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

the species, their body weight and extent of motion [109]. Altogether, these combined drivers have a potential effect on copepod ecology and, consequently, on their biogeographic patterns. Salinity and its variation also affects copepod distribution [110], biomass [111], reproduc- tion [112, 113], growth [114, 115], body size [25] or fatty acid synthesis [115], with various ranges of tolerance that may or may not be favorable, according to their biology and location. This may be relevant since global warming in some areas of the ocean can lead to an increase in salinity (evaporation or greater circulation) or a reduction of the same (greater run-off of fresh water into the ocean, rainfall, or ice melt), producing osmotic stress that may be costly for their physiology. Temperature itself plays a fundamental role in the marine ecosystem, since it controls phys- iological rates [116], metabolism [116], body size [25], reproduction [10], mortality [117] and community structure [24, 118], even at large spatial scales [25, 35, 41, 119]. Thus, it may be thought that temperature also influences the distribution of organisms in the ocean, as it can substantially vary in its three dimensions. Over the meridional axis, temperature exhibits a strong gradient from the equator to the poles, with a similar gradient commonly present from coastal areas to central gyres, whereas in the vertical axis, it has a strong temperature gradient from the upper mixed layer down to the deep, cold waters. These gradients in the water col-

umn structure may contribute to the generation of large-scale spatial trends in the distribution of organisms such as copepods [120]. Although this study did not separate copepods into func- tional groups according certain traits more specifically linked to environmental variations, the fact that composition changed mainly due to oceanographic factors that influence the nesting of functional groups, highlights and support those findings [121], as beta diversity is a proxy of difference in number of species among ecosystems, thus reflecting changes or gradients in the environment as well. Species distribution and the structure of species assemblages may additionally obey some physical processes (e.g., large-scale circulation, currents, eddies, fronts, interaction with the atmosphere) and the effect of local conditions. In this sense, it is possible that in addition to more direct effects on copepod populations, temperature is reflecting variability in water mas- ses distribution controlled by large-scale circulation. The distribution of drifting plankton, including copepods, is largely affected by near-surface currents, allowing species radiation and colonization processes [122], and so influencing diversity patterns. Regarding the structure of the copepod community, we found that some dominant species were representing different current systems, and thus distinct plankton communities. How- ever, we could only assess most occurrences of species (not abundance) for each main surface current system of the SPO (S1 Table). The occurrences distribution of dominant species

seemed to be concordant with patterns documented for copepod diversity on a large scale, with cyclopoids showing greater diversity in the subtropics and calanoids in the temperate zone [25, 123]. Dominant species in the each of the major currents are mostly represented by small Calanoid copepods and the widely distributed Cyclopoid Oithona similis. Some of the recurrent species are Paracalanus parvus and P. indicus. Both species are possibly a single spe- cies traditionally known as P. parvus, but morphologically close to P. indicus, although most likely being a new, non-described species [124]. Lately, it has been referred as P. cf. indicus in the Eastern SPO [125]. The other recurrent species are Calocalanus spp. They are also known as small-sized copepods and mostly found in oceanic waters [126]. The high occurrence of small-sized oceanic copepods, and low presence of large-sized Calanoids, such as Calanus spp., Calanoides spp., and Pleurommama spp., which are abundant in coastal areas of the SPO [38, 93] may indicate that small copepods prevail, or are numerically more abundant than large- sized ones. Also, most data sets do not include harpacticoids or other cyclopoid copepods, such as Oncaea spp. which may be highly abundant in oligotrophic waters of the SPO [127]. PLOS ONE Biodiversity of copepods in the South Pacific PLOS ONE | https://doi.org/10.1371/journal.pone.0306440 July 11, 2024 18 / 28