Chair: Ulf Riebesell

Joy N. Smith(^1,2), Glenn De’ath(²⁾, Astrid Cornils(¹⁾, Claudio Richter(¹⁾, Katharina Fabricius(<sup>2)

1</sup> Alfred Wegener Institute, Bremerhaven, D-27568, Germany
² Australian Institute of Marine Science, Townsville, Queensland, 4810, Australia

Background
Current research evaluating the effects of ocean acidification (OA) on zooplankton is mostly based on single-species laboratory experiments or mesocosm studies. Additionally, most studies have been conducted on calcifying species, which represent a small proportion of the total plankton community, or focus on dominant and robust species of copepods. Very little is known about how entire zooplankton communities may be affected by ocean acidification. In this study, we examined the effects of ocean acidification on demersal zooplankton in communities at two coral reefs in Papua New Guinea, where underwater seeps create natural pH gradients.

Methods
Net tows, emergence traps, and acoustical instruments were used to collect and monitor zooplankton abundances over three separate expeditions, on two separate reefs, and emerging from different types of substrata.

Findings
Results were consistent for all three expeditions and for reefs at two different seep sites. All methods reveal dramatic losses in demersal zooplankton abundances where pH_T was reduced to 7.8 compared to healthy reefs with a normal pH_T of 8.1. We also observed changes in the zooplankton community, and examined each taxonomic group separately to investigate differences in the sensitivity between taxonomic groups.

Conclusions
Laboratory experiments on single-species of zooplankton suggest that many zooplankton, especially copepods, may be robust to ocean acidification. However, our results indicate that even zooplankton species considered tolerant to OA may be in fact be vulnerable to elevated CO₂ under in situ OA conditions where entire zooplankton communities were examined. Laboratory experiments focus on the direct effect of CO₂ on zooplankton physiology, while our results may reflect indirect effects in which habitat loss (i.e. reduced complexity in coral structure) may be an underlying cause for the reduced demersal zooplankton abundance. Reduced zooplankton abundances and altered communities could impact entire marine ecosystems, considering zooplankton are food for many marine organisms.