Ocean acidification has been identified as a risk to marine ecosystems, and substantial scientific effort has been expended
on investigating its effects, mostly in laboratory manipulation experiments. However, performing these manipulations correctly
can be logistically difficult, and correctly designing experiments is complex, in part because of the rigorous requirements
for manipulating and monitoring seawater carbonate chemistry. To assess the use of appropriate experimental design in ocean
acidification research, 465 studies published between 1993 and 2014 were surveyed, focusing on the methods used to replicate
experimental units. The proportion of studies that had interdependent or non-randomly interspersed treatment replicates, or
did not report sufficient methodological details was 95%. Furthermore, 21% of studies did not provide any details of experimental
design, 17% of studies otherwise segregated all the replicates for one treatment in one space, 15% of studies replicated CO2 treatments in a way that made replicates more interdependent within treatments than between treatments, and 13% of studies
did not report if replicates of all treatments were randomly interspersed. As a consequence, the number of experimental units
used per treatment in studies was low (mean = 2.0). In a comparable analysis, there was a significant decrease in the number
of published studies that employed inappropriate chemical methods of manipulating seawater (i.e. acid–base only additions)
from 21 to 3%, following the release of the "Guide to best practices for ocean acidification research and data reporting"
in 2010; however, no such increase in the use of appropriate replication and experimental design was observed after 2010.
We provide guidelines on how to design ocean acidification laboratory experiments that incorporate the rigorous requirements
for monitoring and measuring carbonate chemistry with a level of replication that increases the chances of accurate detection
of biological responses to ocean acidification.