The Humboldt Current System (HCS) extends from southern Chile (~42-45°S) to northern Peru and Ecuador (~4°S) (Montecino and Lange, 2009; Thiel et al., 2007), being the largest of the four main Eastern Boundary Upwelling Systems, including the California, Canary, and Benguela Currents (García-Reyes et al., 2015). The HCS is also considered an important and unique biogeographic province that contains a large proportion of endemic fauna (Briggs and Bowen, 2012; Costello et al., 2017; Spalding et al., 2007, 2012). It represents the equatorward-flowing, eastern portion of the basin-scale South Pacific Subtropical Gyre which, in terms of atmospheric forcing, is mainly influenced by the South Pacific Subtropical Anticyclone or the South Pacific High (Ancapichun and Garcés-Vargas, 2015; Thiel et al., 2007), which spins counter-clockwise and is predominant off the west coast of South America (Schneider et al., 2007; Strub et al., 1998). The South Pacific Subtropical Anticyclone presents seasonal variation, abiding by its northern position (26°S, 86°W) during the late austral fall and winter, when it is also closer to the South American continent and its intensity is weaker. During austral spring and summer, it moves southwest (37°S, 108°W) and shows its maximum intensity (Ancapichun and Garcés-Vargas, 2015). As a result, winds are upwelling-favorable during the spring- summer in the southern region, and all year round in the northern region (Montecino and Lange, 2009). Regarding hydrographic characteristics, four water masses have been found in the upper 500 m of the HCS: Subtropical Water (STW), Subantarctic Water (SAAW), Equatorial Subsurface Water (ESSW), and Antarctic Intermediate Water (AAIW) (Silva et al., 2009). Of these, STW and SAAW are near surface waters containing the mixed layer. STW is found mostly in the northern region (<23°S) and the SAAW in the southern one (>28°S) (Silva et al., 2009). The HCS is also characterized by the presence of a shallow Oxygen Minimum Zone (OMZ) caused by the shoaling of the oxygen-poor ESSW during the upwelling events. Presence of the shallow OMZ results in low concentrations of dissolved oxygen near the surface (Morales et al., 1999). As forced by upwelling, the distribution of the OMZ along the water column in the HCS obeys latitudinal shifts in the upwelling regimes, with a shallower annual average depth of the upper OMZ in northern Chile (permanent upwelling) than in central south Chile (seasonal upwelling) (Yáñez et al., 2012).

Within the zooplankton community in the HCS, copepods are the most representative components (Escribano et al., 2007). These organisms respond rapidly to oceanographic and environmental variations (Escribano et al., 2014; Medellín-Mora et al., 2016; Peterson and Bellantoni, 1987; Pino-Pinuer et al., 2014; Ruz et al., 2018; Yáñez et al., 2012). In the HCS, a large part of the oceanographic variation is controlled by changes in the wind-driven upwelling over a variety of time and spatial scales. Upwelling variation can have a major influence on the ecophysiology and distribution of copepods (Escribano and Hidalgo, 2000; Peterson, 1998), ultimately affecting their diversity patterns (Hidalgo et al., 2010; Rivera et al., 2023). However, the mechanisms underlying the influence of upwelling variation on copepod diversity remain unclear. From a broad ecological perspective, most studies emphasize the importance of deterministic processes based on species niche differences (niche theory), suggesting that environmental factors maintain species richness and originate from environmental heterogeneity (e.g. Chesson, 2000; Tilman, 2004). On the other hand, Pianka’s major hypotheses (Pianka, 1966) recalled the climate stability hypothesis (Fischer, 1960; Klopfer, 1959) to explain observed patterns of diversity, and this is the only one that remains essentially unchanged and relatively unexplored (Fine, 2015; Fjeldså et al., 1997; Guerrina et al., 2024; Schemske and Mittelbach, 2017). Notably, in the HCS, such environmental stability (or instability) appears to be mostly linked to upwelling intensity, which can affect coastal circulation (Marín et al., 2001), water column stratification and temperature (Schneider et al., 2017), distribution of the OMZ and oxygenation (Sobarzo et al., 2007), and availability of food resources (quantity and quality) (Anabalón et al., 2007; Vargas et al., 2006). All these factors can have a fundamental role in the population and community dynamics of copepods in the HCS (Escribano et al., 2012), ultimately influencing their diversity patterns and potentially uploading the relevance of the climate stability hypothesis to the ocean. This is very relevant, considering that initially, the hypothesis was proposed by analyzing diversity patterns on a terrestrial two-dimensions planar scale. Still, the ocean is a three-dimensional (3D) space, and the habitats of different species in seawater are unevenly distributed within it (e.g. Fang et al., 2024). Consequently, evaluating the climate stability hypothesis from a 3D spatial perspective is necessary, revealing whether different depths in the same sea area can exhibit multiple diversity patterns, attributes, and underlying causes (e.g. Moreno et al., 2008).

Furthermore, marine biodiversity can depend on regional biogeographical limits (e.g. Hernández et al., 2005; Miloslavich et al., 2011; Moreno et al., 2006), thus the relevance of the climate stability hypothesis can potentially change depending on these limits which, in the case of HCS, are over a latitudinal environmental gradient. Particularly on the Southeast Pacific, a review by Camus (2001) suggests two primary limits delimiting three principal spatial units: one at the 30°S limiting the Peruvian Province (18°-30°S) and Intermediate area (30°-41°S), and the second at the 41°S delimiting the southern Magellanic Province (41°-56°S).

In this study, we test the climate stability hypothesis as the central modulating mechanism of copepod diversity patterns in the HCS. We used a long-term (17 years) database on copepod species occurrence in the upwelling zone off Chile. Our approach included the estimates of indices for copepod diversity and distribution from available species records within the upper 500 m, comparing upwelling regions with distinct seasonal regimes: a north zone (NZ, 20°-30°S) characterized by a permanent upwelling regime, and a south zone (SZ, 30°-40°S) characterized by a seasonal upwelling regime; and the association with environmental stability estimated from databases on oceanographic variables, including an estimate of kinetic energy in the water column, as driven by changes in upwelling intensity under different climatic forcing conditions. We evaluated the hypothesis that a more stable system would allow to find greater diversity, as a more stable system in terms of upwelling index, temperature and salinity would support greater diversity than more unstable areas. The objectives were a) to evaluate copepod species diversity variability and compare its vertical structure between north and south zones, at different depth ranges, b) to understand the environmental drivers influencing patterns of distribution of Copepoda as representatives of zooplankton in the HCS, c) to determine whether there are differences between strata and between NZ and SZ in terms of diversity, and d) to assess whether observed patterns obeyed to changes in environmental stability forced by upwelling intensity.

The assessment of copepod diversity patterns in the HCS both, over the horizontal and vertical planes, clearly indicated a strong connection between the copepod community and upwelling variation. Changes in upwelling intensity over a seasonal time scale and according to the latitudinal regime can strongly influence the habitat conditions for copepods inhabiting the upper 500 m in the HCS. These changes can affect copepod populations and ultimately the community structure, including biodiversity. It is also important to highlight the strong link between the diversity indices and environmental stability as assessed by temperature stability and kinetic energy (EKE). It has been documented that habitat stability modulates biodiversity along time (Navarro-Mayoral et al., 2024), and that climate change-induced modifications of the mechanisms sustaining ecological stability can result in taxa richness reduction and overall ecosystem malfunctioning (Wang et al., 2023). Then, more stable conditions throughout the year can allow populations to reproduce more continuously and so be present permanently, but also this stability may help maintaining the community composition more stable year-round. In this context, it can be inferred that fewer perturbations or a more predictable environment may also allow that species interactions along with their specific responses to environmental change can function more efficiently (Fischer et al., 2001) to promote a higher species richness.

It is important to stress that both temperature stability and EKE are driven by variation of upwelling intensity. Regarding latitudinal variation, it must be noted that permanent continuous upwelling regime north of 35°S derived from alongshore winds’ climatology ( Figure 2a ) and greater surface EKE ( Figure 3a ) are masked by the delimitation of the zones for this study, as the southern region (SZ) starts at 30°S. However, when considering averages for each zone, a lower monthly standard deviation of wind’s variability ( Figure 2b ) together with greater EKE along depth ( Figures 3 ) can be observed in the northern region (NZ) than in the SZ, indicating more upwelling favorable conditions in the NZ throughout the year, while the SZ shows more variability between seasons, giving lower average values. On the other hand, it has been documented that the SZ (data from 36,4°S) is characterized by a continuous alternation between strong southerly and weak northerlies (UI < 0, downwelling favorable), even at a sub-seasonal scale (Aguirre et al., 2021), with implications on biogeochemical cycles and productivity. In addition, a more poleward location of the South Pacific High during winter has caused more summer-like hydrographic conditions on the continental shelf offshore central Chile (where the SZ was delimited), generating changes in the plankton community (Jacob et al., 2018; Schneider et al., 2017) that may explain increased coastal upwelling in the HCS, and the changes in species dominance associated with it in terms of abundance (Pino-Pinuer et al., 2014), as shifts in the upwelling regimes also affects phytoplankton composition that in turn are determinant for zooplankton reproductive pulses (Vargas et al., 2006). Besides that, wind-forced upwelling can transport naturally occurring low dissolved oxygen waters towards the continental shelf and even cause mortality of different species (Roegner et al., 2011). At the HCS, NZ is characterized by an oxygen minimum zone that is relatively shallow (ca. 10–60 m depth of its upper limit) (Hidalgo et al., 2005; Morales et al., 1999) in the coastal upwelling area and adjacent oceanic waters, generating usually low oxygen concentrations to which organisms may respond in physiological or distributional terms (Morales et al., 1999; Gilly et al., 2012). Despite zooplankton may tend to aggregate above the oxycline associated with more oxygenated surface waters (Tutasi and Escribano, 2020), nauplii, small- and large-sized copepods are able to perform diel vertical migration through the upwelling zone, withstanding severe hypoxia and being able to exhibit a large migration amplitude (∼500 m, large-sized copepods and copepods of the group Eucalanidae), remaining either temporarily or permanently during day or night conditions within the core of the OMZ (Tutasi and Escribano, 2020). Therefore, considering that the OMZ occurs over the whole study area under different conditions and reaches depths of ca. 500 m, it appears that the dominant species described here in terms of occurrence for the HCS (scientific names detailed in Tables 2 and S2 ) may be adapted or have evolved the capacity to cope with low oxygen conditions, temporally through vertical migration (Tutasi and Escribano, 2020) or during more extended periods (Hidalgo et al., 2005). This also could explain the fewer exclusive species found in the 0–100 m depth range at NZ ( Supplementary Table S2 ), as different species may occur along the whole water column, whereas at the SZ more species may tend to aggregate in the upper layer for oxygen (Tutasi and Escribano, 2020) or when the OMZ becomes shallower. The clustering analysis ( Figure 5 ) indicated a similar tendency, with high values of richness in all strata at the north zone (NZ), whereas low species richness values (cold spots) were found at greater depths in the south zone (SZ).

Regarding copepod species diversity, the species common to all strata and zones ( Tables 2 and S2 ) can be considered, in terms of occurrence, dominant for the HCS off Chile as they occur between 0 and 500 m depth and between 20°-40°S. The observed distribution of these species occurrences at this study have been previously cited off Chile ( Table 2 ), and corroborates, complements or broadens the information previously documented.

From the dominant species of the HCS, Calanus chilensis has been considered as a coastal-neritic species endemic to the eastern South Pacific between 10° and 42°S (Marín et al., 1994), highly adapted to the environmental condition of the HCS (Escribano, 1998), and being later proposed as a biological indicator of the intrusion of the Humboldt Current in the Ecuadorian Pacific (Bucheli et al., 2019). More recently, a continuous distribution for the species has been modeled, that ranges from Ecuador to the southernmost area of South America between 0–200 m depth range, whereas the organisms occurring between 200–400 m have a modeled discontinuous distribution with greater suitability for the coast of Chile (Rivera et al., 2023). The neritic zone comprises the shallow coastal waters (down to 100–200 m) overlying the continental shelf (Boaden and Seed, 1985); however, in this study the occurrence of C. chilensis has been registered down to a depth of 500 m, supporting the modeled projection of this species. Together with C. chilensis, Calanus patagoniensis is another species considered endemic to the HCS, but with a more inshore distribution due to biological behavior involving vertical migration that may avoid them to be transported offshore by mesoscale processes (Morales et al., 2010). When revising our data, we can corroborate that for the south zone, that has more data available towards open ocean, C. chilensis occurs along all its extension, while C. patagoniensis occurrences are restricted until the 73°W centroid. Oithona similis, Paracalanus indicus, Pleuromamma gracilis and P. quadrungulata are species that we found to be dominant in terms of occurrence and that have been described in other studies as dominant for the HCS (Escribano et al., 2012; Pino-Pinuer et al., 2014). Here, we found these species occurrence to be restricted until the 73°W centroid as well. P. indicus dominance in terms of abundance has been documented to vary strongly over the south zone, being outnumbered by Drepanopus forcipatus (Pino-Pinuer et al., 2014). In this study, D. forcipatus did not appear to be dominant in terms of occurrence as it only appeared between 0–200 m at NZ and SZ ( Supplementary Table S2 ), and restricted until the 73°W centroid, which is concordant with its known distribution restricted to coastal and shelf areas along southern South America (Park and Ferrari, 2009). Additionally, it has been reported that the regional distribution of C. chilensis secondary production extends much further offshore in the HCS than what is typical for its ecological counterparts in other coastal upwelling systems, extending the area of high productivity which in turn maximizes trophic transfer efficiency towards economically important pelagic fish (Schukat et al., 2021). These aspects involve specific life-strategy traits and life-cycle adaptations (Schukat et al., 2021) that highlights C. chilensis importance as an endemic species adapted to the environmental variability of the HCS.

Acartia tonsa is a widely studied species in the HCS (e.g., Aguilera, 2020; Aguilera and Bednaršek, 2022; Ruz et al., 2015; Yáñez et al., 2018), considered cosmopolitan and characteristic of estuarine and upwelling systems, while also associated to the inshore area (Morales et al., 2010). In this study, this species did not appear to be dominant for the HCS, however it is present in both NZ and SZ between 0–300 m ( Supplementary Table S2 ) and restricted until the 73°W centroid, thus it may be considered an upper-mesopelagic species.

Paraeuchaeta barbata appeared as an exclusive species for the 400–500 m stratum of the NZ ( Supplementary Table S2 ). This calanoid species occurs in deep water throughout the world oceans (Park, 1994), being a very common species between the Southern Ocean and the Arctic basin (Park and Ferrari, 2009) associated with the Antarctic Intermediate Water (AAIW) during austral summer and winter when migrating from and arriving to the Chilean coast, respectively (Yamanaka, 1976). This demonstrates that vertical zonation occurs in the HCS and is associated with seasonal physical-biological processes that in turn may be linked to temperature. Moreover, various species found in the HCS are characteristic of the Southern Ocean (SO) (Park and Ferrari, 2009), such as Metridia lucens and Pleuromamma quadrungulata, deep water species in the SO considered dominant species in this study (found between 0–500 m depth and in both the NZ and SZ, Table 2 ); Gaetanus brevispinus, G. kruppii, Lophothrix frontalis (deep water species in the SO), Metridia gerlachei (very common herbivore calanoid in the SO), found between 0–500 m in the NZ in this study ( Supplementary Table S2 ); Calanus propinquus, Neocalanus tonsus. (epipelagic calanoids of the SO, the latter can perform diapause), Gaetanus tenuispinus (very common deepwater species in the SO), found between 0–500 in the SZ in this study ( Supplementary Table S2 ); Haloptilus oxycephalus, Scolecithricella minor (very common deepwater calanoids of the SO that reach temperate and subarctic regions, respectively), Ctenocalanus citer (epipelagic calanoid endemic to the SO) found between 0–100 m in the SZ in this study ( Supplementary Table S2 ). These examples indicate intrusion and colonization of epipelagic and deepwater copepods from the SO, that are adapted to that productive habitat (Park and Ferrari, 2009), into the HCS. Thus, the HCS must be a suitable habitat for increasing regional diversity, especially that of calanoid copepods.

Generally, when several models produce quite similar AICc values (i.e., within 1 to 2 units of each other), it suggests that there is a reasonable amount of redundancy among predictor variables, as several different combinations of them could be used interchangeably to explain the observed relationship (Anderson et al., 2008). In this study, this was not the case ( Table 4 ), so it can be inferred that the predictors are not inter-correlated. The fact that mean temperature, mean salinity, and oxygen concentration are the main predictors for richness indices such as alpha diversity, Shannon-Wiener index and Hurlbert index relates to the kinetic energy and the environmental stress hypotheses, as higher temperatures are beneficial for ectotherms development, which may lead to higher speciation rates (Fraser and Currie, 1996; Tittensor et al., 2010); whereas environmental stress in terms of temperature and salinity extremes (Fraser and Currie, 1996; Brucet et al., 2009; Hamil et al., 2020; Yuan et al., 2020; Jiménez-Melero et al., 2023) and oxygen depletion (Tittensor et al., 2010) is negatively correlated with diversity. This also reflects the relevance that water masses and the OMZ may have on copepod populations, as the environmental conditions they generate have effects on different aspects of copepods ecology in the HCS (Escribano et al., 2014; Ruz et al., 2018). The importance of temperature stability and EKE as main predictors of species composition and its additive components turnover and nestedness, relates to the climate stability and historical factors hypotheses, that assumes higher diversity in more environmentally stable regions (Tittensor et al., 2010) and provide habitat features that favor extant patterns of dispersal and richness (Fraser and Currie, 1996), respectively. Therefore, mesoscale activity seems to be an important feature at a regional scale for zooplankton, as it has been documented how copepods are transported offshore by eddies, with some species potentially coupling their ontogeny to that process, such as Calanus chilensis (Schukat et al., 2021).

Nevertheless, despite our results exhibiting different patterns between zones and strata for the HCS, we may find a potential limitation for studying biogeographic patterns in this area due to the lack of occurrence data for certain regions, that together with the fact that information is derived from different sampling methods, sampling gears and protocols of analysis, may restrict our understanding of the patterns and processes driving copepod diversity in the HCS. However, future research utilizing species distribution models (SDM) could help mitigate these data gaps.

In summary, it appears that even at regional scale, different aspects related to basal hydrographic conditions, as well as the coupling of physical and biological processes are affecting diversity, with responses to their variations in terms of phenotypic plasticity that are species-specific. This, added to the individual variability (Bi and Liu, 2017), may generate a variety of responses on copepod species. Moreover, within the context of climate change and global warming, it has been described that the effects of temperature variation on different marine communities at different scales are translated into a slowly replacement of cold-tolerant species with warm-tolerant counterparts, with the former being redistributed to greater depths (Burrows et al., 2019), without necessarily decreasing the system’s functionality or shifting horizontal patterns, unless the species interact with the surface for the need of light, or as in the case of copepods, for feeding following DVM. Then, some increased depth range found in this study for some copepod species may be related to warming responses and vertical redistribution. However, this may represent an increase in the metabolic activity for some individuals not adapted for low oxygen conditions or that perform their life cycles associated with processes occurring at shallower depths. Considering the dynamism of the HCS and the increased capacity of plankton to rapidly respond to environmental shifts in temporal and special scales, greater assessments are required for better understanding copepods distribution and their shifts extent, with an associated need for an enrichment of ecological data bases in terms of temporal and spatial coverage, for better implementing macroecological modeling techniques that yield more robust spatial patterns and their associated dominant drivers or processes.