Biodiversity patterns of epipelagic copepods in the South Pacific Ocean: Strengths and limitations of current data bases

at subtropical latitudes and a gradual decrease towards temperate and polar regions [23, 34, 86–90]. However, the presence of hot spots equatorial area off Peru and Ecuador (Fig 4) coin- cides with the Eastern margin biogeographic region where coastal (0–250 Km offshore) and eastern (250–1000 Km offshore) boundary currents that originate from the Humboldt Current and has high levels of production [91]; whereas the hotspots observed in western temperate- subpolar areas off Australia (Fig 4), corresponds, according to their zoning, to the South Sub- tropical Convergence and to the Chatham Rise zones, both frontal zones that marks the inter- section of colder sub Antarctic waters with warmer tropical waters thus supporting substantial production [92]. The latitudinal trend, suggesting a plateau distribution of diversity from trop- ical to temperate regions, can be observed at the spatial distribution of the Shannon index shown in Fig 3, which shows greater diversity over a coastal band from the Equator towards the temperate area in the eastern zone, and higher diversity from subtropical areas towards temperate and subpolar zones at the western zone. Shannon Wiener, however exhibited some discrepancies with species richness. For instance, when comparing the northern vs. the south- ern portion of the eastern SPO (i.e. the Humboldt Current). In this regard, it is important to consider that we estimated the Shannon Wiener based on the species occurrences over the study period, not on species abundance, whereas species richness represents the total number

of species throughout the study period, independently of their frequency of occurrence. There- fore, Shannon Wiener and species richness may show different patterns of diversity, when comparing regions where copepod populations are highly frequent, since their populations are more continuously present during their annual cycles, such as in tropical or subtropical regions, or they have very seasonal life cycles, with low frequency of occurrence, such as in temperate regions [93]. Regarding latitudinal trends, Woodd-Walker et al. (2002) [36] suggested that the decreasing trend with latitude may arise from a more stable seasonal cycle of productivity in the tropical/ subtropical area supporting a stable high-diversity community in comparison with a strongly seasonal productivity in temperate/polar regions which may limit the number of copepods species and so resulting in a low diversity pattern. This latitudinal pattern is reflected in our spatial distribution of Shannon-Wiener index in both sides of the SPO, but it is not clear for the spatial distribution of alpha diversity, that at its western side showed lower values of diver- sity and had a similar tendency with a peak in the tropical and temperate zone, decreasing towards higher latitudes, although the eastern side showed higher values peaking at the tem- perate area that decreased towards the Equator and from the coastal towards the oceanic zone. The zonal pattern on the other hand, remains with much uncertainty due to a poor sam- pling coverage in the oceanic region of the SPO. Our study shows that maximum values of

copepod richness occur around tropical-subtropical areas in both eastern and western sides, but with a strongly reduced diversity in the oceanic region in areas with too few sampled cells. In this regard, Rombouts et al. (2009) [23] suggested that temperature-predicted diversity of copepods should be higher in a subtropical band in both hemispheres, although with no Table 3. GAM+RAC models for species composition. Statistics acronyms are BIC = Bayesian information criterion, ΔBIC = delta BIC (i.e., the difference in BIC score between the best model and the model being compared). Predictors’ acronyms are: Tmean = mean temperature, Tstab = temperature stability, S = salinity, Chla = chlorophyll-a concentra- tion, DO2 = dissolved oxygen concentration, MLD = mixed layer depth, autocovariate = residuals autocovariate. The best model is highlighted in bold. Model BIC ΔBIC 1 Species composition ~ Chla, DO2, S, Tmean + autocovariate 553.32 0 2 Species composition ~ Chla, MLD, DO2, S, Tmean + autocovariate 551.84 1.48 3 Species composition ~ Chla, MLD, DO2, S, Tmean, Tstab + autocovariate 551.68 1.64 https://doi.org/10.1371/journal.pone.0306440.t003 PLOS ONE Biodiversity of copepods in the South Pacific PLOS ONE | https://doi.org/10.1371/journal.pone.0306440 July 11, 2024 16 / 28