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Trophic Cascade Modelling in Bathyal Ecosystems

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Trophic Cascade Modelling in Bathyal Ecosystems Doctoral Thesis Trophic cascade modelling in bathyal ecosystems Author: Supervisor: Valeria Mamouridis Dr. Francesc Maynou A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in the Bio-Economic Modelling Research Group Dept. de Recursos Marins Renovables (ICM-CSIC) and the Dept. de Ingenier´ıa Hidr´aulica, Mar´ıtima y Ambiental (EHMA-UPC) February 2015 Declaration of Authorship I, Valeria Mamouridis , declare that this thesis titled, ’Trophic cascade modelling in bathyal ecosystems’ and the work presented in it are my own. I confirm that: ∎ This work was done wholly or mainly while in candidature for a research degree at this University. ∎ Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated. ∎ Where I have consulted the published work of others, this is always clearly attributed. ∎ Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. ∎ I have acknowledged all main sources of help. ∎ Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself. Signed: Date: iii INSTITUT DE CI ENCIES` DEL MAR (CSIC) UNIVERSITAT POLIT ECNICA` DE CATALUNYA Abstract Dept. de Ingenier´ıa Hidr´aulica, Mar´ıtima y Ambiental (EHMA-UPC) Dept. de Recursos Marins Renovables (ICM-CSIC) Doctor of Philosophy Trophic Cascade Modelling in Bathyal Ecosystems by Valeria Mamouridis In this Ph.D. different aspects of the bathyal ecosystem have been treated with the final aim to investigate trophic cascade mechanisms driven by fishery (top- down control), a phenomenon that in bathyal systems has never been studied. This was exemplified by the in-depth study of the exploited soft-bottom continen- tal slope off Catalonia (NW Mediterranean), where the important fishery of the highly priced red shrimp, Aristeus antennatus , is carried out and the structure of mayor pathways of the trophic web is known. However for the (almost complete) lack of information about the fauna in the sediment, this thesis presents a first quantitative seasonal study on the macrobenthos (infauna) in the studied habitat. Also an analysis on the landings per unit effort (LPUE) of the red shrimp has been developed to define the principal influential variables on this stock. All these studies have in common the investigation of the dynamics of the bathyal system at different levels of organization (population, assemblage and ecosystem) in relation to environmental and fisheries drivers. In Part I (Chapter 2) the infaunal assemblage in two main habitats, inside the canyon and on the adjacent slope, has been described and has been related to en- vironmental and trophic variables. Important seasonal changes have been detected mainly inside the canyon along over the year. This seasonal variability is asso- ciated to the primary production, to river efflux (the Bes´os river near the study area) and to the seasonal occurrence of the Levantine Intermediate Waters (LIW) that affect to the chemical and nutritional value of sediment and near bottom wa- ters. All variables, used as proxy for these three environmental causes, explain the seasonal variability in biomass/abundance and changes in taxonomic composition, vi that reflects, when occurs, the seasonal turnover of trophic types. With respect to the environmental analysis, the canyon was found to be more influenced by sea- sonal variations and the terrigenous provision than the open slope, that conversely was found to be more related to primary production and in general showed less evidence of seasonality. The taxonomic/trophic composition of the infauna we ob- served can be explained with these environmental conditions. In fact the canyon showed high variability in trophic types for the most abundant taxa, whereas the adjacent slope was dominated by sub-surface deposit feeders. Moreover, inside the canyon a temporal turnover of major taxa and trophic habits was observed, whereas the adjacent slope showed one evident difference only in February for the dominance of the genus Prochaetoderma spp. that eats on foraminifera. The ad- vection processes also explain the higher biomass and abundance in our area than in nearby areas (e.g. the Toulon canyon, that is not an extension of any river. This study was mandatory in order to reconstruct the food web, because made possible the flow estimation between this component and other components of the network. Part II (Chapter 3 and 4) concerns a long term study of A. antennatus landings per unit effort (LPUE) performed through regression analysis using both frequen- tist and Bayesian additive models. With the frequentist approach the variability explained by the final model (using a total of six predictors) captured the 43% of the total LPUE variability. The set of fishery-related variables (the daily trips performed by vessels, the gross registered tonnage and the factor vessel) was the most important source, with an ED of 20 .58%, followed by temporal (inter and intra-annual variability, ED = 13 .12%) and finally economic variables (the ex-vessel shrimp price, ED = 9.30%). We found that data derived from fishery, as well as independent data provided by scientific surveys, provide similar indices of the ex- ploited species. The study also showed that the number of trips per month is the most influential variable and that after a certain value it does not correspond to an effective increment in landings per unit effort while high LPUE corresponds to low values of shrimp price, probably the price drops due to the high availability of the product. Thus it may be appropriate reduce the limit of the number of trips per month for a reasonable management. Finally if boats hold a random effect their average effect on the response is null, so there is no need to use them for standardization purposes. For a methodological point of view, in Chapter 4 where the bayesian approach has been used, has been shown that a mixed effect model permits to account for correlated data. vii Part III covers the ecosystem approach of this thesis and is divided into Chap- ter 5 where we built a steady-state model of the bathyal food web (soft bottom community at the continental slope at 600–800 m depth) and Chapter 6 where the occurrence of trophic cascade was investigated. A total of 40 carbon flows among 7 internal and 6 external compartments, were reconstructed using linear inverse modelling (LIM) by merging site-specific biomass data, on-board oxygen consumption rates and published parameters to create population and physiologi- cal constraints. The total carbon flux to the community was 2 .62 mmol C m −2 d−1, entering as vertical (5.2E-03 mmol C m −2 d−1) and advecive organic matter (2.6E- 00 mmol C m −2 d−1). The influx was then partitioned between the total organic matter in sediment, 87.05%) and suspension and filter feeding (12.95%), among zooplankton (1.08%), suprabenthos (3.07%) and macrobenthos (95.74%). The fate of carbon deposited in sediments was its burial, its degradation by prokaryotes or the ingestion by metazoan deposit feeders. The total ingestion of C in sediments by the metazoan community (excluding the meiofauna) was 0.83 mmol C m −2 d−1, corresponding to 31.68% of the total C entering the food web and to 36.34% of the C in sediments, so we deduced that the rest was used by the prokaryotes and nematods (1.58 mmol C m −2 d−1, 69.28%) or trapped in the sediment (0.73 mmol C m −2 d−1, 32.19%). The respiration of the whole community (including the total organic matter, TOM) was 1.89 mmol C m −2 d−1: 83.75% for sediments, in- cluding prokaryotes and meiofauna, 13.34% for macrofauna, 2.86% for megafauna, including the red shrimp Aristeus antennatus . The dynamic simulation was based on a system of ordinary differential equations to predict biomass trends during 5 years after perturbations induced by red shrimp fishery (top-down driver) and by food supply limitations (bottom-up driver). Our simulation demonstrated that trophic cascades induced by fishery cannot occur through major components of the bathyal food web. We search for the incidence of trophic cascade through two and three interactions also including the organic matter in sediment for most complicate trophic cascade hypotheses, but we only found very ephemeral indirect effects persisting less than 10 days, that we considered not enough to demonstrate the occurrence of this mechanism in the system. On the contrary we found indi- rect effects persisting more than one month driven by the source limitation. Our results are in agreement with previous studies in which trophic cascades have not been detected in benthic/detritic food webs, that have some similarities with the bathyal food web, that depends on allochthonous detritus. We explain this lack of trophic cascade with the important role of detritus in the bathyal ecosystems. The dead organic matter or detritus, whereas it is frequently overlooked, is a viii common feature of most ecosystems mainly in benthic ecosystems sustained by allochthonous sources. So, we hypothesised that it is the component controlling the food web dynamics a mechanism known as donor-control. Resumen en espan˜ol: En este Ph.D. diferentes aspectos del ecosistema batial han sido tratados con el objetivo final de investigar mecanismos de cascada tr´ofica impulsados por la pesca (tipo de control top-down), un fen´omeno que en los sis- temas batial nunca ha sido estudiado. Esto fue ejemplificado por el estudio del talud continental de fondo suave adelante de Catalun˜a (Mediterr´aneo norocciden- tal), donde se lleva a cabo la pesquer´ıa de la gamba roja Aristeus antennatus y la estructura de la red tr´ofica es relativamente conocida.
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