Abstract
Understanding the mechanisms maintaining the biodiversity of plankton communities in marine ecosystems subject to a strongly variable ocean has become a critical issue for modern oceanography. Here, we used data on distribution of calanoid copepods in the upper layer of the ocean (0–500 m), a widely distributed taxonomic group in the pelagic realm, to assess the effects of changing oceanographic conditions on their diversity patterns and family and species richness. Copepods abundance and occurrence were evaluated from 2002 to 2015 covering the region extended between the coastal upwelling zone (CUP-Z) and the offshore region of Chile at subtropical and temperate areas. We used spatial analyses of community structure descriptors, such as abundance and diversity (family and species richness), multivariate analysis and General Additive Models (GAMS) in order to study the effect of surface temperature and its gradients, mixed layer depth, salinity and Chlorophyll-a on copepod diversity. Seventeen families were identified comprising 151 species distributed in 3 predefined zones in the onshore-offshore gradient covering the coastal upwelling, the coastal transition and the oligotrophic zones, whereas over the alongshore gradient, same families were majorly linked to the northern and southern portions of the sampled area (20–40°S). Families and species were significantly structured over the zonal gradient, revealing the dominant habitat for each of the families. Spatial patterns revealed the presence of transitional zones comprised by mixed taxa. Over the alongshore gradient this transition zone was linked to the subtropical convergence (30°S). The spatial variation of sea surface temperature (SST) revealed strong environmental zonation of temperature gradients across onshore-offshore and alongshore dimensions. Mean SST combined with mean mixed layer depth explained about 40% and about 29% of variation in family and species richness, respectively over the onshore-offshore axis. We thus conclude that the environmental zonation imposed by SST and its spatial gradients, considered as ecological barriers, is the key driver for maintaining diversity of copepods in the southeast Pacific.
Highlights
The marine pelagic environment presents a high diversity of organisms as part of a complex trophic food web (Lenz, 2012)
Copepod families and species are listed in Supplementary Table S3: Species list
Over the zonal gradient we found from the Akaike information criterion (AIC) value, as the most parsimonious criteria for best-fit Generalized Additive Models (GAMs), that a predictive model incorporated sea surface temperature (SST) and mixed layer depth (MLD) to explain both the family and species richness (Table 2)
Summary
The marine pelagic environment presents a high diversity of organisms as part of a complex trophic food web (Lenz, 2012). Biodiversity and species distribution of zooplankton are known to be affected by changes in oceanographic conditions, such as temperature, oxygenation, salinity and stratification, as it has been demonstrated in other regions of the world (Beaugrand et al, 2002; Beaugrand, 2003; Richardson and Schoeman, 2004; Peterson et al, 2006; Aronés et al, 2009; Gewin, 2010; Seibel, 2011), revealing the strong dependence of these organisms on oceanographic variables due to their limited migration capacity and so reflecting the strong effects of hydrographic patterns on their distribution and that of their prey (McClain and Barry, 2010). These changes are accompanied by variable seasonal regimes from low to high latitudes (Strub et al, 1998)
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