Twilight zone to deep-ocean floor. Developing an understanding of particle dynamics and trophic interactions using a molecular experimental approach

Lead Research Organisation: University of Liverpool
Department Name: Earth Surface Dynamics

Abstract

Photosynthetic primary production is the basis of much of the oceanic food chain. The energy fixed by phytoplankton is transferred to grazers and to higher consumers. The efficiency of the energy transfer between producer and consumer reflects the state of the ecosystem, for example nutrient replete vs nutrient deficient. Through sinking, particulate organic matter (POM) from surface waters is transferred to the deep sea, where it is an important resource for deep-dwelling animals, be they living in pelagic (in the water) or benthic (at the sea floor) environments. We know that in the northern Atlantic Ocean at mid- to high latitude, the deposition of POM is seasonally driven by the surface water spring bloom. However, there is considerable variability in the composition of the POM, which is driven partly by the nature of the phytoplankton, but also by the organisms that graze on them. Much of the POM leaving surface waters is lost in the so-called twilight zone; processes occurring there also presumably modify the chemical composition of the sinking POM. We know virtually nothing about the biological processing of POM that occurs in the twilight zone. The material arriving at the sea floor which feeds the benthic animals is potentially highly altered and depleted in many chemicals, nevertheless, the deep-sea community is highly diverse. In this project, we will work with scientists at the National Oceanography Centre, Southampton and the University of Bristol to try to shed some light on how the energy locked up in organic matter fixed in surface waters is partitioned between consumers living in the deep sea. We will study in detail, the chemistry of particulate material, which includes the phytoplankton and small size organisms that feed on it (bacteria and microzooplankton), the biochemistry of the zooplankton and larger animals living in the deep sea and at the sea floor. We will also carry out incubation experiments using the remotely operated vehicle, Isis, in order to assess how micro-organisms break down organic matter in the twilight zone and how and whether large animals living at the sea floor are able to react to an influx of POM.

Publications

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FitzGeorge-Balfour T (2010) Phytopigments as biomarkers of selectivity in abyssal holothurians; interspecific differences in response to a changing food supply in Deep Sea Research Part II: Topical Studies in Oceanography

 
Description The oceanic biological pump transfers 5 Pg of carbon per year from the atmosphere to the deep sea via phytoplankton photosynthesis and the production of particulate organic matter (POM) in surface waters. The subsequent transfer of part of this POM to deeper waters occurs by sinking of particles following senescence and aggregation, or zooplankton grazing. During sinking, the POM is remineralised through microbial action or by reworking by larger organisms. Below the photic zone, the POM flux is rapidly attenuated through the "Twilight Zone" (TZ), the stratum of the water column where sunlight diminishes to zero (~100 - 1000 m) and which is a region of great significance in biogeochemical cycling. We speculated that there are numerous possible pathways of POM transport/repackaging through the TZ, which in turn influence the chemical signature of material arriving at the sea floor and thus potentially impact the benthic community.



In 2009 during a cruise to the Porcupine Abyssal Plain (PAP, RRS Discovery 341), we collected size-fractionated suspended particles (0.5 - 53 _m and > 53 _m) through the water column using stand-alone pumps and, at similar depths, sinking particles using the NOCS PELAGRA sediment trap system. The chemical composition of the small suspended, large suspended (or slowly sinking) and sinking particles were decoupled, suggesting potentially different sources and remineralisation processes were influencing their composition. For example, a decreasing C/N ratio with increasing water depth for large suspended particles contrasts with an increasing C/N ratio in the small suspended pool. Biological marker data (lipids, amino acids) confirms that the large suspended particles have a likely source from zooplankton exudates and debris increasingly colonized by bacteria at depth, whereas the smaller particles derive largely from phytoplankton and microbial POM. POM concentrations (including labile lipids such as polyunsaturated fatty acids, which have a half life of days, as shown by laboratory degradation studies) do not simply decrease gradually with depth. Instead, there is a consistent suspended (sPOM) maximum between 300 and 600 m. We believe that this accumulation of sPOM, rich in labile and nutritionally valuable lipids at depth most likely reflects active biological transport, involving zooplankton diel migration and excretion of (lipid-rich) fecal pellets.



Surprisingly, composite PELAGRA data from 11 deployments indicate that there is an increasing flux of sinking particles at similar depths. The biochemical composition of these rapidly sinking particles changes little with depth and is similar to the near-surface small suspended particle pool. Notably, the deepest samples do contain distinctive zooplankton-derived biomarkers, which implies that zooplankton occupying the deep daytime resting zone produce new sinking particles at depth, in particular, carnivorous zooplankton (e.g., chaetognaths) feeding on other, lipd-rich zooplankton (e.g., copepods). . Deep-sinking POM recovered from moored sediment traps at the PAP (3900 m, 1000 m above the sea floor) has a markedly different biochemical composition, with little preservation of labile material. While this may reflect poor preservation of POM in the sediment traps, the biochemical composition is nonetheless similar to that of the surficial sediments at PAP and therefore probably reflects the degradation of POM as it sinks below the TZ to the sea floor. This also suggests that most valuable lipids (e.g. polyunsaturated fatty acids) are largely kept within the food web of the upper and intermediate water column, i.e. the TZ. Furthermore, it appears that the lower boundary of the TZ may finally be defined by the lower limit of the zooplankton diel migration and the zone of deep biological activity associated to it.



There is not a consistent supply of POM to the deep-sea floor; we showed that there is temporal variability in the supply of some essential compounds, particularly the carotenoids, to holothurians which dominate the benthic fauna. This may be linked to the reproductive success of certain species (Fitzgeorge-Balfour et al., 2010). Stable isotopic analyses (_13C) of holothurians tissues confirm highly enriched carbon values (~ -14 ‰) which are consistent with those measured in holothurians in other eutrophic deep-sea settings.
Exploitation Route As yet unclear, but the most likely importance will be in understanding the health of the oceans, specifically with respect to ecosystem services. Essentially, the value of this research will be in refining of climate models which currently use a highly simplified and simplistic model for the oceanic biological pump. The identification of clear compositional differences in particles pools of different sinking rates and their different chemistry has important implications for understanding their reactivities.
Sectors Environment