Energy and the factors influencing its utilization have emerged as potential unifying causes of spatial and temporal gradients in biodiversity. Three distinct types of energy have been postulated to play a role in structuring ecological communities; radiation (light), thermal (heat) and chemical (Gibbs free energy in reduced carbon compounds in tissues; [1
]). In the vast deep-sea soft-sediment environment, photosynthetically active radiation is entirely absent. This simplified system of energetic inputs, together with the extremes of both thermal and chemical energy experienced by these organisms, mean that it represents an excellent testbed for species–energy hypotheses.
Particulate organic carbon (POC) flux represents the ‘rain’ of chemical energy available to deep-sea organisms from the euphotic zone. Flux into the ocean's interior decreases with depth as the material is remineralized and with distance seaward from productive coastal waters. This results in an exponential decline in benthic biomass and abundance with increasing ocean depth [2
]. Temperature per se
is not a form of energy, but often interpreted as an expression of thermal energy [1
]. It directly influences the rate processes of population dynamics and biotic interactions, and regulates the biochemical kinetics of metabolism [3
Recently, intriguing evidence has emerged that temporal variation in deep-sea meiofaunal diversity can be related to temperature. Hunt et al
] compared foraminiferan diversity during the last 130 000 years to proxy variables for temperature and productivity. Meta-analysis showed a significant and positive overall temporal relationship with temperature, and no relationship with productivity. Deep-sea ostracod diversity in individual cores has also shown a positive relationship to temporal variation in temperature and a negative relationship to productivity [6
]. Furthermore, temperature is the most consistent predictor of modern broad-scale species diversity for a wide array of taxa in the shallow ocean [8
Bathymetric gradients of benthic molluscan diversity are often unimodal, peaking at mid-bathyal depths [9
]. Diversity appears to be depressed at abyssal depths because extremely low densities make populations vulnerable to chronic local extinction. Causes of lower diversity at upper bathyal depths are much less clear, but may result from accelerated competitive exclusion owing to greater population growth driven by proximity to high coastal production, or guild interactions resulting from bioturbation by large mobile megafauna [10
Here, we use spatially explicit regression models to determine the effects of POC flux, temperature and depth (a potential confounding factor) on diversity in modern deep-sea bivalves [11
] and gastropods [12
], collected from four basins (North American, West European, Guiana and Gambia) in the North Atlantic (). Although previous studies have related diversity to the environment in the deep sea, this is, to our knowledge, the first study to explicitly test for species–energy relationships in the modern day fauna of the deep ocean. Earlier efforts to infer the relationship between diversity and productivity relied on three proxy variables for productivity: depth, animal density and the application of POC flux-depth algorithms to surface production [9
]. We use a newer model developed by Lutz et al
] which integrates global data on satellite imagery with POC flux measured at deep-moored sediment traps to provide a more direct estimate of food supply at depth.
Locations of study samples (white circles) superimposed on particulate organic carbon (POC) flux, temperature and bathymetry.