Benthic trophic structure of a Patagonian fjord (47°S): The role of hydrographic conditions in the food supply in a glaciofluvial system

Journal Pre-proof Benthic trophic structure of a Patagonian fjord (47°S): The role of hydrographic conditions in the food supply in a glaciofluvial system Ilia Cari, Claudia Andrade, Eduardo Quiroga, Erika Mutschke PII:

S0272-7714(19)30057-5

DOI:

https://doi.org/10.1016/j.ecss.2019.106536

Reference:

YECSS 106536

To appear in:

Estuarine, Coastal and Shelf Science

Received Date: 19 January 2019 Revised Date:

2 December 2019

Accepted Date: 11 December 2019

Please cite this article as: Cari, I., Andrade, C., Quiroga, E., Mutschke, E., Benthic trophic structure of a Patagonian fjord (47°S): The role of hydrographic conditions in the food supply in a glaciofluvial system, Estuarine, Coastal and Shelf Science (2020), doi: https://doi.org/10.1016/j.ecss.2019.106536. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author Statement Data will be made available on request

Benthic trophic structure of a Patagonian fjord (47°S): the role of hydrographic conditions in the food supply in a glaciofluvial system

Ilia Caria, Claudia Andradeb, Eduardo Quirogac,d,*, Erika Mutschkeb

a

Magíster en Oceanografía, Facultad de Ciencias del Mar y de Recursos Naturales,

Universidad de Valparaíso, P.O. Box 5080 Reñaca, Avenida Borgoño 16344, Viña del Mar, Chile. b

Instituto de la Patagonia, Universidad de Magallanes, Casilla 113-D, Punta Arenas,

Chile. c

Pontificia Universidad Católica de Valparaíso (PUCV), Escuela de Ciencias del Mar,

Valparaíso, Chile. d

COPAS Sur-Austral, Universidad de Concepción, Concepción, Chile.

Key words: Macrofauna; feeding guild; allochthonous organic matter; stable isotopes; trophic redundancy; sub-Antarctic.

Corresponding author (*): [email protected] (E. Quiroga)

ABSTRACT

Food web studies have provided insight into the dynamics of benthic ecosystems and their stability, stimulating research into the importance of different organic matters (OM) inputs in the ecological and metabolic processes that affect community structure. Using stable isotope analysis (SIA) and Bayesian ellipses, this study examines the influence of terrestrial OM and hydrographic conditions on food web structure and niche width in a glacial system (Baker/Martínez fjord complex). Two ecological groups were identified; one located in the inner and middle fjord and the other one in the outer fjord. Both had isotopically less diverse feeding groups in comparison to non-glacier benthic communities, comprising four trophic levels. In addition, the study area exhibited high trophic redundancy (e.g., species with overlapping trophic niches) associated with chronic natural physical disturbances from glacial sedimentations, which have shaped this non-selective feeding benthic community. The trophic structure metrics suggest that the benthic food web is supported principally by primary production and terrestrial OM pathways, but macroalgal subsidies could be important to detritivores via floating kelp in the inner area of the fjord. The results of this study suggest that glaciofluvial benthic communities have higher trophic redundancy associated with environmental disturbance conducted by sediment load, shaping a benthic community with a lower alpha diversity and fewer trophic levels in comparison to no-glacier environments.

1

1. Introduction Chilean Patagonia (41°S - 56ºS) is one of the most extensive fjord regions in the world, characterized by a highly fragmented coastline comprising numerous islands, channels, and fjords (Silva et al., 2011). This region has strong lateral advection (river-ocean), driven by freshwater input from rivers, runoff and glaciers that control hydrographic characteristics and primary productivity patterns. Indeed, there is a highly stratified estuarine environment with marked longitudinal gradients of suspended particulate material (Dussaillant et al., 2010; Aracena et al., 2011; González et al., 2010, 2011, 2013; Sepúlveda et al., 2011; Silva et al., 2011; Marín et al., 2013; Meerhoff et al., 2019). While hydrographic conditions and environmental variability in this type of ecosystem strongly affect spatial and temporal biological distribution patterns, this is particularly important in environments with glacial influence (Syvitski et al., 1989; Włodarska-Kowalczuk et al., 2005, 2016; Dowdeswell and Vásquez, 2013; Quiroga et al., 2016; Ronowicz et al., 2018). The Baker-Martínez Fjord system (BMF, 47°47'S) is located in central Chilean Patagonia as a confluence of the eponymous Martínez and Baker channels. It is directly influenced by discharges from the Baker river and Pascua river, one of the main environmental and oceanographic forces in the region (Quiroga et al., 2016; Moffat et al., 2018). Moreover, this river basin is acknowledged as having the highest average discharge of all rivers in Chilean Patagonia (1,100 m3 s-1), with high discharge pulses of the Baker river in summer (Dussaillant et al., 2012; Lara et al., 2015). The biogeochemical indicators related to organic matter in sediments (e.g., C/N ratio, stable isotopes of δ13C and δ15N) in this region suggest high contributions from terrestrial origins (allochthonous organic matter, AOM), which are particularly higher at the inner fjord (Sepúlveda et al., 2011; Silva et al., 2011; Vargas et al., 2011; Quiroga et al.,

2

2016; Rebolledo et al., 2018). In general, primary production exhibited well-defined latitudinal patterns in Patagonian fjords (Jacob et al., 2014), which influence the trophic structure of the pelagic communities (González et al., 2013; Meerhoff et al., 2013, 2019) and benthic communities (Quiroga et al., 2012, 2016). For glacially-influenced fjord ecosystems in either hemisphere, macro- and meiobenthic abundance and biomass are negatively affected during periods of glacial melting due to sediment and suspended material transport (Pawlowska et al., 2011; Quiroga et al., 2016; Włodarska-Kowalczuk et al., 2016). The observed patterns agree with the Pearson and Rosenberg model (1978), which describes decreases in species richness and diversity, and the presence of functional groups sensitive to glacial sedimentation (Görlich et al., 1987; Syvitski et al., 1989; Quiroga et al., 2012; Włodarska-Kowalczuk et al., 2005, 2016). Knowledge of benthic organism trophic structure is essential for understanding ecosystem complexity and function. Furthermore, accurate descriptions of trophic structures are necessary to quantify the effects of environmental changes on predatorprey relationships and benthic communities (Grange and Smith, 2013; Quiroga et al., 2014). Although studies on the trophic structure of benthic communities through stable isotopes in Patagonian channels have been carried out at several locations (Andrade et al., 2016; Riccialdelli et al., 2017; Mayr et al., 2011, 2014; Zapata-Hernández et al., 2014, 2016), few have researched environments directly influenced by glacial activity. In the BMF system, Quiroga et al., (2016) and Rebolledo et al. (2018) recorded δ13C variations in surface sediments and benthic organisms, demonstrating seasonal fluctuations of fluvial organic matter. Those studies contribute to the trophic subsidy hypothesis, i.e., terrestrial material contributions mediated by bacterial chemosymbiotic associations that facilitate AOM assimilation (Mcleod and Wing, 2009). The same contributions have been described for other fjord ecosystems in New Zealand

3

(Mcleod and Wing, 2009; Mcleod et al., 2010) and Alaska (Bell et al., 2016). In addition, Andrade et al., (2016) examined benthic community trophic structures at two shallow sub-Antarctic sites in the Strait of Magellan (52-53°S), finding high trophic diversity and, notably, significant differences in isotopic niche width in response to local environmental conditions. Though data on the distribution of hard benthic communities in Patagonian fjords are in most cases limited and descriptive, some well-defined suspension feeder community patterns have been described (Häussermann and Försterra, 2009; Betti et al., 2017; Cárdenas and Montiel, 2015; 2017). Though BMF ecosystem, is an impoverished suspension feeder benthic community, abundant macrofauna has been reported (Quiroga et al., 2012, 2016). These faunistic groups are negatively affected by a combination of enhanced sedimentation rates and accelerated glacier retreat, which contributes to increased discharge of melt-water and frequency of large flood events of turbid glacial melt-water into the coastal fjord and river (Quiroga et al., 2012, 2016). In fact, macrobenthic communities appear to be impacted by large quantities of POM, which are transported into the river and fjord, producing environmental changes in both water column and sediment conditions (i.e., high turbidity) (Quiroga et al., 2016; Rebolledo et al., 2018). Hence, in glacier-influenced regions, studies on environmental variability and its potential impacts on marine food webs are relevant to help understand the effects of climate change on marine and fjord ecosystems (Włodarska-Kowalczuk et al., 2005; Renaud et al, 2007; Grange and Smith, 2013; McGovern, 2016). It is known that benthic communities exhibit changes in their diversity and abundance in response to environmental conditions. In fact, in Arctic fjords benthic communities have been characterized as mature ecosystem with higher alpha diversity and complex food webs,

4

in which these fjord ecosystems have shown remarkable resilience against environmental changes (Węsławski et al., 2017). Thus the present study aims to (i) characterize community trophic structure and isotopic niche widths among functional guilds through measurements of stable isotopes (δ13C and δ15N) and Bayesian ellipses (Stable Isotope Bayesian Ellipses, SIBER, Jackson et al., 2011); and (ii) determine the effect of high sedimentation rates and terrestrial OM input on trophic structure in a fjord system with glacial fluvial influence in Chilean Patagonia.

2. Materials and Methods 2.1. Study area The BMF is almost entirely fed by three major rivers (the Baker, Bravo, and Pascua), delivering a total supply of 5.1 × 104 Km3 y−1 of freshwater to the Baker Basin (Quiroga et al., 2012). This fresh water input influences hydrographic conditions and organic matter exchanges within the estuarine ecosystem (Quiroga et al., 2012; 2016). Flow rates in these rivers are very seasonal, with maximum flow rates in summer, and minimum, in spring. This corresponds to respective maximum and minimum ice-melt periods. The Baker River accounts for about 1000 m3s−1 of total input into the area, and is also the major source of fine sediment, organic matter, and nutrients such as phosphates and silicates (Quiroga et al., 2012; Marín et al., 2013). The contribution of the Baker River to the total annual load of fine suspended sediment and particles of mostly silt and clay into the Martínez channel is approximately 5.3×106 ton y−1 (Quiroga et al., 2012).

2.2. Sampling methods To characterize ecological processes in the study area, we obtained environmental and 5

biological seasonal data in the Martínez channel aboard the Research Vessel (RV) "Sur Austral", of the University of Concepción, as detailed in table 2. Environmental and biological data were obtained from March 2014 to December 2015 (Quiroga et al., 2016). In addition, onboard the Oceanographic RV “Cabo de Hornos”, five stations were sampled in the Baker channel and in oceanic waters near the Gulf of Penas in October 2014 during the CIMAR 20 and CIMAR 23, Expeditions. These were undertaken under the umbrella of the Research Program for the oceanographic study of austral channels and fjords, organized by Chilean National Oceanographic Committee. (Silva and Palma, 2008). In total, 10 stations were sampled between latitudes 47-48ºS and longitudes 75-73ºW (Fig. 1; Table 1). Hydrographic conditions were determined from a set of conductivity, temperature, and depth (CTD) profiles (SBE-25, Seabird Electronics). Water samples were collected in Niskin bottles (5 L) to measure chlorophyll-a (Chla) and total suspended particulate organic matter (SPOM) at surface, 5, 10, 20, and 30 m depth. Duplicate Chla samples were filtered (GF/F glass fiber filters) and frozen (−20°C) prior to fluorometry (Turner design, TD-700) following Holm-Hansen et al. (1965). Duplicate total SPOM samples were vacuum-filtered from 200 mL water on pre-weighed Whatman GF/F glass microfiber filters (0.7 mm pore size) and rinsed with distilled water (Zajączkowski and Włodarska-Kowalczuk, 2007). Oceanographic sections (Chla and salinity) along the channels Baker and Martínez were observed with Ocean Data View (Schlitzer, 2012) to compare among locations. Sediment samples were collected from independent replicates using a gravity corer (50 mm internal diameter, and 1000 mm of length) and Van Veen grab (0.052 m2). Surface sediment samples were kept frozen (−20°C) prior to analyses of sediment organic matter (SOM) and carbon and nitrogen stable isotopes ratios. The biological material for stable isotopes analyses were collected using gravity corers, Van Veen

6

grabs, a modified Agassiz trawl (AGT), with a beam width of 1.2 m x 0.5 m high and mesh size of 1 cm internal and 4 cm external diameter, crab traps (1 cm internal and 4 cm external diameter) and bongo plankton nets (300 µm; Table 1). SOM represented by surface sediment and organisms were stored whole at -20ºC immediately after collection and prior to analysis at the laboratory. Small specimens were rinsed with distilled water for 24 hours to evacuate gut contents (Levin and Currin, 2012). Muscle tissues were dissected from large individuals (e.g., fishes), while the whole body was used in small individuals (e.g., isopods, polychaetes). Zooplankton was separated into size classes (mesh sizes: 2000 µm, 1000 µm, 500 µm, 300 µm and 150 µm), dried in an oven (60ºC) for 48 hours, and then ground into a fine powder with a mortar and pestle. Small amounts were placed into Eppendorf capsules and stored in a desiccators for later stable isotope analyses. Stable isotopes analysis, including carbon and nitrogen concentration measurements, was carried out at the Laboratory of Applied Stable Biogeochemistry and Isotopes (LABASI, PUC, Chile) with an Isotope Ratio Mass Spectrometer (Thermo Fisher Scientific, Delta V Advantage IRMS) coupled with an Elemental Analyzer (Flash, EA2000). Stable isotope ratios were expressed in delta notation, δ13C and δ15N: for carbon, as the deviation from conventional standard Vienna Pee Dee Belemnite (VPDB); and for nitrogen, atmospheric N2 per mille (‰). Fish δ13C data were corrected for lipid variation (Kiljunen et al., 2006). Consumer trophic positions (TP) were calculated following the Vander Zanden and Rasmussen (1999) equation:

TPconsumer = 1+ ( δ15Nconsumer - δ15NSOM)/3.4

where TPconsumer is the estimation of the trophic position of the consumer, δ15Nconsumer is

7

the measured δ15N value of the consumer analyzed, and δ15NSOM is the nitrogen isotope of the food source selected as baseline value. This latter variable was calculated as mean δ15N for all SOM samples but also values were taken from Rebolledo et al., (2018) who compiled more information of SOM in the study area (see Suppl. Data. 1). We selected SOM as baseline for each site under the assumption that this is the main nutritional food source for primary consumers in the BMF. The constant 1 value corresponds to the position of primary sources in the food web (Vander Zanden and Rasmussen, 1999). A value of 3.4 ‰ was set as average δ15N enrichment per trophic level following Post (2002). Though a constant trophic enrichment factor could otherwise be considered a simplification or an underestimate of consumer trophic position, the mean value here has been tested in other fjord systems (Zapata-Hernández et al., 2014; 2016).

2.3. Data analysis Based on field observations of feeding behavior and the literature (McDonald et al., 2010; Häussermann and Försterra, 2009), feeding guild assignments were made for each taxon (Table 1). Bi-plots of δ13C versus δ15N (mean values of each functional guild and carbon sources) provide a general overview of trophic structures and identify possible trophic relations between food sources and consumers. The relative contribution from diet sources was calculated using the Bayesian isotope mixing model in the SIAR program, a complementary package in R software (Parnell et al., 2010). The trophic enrichment factor (TEF) used for δ13C was 1 ± 0.4‰ and 3.4 ± 1‰ for δ15N (Post, 2002; Vander Zanden and Rasmussen, 1999). Analysis of isotopic niche width (δ13C and δ15N) were grouped into feeding guilds. Niche width and the overlap (95% Bayesian ellipses) was then estimated among feeding guild groups with enough data at all sites and ellipses were used to plot isotopic niches. Niche size of group as the

8

standard ellipse area (SEAc) were corrected for small samples sizes (Jackson et al., 2011) to be less sensitive to sample sizes (Syväranta et al., 2013). Overlapping between functional feeders was calculated in SIBER (R, version 4.2: Jackson et al., 2011) using the corresponding Bayesian estimates for the overlap between the 95 % ellipses and expressed as a proportion of the non-overlapping area of the two respective ellipses. Bray–Curtis dissimilarities among organism groups were classified using double-root transformed δ13C, and the SIMPROF test, in order to analyze hierarchical clusters (Clarke et al., 2008). Calculations were carried out with statistical package PRIMER-E, v6.0 (Clarke et al., 2006). Additionally, a simplified mixing model was used to check the relative importance of the allochthonous contribution to the diet of benthic species, applying a two-source mixing model modified (Bianchi, 2007). For this analysis, the crab Peltarion spinosulum was selected as target species for diet reconstruction since this crustacean appeared to be common in all locations in the study area. The percent of organic carbon assimilated (OCA) by P. spinosulum was calculated as:

%OCAP spinosulum = [(δ13CP. spinosulum [fjord] - δ13C SOM) - (δ13CP. spinosulum [marine] - δ13C SOM)]

where δ13CP. spinosulum [fjord] is the isotopic composition of P. spinusulum in the inner fjord area; where δ13CP.

spinosulum [marine]

is the isotopic composition of P. spinusulum from

marine environment (Andrade et al., 2016) and δ13CSOM is the isotopic composition of the SOM in the inner fjord (Rebolledo et al., 2018).

3. Results 3.1. Hydrographic conditions

9

Temperature along the Martínez channel varied in spring (December 2015), fluctuating between 8.8 ºC and 12.1 ºC, with higher temperatures in the outer fjord (Fig. 2a). Surface salinity exhibited a longitudinal gradient along the channel, ranging from 0 near the mouth of the Baker River to values closer to 30 in the outer fjord. In deeper, quasihomogeneous waters, salinities of >31 were recorded (Fig. 2b). Hydrographic conditions for the Baker channel (October 2015) included a cold layer (< 9º C) over the first 10 m depth, with warmer waters recorded at 35 m depth (Fig. 2c). Unlike the Martínez channel, salinity was measured at a shorter longitudinal gradient (<50 km); the lowest salinity (<6) was recorded within the first 10 m of the water column, while salinities >31 occurred in deeper waters farther along the transect (Fig. 2d).

3.2. Stable Isotope composition of OM food sources Three potential food sources were analyzed: sediment organic matter (SOM), suspended particulate organic matter (SPOM), and river particulate organic matter (POM river) (Table 2). In general, a well-defined spatial gradient in the carbon stable isotopes was observed, particularly in the Martínez channel due to the freshwater input and sediment load from the Baker River. The SOM was depleted in 13C at inner fjord stations (-26.58 ± 1.07 ‰), while less negative values were recorded in the outer fjord section (-21.65 ± 0.46 ‰), exhibiting significant differences among locations (Kruskal-Wallis test, Hc =25.4, p<0.001). In the case of SPOM, the most negative values of δ13C of were also recorded at the inner fjord (-27.30 ± 1.60 ‰), with less negative values in the Baker channel (-25.89 ± 1.0 ‰) (Table 2). The POM River was measured only at the inner fjord /mouth of the Baker River, giving δ15N values of 2.40 ± 1.00 ‰, and for δ13C, values of -27.21 ± 0.96 ‰ (Table 2).

10

3.3. Isotopic composition of taxa A total of 43 taxa belonging to seven phyla of mainly benthic macroinvertebrates, bony fishes, and cartilaginous fishes, were identified. Fractionated zooplankton samples (150 - 2000 µm) were collected and analyzed by size classes (Table 2). In the inner section of the fjord, 15 taxa were identified (crustaceans, polychaetes, molluscs, echinoderms, bony fish and sharks). In the middle fjord for Martínez and Baker channels, 33 taxa were identified. In contrast, in the outer fjord, we collected 3 taxa (Table 2). The cluster analysis shown faunistic associations with high similarity for inner and middle fjord sites (Fig. 3). In addition, benthic and pelagic organisms in the inner fjord were the most depleted in

13

C, compared to those collected in the middle and outer fjords (Kruskal-

Wallis test, Hc = 11.7, p<0.01). In terms of species, δ13C and δ15N for BMF consumers over the entire trophic structure are shown in Fig. 4. Among the benthic consumer community inhabiting the inner fjord, Lumbrineridae unidentified (Polychaeta) and Tripylaster philippii (Echinodermata) registered the most negative values of δ13C with 24.23 and -23.96 ±0.50‰, respectively. Higher values of δ15N (14.66 ± 0.11‰), and providing the highest value of trophic position in the study area (TP = 4.23) was recorded in the pink cusk-eel Genypterus blacodes (Pisces: Ophidiidae) (Table 2). In the Martínez channel, the most negative carbon value (-25.17 ‰) was recorded in the mesoplankton fraction of zooplankton (500 µm), and the lowest nitrogen value (9.31 ‰), for microplankton (300 µm). The least negative carbon value was recorded for a Perciforme of the Malacanthidae family (-14.72 ± 0.21 ‰) along with the highest δ15N (15.81 ± 0.05 ‰). In the Baker channel, the most negative carbon value (23.15 ± 0.67 ‰) and coincidentally the lowest nitrogen value (7.00 ± 2.11 ‰) was recorded for the hydrocoral Errina antarctica (cnidarian). The least negative carbon

11

value was recorded in ophiuroid Ophiura (Ophiuroglypha) lymani (-12.09 ± 0.21 ‰), and the highest nitrogen value was recorded for the decapod Cancer edwarsii (14.10 ‰), with the highest estimated TP value for the Baker channel (TP = 3.52). At stations located in the outer fjord, the most negative carbon values were recorded for polychaete Sternapsis sp. (-19.44 ± 0.20 ‰), which also had the lowest nitrogen values (12.02 ± 0.17 ‰). The highest estimated TP value in the outer fjord was recorded for the cnidaria Anthozoa unidentified (TP = 2.76).

3.4. Functional guilds The BMF complex system was shown to consist of six functional groups (Table 2), with omnivorous and carnivorous predators as the functional group with the largest quantity of species at the inner zone of the fjord (11 species); and to a lesser extent, a macro- and mesozooplankton group, and a deposit feeder and grazing group (3 and 1 species, respectively). The Martínez and Baker channels were dominated by predators (14 species), suspension feeders (6 species), and deposit feeders (5 species) (Table 2). The inner fjord section had a wide range of isotopic values for predators, ranging from -16.68 ‰ to -24.23 ‰ for δ13C; and from 5.27 ‰ to 14.66 ‰ for δ15N. The Martínez channel had narrower ranges than did the Baker channel: predator δ13C was between -14.72 ‰ and -17.67 ‰ (Average ±SD; -16.41 ±1.02‰); in the Baker channel, -15.65 ‰ and -19.23 ‰ (-17.29 ±1.73‰); and for nitrogen, between 12.38 and 14.70 ‰ (10.85 ±9.38‰) and 11.31‰ and 14.10‰ (13.75 ±0.43‰), respectively. The zooplankton detritivores components slightly higher average values for δ15N in the Martínez channel (10.68 ±0.90‰) and more negative δ13C (-23.95 ±1.65‰) compared to the inner fjord (δ15N = 9.94 ±0.51‰; δ13C = -21.96 ±0.56‰). In contrast, for deposit feeders, δ15N values in the inner zone ranged from 5.27 ‰ to 10.95 ‰ (8.76 ±3.06‰)

12

and, while in the Martínez and Baker channels the values were higher (9.66 ‰ to 13.21 ‰) (11.06 ±1.37‰). The δ13C values within the inner fjord ranged between -17.82 ‰ and -20.17 ‰ (-19.11 ±2.50‰), narrower than those of the Martínez and Baker channels, between -12.09 ‰ and -20.51 ‰ (-17.03 ±3.28‰). Grazers recorded higher average δ15N in the middle zone of the fjord (9.85 ±0.53‰) as compared to the inner zone (8.85 ±0.05‰). As for the values of δ13C, the inner zone was on average more negative (-23.96 ±0.50‰) compared to the middle zone (-14.83 ±1.86‰) (Table 2). Potential food sources contributed in different proportions to consumers living in the middle zone of the fjord. However, consumers used a combination of sediment organic matter, SPOM, and POM River in the inner and middle zones of the fjord (Fig. 6).

3.5. Isotopic niche widths and trophic position Analysis of the isotopic niche width included the standard ellipse area (SEAc) for the functional groups in the study area and the overlap (95 % Bayesian ellipses) among most functional groups (Fig. 5). The SEAc corrected show that deposit feeders had the highest isotopic niche width (SEAc = 21.27), followed by grazers (SEAc = 14.14), predators (SEAc = 10.41), suspension feeders (SEAc = 10.15) and micro- and mesozooplankton (SEAc = 2.53). For overlap of isotopic niches, predators overlapped all functional groups present in the BMF, and in particular with suspension feeders (% overlapping = 39.19 [minimum – maximum; 24.37 - 64.19]) followed by deposit feeders (% overlapping = 38.56 [23.36 - 52.47]). Bayesian overlapping was smaller for predators and grazers (% overlapping = 14.41 [8.55 - 27.33]), but not for predators and zooplankton detritivores (% overlapping = 7.54 [2.68 - 20.42]). Also, suspension feeders overlapped with deposit feeders (% overlapping = 38.55 [19.14 - 57.86]) and grazers (% overlapping = 21.92 [11.61 - 42.30]), while Bayesian overlapping was

13

smaller between suspension feeders with zooplankton detritivores (% overlapping = 8.87 [3.38 - 23.98]). Bayesian overlapping between grazers and suspension feeders was small (% overlapping = 14.57 [8.43 - 27.97]), and even smaller between deposit feeders and zooplankton detritivores (% overlapping = 7.07 [1.61 - 18.95]). The zooplankton detritivores all most did not overlapped with grazers (% overlapping = 0.08 [0 - 0.17]). The estimated relative trophic positions (TP) of consumers living in the BMF are summarized in Table 2. The inner zone of the fjord had the highest number of organisms classified as primary consumers (TP ranging from 2.08 to 2.83, excluding zooplankton detritivores), with less secondary consumers (TP ranging from 3.14 to 3.57; 3 species) and one tertiary consumer with a TP equal to 4.23. Less trophic diversity was recorded in the Martínez channel, characterized by the dominance of the zooplankton detritivores as the only primary consumers, and a reduced number of species for secondary (TP ranging from 3.05 to 3.75; 10 species) and one tertiary consumer with a TP equal to 4.08. The Baker channel was characterized by primary consumers (6 species with TP ranging from 2.09 to 2.70) and secondary consumers (6 species with TP ranging from 3.20 to 3.52). Results showed that communities were represented mainly by primary and secondary consumers (TP = 2 and 3) and to a lesser extent by higher order consumers (TP = 4).

4. Discussion 4.1. The influence of environmental stress on the trophic structure Fluvial discharges significantly change environmental conditions in the BMF complex system in ways that modify the hydrographic and sediment conditions, which in turn affects trophic structure of benthic communities. Two ecological groups were identified; one located in the inner and middle fjord and the other one is located in the outer fjord.

14

Both had isotopically less diverse feeding groups, comprising at least four trophic levels. In addition, a benthic community with overlapping trophic niches was identified, associated with chronic natural physical disturbances from glacial sedimentations, giving rise to this non-selective feeding benthic community. Sediments in the BMF complex system where shown to be characterized by variable amounts of terrestrial and marine OM. Indeed, bulk C/N and δ13C have been used equally in estimations of terrestrial OM contributions to benthic systems. High levels of chlorophyll-a, derived from primary production, were registered in summer periods, particularly in the outer fjord. In addition, a mixture of emergent vascular terrestrial woody plants and freshwater phytoplankton was shown to provide an OM subsidy to the inner fjord benthic ecosystem, producing a well-defined gradient along the fjord characterized by distinct proportions of OM. These results are consistent with previous findings (Quiroga et al., 2016; Rebolledo et al., 2018) and illustrate how the composition and distribution of benthic communities may reflect along-fjord changes in environmental conditions, driven mostly by seasonal changes in freshwater inputs, as well as small-scale spatial variations in flow along the fjord (Meerhoff et al., 2019). It is worth mentioning that the absence of macroalgal assemblages as an OM source (red and green algae and brown algae or kelps) is coherent with the light conditions and the presence of a freshwater layer in the upper stratum of the water column associated with the river discharges in Patagonian environments (Betti et al., 2017). The freshwater layer and particulate suspended materials from glacial origins appear to limit primary production and macroalgal colonization (kelps and green and red algae), resulting in less negative carbon values for basal sources as shown in the bi-plot of δ13C versus δ15N (Fig. 4).

15

The trophic structure metrics in our study area suggest that the benthic food web is supported principally by phytoplankton production and terrestrial organic matter pathways. Previous analyses of other Chilean Patagonian fjords have shown that primary production does not sustain the benthic trophic structure (Mayr et al., 2011; Zapata-Hernández et al., 2014, 2016; Andrade et al., 2016; Quiroga et al., 2016). Andrade et al., (2016) described the trophic structure of benthic communities, identifying three heterotrophic levels with macroalgae and suspended organic matter as the main food sources. In addition, estimates of potential food source contributions for benthic consumers suggest that kelps might be shown an important dietary item, e.g., for the irregular echinoid (T. philippii), shown in previous studies to constitute up to 35.1% and 42.5% of their diet (Zapata-Hernández et al., 2016). Macroalgal subsidies can be important to detritivores (Fraser et al., 2018), and they may contribute as trophic subsidy in the study area via floating kelps (Wichmann et al., 2012). Regardless, this study found SOM to be one of the most important basal resources in the study area. The Bianchi model and isotopic values of decapod P. spinulosum were used in order to estimate the terrestrial OM contributions to benthic consumer diets (Table 3). It was calculated that about 31.86% of its diet is sustained from SOM, which is similar to the contribution of basal resources (SOM) estimated from the mixed model estimates of potential food sources (Fig. 6). In fact, terrestrial OM contributions were found to play an important ecological role in the benthic and pelagic food webs, accounting for 5090% of total organic matter (Lafon et al., 2014; Quiroga et al., 2016; Meerhoff et al., 2019). In this study, the only pelagic taxon classifiable as an omnivore was the gregarious squat lobster, Munida gregaria, a key food source for numerous fish and marine mammal species in Patagonian fjord trophic webs, where it has been found in

16

higher abundances (Meerhoff et al., 2019). Here, it was especially enriched in δ15N but less so in δ13C, in comparison to the other benthic feeding guilds. M. gregaria appear to be closely related to river discharge dynamics in the region, with higher abundances as likely responses to terrestrial OM input via rivers or glacial lake outburst floods (Meerhoff et al., 2019). This species shows plasticity in its feeding mode, depending on the period of its life-cycle or presence or absence of resources. In general, the zooplankton were depleted in

13

C relative to bulk SPOM, suggesting that they play an

important role in the food web in the study area, connecting the organic carbon advected from rivers and thus increasing the efficiency of OM from detritus inputs from rivers. Indeed, zooplankton detritivores (> 300 um) were largely depleted in

13

C (-25.17 to -

21.07 ‰) throughout the BMF complex system (Ortíz, 2016), which have been also recorded for copepods in the Steffen channel (Vargas et al., 2011). These results suggest the importance of sources of terrestrial carbon in benthic and planktonic communities, which could maintain high secondary production through different trophic subsidies (Vargas et al., 2011).

4.2. Trophic structure and isotopic niche It is known that benthic food webs depend on primary production, fresh, reworked settled OM or terrestrial OM input (Gillies et al., 2013). In general, there are high temporal and spatial variability in primary food sources, where suspension feeders could have variable diets in response to environmental gradients in food supply, in the study area; benthic fauna seem to be characterized by lower trophic levels in comparison to no-glacier environments, which would be more resilient to changing seasonal conditions. In this regard, this study has clearly demonstrated that the trophic structure of benthic communities in fjord environments are correlated to physical

17

hydrographic conditions; differences among locations are likely due to terrestrial input, such as POM advection or resuspension that contribute terrestrial OM to benthic systems (McLeod and Wing, 2009; Lafon et al., 2014; Quiroga et al., 2016; Meerhoff et al., 2019). Suspension feeder δ13C were lower in our study area than those of suspended particulate OM reported for Sub-Antarctic and Arctic fjords (Renaud et al., 2011; Gillies et al., 2013; Tu et al., 2015) or Antarctic ecosystems (e.g., Jacob, 2005; Mintenbeck et al., 2007; Quiroga et al., 2014). These results reinforce terrestrial carbon sources being incorporated by deposit feeders and suspension feeders. In the study area, melt-water discharge from the Baker River was also shown to influence the trophic structure of benthic communities, with large quantities of POM transported into the river and fjord. These environmental conditions affect the isotopic composition of particulate organic matter, which reflects the depositional process that results in discrete zones of organic matter accumulation or resuspension (Tu et al., 2015). Under the above conditions, in the study area, benthic community is dominated by a few disturbance-tolerant species, characterized by trophic guilds with a high proportion of species with overlapping trophic niches or higher trophic redundancy, according to Layman et al. (2007). This study confirms previous data on the direct effects of high sedimentation rates and terrestrial OM input on benthic and pelagic communities, which produces changes in the community structure and trophic ecology (Lafon et al., 2014; Quiroga et al., 2016; Meerhoff et al., 2019). The BMF system, as a type of glacial- and freshwater-influenced environment, however, has trophic structures dissimilar to those described for other fjords (Zapata-Hernández et al., 2014; 2016; Andrade et al., 2016). The BMF has been shown to exhibit high trophic redundancy or species with overlapping trophic niches of species, similar to that benthic communities described in submarine canyons (Demopoulos et al., 2017). Other zones in the

18

Patagonian region have reportedly non-selective benthic communities with trophic relationships that depend on either local availability of intertidal prey (Andrade et al., 2016; Riccialdelli et al., 2017) or trophic subsides from local primary production and macroalgae, such as found in inner areas of Chiloé (Zapata-Hernández et al., 2016). The ecological stress caused by the river Baker show that SOM is the most important environmental factor that affect the trophic structure of the benthic and pelagic communities (Quiroga et al., 2016; Meerhoff et al., 2019). In this study, echinoderms had a wider niche due to additional food items in their diet. In contrast, suspension feeders exhibited a narrow isotopic niche, which was associated suspended particulate organic matter. Grazers exhibited broad isotopic niche based on carbon values, suggesting wide food resource variety. This latter is particularly evident given carbon isotopic values for T. philippii (-23.96 ‰) and N. magellanica (-14.86 ‰). Estimated consumer TPs in the study area shows that BMF complex exhibited low trophic diversity. The above further suggests that glaciofluvial environments may promote lower trophic diversity and higher trophic redundancy. In summary, the benthic food web under study was found to be principally supported by primary production and terrestrial organic matter pathways. However, macroalgal subsidies could also be important to detritivores and herbivores via floating kelps in the inner area of the fjord (Ronowicz et al., 2018). In fact, most suspensionfeeding bivalves had isotopic signals consistent with more than a 50% contribution from kelps and rockweeds in Arctic fjords (Renaud et al., 2015). The results of this study suggest that glaciofluvial benthic communities have higher trophic redundancy associated with environmental disturbance conducted by sediment load, shaping a benthic community with a lower alpha diversity and fewer trophic levels in comparison to no-glacier environments. In view of climate change, these results help us to

19

understand the possible impacts on benthic ecosystems if any shifts in trophic structure occur due to ongoing temperature increases and ice-cover reduction, increasing the sedimentation loads, limiting the availability of food supply, with negatives effects on biodiversity and fjord ecosystem functions.

Acknowledgements This research was funded by a Regular FONDECYT Grant 1130691 (CONICYT) and project CONA C23F 17-09 (Granted to E. Quiroga and N. Olguín). Additional funds are thanked to Research Department of the PUCV (No. DII-PUCV 223.730/2016) and the project CONA C20F 14-04 (Granted to E. Mutschke and C. Aldea, Universidad de Magallanes). We thank also to Dr. Américo Montiel (Universidad de Magallanes) as part of isotopic analysis were covered by his research project UMAG-DI (No. PR-F102-IP-15). Special thanks go to Jim McAdam for the English improve. We are also grateful for the financial and logistical support of Center for Oceanographic Research COPAS Sur-Austral (CONICYT PIA PFB31 and CONICYT PIA APOYO CCTE AFB170006). Thanks also to Rodrigo Mansilla (COPAS Sur-Austral), Paula Ortíz (CIEP Center), Carlos Pineda, Nicole Salinas and the crew of RV Sur Austral (UDEC) for their important assistance on each of the fieldworks. This is the second contribution of the LOBOS Team (Laboratorio de Oceanografía y Bentos) from PUCV.

References Andrade, C., Brey, T., 2014. Trophic ecology of limpets among rocky intertidal in Bahía Laredo, Strait of Magellan (Chile). Anales del Instituto de la Patagonia 42 (2), 65–70. Andrade, C., Ríos, C., Gerdes, D., Brey, T., 2016. Trophic structure of shallow-water

20

benthic communities in the sub-Antarctic Strait of Magellan. Polar Biology 39, 2281–2297. Aracena, C., Lange, C.B., Iriarte, J.L., Rebolledo, L., Pantoja, S., 2011. Latitudinal patterns of export production recorded in surface sediments of the Chilean Patagonian fjords (41-55°S) as a response to water column productivity. Continental Shelf Research 31, 340–355. Bell, L., Bluhm, B.A., Iken, K., 2016. Influence of terrestrial organic matter in marine food webs of the Beaufort Sea shelf and slope. Marine Ecology Progress Series 550, 1–24. Betti, F., Bavestrello, G., Bo, M., Enrichetti, F., Loi, A.,Wanderlingh, A., Perez-Santos, I., Daneri, D., 2017. Benthic biodiversity and ecological gradients in the Seno Magdalena (Puyuhuapi Fjord, Chile). Estuarine, Coastal and Shelf Science 198, 269–278. Bianchi, T. S., 2007. Biogeochemistry of estuaries. Oxford University Press on Demand, 720 pp. Cárdenas, C.A., Montiel, A., 2015. The influence of depth and substrate inclination on sessile assemblages in subantarctic rocky reefs (Magellan region). Polar Biology 38 (10), 1631–1644. Cárdenas, C.A., Montiel, A., 2017. Coexistence in Cold Waters: Animal Forests in Seaweed-Dominated Habitats in Southern High Latitudes. In: Rossi, S., Bramanti, L., Gori, A., Orejas, C. (Eds.) Marine Animal Forests. Springer, Cham, pp 257– 276. Clarke, K.R., Somerfield, P.J., Chapman, M.G., 2006. On resemblance measures for ecological studies, including taxonomic dissimilarities and a zero-adjusted BrayCurtis coefficient for denuded assemblages. Journal of Experimental Marine

21

Biology and Ecology 330, 55–80. Clarke, K.R., Somerfield, P.J., Gorley, R.N., 2008. Testing of null hypotheses in exploratory community analyses: similarity profiles and biota-environment linkage. Journal of Experimental Marine Biology and Ecology 366, 56–69. Demopoulos, A.W. J., McClain-Counts, J., Ross, S.W., Brooke, S., Mienis, F., 2017. Food-web dynamics and isotopic niches in deep-sea communities residing in a submarine canyon and on the adjacent open slopes. Marine Ecology Progress Series 578, 19–33. Dowdeswell, J.A., Vásquez, M., 2013. Submarine landforms in the fjords of southern Chile: Implications for glacimarine processes and sedimentation in a mild glacierinfluenced environment. Quaternary Science Review 64, 1–19. Dussaillant, A., Benito, G., Buytaert, W., Carling, P., Meier, C., Espinoza, F., 2010. Repeated glacial-lake outburst floods in Patagonia: An increasing hazard? Natural Hazards 54, Dussaillant J., A., Buytaert, W., Meier, C., Espinoza, F., 2012. Hydrological regime of remote catchments with extreme gradients under accelerated change: the Baker basin in Patagonia. Hydrological Science Journal 57, 1530–1542. Fraser, C.I., Morrison, A.K., Hogg, A.M., Macaya, E.C., van Sebille, E., Ryan, P.G., Padovan, A., Jack, C., Valdivia, N., Waters, J.M., 2018. Antarctica’s ecological isolation will be broken by storm-driven dispersal and warming. Nature Climate Change 8, 704–708. Gillies, C.L., Stark, J.S., Johnstone, G.J., Smith, S.D.A., 2013. Establishing a food web model for coastal Antarctic benthic communities: a case study from the Vestfold Hills. Marine Ecology Progress Series 478, 27−41. González, H.E., Calderón, M.J., Castro, L., Clement, A., Cuevas, L.A., Daneri, G.,

22

Iriarte, J.L., Lizárraga, L., Martínez, R., Menschel, E., Silva, N., Carrasco, C., Valenzuela, C., Vargas, C.A., Molinet, C., 2010. Primary production and plankton dynamics in the Reloncaví Fjord and the Interior Sea of Chiloé, northern Patagonia, Chile. Marine Ecology Progress Series 402, 13–30. González, H.E., Castro, L., Daneri, G., Iriarte, J.L., Silva, N., Vargas, C.A., Giesecke, R., Sánchez, N., 2011. Seasonal plankton variability in Chilean Patagonia fjords: Carbon flow through the pelagic food web of Aysen Fjord and plankton dynamics in the Moraleda Channel basin. Continental Shelf Research 31, 225–243. González, H.E., Castro, L.R., Daneri, G., Iriarte, J.L., Silva, N., Tapia, F., Teca, E., Vargas, C.A., 2013. Land-ocean gradient in haline stratification and its effects on plankton dynamics and trophic carbon fluxes in Chilean Patagonian fjords (4750°S). Progress in Oceanography 119, 32–47. Gorlich, K., Westawski, J.M., Zajaczkowski, M., 1987. Suspension settling effect on macrobenthos biomass distribution in the Hornsund fjord, Spitsbergen. Polar Research 5, 175–192. Grange, L.J., Smith, C.R., 2013. Megafaunal communities in rapidly warming fjords along the West Antarctic Peninsula: Hotspots of abundance and beta diversity. PLoS One 8 (12), e77917. Häussermann, V., Försterra, G., 2009. Marine Benthic Fauna of Chilean Patagonia, Nature in Focus, Chile, 1000 pp. Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W., Strickland, J.D., 1965. Fluorometric determination of chlorophyll. ICES Journal of Marine Science 30 (1), 3−15. Jackson, A.L., Inger, R., Parnell, A.C., Bearhop, S., 2011. Comparing isotopic niche widths among and within communities: SIBER - Stable Isotope Bayesian Ellipses in R. Journal of Animal Ecology 80, 595–602.

23

Jacob, U. 2005. Trophic dynamics of Antarctic shelf ecosystems—food webs and energy flow budgets. PhD thesis, Alfred Wegener Institute for Polar and Marine Research. Jacob, B., Tapia, F., Daneri, G., Iriarte, J.L., Montero, P., Sobarzo, M., Quiñones, R., 2014. Spring time size-fractionated primary production across hydrographic and PAR-light gradients in Chilean Patagonia (41–50°S). Progress in Oceanography 129, 75–84. Kiljunen, M., Grey, J., Sinisalo, T., Harrod, C., Immonen, H., Jones, R.I., 2006. A revised model for lipid‐normalizing δ13C values from aquatic organisms, with implications for isotope mixing models. Journal of Applied Ecology 43 (6), 1213– 1222. Lafon, A., Silva, N., Vargas, C.A., 2014. Contribution of allochthonous organic carbon across the Serrano River Basin and the adjacent fjord system in Southern Chilean Patagonia: Insights from the combined use of stable isotope and fatty acid biomarkers. Progress in Oceanography 129, 98–113. Layman, C., Arrington, D.A., Montana, C.G., Post, D.M., 2007. Can stable isotope ratios provide for community-wide measures of trophic structure? Ecology 88, 42–48. Lara, A., Bahamóndez, A., González-Reyes, A., Muñoz, A.A., Cuq, E., Ruíz-Gómez, C., 2015. Reconstructing stream flow variation of the Baker River from tree-rings in Northern Patagonia since 1765. Journal of Hydrology 529, 511–523. Levin, L.A., Currin, C.,

2012. Stable Isotope Protocols: Sampling and Sample

Processing. UC San Diego: Scripps Institution of Oceanography. Retrieved from https://beta.escholarship.org/uc/item/3jw2v1hh. Macdonald, T.A., Burd, B.J., Macdonald, V.I., Van Roodselaar, A., 2010. Taxonomic

24

and feeding guild classification for the marine benthic macroinvertebrates of the Strait of Georgia, British Columbia. Canada Technical Report. Fisheries and Oceans Canada, Sidney, 69 pp. Marín, V.H., Tironi, A., Paredes, M.A., Contreras, M., 2013. Modeling suspended solids in a Northern Chilean Patagonia glacier-fed fjord: GLOF scenarios under climate change conditions. Ecological Modelling 264, 7–16. Mayr, C., Försterra, G., Häussermann, V., Wunderlich, A., Grau, J., Zieringer, M., Altenbach, A.V., 2011. Stable isotope variability in a Chilean fjord food web: Implications for N- and C-cycles. Marine Ecology Progress Series 428, 89–104. Mayr, C., Rebolledo, L., Schulte, K., Schuster, A., Zolitschka, B., Försterra, G., Häussermann, V., 2014. Responses of nitrogen and carbon deposition rates in Comau Fjord (42°S, southern Chile) to natural and anthropogenic impacts during the last century. Continental Shelf Research 78, 29–38. McGovern, M., 2016. Hyperbenthic Food-Web Structure in Kongsfjord: A Two-Season Comparison using Stable Isotopes and Fatty Acids. Master´s thesis, UiT Norges arktiske universitet, unpublished. McLeod, R.J., Wing, S.R., 2009. Strong pathways for incorporation of terrestrially derived organic matter into benthic communities. Estuarine, Coastal and Shelf Science 82, 645–653. McLeod, R.J., Wing, S.R., Skilton, J.E., 2010. High incidence of invertebratechemoautotroph symbioses in benthic communities of the New Zealand fjords. Limnology and Oceanography 55, 2097–2106. Meerhoff, E., Castro, L., Tapia, F., 2013. Influence of freshwater discharges and tides on the abundance and distribution of larval and juvenile Munida gregaria in the Baker river estuary, Chilean Patagonia. Continental Shelf Research 62, 1–11.

25

Meerhoff, E., Castro, L.R., Tapia, F.J., Pérez-Santos, I., 2019. Hydrographic and Biological Impacts of a Glacial Lake Outburst Flood (GLOF) in a Patagonian Fjord. Estuaries and Coasts 42, 132–143. Mintenbeck, K., Jacob, U., Brey, T., Knust, R., Arntz. W.E., 2007. Depth-dependence in stable isotope ratio δ15N of benthic POM consumers: a potential bias in isotope based food web studies. Deep Sea Research Part II 54, 1015−1023. Moffat, C., Tapia, F. J., Nittrouer, C. A., Hallet, B., Bown, F., Boldt Love, K., Iturra, C., 2018. Seasonal evolution of ocean heat supply and freshwater discharge from a rapidly retreating tidewater glacier: Jorge Montt, Patagonia. Journal of Geophysical Research: Oceans 123 (6), 4200−3223. Ortíz, P., 2016. Escalas de variabilidad en la composición de tamaños y estructura comunitaria del componente planctónico en un sistema de fiordo con influencia glacial (Patagonia Central, 47°S). Master thesis of oceanography. Pontifical University of Catholic of Valparaíso, 88 pp. Parnell, A. C., Inger, R., Bearhop, S.,Jackson, A. L., 2010. Source partitioning using stable isotopes: coping with too much variation. PloS one, 5(3), e9672. Pawłowska, J., Włodarska-Kowalczuk, M., Zajączkowski, M., Nygård, H., Berge, J., 2011. Seasonal variability of meio-and macrobenthic standing stocks and diversity in an Arctic fjord (Adventfjorden, Spitsbergen). Polar Biology 34 (6), 833–845. Pearson, T.H., Rosenberg, R., 1978. Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanography and Marine Biology: An Annual Review 16, 229–311. Post, D.M., 2002. Using stable isotopes to estimate trophic position: models, methods, and assumptions. Ecology 83, 703–718. Quiroga, E., Ortíz, P., Gerdes, D., Reid, B., Villagran, S., Quiñones, R., 2012. Organic

26

enrichment and structure of macrobenthic communities in the glacial Baker Fjord, Northern Patagonia, Chile. Journal of the Marine Biological Association of the United Kingdom, 92 (1), 73–83. Quiroga, E., Gerdes, D., Montiel, A., Knust, R., Jacob, U., 2014. Normalized biomass size spectra in high Antarctic macrobenthic communities: linking trophic position and body size. Marine Ecology Progress Series 506, 99–113. Quiroga, E., Ortíz, P., González-Saldías, R., Reid, B., Tapia, F.J., Pérez-Santos, I., Rebolledo, L., Mansilla, R., Pineda, C., Cari, I., Salinas, N., Montiel, A., Gerdes, D., 2016. Seasonal benthic patterns in a glacial Patagonian fjord: The role of suspended sediment and terrestrial organic matter. Marine Ecology Progress Series 561, 31–50. Rebolledo, L., Bertrand, S., Lange C.B., Tapia, F.J., Quiroga, E., Troch, M., Silva, N., Cárdenas, P., Pantoja, S., 2018. Compositional and biogeochemical variations of sediments across the terrestrial-marine continuum of the Baker-Martínez fjord system (Chile, 48°S). Progress in Oceanography 174, 89–104. Renaud, P.E., Włodarska-Kowalczuk, M., Trannum, H., Holte, B., Wȩsławski, J.M., Cochrane, S., Dahle, S., Gulliksen, B., 2007. Multidecadal stability of benthic community structure in a high-Arctic glacial fjord (van Mijenfjord, Spitsbergen). Polar Biology 30, 295–305. Renaud, P.E., Tessmann, M., Evenset, A., Christensen, A.N., 2011. Benthic food-web structure of an Arctic fjord (Kongsfjorden, Svalbard). Marine Biology Research 7 (1), 13–26. Renaud, P.E., Løkken, T.S., Jørgensen, L.L., Berge, J., Johnson, B.J. 2015. Macroalgal detritus and food-web subsidies along an Arctic fjord depth-gradient. Frontiers in Marine Sciences 2:31. doi: 10.3389/fmars.2015.00031

27

Riccialdelli, L., Newsome, S.D., Fogel, M.L., Fernández D.A., 2017. Trophic interactions and food web structure of a subantarctic marine food web in the Beagle Channel: Bahía Lapataia, Argentina. Polar Biology 40, 807–821. Ronowicz, M., Kukliński, P., Włodarska-Kowalczuk, M., 2018. Diversity of kelp holdfast-associated fauna in an Arctic fjord - inconsistent responses to glacial mineral sedimentation across different taxa. Estuarine, Coastal and Shelf Science 205, 100–109. Sepúlveda, J., Pantoja, S., Hughen, K.A., 2011. Sources and distribution of organic matter in northern Patagonia fjords, Chile (~44-47°S): A multi-tracer approach for carbon cycling assessment. Continental Shelf Research 31, 315–329. Silva, N., Palma, S., 2008. The CIMAR Program in the austral Chilean channels and fjords. In: Progress in the oceanographic knowledge of Chilean interior waters, from Puerto Montt to Cape Horn. N. Silva & S. Palma (eds.). 2008 Comité Oceanográfico Nacional - Pontificia Universidad Católica de Valparaíso, Valparaíso, pp 11–15. Silva, N., Vargas, C.A., Prego, R., 2011. Land-ocean distribution of allochthonous organic matter in surface sediments of the Chiloé and Aysén interior seas (Chilean Northern Patagonia). Continental Shelf Research 31, 330–339. Schlitzer, R. 2012. Ocean Data View. http: //odv.awi.de. Syväranta, J., Lensu, A., Marjomäki, T.J., Oksanen, S., Jones, R.I., 2013. An empirical evaluation of the utility of convex hull and standard ellipse areas for assessing population niche widths from stable isotope data. PLoS One 8 (2), e56094. Syvitski, J.P.M., Farrow, G.E., Atkinson, R.J. a, Moorep, G., Andrews, J.T., 1989. Baffin Island Canada fjord macrobenthos bottom communities and environmental significance. Arctic. 42 (3), 232–247.

28

Tu, K.L., Blanchard, A.L., Iken, K., Horstmann-Dehn, L., 2015. Small-scale spatial variability in benthic food webs in the northeastern Chukchi Sea. Marine Ecology Progress Series 528, 19–37. Vander Zanden, M.J., Rasmussen, J.B., 1999. Primary consumer δ13C and δ15N and the trophic position of aquatic consumers. Ecology 80 (4), 1395–1404. Vargas, C.A., Martínez, R.A., San Martín, V., Aguayo, M., Silva, N., Torres, R., 2011. Allochthonous subsidies of organic matter across a lake-river-fjord landscape in the Chilean Patagonia: Implications for marine zooplankton in inner fjord areas. Continental Shelf Research 31, 187–201. Węsławski, J.M., Buchholz, F., Głuchowska, M., Weydmann, A., 2017. Ecosystem maturation follows the warming of the Arctic fjords. Oceanologia 59 (4), 592– 602. Wichmann, C.S., Hinojosa, I., Thiel, M., 2012. Floating kelps in Patagonian Fjords: an important vehicle for rafting invertebrates and its relevance for biogeography. Marine Biology 159, 2035–2049. Wlodarska-Kowalczuk, M., Pearson, T.H., Kendall, M.A., 2005. Benthic response to chronic natural physical disturbance by glacial sedimentation in an Arctic fjord. Marine Ecology Progress Series 303, 31–41. Wlodarska-Kowalczuk, M., Mazurkiewicz, M., Jankowska, E., Kotwicki, L., Damrat, M., Zajaczkowski, M., 2016. Effects of fluvial discharges on meiobenthic and macrobenthic variability in the Vistula River prodelta (Baltic Sea). Journal of Marine Systems 157, 135–146. Zajączkowski,

M.,

Włodarska-Kowalczuk,

M.,

2007.

Dynamic

sedimentary

environments of an Arctic glacier-fed river estuary (Adventfjorden, Svalbard). I. Flux, deposition, and sediment dynamics. Estuarine, Coastal and Shelf Science 74

29

(1-2), 285–296. Zapata-Hernández, G., Sellanes, J., Mayr, C., Muñoz, P., 2014. Benthic food web structure in the Comau fjord, Chile (~42°S): Preliminary assessment including a site with chemosynthetic activity. Progress in Oceanography 129, 149–158. Zapata-Hernández, G., Sellanes, J., Thiel, M., Henríquez, C., Hernández, S., Fernández, J.C., Hajdu, E., 2016. Community structure and trophic ecology of megabenthic fauna from the deep basins in the Interior Sea of Chiloé, Chile (41–43°S). Continental Shelf Research 130, 47–67.

30

Figures and Tables Captions

Fig. 1 Location of sampling stations in the Baker-Martínez fjord complex system in Chilean Patagonia. White dots by sampling stations for March 2014 and December 2015; black dots by sampling stations for October 2014.

Fig. 2 The cross-sections of (a) temperature and (b) salinity in the Martínez channel and (Left) and (c) temperature and (d) salinity in Baker channel (right), ranging from 0 near the mouth of the river.

Fig. 3 Cluster analysis based on δ13C values considering organisms in the Martínez-Baker fjord complex system. Groups of organisms were classified using hierarchical cluster analysis of Bray–Curtis dissimilarities. (Inner-fjord by green color; Middle-fjord Martínez channel by blue color; Middle-fjord Baker channel by red color and Outerfjord by pink color).

Fig. 4 Plot of δ15N and δ13C in benthic consumers and potential food sources in the BakerMartínez fjord complex system. SOM: Sedimentary organic matter; SPOM: Suspended particulate organic matter; POMriver: Particulate organic matter from river sources.

31

Fig. 5 Isotopic niche widths of functional guilds in the study area. Solid lines enclose the standard ellipse area (SEA), representing consumer isotopic niches. Dotted lines are the convex hulls representing the total niche widths of different consumers.

Fig. 6 Mixed model estimates of potential food source (POM river, SOM, SPOM) contributions (%) to diets of predators (P), deposit feeders (DF), grazers (G), scavengers (S) , suspension feeders (SF) and zooplankton detritivores (Z) at the Baker- Martínez Fjord complex system.

Table 1. List of stations, locations, depth, season, and sampling gear used during the study.

Table 2. Information on δ13C and δ15N isotope ratios (mean and SD) for sediment, invertebrates, and bony fishes in the inner, middle and outer fjords.

Table 3. Contribution of terrestrial organic carbon to isotopic composition using the Bianchi mixing model for decapod P. spinosulum in the study area.

Supplementary material 1. Information of sites, latitude, longitude, depth, C/N ratio, δ13C and δ15N for sediment organic matter in the Baker-Martínez fjord complex system.

32

33

Station

Date

Latitude (⁰S)

Longitude (⁰W)

Depth (m)

Fjord Section

Location

Season

Sampling gear

1 2 3 4 5 6 7 8 9 10

March 25, 2014 December 15, 2015 August 1, 2014 October 8, 2014 december 12, 2015 December 14, 2015 April 10, 2014 December 13, 2015 October 15, 2014 October 15, 2014

47⁰53.19' 47⁰47.49' 47⁰46.08' 47⁰57.60' 47⁰45.25' 47⁰52.50' 47⁰53.12' 47⁰53.50' 47⁰44.08' 47⁰22.00'

73⁰21.19' 73⁰35.52' 73⁰41.62' 74⁰00.83' 74⁰13.21' 74⁰27.24' 74⁰ 32.69' 74⁰ 40.76' 74⁰45.29' 75⁰38.00'

50 38 252 120 51 90 50 63 250 142

Inner Inner Inner Middle Middle Middle Outer Outer Outer Outer

Baker River mouth Baker River mouth Steffen fjord mouth Baker channel/Tito island Berta Island Punta Baker Scout Island Porcias-Scout Islands Martínez channel Gulf of Penas

Late summer Summer Spring Spring Summer Summer Spring Summer Spring Spring

Trap/gravity corer/Plankton net Trap/gravity corer Agassiz trawl Agassiz trawl Trap Van Veen grab/Plankton net Agassiz trawl Van Veen grab/traps/intertidal Agassiz trawl Agassiz trawl

Phyllum taxon

Inner fjord (Martínez channel) δ15N δ13C n Mean SD Mean

Crustacea Lithodes santolla 3 Libidoclaea grenaria * 2 Libidoclaea grenaria ** 4 Peltarion spinosulum * 1 Peltarion spinosulum** 4 Munida gregaria n.d. Pagurus gaudauchi Paralomis granulosa Cancer edwarsii (male) Cancer edwarsii (female) Munida subrugosa Stereomastis suhmi Amphipoda indet. 1 Cirolana albinota 1 Polychaeta Nephtyidae indet. 1 Terebellidae indet. 1 Lumbrineridae indet. 1 Sternaspis sp. Maldanidae indet. Nemertea Nemertinea indet. 1 Mollusca Fusitriton cancellatus 2 Nacella magellanica (ST5*) Mytilus chilensis Aulacomya atra Tegula atra Nacella magellanica (ST8*) Zooplankton Macroplankton (2000 µm) n.d. Mesoplankton (1000 µm) n.d. Mesoplankton (500 µm) n.d. Microplankton (300 µm) n.d. Microplankton (150 µm) n.d. Echinodermata Tripylaster philippii 3 Gorgonocephalus chilensis Ophiocten amitinum Ophiura (Ophiuroglypha) lymani (ST8**) Ophiura (Ophiuroglypha) lymani (ST9**) Labidiaster radiosus Cnidaria Corallium rubrum Hydrozoa indet. Errina antarctica Anthozoa indet. Chondrichthyes Schroederichthys chilensis 2 Osteichthyes Genypterus blacodes 3 Malacanthidae indet. Oncorhynchus mykiss Basilichthys australis Sebastes oculatus SOM 15 SPOM 2 POM river

9.12 8.83 7.35 6.87 7.74 10.95

0.04 0.13 0.72 0.72

-21.41 -20.74 -22.98 -23.24 -22.64 -19.34

SD

TP

0.11 0.02 0.79

2.60 2.52 2.08 1.94 2.20 3.14

0.79

Middle fjord (Martínez channel) δ15N δ13C n Mean SD Mean SD

TP

2

12.83

-15.50 0.18

3.20

1

12.38

-16.93

3.07

1 1

13.21 13.86

-15.67 -17.67

3.31 3.50

3 10.05 7.98

-20.17 -21.46

2.88 2.27

11.78 5.27 7.72

-17.38 -17.82 -24.23

3.38 1.47 2.19

12.31

0.03

0.56

-18.37 0.49

Middle fjord (Baker channel) δ15N δ13C n Mean SD Mean

n.d.

11.31

1 2 3

14.10 13.89 9.66

0.06 0.51

SD

Outer fjord δ15N n Mean

TP

-17.82

2.70

-16.59 -15.91 0.06 -19.03 0.63

3.52 3.46 2.21

9.91

0.07

-22.01

1.80

-19.93 0.18

2.83

8.85

0.05

-21.14 -21.77 -22.61 -22.32 -21.95

3.03 2.92 2.79 2.63 2.85

-23.96 0.50

2.52

12.43

0.13

-18.84 0.02

3.57

14.66

0.11

-16.68 0.21

4.23

3.67 3.50 2.40

2.01 0.60 1.10

-26.58 1.07 -27.30 1.60 -27.21 0.96

11.67 11.26 10.52 9.31 10.62

-24.39 -24.28 -25.17 -24.87 -21.07

13.86 2 3 2 2 2 6 1

13.37 15.81 13.69 13.52 14.70 5.34 4.12

-16.60 -14.72 -17.47 -17.08 -15.30 -23.64 -26.00

TP

9.24 8.82 8.27 10.11 10.21

0.01 0.04 0.10 0.01

-14.86 -19.10 -19.47 -16.68 -12.96

0.11 0.09 0.08 0.07

12.02 12.46

0.17 0.03

-19.44 0.20 -17.56 0.20

2.14 2.27

2.09 1.97 1.81 2.35 2.38

2.86 2.74 2.52 2.17 2.55

-16.42 0.03 0.05 0.12 0.05 0.02 2.39 1.00

SD

3.05

2 2 n.d. 2 3 10.58 10.19 9.77 9.21 9.95

δ13C Mean

#

3 3 6.39

SD

0.05 0.09 0.02 0.29 0.03

2.62 1.00

Diet

Feeding mode

Om Ca Ca Ca Ca Ca Om Om Ca Ca Ca Om Om Ca

Predator Predator Predator Predator Predator Deposit feeder Deposit feeder Predator Predator Predator Deposit feeder Scavenger Deposit feeder Predator

Ca Om Ca Om Om

Predator Deposit feeder Predator Deposit feeder Deposit feeder

Om

Predator

Ca Om Om Om He He

Predator Grazer Suspension feeder Suspension feeder Grazer Grazer

Om Om Om Om Om

Zooplankton Zooplankton Zooplankton Zooplankton Zooplankton

3 3 3 3 3

13.83 9.41 10.14 11.01 13.01

0.18 0.56 0.62 0.64 0.37

-19.06 -20.87 -20.51 -12.09 -15.65

0.34 0.86 0.33 0.21 0.52

3.44 2.14 2.35 2.61 3.20

He Ca Om Om Om Ca

Grazer Predator Suspension feeder Deposit feeder Deposit feeder Predator

2 4 3 3

13.38 13.26 7.00 13.92

0.04 0.48 2.11 0.15

-19.26 -20.29 -23.15 -19.23

0.55 0.45 0.67 0.01

3.31 3.27 1.43 3.47

Om Om Om Ca

Suspension feeder Suspension feeder Suspension feeder Predator

3.51

Ca

Predator

3.36 4.08 3.46 3.41 3.75

Ca Ca Ca Ca Ca

Predator Predator Predator Predator Predator

11 1

5.53 1.82

3.03 1.00

-24.61 1.21 -25.89 1.00

2

10

14.13

8.15

0.41

0.68

-18.13 0.33

-21.65 0.46

2.76

*=Late spring; **= late summer Diet: Ca= Carnivorous; Om= Omnivorous; He= Herbivorous

Variable

δ13C

Location

P. spinosulum [a] P. spinosulum [b] Sediment organic matter (SOM)

-22.94 Martínez channel -14.30 Bahía Laredo -26.98 Martínez channel

References

Present study Andrade et al. (2016) Rebolledo et al. (2018)

Model = (δ13C P. spinosulum [a] - δ13C SOM) - (δ13C P. spinosulum [b] - δ13C SOM) % organic carbon assimilated by P. spinosulum Note: data from carbon stable isotopes were averaged

31.86%

1 Highlights

Benthic food-web in fjords is supported by primary production and terrestrial OM. Kelps provide a trophic subsidy to benthic food web in the inner area of the fjord. Glaciofluvial benthic communities have lower diversity and fewer trophic levels. Glaciofluvial benthic communities have higher trophic redundancy.

Declaration of interests ☐The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

No Conflict of interest existed