Temporal change of plankton size structure preserved by Lugol’s solution: a FlowCAM study

Plankton size structure is crucial for understanding marine ecosystem dynamics and the associated biogeochemical processes. A fixation step by acid Lugol’s solution has been commonly employed to preserve plankton samples in the field. However, the acid Lugol’s solution can bias the estimation of size structure and the preserved plankton size structure can vary with time. Here, we explore the impact of sample storage time on the size-structure of the plankton community preserved by Lugol’s solution. Two short-term experiments and one long-term experiment were conducted to explore the change of plankton community size structure with the storage time: covering from a week to a month, and to nearly seven months based on particle-size data obtained by continuous Flow Cytometer and Microscope (FlowCAM) measurements. We found a linear change of plankton size with the storage time in short-term periods (less than 3 months) with a decrease of the slope but an increase of the intercept for the normalized biomass size spectrum (NBSS). However, there were opposite trends for NBSS with increasing slope but decreasing intercept after 3 months. The potential causes of the distinct patterns of the NBSS parameters are addressed in terms of the interplay between particle aggregation and fragmentation. We found large changes in plankton biovolume and abundance among different size classes, which may indicate a distinct effect of acid Lugol’s solution on various plankton size classes. The mechanism driving temporal change in the size-structure of the Lugol-fixed plankton community was further discussed in terms of particle aggregation and fragmentation. Finally, we emphasize that the effect of storage time should be taken into account when interpreting or comparing data of plankton community acquired from samples with various storage durations.