ENSO and anthropogenic impacts on phytoplankton diversity in tropical coastal waters

•Phytoplankton diversity is well reflected anthropogenic and climate impacts.•Taxonomic diversity indices are better used to assess environmental impacts.•ENSO effects on taxonomic diversity can be predictable in less human impacted waters.

16-year phytoplankton data were analysed to assess ENSO and anthropogenic impacts on biodiversity and community structure at 3 locations (Nha-Trang and Phan-Thiet Bays and near Phu-Qui Island) in South Centre Viet Nam to understand (1) the primary scales of change in phytoplankton community structure, and traditional and taxonomic diversity indices; (2) the significance of environmental changes and/or climate variability on phytoplankton diversity; and (3) the usefulness of these long-term data for analysing future impacts of anthropogenic and climate changes. Traditional and taxonomic diversity indices were compared and tested in linkage with environmental conditions and ENSO. Nutrient data indicated stronger environmental impacts in Phan-Thiet Bay, milder in Nha-Trang Bay and less noticeable near Phu-Qui Island. There were measurable impacts of both anthropogenic and ENSO on phytoplankton at different locations in various parameters, e.g. species number, diversity and community structures. The lowest diversity was recorded in the most anthropogenically impacted site, Phan-Thiet Bay. Although a stronger impact on phytoplankton was recorded in ENSO year in Phan Thiet Bay, quantitative separation between anthropogenic and ENSO impacts using phytoplankton biodiversity indices was impossible. In the waters with less anthropogenic impacts, ENSO effects on taxonomic diversity was better indicated by negative phytoplankton responses to the ONI index (Nha-Trang Bay) and recovery of phytoplankton after the ENSO events (near Phu-Qui Island). Among the diversity indices, the taxonomic diversity indices (e.g. Δ+ and Λ+) better described impacts of ENSO than the traditional ones.

Graphical abstract(a) Correlation between average taxonomic distinctness index of phytoplankton in Nha Trang Bay and ONI index from covariant analysis. Solid circle is individual Δ+ value with corresponding ONI index, solid line is regression line (p = 0.0035, ANOVA). (b) Increasing of average taxonomic distinctness (Delta+, fill circles) and decreasing of taxonomic variation (Lambda+, fill circles) between 2001 and 2008 vs. between two strong ENSO events (ONI index, fill bars) reflecting the phytoplankton diversity was collapsed and recovery with environmental disturbances (ENSO). (Dashed lines are linear regressions, p = 0.0025 and 0.0001 for Δ+ and Λ+, respectively.)Download high-res image (133KB)Download full-size image