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Normalized Biomass Size Spectra in High Antarctic Macrobenthic Communities: Linking Trophic Position and Body Size

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Normalized Biomass Size Spectra in High Antarctic Macrobenthic Communities: Linking Trophic Position and Body Size Vol. 506: 99–113, 2014 MARINE ECOLOGY PROGRESS SERIES Published June 23 doi: 10.3354/meps10807 Mar Ecol Prog Ser Normalized biomass size spectra in high Antarctic macrobenthic communities: linking trophic position and body size Eduardo Quiroga1,*, Dieter Gerdes2, Américo Montiel3, Rainer Knust2, Ute Jacob2 1Pontificia Universidad Católica de Valparaíso (PUCV), Escuela de Ciencias del Mar, Av. Altamirano #1480, Casilla 1020, Valparaíso, Chile 2Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany 3Instituto de la Patagonia, Universidad de Magallanes, Av. Bulnes #01855, Casilla 113-D, Punta Arenas, Chile ABSTRACT: Normalized biomass size spectra (NBSS) and the stable nitrogen isotopic composi- tion of food webs were analyzed to determine the inter-specific relationships between trophic level (measured as δ15N) and body size in macrobenthic communities living on the continental shelf of the southeastern Weddell Sea (SEWS). We found that the relationship between trophic level and body size for the whole macrobenthic community was not significant (r2 = 0.08, p > 0.05), probably associated with biomass accumulated in the larger body size fractions, which are repre- sented particularly by suspension feeders such as sponges and tunicates. We used an alternative method of studying trophic structure of the aquatic communities based on the distribution of resid- uals of the NBSS. Here we demonstrate how residual distribution exhibited dome-like patterns, which may offer an additional quantitative tool for studying the relationship between trophic level and body size. These domes of biomass represent trophic positions derived from relationships between the body size distribution of the predators in one trophic position and their prey in another. We found 4 well defined domes of biomass, which appear when large benthic species are abundant, especially favored in an environment with high organic matter flux. A significant posi- tive relationship (r2 = 0.59, p < 0.05) between the trophic level and residuals derived from NBSS suggests that these domes might be considered as functional groups comprising organisms from different trophic levels. We suggest that dome-like patterns in the biomass size spectra can pro- vide a robust framework for conceptualizing and statistically modeling trophic levels of macro- benthic communities. KEY WORDS: Macrofauna · Trophic spectrum · Trophic level · Stable isotopes Resale or republication not permitted without written consent of the publisher INTRODUCTION al. 2008, Gutt et al. 2011). Macrobenthic diversity and distribution patterns as well as environmental The southeastern Weddell Sea (SEWS) shelf in the conditions have been extensively studied on the high Antarctic is rich and diverse in macrobenthic SEWS shelf (Arntz et al. 1994, Brey & Gerdes 1998, species which are maintained by high levels of pri- Clarke & Johnston 2003, Gutt 2007). In general, mary production (PP), particularly during the austral trophic relationships and food web structure in Ant - summer (e.g. Arntz et al. 1994, Gutt et al. 1998). In arctic coastal benthic communities are affected by addition, this region is affected continuously by ice- strong coupling of the physical environment and PP berg scouring, enhancing the environmental com- (Norkko et al. 2007). The SEWS shelf experiences plexity and diversity (e.g. Gutt 2001, 2007, Gerdes et high sedimentation rates of fresh organic matter *Corresponding author: [email protected] © Inter-Research 2014 · www.int-res.com 100 Mar Ecol Prog Ser 506: 99–113, 2014 (OM), composed of diatom aggregates and fecal pel- nitrogen and carbon isotope compositions of natural lets that are accumulated on the sediment as green samples are given as delta values (δ), representing mats or food banks (Bathmann et al. 1991, Isla et al. the isotope ratio of the heavy to the light isotope (‰) 2009, Sañé et al. 2012). Furthermore, tidal currents relative to an international standard (Michener & resuspend and redistribute the OM, which can per- Lajtha 2007). The carbon isotope composition (δ13C) sist several weeks with a high nutritional value after of an organism is used to distinguish isotopically the spring−summer pulse (Rossi et al. 2013). different sources in food webs (DeNiro & Epstein Many authors have stressed the importance of 1978). In contrast, nitrogen isotopes in consumers body size in benthic marine fauna, but very little is become enriched in δ15N by 3 to 4 times, which is known about the processes that determine size distri- used to predict an organism’s trophic level (Mich- bution in marine invertebrates (Roy 2002). Studies ener & Lajtha 2007). have focused on bathymetric patterns in relation to Normalized biomass size spectra (NBSS) are the availability of food (Thiel 1975, Gage & Tyler widely used to describe the structure of marine ben- 1991), the physical characteristics of the sediment thic ecosystems (e.g. Drgas et al. 1998, Saiz-Salinas & (Drgas et al. 1998), the gradient of OM (Schwing- Ramos 1999, Quiroga et al. 2005, Akoumianaki et al. hamer 1985), the life-history strategies of dominant 2006, Sellanes et al. 2007, Hua et al. 2013). The NBSS taxa (Warwick 1984), and oxygen levels (Quiroga et coefficients (slope and intercept) are extracted by al. 2005). Gage & Tyler (1991) argued that large ben- performing a least-squares linear regression on the thic organisms in deep waters are mainly opportunis- log-transformed values of normalized biomass and tic scavengers, although some deposit feeders and body size. These coefficients have been proposed as suspension feeders are also present, probably associ- quantitative indices of aquatic ecosystem structure, ated with the possibility of reducing predation risk or which allows a more dynamic interpretation of an the ability to forage over a wide area. On the SEWS ecosystem because the slope of the spectra seems to shelf, large sessile and sedentary suspension feeders be related to energy utilization within the whole with associated mobile fauna dominate macroben- community (Sprules & Munawar 1986, Kerr & Dickie thic communities (Gutt 2007). These suspension 2001). In fact, theoretical models regarding the distri- feeders can feed on a broad spectrum of prey items, bution of biomass by size classes have been devel- which range from bacteria to zooplankton and detri- oped in order to explain ecosystem functioning (Kerr tal particulate OM (Gili et al. 2001). For instance, & Dickie 2001). Quintana et al. (2002) suggested an hydrozoans and gorgonians feed on zooplankton, alternative method of studying trophic structure of while sponges prefer diatoms, probably due to favor- the aquatic communities based on the distribution of able bottom currents which also support suspension residuals of the NBSS: using this approach, Quintana feeders and suspension feeding epibionts by produc- et al. (2002) described dome-like patterns over par- ing an advective food supply in the nepheloid layer ticular ranges of body size, which seem to be formed (Gaino et al. 1994, Coma et al. 1998, Gili et al. 2001). by different trophic groups (Dickie et al. 1987). Each Few studies have investigated benthic food webs dome in the biomass size spectra represents a trophic across similar communities from different regions in position derived from relationships between the body the Antarctic (Jacob 2005, Mintenbeck et al. 2007, size distribution of the predators in one trophic posi- Norkko et al. 2007, Gillies et al. 2013). In the benthic tion and their prey in another, according to pre dator− system, the trophic level is expected to increase with prey theory (Kerr & Dickie 2001). Thus, do me-like increase of body size (see France et al. 1998, Jen- patterns have been related to trophic orga nization of nings et al. 2002), but empirical evidence describing organisms living in communities as a consequence of the relationships between body size and trophic level the dependence of the production: biomass ratio (P:B is limited and also difficult to quantify (France et al. ratio) on body size, and they are used to describe the 1998, Jennings et al. 2002). trophic structure of marine and freshwater communi- The stable isotopes of nitrogen (δ15N) and carbon ties (e.g. Dickie et al. 1987, Kerr & Dickie 2001, Quin- (δ13C) are an efficient means for tracing carbon flow tana et al. 2002, Quiñones et al. 2003, Quiroga et al. and trophic position in food webs (Michener & 2005). Lajtha 2007, France 2014). Several studies have In order to understand the functioning of the ben- used stable isotopes to determine trophic relation- thic ecosystem it is necessary to center our attention ships in Antarctic coastal benthic systems (e.g. not only on community structure analysis, but also on Nyssen et al. 2002, Jacob 2005, Mintenbeck et al. the empirical study of trophic structure of marine 2007, Norkko et al. 2007, Gillies et al. 2013). The communities from an allometric point of view. Here Quiroga et al.: Biomass size spectra in Antarctic macrobenthic communities 101 we explored the relationship between dome-like patterns derived from NBSS and trophic level using δ15N in order to describe the trophic struc- ture of benthic communities. We hypo thesized that if these domes of biomass provide a surrogate for trophic level, then analysis of size spectra can be used to describe changes in the trophic struc- ture of marine benthic communities as has been proposed by Jennings et al. (2002). The aims of this study were (1) to analyze the body size distri- bution of the macrobenthic community on the con- tinental shelf of the SEWS, (2) to describe the trophic structure using stable isotopes, and (3) to determine whether domes of biomass are good descriptors of the trophic structure and trophic level of groups of organisms living in macroben- thic communities on the continental shelf of the SEWS. MATERIALS AND METHODS Sampling Macrofaunal samples were collected at 14 sta- tions (88 cores) on the continental shelf of the SEWS during the Benthic Disturbance Experiment (BENDEX) Expedition ANT XXI/2 in 2003 and the Change in Antarctic Marine Biota (CAMBIO) Ex pedition ANT XXVII/3 in 2011 onboard RV ‘Polarstern’ (Table 1, Fig.
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