• Aim

The latitudinal diversity gradient (LDG) is a consequence of evolutionary and ecological mechanisms acting over long history, and thus is best investigated with organisms that have rich fossil records. However, combined neontological-paleontological investigations are mostly limited to large, shelled invertebrates, which keeps our mechanistic understanding of LDGs in its infancy. This paper aims to describe the modern meiobenthic ostracod LDG and to explore the possible controlling factors and the evolutionary mechanisms of this large-scale biodiversity pattern.

• Location

Present-day Western North Atlantic

• Taxon

Ostracoda

• Methods

We compiled census data from ostracods living in shallow marine environments of the western North Atlantic Ocean. Using these data, we documented the marine LDG with multiple metrics of alpha, beta (nestedness and turnover), and gamma diversity, and we tested whether macroecological patterns could be governed by different environmental factors, including temperature, salinity, dissolved oxygen, pH and primary productivity. We also explored the geologic age distribution of ostracod genera to investigate the evolutionary mechanisms underpinning the LDG.

• Results

Our results show that temperature and climatic niche conservatism are important in setting LDGs of these small, poorly-dispersing organisms. We also found evidence for some dispersal-driven spatial dynamics in the ostracod LDG. Compared to patterns observed in marine bivalves, however, dispersal dynamics were weaker and they were bi-directional, rather than following the “out-of-the-tropics” model.

• Main Conclusions

Our detailed analyses revealed that meiobenthic organisms, which comprise two-thirds of marine diversity, do not always follow the same rules as larger, better-studied organisms. Our findings suggest that the under-studied majority of biodiversity may be more sensitive to climate than are well-studied, large organisms. This implies that the impacts of ongoing Anthropocene climatic change on marine ecosystems may be much more serious than presently thought.

1.  Present-day shallow marine ostracod data

We constructed a comprehensive, equator-to-pole compilation of ostracod censuses from the western North Atlantic and the Arctic Oceans. We standardized ostracod taxonomy and integrated census data from previously published studies, which involved restudy of previous collections. We obtained complete ostracod census data from the original faunal slides from the US Geological Survey’s collections, including those of Hazel (1970), Valentine (1971), Cronin (1983), Cronin (1990) and Lyon (1990). Taxonomy was based on Cronin (1990) with additional information for high latitude species from Yasuhara et al. (2012) and Gemery et al. (2015).

In addition to the census data from the USGS collections, census data from Kontrovitz (1976), Teeter (1975), and the Arctic Ostracode Database (Gemery et al., 2015) were added to our dataset after standardizing taxonomy using SEM pictures and descriptions from the literature. Locality information (longitude, latitude and water depth) was obtained from the original cruise reports and literature.

2. Environmental parameters

Environmental parameters were obtained from various databases (Appendix 2 and Table S3). Mean annual values (1955 – 2012) for temperature (°C), dissolved oxygen (ml/L) , and salinity were obtained from the World Ocean Atlas (Baranova, 2015). pH data were obtained from the Global Ocean Data Analysis Project Bottle Data (version 2) (Olsen et al., 2016). Mean sea surface net primary productivity (NPP, in mgC/m²/day) data (1998-2014) were obtained from Oregon State University Ocean Productivity Centre (http://www.science.oregonstate.edu/ocean.productivity/) (Behrenfeld and Falkowski, 1997). Sea bottom environmental parameters were interpolated for each standard depth [based on World Ocean Atlas (Baranova, 2015)] using the Triangulated Irregular Network (TIN) interpolation from the plugin “interpolation” of QGIS (QGIS Development Team, 2016) with geographic range: latitude = 0 – 90°N; longitude = 100°W – 30°E. Interpolated environmental raster layers were overlaid with ostracod sites in QGIS (QGIS Development Team, 2016) and Point Sampling Tool was used to extract the values of environmental parameters at each site.