The effect of temperature change and oxygen reduction on zooplankton composition and vertical distribution in a semi-enclosed marine system

Abstract

This study sheds light on the influence of dissolved oxygen (DO) and temperature on fine-scale zooplankton vertical distribution in a seasonally stratified, enclosed marine lake on the Croatian island of Mljet. Unusually high precipitation during the spring and summer of 2014 caused extended hypoxia above the bottom in the autumn. Nauplii, copepodites, Oithona nana and appendicularians strictly avoided the layers of reduced DO. Copepods Oithona similis, Mesaiokeras hurei, oncaeids, and polychaeta trochophore comprised a community adapted to the hypoxic bottom layer (10%-14% dissolved oxygen). Oncaeids were the most abundant and were presented by two small species: Oncaea zernovi and the more numerous Monothula subtilis. Polychaeta larvae were extraordinary numerous, too. Proliferation of ocaeids and polychaeta larvae in hypoxic zone could be related to their ability to feed on detrital matter and associated microbial community. Contrary, low abundance of other copepod developmental stages were found in the whole water column suggesting that the high load of organic material and hypoxic events inhabited their reproduction. Our study has documented the dramatic impact of local environmental changes on the planktonic community, providing additional evidence that low oxygen has an effect on the distribution and abundance of zooplankton.