Mapping Antarctic suspension feeder abundances and seafloor food-availability, and modelling their change after a major glacier calving

Seafloor communities are a critical part of the unique and diverse Antarctic marine life. Processes at the ocean-surface can strongly influence the diversity and abundance of these communities, even when they live at hundreds of meters water depth. However, even though we understand the importance of this link, there are so far no quantitative spatial predictions on how seafloor communities will respond to changing conditions at the ocean surface. Here, we map patterns in abundance of important habitat-forming suspension feeders on the seafloor in East Antarctica, and predict how these patterns change after a major disturbance in the icescape, caused by the calving of the Mertz Glacier Tongue. We use a purpose-built ocean model for the time-period before and after the calving of the Mertz-Glacier Tongue in 2010, data from satellites and a validated food-availability model to estimate changes in horizontal flux of food since the glacier calving. We then predict the post-calving distribution of suspension feeder abundances using the established relationships with the environmental variables, and changes in horizontal flux of food. Our results indicate strong increases in suspension feeder abundances close to the glacier calving site, fueled by increased food supply, while the remainder of the region maintains similar suspension feeder abundances despite a slight decrease in total food supply. The oceanographic setting of the entire region changes, with a shorter ice-free season, altered seafloor currents and changes in food-availability. Our study provides important insight into the flow-on effects of a changing icescape on seafloor habitat and fauna in polar environments. Understanding these connections is important in the context of current and future effects of climate change, and the mapped predictions of the seafloor fauna as presented for the study region can be used as a decision-tool for planning potential marine protected areas, and for focusing future sampling and monitoring initiatives.

Seafloor communities are a critical part of the unique and diverse Antarctic marine life. Processes at 10 the ocean-surface can strongly influence the diversity and abundance of these communities, even 11 when they live at hundreds of meters water depth. However, even though we understand the 12 importance of this link, there are so far no quantitative spatial predictions on how seafloor 13 communities will respond to changing conditions at the ocean surface. 14 Here, we map patterns in abundance of important habitat-forming suspension feeders on the seafloor 15 in East Antarctica, and predict how these patterns change after a major disturbance in the icescape, 16 caused by the calving of the Mertz Glacier Tongue. We use a purpose-built ocean model for the time-17 period before and after the calving of the Mertz-Glacier Tongue in 2010, data from satellites and a 18 validated food-availability model to estimate changes in horizontal flux of food since the glacier 19 calving. We then predict the post-calving distribution of suspension feeder abundances using the 20 established relationships with the environmental variables, and changes in horizontal flux of food. 21 Our results indicate strong increases in suspension feeder abundances close to the glacier calving site, 22 fueled by increased food supply, while the remainder of the region maintains similar suspension 23 feeder abundances despite a slight decrease in total food supply. The oceanographic setting of the 24 entire region changes, with a shorter ice-free season, altered seafloor currents and changes in food-25 availability. 26 Our study provides important insight into the flow-on effects of a changing icescape on seafloor 27 habitat and fauna in polar environments. Understanding these connections is important in the context 28 of current and future effects of climate change, and the mapped predictions of the seafloor fauna as 29 presented for the study region can be used as a decision-tool for planning potential marine protected 30 areas, and for focusing future sampling and monitoring initiatives. 31

Introduction 32
Primary productivity is at the base of most marine ecosystems. In Antarctica, primary production is 33 highly seasonal and intricately tied to the location, timing and duration of sea-ice and ice-free areas 34 such as polynyas (Arrigo and van Dijken, 2003). The collapse of large ice-shelves or calving of 35 massive icebergs, and the retreat of sea-ice that is mainly observed around the Western Antarctic 36 Peninsula in recent years (Parkinson and Cavalieri, 2012), can dramatically alter the oceanographic 37 setting with down-stream effects on the pattern of primary production hotspots and on Southern 38 Ocean For most seafloor communities living below the photic zone (~200 m), surface-derived primary 43 production represents their main food source (Dayton and Oliver, 1977 their distribution might change due to a changing icescape at the ocean surface. One of the reasons 50 for the lack of quantitative studies is that although surface-derived food is one of the main drivers, it 51 is only recently that the nature and strength of this relationship has been quantified on the Antarctic 52 shelf using a so-called Food-Availability-Model (FAM) . Combining surface-53 productivity and ocean currents with particle-tracking, FAMs estimate the distribution of surface-54 derived food at the seafloor, and evaluate the estimates against data from sediment cores. Jansen et al. 55 (2018) demonstrated a strong link between modelled flux of suspended food along the seafloor and 56 abundances of sessile suspension feeders, providing a framework that allows to estimate the 57 distribution of key elements of the seafloor community and to predict how they may change with 58 changing ocean productivity and currents. 59 One Antarctic region that has recently undergone drastic environmental changes is the George V 60 shelf been observed to influence the community structure of shallow-water benthos (Clark et al., 2015). 67 However, the effect of the MGT-calving on the seafloor across the region has so far neither been 68 assessed nor observed, and so its impact on benthic communities across the continental shelf is still 69 unknown. Obtaining this knowledge, however, is crucial for meaningful assessment of the 70 comprehensiveness, effectiveness and representativeness of the proposed marine protected areas in 71 this region. 72 Here, we (i) quantify differences in the environmental setting on the George V shelf that will affect 73 the supply of food to the benthos. In our modelling, we apply a recently developed FAM (Jansen et  74 al., 2018) on two 5-year climatologies of remotely sensed surface chlorophyll-a for the period before 75 and after the glacier calving, and use ocean current velocities from a purpose-built oceanographic 76 model (Cougnon et al., 2017) (more details can be found in the Methods). We then (ii) map the 77 distribution of benthic suspension feeder abundances before the glacier calving, using faunal 78 abundances derived from underwater camera images and environmental predictor variables. Using 79 the pre-calving statistical model for the suspension feeder abundances and the change in 80 environmental conditions after the glacier calving, we then (iii) predict changes in suspension feeder 81 abundances across the region, revealing the strong impact of the changing icescape on the seafloor 82 ecosystem ( Mertz Bank, in the deep George V Basin and below the iceberg B09B at the North-Western edge of 103 Commonwealth Bay (Fig. 3f). 104 The Food-Availability-Model (FAM) tracks and quantifies three components of surface-derived food 105 particles: the sinking component captures the advection of phytodetrital matter by currents as it sinks 106 through the water column until it reaches the seafloor; the flux component represents the horizontal 107 flux of food particles along the seafloor before sedimentation; the settling component represents the 108 final location of advected particles after taking into account the redistribution by seafloor currents. 109 Sinking and settling particles follow similar patterns to the other environmental variables mentioned 110 before, with an eastward shift for the peak number of sinking and settling particles, and absence of 111 sedimentation on large parts of the Mertz Bank (closest to the former tip of the MGT) due to 112 increased current speeds ( Supplementary Fig. 3). Horizontal food flux along the seafloor, which is 113 dependent mainly on the interaction between the distribution of surface productivity and seafloor 114 current speeds, increases 20-50 fold on wide sections of the Mertz Bank (Fig. 3i). In contrast, 115 changes are more patchy on the Adélie Bank, where increases in flux are mostly restricted to the 116 inner section of the bank and the shelf break, while the edges of the bank experience decrease in 117 flux. Further, most of the deeper sections of the shelf experience lower flux than before the calving. 118

Predicted changes in suspension feeder abundances 119
Mapped predictions of suspension feeder (SF) cover are based on the statistical relationship between 120 pre-calving cover estimated from still-images, and the environmental covariates depth and log 121 (horizontal flux) (Supplementary Table 1, deviance-explained = 44 %), which are selected as the best 122 predictor variables by the stepwise regression process (Supplementary Table 2). There is a good fit 123 between the predicted values from the statistical model and the observed values at the sampling sites, 124 with a slight underestimation of high cover values (Fig. 4). SF-cover before the calving is high on 125 most of the shallower sections of the shelf (<500 m depth) ( Bank, while the Adélie Bank and other areas retain similar SF-cover as previously. The stark 132 difference in predicted SF-abundance between the Mertz Bank in the east and the Adélie Bank in the 133 west stems from the 20-50-fold increase in particle flux on the Mertz Bank that is a direct result of 134 both increased surface production and stronger tidal currents. 135

3
Discussion 136 We predict that the calving of massive icebergs will have far-reaching effects on benthic 137 communities mediated through the mechanism of pelagic-benthic coupling, and that changes occur 138 even hundreds of kilometers away from the glacier tongue. While previous studies have shown that 139 calving events can have localized negative impacts on the benthos through iceberg scouring (Gutt et 140 al., 1996), here we predict that the combination of changes in local oceanography and surface 141 production influences patterns of seafloor food-availability at much larger scales. the Mertz Bank now seem much more favorable for suspension feeders (SFs) than before the calving. 146 Our modelling suggests that there will be a strong, but locally confined increase in SF-abundance on 147 the Mertz Bank of up to 40 %. Further away, near the Adélie Bank, increases in bottom current 148 speeds seem to compensate for the overall decrease in food supply, resulting in a prediction of only 149 marginal changes in SF-abundance. The distribution of surface production around the newly formed 150 polynya on the leeward side of the grounded iceberg B09B (Fogwill et al., 2016) seems to slightly 151 influence SF-abundances with relatively stable cover predicted beneath the polynya in contrast to 152 decreasing cover in the ice-covered area directly north of the iceberg B09B. Close to and below the 153 position of the tip of the MGT before it broke away, where Beaman and Harris (2005) have  154 previously found a high number of macrobenthic species, including many sponges and bryozoans, 155 our model predicts a substantial decrease in SF-abundance, due to a decrease in floor current speed 156 affecting the horizontal food-flux. However, we caution that little confidence should be placed in this 157 result; the environmental conditions in this area might be unique due to the glacier tongue and we 158 lack biological samples for this area. Further, we also lack confidence in the food-availability-data 159 because of missing data in the remotely sensed surface chl-a dataset (see Methods section 4.2.2). 160 Unfortunately, the shallower sections of the Mertz Bank and the western part of the Adélie Bank, 161 which we show here are particularly interesting areas, have not been physically sampled as part of the 162 survey (see Fig. 2). However, because the survey was designed to cover a wide range of depths and 163 geomorphologies (Hosie et al., 2011), because of the high confidence in the predicted values from the 164 statistical model (Fig. 4), and because of the similarity in environmental conditions between the 165 shallow banks prior to the glacier calving, we are confident the relationship between environmental 166 variables and distributional patterns of suspension feeder abundances is consistent across the region. 167 Regions around ice-shelfs and glacier tongues provide valuable insight into the dynamic environment 168 of the Antarctic shelf. When an ice-shelf calves a massive iceberg or collapses entirely, the marine 169 environment, to which species might have acclimated to for many years, can transition quickly 170 between a food-poor and a food-rich system (Gutt et al., 2011a). The MGT is thought to calve 171 massive icebergs in a ~70 year cycle (Giles, 2017), meaning that there are likely also differences in 172 the long-term stability of environmental conditions, and in the frequency of iceberg-scour between 173 the Mertz Bank near the MGT and the Adélie Bank in the West of the George V shelf. Studies on the 174 West Antarctic Peninsula suggest that at least some components of Antarctic benthic communities on 175 the shelf, such as glass sponges and pioneering species, can increase rapidly in areas that are newly 176 ice-free, fueled by higher export of surface production (Gutt et  The study area is located on the relatively deep (500-700 m) East Antarctic continental shelf and 205 slope between latitudes 139°E and 147°E (Fig. 2).

Biological data collection: 298
We use the same dataset of benthic images as used by Jansen et al. (2018) during the CEAMARC were designed to cover a wide range of depths and geomorphologies in the 303 region and therefore can be considered representative of the area modelled. A forward facing 8 304 megapixel Canon EOS 20D SLR with two speedlight strobes was mounted on a beam trawl and 305 pictures were taken every 10 seconds. 32 sites were sampled with transect length mostly between 4-306 6 km, with exceptions ranging between 3-16 km. The trawl was controlled using a deck winch. 307 Benthic fauna were identified to the lowest taxonomic resolution possible and, where species 308 identification was not possible, specimens with similar overall appearance were grouped into 309 morphotypes. The bottom third of each image was scored. For each image, the abundance of each 310 species/morphotype was estimated within 5% bins from 0% to 50% and 10% bins from 50% to 311 100%. Using taxonomy and body-type along with expert knowledge, the abundance of the 312 suspension feeding fauna in each picture was calculated. predictor variables. We found that using a negative binomial generalized linear model (compared to a 325 linear model in the previous study) did not affect the selection of model terms. Therefore, the change 326 in selected model terms is likely to come from the difference in the ocean model or the surface 327 productivity. The pre-calving statistical model showed a good fit between the predicted and the 328 observed values at the sample sites, with possibly a slight underestimation of suspension feeders at 329 high abundances (Fig. 5). 330 We then used the pre-calving statistical model to predict the spatial distribution of SF-cover in both 331 the pre-calving and the post-calving environment. The difference between the resulting maps was 332 used to make inferences about areas with expected increases and decreases in the abundance of 333 suspension feeders. Further, we bootstrapped the parameters of the pre-calving statistical model to 334 obtain estimates for the standard-deviation of the predictions. 335 For the statistical analysis, we used R Version 3.3.1 (R Core Team, 2016) and the packages "raster" 336 (Hijmans and 2015), "MASS" (Venables and Ripley, 2002), "maptools" and "modEvA". 337

Data availability: 338
Estimates of suspension feeder abundances from benthic images are available through the Australian 339 Antarctic Division Data Centre . Raster files containing mapped predictions of 340 food-availability and suspension feeder abundances presented in this study, from before and after the 341 glacier calving are available through the Australian Antarctic Data Centre (Jansen, 2018 (sea-520 ice, surface-chl-a, ocean current speeds, food-export) and seafloor fauna due to the calving of the 521 Mertz Glacier Tongue (MGT) in 2010. The graphics shows a cross-section of the George V 522 continental shelf approximately 80 km off the coast, looking South towards the Antarctic continent. 523 The top graphic shows pre-calving environmental conditions and displays abundances of suspension 524 feeders as observed from towed camera images. The bottom graphic shows observed changes in sea-525 ice, surface-chl-a and the position of the grounded iceberg B09B, as well as modelled changes in 526 ocean current speeds, food-availability and suspension feeder abundances. shows the location of the study area as highlighted by the red box. 535