Martin J. Cox

Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007–2010. The MAR, 3,704,404 km2 in area, accounts for 44.7% lower bathyal habitat (800–3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.

Shapes of Krill Swarms and Fish Schools Emerge as Aggregation Members Avoid Predators and Access Oxygen

Many types of animals exhibit aggregative behavior: birds flock, bees swarm, fish shoal, and ungulates herd. Terrestrial and aerial aggregations can be observed directly, and photographic techniques have provided insights into the behaviors of animals in these environments and data against which behavioral theory can be tested. Underwater, however, limited visibility can hamper direct observation, and understanding of shoaling remains incomplete. We used multibeam sonar to observe three-dimensional structure of Antarctic krill shoals acoustically. Shoal size and packing density varied greatly, but surface area:volume ratios (roughnesses) were distributed narrowly about ∼3.3 m(-1). Shoals of clupeid fish (e.g., sardine, anchovy) from geographically and oceanographically diverse locations have very similar roughnesses. This common emergent shape property suggests common driving forces across diverse ecosystems. Group behavior can be complex, but a simple tradeoff--that we model--in which individual fish and krill juggle only their access to oxygen-replete water and exposure to predation can explain the observed shoal shape. Decreasing oxygen availability in a warming world ocean may impact shoal structure: because structure affects catchability by predators and fishers, understanding the response will be necessary for ecological and commercial reasons.

Biogeography of the Global Ocean’s Mesopelagic Zone

The global oceans near surface can be partitioned into distinct provinces on the basis of regional primary productivity and oceanography [1]. This ecological geography provides a valuable framework for understanding spatial variability in ecosystem function but has relevance only partway into the epipelagic zone (the top 200 m). The mesopelagic (200-1,000 m) makes up approximately 20% of the global ocean volume, plays important roles in biogeochemical cycling [2], and holds potentially huge fish resources [3-5]. It is, however, hidden from satellite observation, and a lack of globally consistent data has prevented development of a global-scale understanding. Acoustic deep scattering layers (DSLs) are prominent features of the mesopelagic. These vertically narrow (tens to hundreds of m) but horizontally extensivexa0(continuous for tens to thousands of km) layers comprise fish and zooplankton and are readily detectable using echosounders. We have compiledxa0a database of DSL characteristics globally. We show that DSL depth and acoustic backscattering intensity (a measure of biomass) can be modeled accurately using just surface primary productivity, temperature, and wind stress. Spatial variability inxa0these environmental factors leads toxa0axa0natural partition of the mesopelagic into tenxa0distinct classes. These classes demark a more complex biogeography than the latitudinally banded schemes proposed before [6, 7]. Knowledge of how environmental factors influence thexa0mesopelagic enables future change to be explored:xa0we predict that by 2100 there will be widespread homogenization of mesopelagic communities andxa0that mesopelagic biomass could increase byxa0approximately 17%. The biomass increase requires increased trophic efficiency, which could arise because of ocean warming and DSL shallowing.

Fewer but Not Smaller Schools in Declining Fish and Krill Populations

Many pelagic species (species that live in the water column), including herring and krill, aggregate to form schools, shoals, or swarms (hereafter simply schools, although the words are not synonyms). Schools provide benefits to individual members, including locomotory economy and protection from predators that prey on individuals, but paradoxically make schooling species energetically viable and commercially attractive targets for predators of groups and for fishers. Large schools are easier to find and yield greater prey/catch than small schools, and there is a requirement from fields as diverse as theoretical ecology and fisheries management to understand whether and how aggregation sizes change with changing population size. We collated data from vertical echosounder surveys of taxonomically diverse pelagic stocks from geographically diverse ecosystems. The data contain common significant positive linear stock-biomass to school-number relationships. They show that the numbers of schools in the stocks change with changing stock biomass and suggest that the distributions of school sizes do not change with stock biomass. New data that we collected using a multibeam sonar, which can image entire schools, contained the same stock-biomass to school-number relationship and confirm that the distribution of school sizes is not related to changing stock size: put simply, as stocks decline, individuals are distributed among fewer schools, not smaller schools. Since school characteristics affect catchability (the ease or difficulty with which fishers can capture target species) and availability of prey to predators, our findings have commercial and ecological implications, particularly within the aspirational framework of ecosystem-based management of marine systems.

A method for identifying Sound Scattering Layers and extracting key characteristics

Mid-trophic level water-column (pelagic) marine communities comprise millions of tonnes of zooplankton and micronekton that form dense and geographically extensive layers, known as sound scattering layers (SSLs) when observed acoustically. SSLs are ubiquitous in the global ocean, and individual layers can span entire ocean basins. Many SSLs exhibit clear diel vertical migration behaviour. Vertical migrations contribute substantially to the ‘biological pump’, such that SSLs have important global biogeochemical roles: SSLs are important conduits for vertical energy and nutrient flow. Ship-based remote sensing of SSLs using acoustic instruments (echosounders) enables their shape and density to be quantified, but despite SSLs being discovered in the 1940s, there is no consistent method for identifying or characterising SSLs. This hampers ecological and biogeographical studies of SSLs. We have developed an automated and reproducible method for SSL identification and characterisation, the sound scattering layer extraction method (SSLEM). It functions independently of echosounder frequency and the spatial scale (vertical and horizontal) of the data. Here we demonstrate the SSLEM through its application to identify SSLs in data gathered to a depth of 1000xa0m using 38 kHz hull-mounted echosounders in the south-west Indian Ocean and Tasman Sea. SSLs were identified in the water column as horizontally extensive echoes that were above background noise. For each identified SSL, a set of 9 quantitative ‘SSL metrics’ (describing their shape, dynamics and acoustic backscattering distribution) were determined, enabling inferences to be made concerning the spatial arrangement, distribution and heterogeneity of the biological community. The method was validated by comparing its output to a set of visually derived SSL metrics that were evaluated independently by 8 students. The SSLEM outperformed the by-eye analysis, identifying three times the number of SSLs and with greater validity; 95% of SSLs identified by the SSLEM were deemed valid, compared to 75% by the students. In the same way that data obtained from satellites have enabled the study and characterisation of global phytoplankton distribution and production, we envisage that the SSLEM will facilitate robust, repeatable and quantitative analysis of the growing body of SSL observations arising from underway-acoustic observations, enhancing our understanding of global ocean function.