Macrofaunal and foraminiferal community structure and their response to simulated phytodetrital food pulses

Abstract

To learn more about deep-sea ecosystem functioning, this thesis has focused on elucidating the role of macrofauna and Foraminifera in deep-sea C-cycling. The first manuscript describes the composition of, and for the first time, the response by macrofauna to a simulated organic matter (OM) pulse at an abyssal Station in the NE Pacific. In each experiment, 1.2 g C m-2 of 13C-labelled Skeletonema costatum was deposited onto the seafloor using a benthic chamber lander. Macrofaunal abundance and biomass were found to be significantly higher at 0-5 cm depth compared to 5-10 cm, and were dominated by the Nematoda and Crustacea, respectively. Twenty-five percent of the macrofauna specimens showed 13C-signatures indicative of label ingestion, but specific uptake and C-turnover rates varied strongly between and within taxa. Two organisms, a single cumacean from chamber two and a paraonid polychaete from chamber three, were responsible for the majority of C-turnover, and had ingested up to 2.3 % of their body weight in C. Macrofaunal C-turnover was much lower than recorded in the abyssal NE Atlantic, which is most likely due to differences in the timing of the experiments relative to the spring/ summer bloom, different experimental durations and disparities in macrofaunal community structure. These results emphasize the degree of plasticity inherent in the macrofaunal response to a food pulse and stress the need for comprehensive in-situ investigations to further our understanding of deep-sea benthic ecosystem functioning. The second manuscript of this thesis describes the response by bathyal macrofauna from a deep western Norwegian fjord to a simulated OM pulse. In each experiment, 1 g Corg m-2 of 13C-labelled Skeletonema costatum was deposited onto intact sediment cores collected from 688 m water depth and incubated ex-situ for 2, 7 and 14 d. Macrofaunal abundance and biomass estimates were comparable to other deep-sea continental margin sediments of similar depths but in contrast to previous fjord studies, the macrofaunal community was numerically dominated by ostracods. Tracer experiments revealed highest uptake of tracer after 7 and 14 d compared to 2 d. Of the 7 deposit feeding polychaete families, only the Paraonidae&Cirratulidae - together with the largely carnivorous Lumbrineridae - showed a significant response to our labelled C-source. The lack of response by the majority of deposit feeders, and the unexpected feeding mode of the Lumbrineridae may be attributable to species- rather than family-specific feeding-ecologies and/ or ontogenetic changes in diet/ feeding mode. Total macrofaunal C-turnover was much lower than recorded in the deep Sognefjord in a 3 d feeding experiment, and is most likely a result of 1) distinct differences in macrofaunal community composition between the two sites, with a predominantly sub-surface feeding macrofaunal assemblage in the Korsfjorden as opposed to a surface-feeding community in the Sognefjord and/ or 2) variations in OM supply and demand. In conclusion, this investigation highlights the importance of ecological information on species level for a detailed understanding of C-cycling and early diagenesis in marine sediments. Finally, the third manuscript describes the response by Foraminifera to labile OM from the same study as that described in chapter 2. The Foraminiferal community ( 250 micron) from the Korsfjordenen was largely dominated (91 %) by the deep-dwelling species Globobulimina turgida and Melonis barleeanum, aswell as the shallow infaunal species Hyalinea balthica. With the exception of H. balthica, Foraminiferal average living depths (ALD) were continuously deep, indicating that the majority of Foraminifera were not vertically migrating to the surface to feed on the fresh organic matter pulse. Hyalinea balthica did migrate towards the sediment surface, but did not feed on our 13C-tracer, suggesting that migration was due to another factor, possibly microhabitat restabilization following initial sub-optimal living conditions. No Foraminifera were involved in C-turnover during the first 7 d. After 14 d, 3 % of the Foraminifera samples possessed delta 13C-signatures indicative of C-uptake, but uptake was confined to G. turgida. Foraminifera contributed 2.4 % to faunal C-turnover (Foraminifera plus macrofauna), despite them making up 24 % of the combined biomass. The dominance of deep-infaunal species such as G. turgida and M. barleeanum (68 %) that prefer degraded organic matter over more labile material, as well as the coarse size fraction of Foraminifera analysed, which are known to respond slower to phytodetritus deposition than smaller, more opportunistic taxa, are likely reasons for the retarded response to food input observed in this study.