ATLAS Deliverable 2.2: Integrated physiological experiments and models

A major objective of ATLAS is to produce a new class of predictive modelling tools that integrates food supply and biogeochemical cycling for mapping deep-sea species and ecosystems at management relevant spatial scales. To produce these models it is necessary to integrate field data on hydrodynamic conditions, organic matter concentration and settling flux to the seafloor with ex situ aquarium studies conducted to determine how deep-sea organisms respond to altered food supply regimes and changes in ocean conditions, such as ocean acidification. Here we report results of the ex-situ experiments using scleractinian corals, black corals, octocorals, bivalves and sponges conducted by different ATLAS partners and then use this data together with data available from the literature (DL 2.1) to produce physiological models for selected cold-water coral and sponge species.
Four types of experiments were carried out to:
(1) Assess the capacity of selected CWC species to capture live zooplankton prey under different hydrodynamic conditions (section 3.1). For this purpose, a set of experiments were performed in the Azores (section 3.1.1) and Norway (section 3.1.2). Experiments in the Azores tested capture rate efficiency of the black coral Antipathella wollastoni and the gorgonians Viminella flagellum and Dentomuricea meteor using the rotifer Branchionus plicatilis as prey under flow velocities of 1.5, 4 and 6 cm/sec for the black coral and 3, 6 and 9 cm/sec for the gorgonians, using laminar flow experimental flumes. Results obtained for A. wollastoni show the capacity for this species to consume high amounts of prey and this capacity seems to be enhanced in intermediate flow velocities (4cm/sec). This explain the natural abundance of A. wollastoni in crevices protected from strong currents. Experimental results obtained for the gorgonians, demonstrated higher capture rates per polyp for V. flagellum, with maximum capture rates at 6 cm/sec, compared with D. meteor with maximum capture rates at 9cm/sec. These results may be related to the larger size of polyps of V. flagellum and might reflect different feeding strategies by the two species due to their different morphological shape. All species studied showed lower capture rates per polyp when compared with the reef-forming Lophelia pertusa likely related with the smaller polyp size of black corals and gorgonians but preference for higher flow rates. These species-specific effects corroborate the hypothesis that flow regime can be a very important factor in describing species presence, density and success among different areas and may an important prediction factor of the distribution of species under present and future conditions.
Studies conducted in Norway tested capture rate efficiency for the reef-forming species Lophelia pertusa, the sponges Geodia barretti, Stryphnus sp. and Phakellia ventilabrum, and the bivalve Acesta excavata under 2 natural seston concentrations and water flow treatments using a flow through chamber. Results of these studies found higher consumption rates for L. pertusa and A. excavata when compared with sponges. Interestingly, the sponge P. ventilabrum, a species characterized as having low abundance of associated microbial organisms (Low Microbial Abundance (LMA) sponges), had higher consumption rates when compared with Geodia barreti and Stryphnus sp, which characteized as High Microbial Abundance (HMA) sponges. This may be due to the fact that HMA sponges may complement their diet with their own symbionts and dissolved organic matter. Furthermore, L. pertusa and A. excavata showed a wider range of food sources consumed when compared with sponges which preferentially fed on algal and bacterial particles.
(2) ) Determine the feeding preferences of selected habitat forming CWC species in the Azores (black corals and gorgonians) to different food source using live algae, zooplankton and DOC fed with isotopically labelled OM (13C and 15N) ; and estimate the incorporation of carbon and nitrogen and metabolic activity under ingestion of these different food sources (section 3.1.3). Results of these experiments showed that the black coral A. wollastoni and gorgonians V. flagellum and D. meteor can feed on a variety of food sources (DOC, algae and zooplankton), but preferentially fed on zooplankton. This was translated into higher incorporation of carbon and nitrogen in to coral tissues and higher oxygen consumption rates in coral feeding on zooplankton, suggesting that this food source is of high importance for the growth and metabolism of black corals and gorgonians, as it has been already documented for CWC scleractinian species. The gorgonian D. meteor showed higher tracer incorporation and oxygen consumption for all food sources consumed when compared with V. flagellum, suggesting different metabolic strategies between these two co-existing gorgonians. While fragments of D. meteor appeared to be more efficient in removing prey than V. flagellum (section 2.1.1.) under similar flow velocities, further studies are needed to determine if the lower values displayed by V. flagellum can be attributed to lower capture and ingestion of prey or if ingested prey was used for other metabolic processes such as excretion, calcification or growth of the main skeletal axis, which was not considered in this study.
(3) Understand competition interactions of two co-occurring octocoral species D. meteor and V. flagellum under different water flow conditions. Results showed that both species have higher incorporation rates under 4cm s-1 in comparison to 2cm s-1 likely because of higher prey encounter rate. D. meteor had higher incorporation of C and N tracers for all treatments when compared to V. flagellum likely related to the more branching and fan shape colony morphology of D. meteor that can maximize prey capture (see also 3.1.2). Viminella flagellum is a whip coral that stands higher in the water column and likely gets access first to food particles. These different feeding strategies may contribute to their co-existence in the same coral garden. However if food becomes scarce, D. meteor may gain a competitive advantage in relation to V. flagellum.
(4) Understand the impacts of OA on coral garden forming octocorals in Azores by testing the interactive effects of OA and food availability. Specific objectives of this experiment were to determine if and how food concentration alters the impacts of OA on the physiology of the tested species; and to assess the impact of OA on food uptake and food assimilation. Viminella flagellum displayed a decrease in polyp activity under OA conditions for all food treatments although this was more pronounced under starvation. This can consequently lead to lower energy input, although during the course of the experiments main metabolic rates such as oxygen consumption and tracer C respiration remained stable or even increased under acidified conditions. The increased oxygen consumption and tracer C respiration observed under acidified conditions are indications of increased metabolic demands. Nevertheless, the uncoupling between the patterns in tracer C respiration and oxygen consumption in the case of V. flagellum fragments under acidified conditions is a strong indication of a change in metabolic processes among fragments under low and high conditions, i.e. under low food concentrations energy may be diverted towards survival physiological mechanisms instead of growth and reproduction. Negative impacts of acidification on the species physiology, such as the one described for V. flagellum, may not directly affect their survival but are expected to impact other processes such as long-term growth and reproduction with possible detrimental effects on the population level which can cascade to the whole community.
(5) Determine the physiological impacts of crude oil and dispersants on the marine shallow-water sponge Halichondria panicea as model species. Respiration rates from the single concentration experiment and the dose-response experiments were highly variable and did not seem to change with exposure to hydrocarbons. However, clearance rates decreased sharply in sponges exposed to crude oil fractions or chemically enhanced oil fractions, even at low oil loading. It is likely that stopping its filtration activity for extensive periods of time will strongly impact survival of H. panicea. The capacity of sponges to survive longer exposure periods should therefore be further investigated.
In the last section of the report (section 4), physiological models for passive (i.e. CWC) and active (i.e. sponges) suspension feeders were constructed based on literature and experimental data (see section 3) using physiological parameters such as respiration and assimilation efficiency. For illustrative purposes, these models linked the respective physiological models to theoretical trajectories of variable food supply using L. pertusa as representative passive filter feeder of the Rockall Bank and the sponge Geodia sp. as a representative of a suspension active filter feeder from Davis Strait. These physiological models will be coupled in D2.5 to the environment using the hydrodynamic model (developed in D2.4) factors and calibrated by field data in D2.3.
Overall, experiments conducted in this DL have contributed to a better understanding of the feeding ecology and ecophysiology of understudied habitat-forming species such as black corals, octocorals, bivalves and sponges. It has also elucidated how octocorals, and the coral gardens they form, may be impacted by future predicted changes in seawater chemistry (OA) and food supply (OM concentration) and how sponges may be impacted by anthropogenic impacts such as oil spills. We have also demonstrated how this data can be integrated in physiological models of CWC and DWS as a proxy for the environmental status of the ecosystems.