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

of conservation purposes and preserving ocean biodiversity, it becomes clear that the variation in the biogeographic regions from time series observations [4], as well as from geological time scale analyses [5], requires a better understanding of underlying mechanisms driving the spa- tial and temporal biodiversity patterns [6]. Zooplankton are a significant component of the marine ecosystem; they are the most wide- spread form of animal life on Earth, with the longest history of evolutionary continuity [7]. These organisms are also the foundation of life in the ocean, acting as the trophic link between primary producers and upper trophic levels [8], being important components of marine bio- geochemical cycles [9]. They are also known to rapidly respond to oceanographic and environ- mental variations, such as fluctuations on temperature [10], oxygenation [11], acidification [12, 13], stratification [14, 15], primary production [16, 17], upwelling [10, 18, 19], circulation [17] and advection [20], thus providing themselves essential environmental sensors for a changing ocean. Copepods are the taxon with the largest amount of accepted species described and docu- mented in the World Register of Marine Species (WoRMS) [21]. It is also a well-represented taxonomic group in several data bases for marine species, such that a relatively thorough data- base on their distribution (horizontal and vertical) is available for the South Pacific Ocean [22]. Most copepod data bases only report species occurrences, or at higher taxonomic levels,

and with emphasis on Calanoid copepods. Even that, such data can facilitate studies on the drivers controlling copepods species richness, how they are spatially structured, and the way they might respond to a changing environment. However, the study of large-scale copepod diversity patterns and their fluctuations driven by environmental forces have been scarcely conducted [23–25]. Moreover, open ocean studies on biogeography patterns of copepods are especially scarce in the Southern Hemisphere. For the vast South Pacific Ocean (SPO), that represents the largest yet the least-known marine ecosystem in the world, available data on copepods are mostly compiled in the Ocean Biodiversity Information System (OBIS) Portal [26]. An important task, however, to use these data sets for studies on copepods distribution and diversity, is the assessment of their quality in terms of resolution and coverage, both in time and space. With respect to global distribution of plankton species richness, efforts have been made to test hypotheses explaining spatial patterns [23, 27]. For instance, the species-energy hypothesis [28, 29], stating that available energy can regulate species diversity, has been widely supported [30], but how diversity becomes linked to available energy is matter of debate [23]. In the same context, the best predictor of plankton diversity in the ocean appears to be temperature, exhib- iting the greatest correlation or highest explanatory power as reported in many works [3, 23,

25, 30–35], although, when analyzing latitudinal patterns of copepod diversity in the Atlantic Ocean, Woodd-Walker et al. (2002) [36] found that species diversity was fundamentally related to variability in seasonality from the equator to polar regions rather than to tempera- ture. The explanation was that more stable conditions and continuous primary production at low latitude can promote a higher diversity, while a highly variable and strongly seasonal cycle of primary production in high latitudes can favor the dominance of fewer species. In any case, the positive correlation between sea temperature and diversity of zooplankton continues to be a key issue for predicting large-scale spatial variability of plankton communities [37, 38]. Tem- perature is one of the two most important properties of seawater controlling its density and thus governing the dynamics and circulation of the water masses [39]. Temperature also plays a key role in the range of distribution of various marine organisms [34, 40]. For instance, cope- pods exhibit a strong dependence in their vital rates with temperature [17], since they are ecto- therms, so that their population dynamics is strongly influenced by this factor, with major implications on their species composition and diversity patterns [18, 41]. PLOS ONE Biodiversity of copepods in the South Pacific PLOS ONE | https://doi.org/10.1371/journal.pone.0306440 July 11, 2024 2 / 28 and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.