Sampling errors arising from carousel entrainment and insufficient flushing of oceanographic sampling bottles

Collection of representative water samples is necessary for limnology and oceanography. Unfortunately, some common sampling practices using current technology have the potential to introduce significant sampling errors. For example, modern carousel hardware and software can permit closing of sampling bottles as soon as the bottle reaches the desired depth rather than allowing sufficient time (i.e., soak time) for ambient water to flush the sampling bottles. The large size of many conductivity-temperature-depth (CTD)/carousels and their associated instrumentation also increase the impacts of water entrainment as the equipment travels within the water column. Finally, some modern sampling bottles have small openings relative to their volumes, a factor that inhibits bottle flushing, particularly if the bottle closures are not completely open. Inspection of data from selected research cruises suggests that insufficient soak times can produce biased water samples. In this study, we undertook field experiments that help to quantify the errors that can arise from CTD carousel entrainment and insufficient bottle flushing. The experiments demonstrate that under stratified conditions, soak times of more than 2 min may be required to collect representative water samples. The experiments also demonstrate the occurrence of stratification within sample bottles. Some protocols that may reduce sampling errors are suggested.