Differences in the carbon flows in the benthic food webs of abyssal hill and plain habitats

Inputs of detritus from the surface ocean are an important driver of community dynamics in the deep sea. The assessment of the flow of carbon through the benthic food web gives insight into how the community is sustained, and its resilience to fluctuations in food supply. We used a linear inverse model to compare the carbon flow through the food webs on an abyssal hill and the nearby plain at the Porcupine Abyssal Plain sustained observatory (4850 m water depth; northeast Atlantic), to examine the partitioning of detrital input in these substantially different megafaunal communities. We found minimal variation in carbon flows at the plain over two years, but differences in the detrital inputs and in the processing of that carbon input between the hill and plain habitats. Suspension feeding dominated metazoan carbon processing on the hill, removing nearly all labile detritus input to the system. By contrast, half of all labile detritus was deposited and available for deposit feeders on the abyssal plain. This suggests that the biomass on the hill is dependent on a more variable carbon supply than the plain. The presence of millions of abyssal hills globally suggests that the high benthic biomass and respiration, and reduced deposition of detritus may be pervasive, albeit with varying intensity. Community dynamics in the deep-sea are mainly driven by the input of detritus from the surface ocean (Gooday et al. 1990; Pfannkuche 1993; Billett et al. 2010; Smith et al. 2013). The benthic community is very food-limited as this major energy input is seasonal with substantial interannual (Lampitt et al. 2010b) and climatically driven (Ruhl et al. 2008) variation. Consequently, competition for this resource is thought to be high and the benthic food web is complex (Iken et al. 2001). The resulting partitioning of this detrital input between different components of the food web structure the community, in particular feeding groups, as confirmed by the examination of gut contents (e.g., Witbaard et al. 2001; Amaro et al. 2009; FitzGeorge-Balfour et al. 2010) and through isotopic analysis (e.g., Iken et al. 2001; Gutierrez-Rodriguez et al. 2014). The assessment of the flow of carbon through the food web gives insight into how the community is sustained, and the resilience of a community to fluctuations in food supply. This may be particularly important to understanding the dynamics of a community known for “boom and bust” cycles (e.g., Billett et al. 2010). Photographic surveys have changed our understanding of the abyssal benthic community at the Porcupine Abyssal Plain (PAP; 4850 m; northeast Atlantic), one of the best-studied abyssal sites on Earth. Previous studies assessed the benthic megafaunal assemblage using trawls, and found echinoderms to be the most common fauna (Billett et al. 2001, 2010). Recent photographic surveys, using a towed camera platform (Durden et al. 2015a) and an autonomous underwater vehicle (Morris et al. 2014, 2016; Milligan et al. 2016) have found the densities and biomass of megafauna to be significantly greater than those estimated by trawling: the estimated megafaunal density increased 1800% between that estimated by trawling and the first of these photographic surveys, equating to a 2084% increase in megabenthic standing stock. Furthermore, a small cnidarian with mixed feeding modes has been discovered to be the dominant megafaunal species at the site (Durden et al. 2015b). These new discoveries are a result of the increased ability to detect smaller and delicate fauna either undersampled or not caught using the trawl. These photographic surveys also revealed substantial differences in the megafaunal communities on the abyssal hills *Correspondence: [email protected] This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 1 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 00, 2017, 00–00 VC 2017 The Authors Limnology and Oceanography published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10532 and surrounding plain (Durden et al. 2015a; Morris et al. 2016). The total density and biomass were substantially higher on the hills than the plain: in the first study, density and fresh wet weight biomass on the highest hill were 1.5 and 3.1 times the values on the plain, respectively. This biomass was also distributed differently among feeding modes. The biomass of suspension feeders on the highest hill was 17 times higher than that on the plain, while the biomass of surface deposit feeders was only 23% of that on the plain. The higher standing stocks on the hill existed with only a small difference in visible detritus being found on the seabed between hills and plain (Morris et al. 2016), suggesting that the structure of the food web is likely different. However, the consequences of this different community structure for detritus partitioning within the food webs of these contrasting abyssal settings has not been examined. An initial analysis of the benthic carbon flows at the PAP was completed by van Oevelen et al. (2012). The approach combined a linear inverse model (LIM) to determine the carbon flows using a likelihood approach (van Oevelen et al. 2010), with a dynamic model based on isotopic tracer uptake data, which allowed the mechanisms of carbon transfer to be examined. This “original model” (van Oevelen et al. 2012) found that bacteria dominated the processing of carbon, and that the foraminifera, meiofauna, and macrofauna were strongly dependent on semi-labile detritus. This suggested that the benthic community largely uses a comparatively temporally stable food source, rather than depending on the periodic labile detritus input, as found at another abyssal plain in the Pacific (Dunlop et al. 2016). The megafauna were not anticipated to remove much of this labile detritus input, although it was acknowledged that better estimates of their biomass would be needed to better constrain their role in detritus processing. The aim of this study is to update the original food web model with new data from the PAP long time-series location collected in 2011 (“2011 Plain model”), to provide a more refined representation of the benthic carbon flows, and to reassess the carbon transfer pathways with particular focus on the role of the megafauna in the community. Interannual variations in carbon flows are assessed by comparing 2011 to 2012 (“2012 Plain model”). The model is then applied to assess differences in the carbon flow between the hills (“2011 Hill model”) and the plain (“2011 Plain model”), to examine the partitioning of detrital input in these contrasting abyssal settings with substantially different megafaunal communities.