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

contrasting differences between coastal and oceanic regions. Such patterns have also been found in other planktonic groups, such as euphausiids [34], tintinnid ciliates [94], foraminifera [34, 95]; as well as benthic marine invertebrates, such as prosobranch gastropods [32]; and higher trophic level organisms, fishes [34, 96], sharks [34], squids [34] and cetaceans [34]. For instance, in a recent work [19] a total number of 121 species of copepods was reported from a single cruise carried out in the South Pacific Subtropical Gyre, indicating this area can be com- parable to the North Pacific Subtropical Gyre in which 125 species were reported by McGowan and Walker (1979) [97], although more recently Vereshchaka et al. (2017) [98] indicated that more than 240 species can be found in the central-south Pacific region. Therefore, the species records available at OBIS may not represent the potentially rich copepod community in the central gyre of the SPO; and that is why we opted for dividing the basin into a eastern and western side and exclude the longitudinal band between -110˚−-170˚ for performing our anal- yses, as this area had too few sampling. Temperature and its stability were the main factors, initially proposed in our hypothesis, as predictors of copepod biodiversity patterns in this basin. However, hot spots were also found off Chile, Peru and Ecuador along the main coastal upwelling system, which might be associ- ated to 1) the habitat features hypothesis [34] related to ecosystem size and mesoscale pro-

cesses, that are known to influence positively copepod diversity [34, 86, 90]; and 2) the productivity-richness hypothesis that predicts a positive effect of primary productivity on rich- ness [34], supported by other studies finding a positive relationship between copepod diversity and chlorophyll-a concentration [86, 87, 89, 99]. Biogeographic patterns can also be explained on the basis of evolutionary [100, 101] and spatial processes [102] which are considered as the historical context for observed spatial distribution. Nevertheless, as stressed above, limited data coverage precludes a clear conclusion on underlying mechanisms explaining observed patterns of diversity. Considered the above-mentioned potential mechanisms, we selected environmental vari- ables to evaluate factors explaining copepod species richness and species composition. All vari- ables were correlated to each other with a Spearman’s rank correlation coefficient of |ρ <0,7|, except for Tmean and O2, that showed ρ = -0.99. In general, variables with correlation values of |ρ| less than 0.7 are not recommended for selection [103]; nevertheless, despite this high value of |ρ|, we chose to include both variables instead of conducting further testing. This is because dissolved gases in the ocean tend to decrease their concentration with higher temperature [104], and so this relationship would not reflect a systematic bias. The oceanographic factors explaining copepod species richness (Chla, S, Tmean) and species composition (Chla, O2, S,

Tmean) in the SPO are of significance in a lesser or greater degree at different zones of the study area; and, they have also been documented to be of importance on a macroscale basis for cope- pod abundance and body size [25]. Moreover, the GDM-based spatial analysis conducted for evaluating species composition showed that the stronger predictors of the observed dissimilar- ities were the same variables (S, O2, Tmean, Chla; S3 Fig). It has been found that chlorophyll strongly correlates with copepod abundance in the ocean [34, 86, 99, 105]. Oxygen is also a significant factor influencing copepod distribution in the ocean, because its availability is a major driver for temperature-size responses in aquatic organisms [106], and the variation in its concentration, for example, due to deoxygenation, can affect copepod physiology, with drastic impacts in both coastal [11, 107] and oceanic [108] species. Warmer water increases copepod growth, development and molting rates [107], with an associated increase in the oxygen demand. This can lead to a reduction in population growth by limiting the hatching success and naupliar growth in early life stages [11], as well as reducing egg production and somatic growth in adults [107], together with shifts in their depth distribution [108]. This involves physiological adaptations that may vary according to PLOS ONE Biodiversity of copepods in the South Pacific PLOS ONE | https://doi.org/10.1371/journal.pone.0306440 July 11, 2024 17 / 28