Vast cold-water coral (CWC) reefs occur in temperate regions, where strong seasonality in temperature and light leads to a short but highly productive spring period. How CWCs respond physiologically to this strong seasonal forcing remains unclear, due to the remoteness of their deep-sea habitats. In an in situ transplantation study at Nakken reef, Norway, we investigated a full seasonal cycle of (1) temperature and food availability, (2) diet, (3) biomass and tissue reserves, (4) oxygen consumption, and (5) linear growth of the reef-building coral Lophelia pertusa. All investigated variables showed a distinct seasonality.An increase in the organic carbon and amino acid content, linear extension and budding rate from February to late May, at a simultaneous increase of phytoplankton-and zooplanktonfatty acid trophic markers (FATMs), and δ 15 N-derived trophic level, indicates an efficient exploitation of the spring phytoplankton-and the subsequent zooplankton bloom. A pool of neutral-lipid-derived fatty acids (NLFAs), indicative of energy storage and gametogenesis, was formed from May to October, accompanied by increased oxygen consumption, i.e., metabolic activity. In late autumn and early winter (October to December), tissue reserves were maintained, in spite of low sPOM and zooplankton food availability, and the lower tissue δ 13 C and higher contribution of bacterial FATMs suggest increased reliance on more degraded material. The concurrent reduction in linear growth further suggests a lower energy availability at this time of the year. A large (>50%) drop of all tissue pools between December and February coincided with the spawning season of L. pertusa, and demonstrates a high energetic cost of reproduction. Our results show for the first time a strong seasonal control of critical life history traits such as growth patterns and timing of reproduction in this prominent deep-sea species.
cold-water coral (cWc) reefs are one of the most diverse and productive ecosystems in the deep sea. especially in periods of seasonally-reduced phytodetritus food supply, their high productivity may depend on the recycling of resources produced on the reef, such as dissolved organic matter (DoM) and bacteria. Here, we demonstrate that abundant suspension feeders Geodia barretti (high-microbialabundance sponge), Mycale lingua (low-microbial-abundance sponge) and Acesta excavata (bivalve) are able to utilize 13 c-enriched (diatom-derived) DoM and bacteria for tissue growth and respiration. While DoM was an important potential resource for all taxa, utilization of bacteria was higher for the sponges as compared to the bivalve, indicating a particle-size differentiation among the investigated suspension feeders. interestingly, all taxa released 13 c-enriched particulate organic carbon, which in turn may feed the detritus pathway on the reef. especially A. excavata produced abundant (pseudo-)fecal droppings. A second stable-isotope tracer experiment revealed that detritivorous ophiuroids utilized these droppings. The high resource flexibility of dominant reef suspension feeders, and the efficient recycling of their waste products by the detritivore community, may provide important pathways to maintain the high productivity on cold-water coral reefs, especially in periods of low external food supply. Cold-water coral (CWC) reefs 1 rank amongst the most metabolically-active ecosystems of the deep sea 2,3. During the spring phytoplankton bloom, the CWC reef community is supported by the export of phytodetritus from the surface ocean 4-6 , but for significant parts of the year, the deep reefs are limited in particulate food sources 5,7. Dissolved organic matter (DOM) could then act as an alternative C source (dissolved organic carbon, i.e. DOC). DOM is permanently present in the deep sea, but typically at low concentrations of <50 μΜ DOC 8. However, mucus production by the CWCs and the metabolic activity of the dense reef epifauna, significantly increases the DOM concentration in the reef water 9-11. The elevated, labile DOM measurably stimulates bacterial abundance and productivity 9,11,12. Utilization of permanently present and enhanced DOM and bacteria within the reef community could retain energy on the reef that otherwise would be lost (Fig. 1a). The faunal community on CWC reefs is characterized by a high abundance of active suspension feeders 13,14. Three abundant species, especially on Norwegian CWC reefs, include the emergent high-microbial-abundance (HMA) sponge Geodia barretti, the emergent low-microbial-abundance (LMA) sponge Mycale lingua, and the bivalve Acesta excavata. Mycale lingua colonizes the live coral framework 15 , G. barretti lives on the dead reef framework or in sponge grounds neighbouring the reefs, and A. excavata forms clusters within grooves or beneath overhangs of the reef framework 15. With their high water processing rates 16,17 , these active suspension feeders may readily access resource...
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