Phytoplankton collection methods in three bays

Phytoplankton are diverse unicellular organisms vital for regulating and maintaining aquatic food webs and global biogeochemical processes. Understanding the phytoplankton diversity improves monitoring and the sustainable use of the environment. Numerous methods exist for the collection of phytoplankton, but the lack of cross-method comparisons makes it difficult to compare results across space and time. This study provided insights into the effects of collection techniques on phytoplankton enumeration across three spatially distinct bays. Water samples (grab) and two different net mesh sizes, i.e. 20 µm and 55 µm nets and vertical and horizontal net tows, were used to test their effectiveness for phytoplankton community assessment. We found that the method of collection affects the species count, abundance, evenness, dominant and rare taxa captured with our study dominated by diatoms followed by dinoflagellates, flagellates and silicoflagellates. The strengths and limitations of each collection method demonstrate how methodological biases can influence the magnitude and resolution of phytoplankton community characterization. The results revealed that geographic location is a dominant driver in phytoplankton community composition and structure, however, sampling collection methods consistently affect which taxa are represented and this is context-dependent based on the sampled environment. We propose that vertical net tows should be the backbone of monitoring programs as they provide a representation of the phytoplankton community structure and abundance. However, we recommend that a combined approach of water grabs and at least vertical nets be used tailored to regional conditions, as this will provide the most accurate and ecologically meaningful assessments.