species of fish, especially in the sense that they can tolerate much lower oxygen concentrations. Oxygen deficiencies in the oceans, in turn, occur increasingly as a result of eutrophication and increasing concentrations of organic matter in the seas. As a result, more organic material sinks into the deep water where it is decomposed by oxygen-consuming microorganisms, a general decrease in oxygen and formatting of hypoxic events. Climate change, which causes warming of the ocean surface, can exacerbate this situation, and only small amounts of the oxygen consumed by microorganisms at greater depths are replaced. Thus, increasing jellyfish scourges could therefore be a result of combined stressors that lead to a restructuring of the marine ecosystems, which will disadvantage the classical food chain “phytoplankton – zooplankton – fish”. From these newly-created conditions of synergistic effects of overfishing and increased eutrophication, on the other hand benefits jellyfish abundance