Trophic relationships of benthic fauna at the Oman Margin: the impact of mass jelly falls.

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

Abstract

The Arabian Sea is one of the most biologically productive areas of the world's oceans. It is also characterised by an oxygen minimum zone (OMZ); OMZ's are areas of water that have very low oxygen concentrations. Our study area is the Oman continental margin, here the OMZ impinges on the sediment community. This has implications for the fauna living here. Where oxygen concentrations are low, the faunal community will be characterised by soft bodied animals that are able to withstand these conditions e.g. polychaete worms and not molluscs or crustaceans. The Oman margin is subject to two monsoon seasons; these lead to mixing of the water column which brings nutrients to the surface waters feeding microscopic plants, which when they sink and die form a food source for the sediment faunal community. Additionally under conditions of low oxygen concentrations, bacteria can use sulphur to fix carbon for energy. This is known as chemosynthesis and these bacteria also provide a potential food source for the sediment community. The bacteria can be free living, or can form mats on the sediment surface for the fauna to graze on. The balance between food sources and oxygen concentrations is likely to be critical to the benthos and any perturbation may lead to a shift in community structure. We intend to investigate the food sources available to the sediment community and to understand the trophic linkages that exist within the sediment community. We believe that where oxygen concentrations are low, bacterial food sources may play an important role in trophic dynamics and where oxygen is high the dominant food source will be the microscopic plants. We have a unique opportunity to study the impact of enhanced hypoxia on sedimentary communities, as the result of a huge fall of decaying jellyfish across a large area of the Oman Margin, within and below the OMZ which led to the development of chemosynthesis and a potential food source for the benthic animals. We will compare the benthic community structure with that at the Pakistan Margin, where we have an existing data set for an OMZ where samples were collected at a similar time, but where there was falls of jelly fish were absent.

Publications

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Jeffreys R (2015) The trophic and metabolic pathways of foraminifera in the Arabian Sea: evidence from cellular stable isotopes in Biogeosciences

 
Description Naturally occurring oxygen minimum zones (OMZ, dissolved oxygen concentrations < 22 x 10-6 M) cover an area of over one million km2 of continental shelf and bathyal seafloor. Anthropogenic activity is increasing the area of hypoxic seafloor, for example in the Gulf of Mexico, the Black Sea and Baltic Sea. As a result of global climate change, large increases in biological productivity (> 300 %) have been observed in the Arabian Sea which has led to large blooms and subsequent kills of fish, jelly fish and pelagic crabs. Mass deposition of jelly fish on the Oman margin led to the development of bacterial mats and a potential chemosynthetic food source for the benthic fauna. Understanding the balance between food supply, its quality and oxygen concentrations on benthic community structure is vital in determining change in extreme environments. We studied a natural OMZ at the Oman Margin in the Arabian Sea, along a gradient of increasing faunal densities and compared this with an existing data set for the Pakistan Margin. We hypothesised that:

i) At low O2 concentrations (<22 x 10-6 M) both chemosynthetic and photosynthetic food sources are present and are utilised by the fauna, and that there is greater trophic diversity in the benthos than in areas where oxygen concentrations are > 22 x 10-6 M and photosynthetically-derived food sources are the main food source and trophic redundancy will prevail.

ii) Differences in food quality available to the benthos across the Oman margin, together with oxygen concentrations, govern benthic community structure.



Sediment cores and megafauna were collected at 12 depths (300 to 3200m) spanning the OMZ at the Oman Margin on CD143 following the SW monsoon during November 2002. Sediment cores were sliced and sieved on a 300 micron sieve and the macrofauna were sorted on ice to the lowest possible taxonomic level using a stereo microscope. Sediment samples were also taken for protozoan meiofauna (foraminiferans) and bacteria. Samples were then prepared and analysed for total organic carbon and nitrogen and for stable isotopic analyses (SIA) of the organisms and their food sources (surrounding sediments).



We confirmed hypothesis i), showing that there was evidence of utilization of both chemosynthetic and photosynthetic sources of carbon and nitrogen within the OMZs at the Oman and Pakistan Margins in all of the size classes. For example, in the foraminiferans, at 363m water depth, within the Oman Margin OMZ, miliolid and agglutinated foraminifera have enriched _13C signatures (2.10 & -0.53 ‰, respectively) implying carbon fixation by either Form II Rubisco or reverse-tricarboxylic acid (r-TCA) pathways. Depleted _15N (-12 to 5 ‰) signatures in the OMZ at both margins also imply the ingestion of chemosynthetic nitrifying bacteria by certain foraminifera. Most strikingly, at 2030m at the Oman Margin, well below the OMZ, foraminifera tentatively identified as Ammolagena clavata, were also highly enriched in 13C (_13C = -2.73 ‰), implying that they also feed on chemosynthetic food sources, perhaps associated with bacterial mats colonising the mass jelly fall.



With respect to hypothesis ii), differences in food quality between the Oman and Pakistan Margins are evident, so that % TOC and %N contents of surficial sediments are ~1 % and ~0.2 % higher at Oman cf. Pakistan Margin, respectively. Hydrogen indices (a proxy for bound lipid) of Oman Margin surface sediments were also significantly higher than those of the Pakistan Margin, which is consistent with higher OM quality at Oman. However, at Oman, food quality remains high even below the OMZ and so we speculate that the principal control on the benthic community is imposed by oxygen stress. There is some evidence that the jelly falls imposed local anoxia and that the resultant chemosynthetic environments provided a food source for certain opportunistic benthic species.



These findings represent the first direct comparison of the trophic structure of the benthic community at the OMZs of the Oman and Pakistan Margins of the Arabian Sea. The study reveals some substantial differences between them, but signals the relative importance of chemosynthetic food sources at both margins. We will submit three manuscripts for publication in International Journals and have presented our work through invited presentations at National and International Meetings.
Exploitation Route Not clear at present, although likely to be of significance in the health of the oceans and in policy pertaining to management of low oxygen ecosystems. The oxygen balance in the oceans is crucial as it is tied into the functioning of all ecosystems and into the cycling of important greenhouse gases (carbon dioxide, nitrous oxide and methane). Furthermore, impacts of deoxygenation are clear through mass kills of marine organisms, which impact local fisheries etc. Finally, the ecosystem services of the benthos are poorly understood. The value of this small-scale study are therefore in enhancing our understand, which will ultimately feed into better models of these environments.
Sectors Communities and Social Services/Policy,Environment