Biodiversity's big wet secret: the global distribution of marine biological records reveals chronic under-exploration of the deep pelagic ocean

Abstract

Background: Understanding the distribution of marine biodiversity is a crucial first step towards the effective and sustainable management of marine ecosystems. Recent efforts to collate location records from marine surveys enable us to assemble a global picture of recorded marine biodiversity. They also effectively highlight gaps in our knowledge of particular marine regions. In particular, the deep pelagic ocean--the largest biome on Earth--is chronically under-represented in global databases of marine biodiversity.

Methodology/principal findings: We use data from the Ocean Biogeographic Information System to plot the position in the water column of ca 7 million records of marine species occurrences. Records from relatively shallow waters dominate this global picture of recorded marine biodiversity. In addition, standardising the number of records from regions of the ocean differing in depth reveals that regardless of ocean depth, most records come either from surface waters or the sea bed. Midwater biodiversity is drastically under-represented.

Conclusions/significance: The deep pelagic ocean is the largest habitat by volume on Earth, yet it remains biodiversity's big wet secret, as it is hugely under-represented in global databases of marine biological records. Given both its value in the provision of a range of ecosystem services, and its vulnerability to threats including overfishing and climate change, there is a pressing need to increase our knowledge of Earth's largest ecosystem.

Figure 1. The depth distribution of OBIS records of global marine biodiversity.

A. Number of OBIS records against ocean depth (grey symbols); the general trend is illustrated with a lowess smooth (solid line). Horizontal dashed lines indicate the divisions into regions A-E defined by depth at 200, 1000, 4000 and 6000m (see Table 1). B. The proportion of all OBIS records occurring in the different depth zones identified in Table 1, against the proportion of the global ocean that occurs at those depths. The 1∶1 line identifies those areas of the ocean with proportionately more (points above the line) or fewer (points below the line) records than expected given their area. This gives a conservative view of under- and over-representation based on the volume of each habitat.

Figure 2. Global distribution within the water column of recorded marine biodiversity.

The horizontal axis splits the oceans into five zones on the basis of depth (see Table 1), with the width of each zone on this axis proportional to its global surface area. The vertical axis is ocean depth, on a linear scale. This means that area on the graph is proportional to volume of ocean. For instance, in the deep sea each cell of 200m depth represents c. 3.5×10⁶ km³ (see cell drawn separately for scale). The number of records in each cell (each unique combination of sample and bottom depth, following the scheme in Table 1) is standardised to the volume of water represented by that cell, and then log₁₀-transformed. The inset shows in greater detail the continental shelf and slope, where the majority of records are found.

Figure 3. The proportion of recorded marine biodiversity originating from the midwater pelagic ecosystem.

The ocean is split into the depth zones defined in Table 1. Midwater is defined as all of the water column except the 10m nearest the surface or the 10m (for the continental shelf) or 100m (for the mesopelagic continental slope) above the sea bed, and for the other ocean zones as the water column excluding the 100m nearest the surface and the 200m (for the bathypelagic zone and the abyssal plain) or 1000m (for the hadal zone) above the sea bed. For each zone, the plot shows the median, interquartile range and total range of observed proportions. The shallowest two depths (0—10m and 10—20m) are excluded, as there is no midwater according to our definition.