1 INTRODUCTION

The sediments of the deep oceans have been long-heralded as surprisingly diverse environments (Grassle & Maciolek, 1992; Sanders, 1968), and the maintenance of this diversity has been the subject of many studies through the years (reviewed in McClain & Schlacher, 2015; Rex & Etter, 2010). One of the most prominent theories explaining this diversity is the “patch-mosaic hypothesis” of Grassle and Sanders (1973), which suggests that in the deep sea, many small, localized disturbances on the sea floor create patches of communities. These communities proceed through succession states out of sync with neighboring patches, resulting in a “patch mosaic” of community assemblages across the sea floor. This idea was later expanded to include both physical disturbance and heterogeneity in food resources, i.e., organic carbon content and quality, as determinants of patches (Etter & Mullineaux, 2001; Grassle, 1989). These mechanisms create a mosaic that persists over longer temporal spans than in shallow water due to comparatively lower rates of bioturbation and slower currents (Etter & Mullineaux, 2001).

Energetic heterogeneity is recognized to strongly influence beta-diversity of deep-sea communities (Danovaro et al., 2013; Etter & Grassle, 1992; Gage, 1996; Guilini et al., 2011). Experimental evidence utilizing enrichment of colonization trays indicates that both the presence of species and the relative abundance of species are influenced by carbon availability (Gallucci et al., 2008; Levin & Smith, 1984; Snelgrove et al., 1992, 1994). Naturally occurring changes in carbon availability and sediment types across the sea floor are also documented to impact diversity, abundance, and composition of species across scales (reviewed in McClain & Schlacher, 2015; McClain et al., 2020; Rex & Etter, 2010). This variability in carbon resources across deep-sea patches is in part the result of heterogeneity in the rain of food from above. The quality and amount of particulates, “marine snow,” changes with seasons and time of day (Graham et al., 2000; Lampitt et al., 1993), latitude (Gorsky et al., 2003; Weber et al., 2016), and depth (Shanks & Trent, 1980). Enhanced local accumulation of particulates may promote diversity of sediment infauna, for example around scleractinian corals (Demopoulos et al., 2014). Larger aggregates of food arrive in random pulses as “food falls,” which occur as animal carcasses or plant material. Food falls are larger than marine snow aggregates and sink quickly due to their size thus circumventing heavy recycling by pelagic bacteria and fauna, arriving to the sea floor as high-quality, high-quantity food (Smith & Baco, 2003; Smith et al., 2015).

These food falls play a substantial role in contributing to the patch-mosaic of biodiversity in the deep sea (Grassle & Morse-Porteous, 1987; Stockton & DeLaca, 1982). First, food falls serve as disturbance events for sediment infauna both when the fall itself reaches the sea floor and during ensuing activity by scavengers and other fauna (Smith et al., 1998). Secondly, food falls create patches of increased organic carbon and can also provide unique food sources for bone/wood obligates and sulphophiles (Baco & Smith, 2003; Turner, 1973). The organic enrichment of local sediments around some food falls enhances abundance and diversity of macrofaunal communities (Bernardino et al., 2010). During degradation, food fall communities utilize chemosynthetic pathways atypical of soft-bottom communities and thus may enrich beta-diversity through unlocking novel niche space and hosting chemoautotrophic species shared with reducing environments such as hydrothermal vents and seeps (Alfaro-Lucas et al., 2018; Bernardino et al., 2012; Bernardino et al., 2010).

Food falls span several orders of magnitude in size, from algal aggregates to whale carcasses. Food falls in the size range of fish and jellyfish and large whales have received the most attention, but less is understood about food falls in the intermediary size range, for example seals/sea lions, porpoises, dolphins, and reptiles (Figure S1). For the sake of this study, we define three food fall size groups by mass. Small food falls here are those which are 0–10 kg, e.g., plant detritus, jellyfish, and fish. Intermediately sized food falls are those which are 10–1000 kg, e.g., seals/sea lions, porpoises, dolphins, and reptiles. Large food falls are those which are >1000 kg, e.g., whales. Note that the definitions of these size groups are not fixed. For example, “medium” food falls have been defined by length rather than mass by Scheer et al. (2022). However, defining by mass is useful for comparisons of the results of this study to food falls within other size groups. This is particularly important given that these size categories are expected to vary in their contribution to diversity maintenance because of their difference in carbon content and accessibility, their ability to generate and sustain chemosynthetic pathways, and their hypothesized frequency of occurrence.

Research on the response of sediment communities to large (>1000 kg) food falls has primarily centered on marine mammals. Whale falls can deliver the equivalent of up to 2000 years of background carbon flux in one pulse, drawing in opportunistic scavengers, hosting multiple suites of endemic species through several successional stages, and overall driving increased alpha- and beta-diversity for time-scales on the order of years to decades and spatial scales up to 100 m² (Smith & Baco, 2003; Smith et al., 2015). Whale falls also have intense impacts on the abundance and composition of sediment communities, likely because of the magnitude of the associated organic enrichment (Smith et al., 2014). The impressive impacts of whale falls are typically credited to their large size (>1000 kg) and/or high lipid content of the bones (Smith & Baco, 2003).

The influence of small (0–10 kg) food falls is understood primarily through studies of plant and algal material falls or opportunistic observations of small animals. Alhough non-animal food falls such as wood logs, kelp, and Sargassum mats are individually typically small, their collective contribution to the carbon budget of deep communities is hypothesized to be relatively large due to their frequency and abundance (e.g., Baker et al., 2018; Harbour et al., 2021; Vetter & Dayton, 1998; Vilas et al., 2020). Carcasses of a fish (Soltwedel et al., 2003) and a decapod (Klages et al., 2001) suggested that carcasses of this size are utilized by opportunistic scavengers, particularly amphipods, very quickly (hours), such that the sphere of influence does not reach into sediment infauna communities. Similarly, jellyfish carcasses individually do not appear to enrich sediment communities, but because jellyfish are short-lived and abundant, “jelly falls” collectively may contribute to patchiness and sediment community structure of some regions, especially fjords (Sweetman & Chapman, 2011).

While the impact of large and small food falls has received considerable interest, effects of intermediately sized (10–1000 kg) animal carcasses remain comparatively poorly understood. Studies of carcasses of marine mammals other than whales are limited, but there are some examples of these food falls strongly influencing abundance and diversity of infauna communities, likely due to high lipid content of the bones (e.g., porpoises; Soltwedel et al., 2003). There are particularly few records of evidence of the effects of non-mammalian vertebrate food falls, perhaps because natural observations of these food falls in situ are quite rare. An opportunistic study of one Mobula sp. (ray) carcass suggested that food falls in this size range may increase local diversity of scavenging communities, particularly fish, but do not persist long enough for sediment infauna communities to benefit substantially from organic enrichment (Higgs et al., 2014). Organic enrichment of surrounding sediments is thought to require longer residence times of food falls, thus, individually, small and intermediately sized carcasses, which are quickly and efficiently exploited by mobile scavengers have been hypothesized to have little impact on sediment infauna communities (Higgs et al., 2014).

Although food falls of many size scales are recognized as conduits of carbon from surface waters to the deep, it remains unclear how, or even if, all but the largest (>1000 kg) vertebrate falls contribute to overall soft-bottom sediment community diversity. It is reasonable to expect that there is a higher frequency of small (0–10 kg) and intermediately sized (10–1000 kg) animal food falls relative to large whales as smaller animals typically have larger populations than animals with larger body sizes (Damuth, 1981; Kerr & Dickie, 2001). Consequently, these smaller food falls may collectively have an outsized impact on biodiversity of the deep oceans. Moreover, the role of reptiles in these processes is unexplored, although these animals are abundant and diverse along coast lines and were once dominant fauna of the Mesozoic oceans (Kelley & Pyenson, 2015). Here, we address the role of an intermediately sized vertebrate carcass of the reptile Alligator mississippiensis in structuring sediment macrofaunal communities. Specifically, we ask: (1) Can an intermediately sized (19.5 kg) reptilian carcass enrich surrounding sediments with organic carbon, and (2) Does an intermediately sized reptilian carcass impact the alpha-, beta-, and gamma-diversity of sediment macrofauna?

2 METHODS

As part of a broader food fall experiment (McClain et al., 2019), sediment core samples were taken around one alligator fall. The carcass of Alligator mississippiensis (175.3 cm length, 19.5 kg; McClain et al., 2019) was obtained from the Louisiana Department of Wildlife and Fisheries, Alligator Management Program (LDWF Tag 898310). The alligator carcass was frozen immediately post-mortem for later deployment. The carcass was deployed at 2034 m on 20 February 2019 at 27.3126°N, 88.9270°W in the northern Gulf of Mexico (Figure 1). The site is characterized by soft sediment and has a bottom temperature of 3.93°C, a current velocity of 0.001 m/s, a dissolved oxygen concentration of 198.21 μmol m⁻³, and a salinity of 34.9 as determined using the Bio-ORACLE v2 database (Assis et al., 2017; Tyberghein et al., 2012). Additionally, the site receives an annual mean POC flux of 1.48 gC m⁻¹d⁻¹ as determined using the Lutz et al. (2007) model. The carcass was deployed with a remotely operated vehicle (ROV Global Explorer, operated from the R/V Pelican) by an attached polypropylene harness, and a 20 kg weight attached to the underside of the harness anchored the individual in place on the sea floor (Figure 2). Sediment sampling occurred at 51 days post-deployment on 12 April 2019, by which time the carcass had been completely stripped of soft tissue by scavengers and only a skeleton remained (Figure 2).

This food fall experiment was conducted as “science of opportunity” and logistical constraints allowed for no additional sampling or observation time during the experimental period of 51 days. Thus, the precise time post-deployment at which the soft tissue was fully consumed can only be estimated as less than 51 days, the maximum time of consumption. As part of the broader experiment, an additional alligator carcass, deployed 100 km from the carcass detailed here, was observed at 1-day post-deployment. At this time, giant isopods (Bathynomus giganteus) had already broken through the tough hide and were feeding on the flesh of the carcass (McClain et al., 2019). This observation indicates that it is possible that the soft tissue had been fully consumed within a few days, and the carcass may have been in skeletal form for some time prior to sediment sampling.

Push cores (7 cm diameter) were used to collect sediment samples in six transects perpendicular to the alligator fall. Each transect consisted of paired cores taken at two distance groups from the alligator fall: “inner” (directly against the exposed vertebrae) and “outer” (0.5 m away). Six additional cores were taken ~5 m away from the carcass in a tight 3 × 2 grid pattern to account for natural patchiness in the background community and thus create the “background” distance group. For this size carcass, 5 m is sufficiently far away to fall outside the influence of enrichment from the food fall; for a whale fall >1000× larger than this alligator carcass, enrichment of the sediment was minimal at >3 m distance from the carcass (Smith et al., 2014). The top 5 cm of the sediment cores were extruded and sieved through 300 μm mesh. In the deep Gulf of Mexico, 82% of macrofauna occur in this fraction (Montagna et al., 2017), and 65%–98% of macrofauna occur in the top 3 cm at sites nearby to this study (Demopoulos et al., 2016). Evidence from an experimental pulse study also suggests that macrofaunal uptake of carbon occurs within the top 0–5 cm, even after 23 days (Witte et al., 2003). The usage of a 300 μm mesh was based on prior methodological findings (Montagna et al., 2017) that larger mesh sizes undersampled smaller macrofaunal components. All material retained was preserved in 80% ETOH. Each sample was stained with Rose Bengal for at least 24 h. All individuals were counted and sorted to morphospecies.

Two 2-cc sediment plugs were subsampled from the surface (top 1–2 cm) of each core using a 10-cc syringe. Sediment plugs were frozen for later analysis. Frozen sediment was homogenized and dried at 60°C for 48 h and initial and final weights were taken to determine water content. Dried samples were ground using a mortar and pestle and placed in a desiccator with concentrated fuming hydrochloric acid for 48 h to remove inorganic carbon. Samples were dried post fumigation for 48 h and then analysed for total organic carbon (TOC) using an elemental analyser (Costech 1040 CHNOS Elemental Combustion Systems). The analytical error associated with carbon analysis was <1% as determined by the Wetlands Biogeochemistry Analytical Services (WBAS) at Louisiana State University.

All analyses were conducted in R (v. 4.0.2). Total abundance and species richness were calculated for each sample using the package “vegan” (Oksanen et al., 2019). Gamma-diversity was partitioned into alpha- and beta-diversity using the package “vegetarian” per the methods of Jost (2007); for richness, H_gamma = H_alpha * H_beta. One-way analysis of variance (ANOVA) was used to test variation in abundance, species richness, and TOC across distance groups. Diversity and abundance were predicted to be highest near the carcass (the cores from the inner group), or to be highest at intermediate distance from the carcass (the cores from the outer group), so ANOVAs were performed with a priori planned contrasts to increase statistical power. ANOVA assumptions of normality and equal variance were verified for each model using the Shapiro–Wilk test and Bartlett's test, respectively. The effect of TOC on abundance and species richness was tested using linear regression.

Variation in community structure, i.e., changes in both taxonomic composition and the distribution of individuals among species, across the three distances groups was quantified using a constrained ordination method. Bray-Curtis distance, on log-transformed abundances, was used to quantify dissimilarities in community structure between cores. Bray-Curtis distance was chosen because of its robust monotonic and linear proportional relationship with ecological distances (Faith et al., 1987). Redundancy Analysis (RDA) extends multiple regression to allow for multiple response variables on multiple explanatory variables. Here, a Constrained Analysis of Principal Coordinates (CAP) was utilized. CAP is similar to RDA, but allows for non-Euclidean dissimilarity indices, such as Bray-Curtis distance. The CAP was performed using the “capscale” function in the package “vegan” (Oksanen et al., 2019). Variation in community structure among distance groups was evaluated using a PERMANOVA on the CAP scores.

3 RESULTS

Total organic carbon of sediments decreased significantly with distance from the alligator carcass (Table 1; Figure 3a). Mean TOC of the inner cores was 4.99%, decreasing to 3.09% in the outer cores and 1.98% in the background cores. This represents a 2.5x increase from the background cores to the inner cores (Figure 3a). Across all cores (n = 18), 120 individuals across 44 species were identified. Polychaetes were the most abundant and speciose taxon, representing 66.7% of the total number of individuals and 61.4% of the total species observed. Echinoderms and priapulids were the least abundant and least speciose taxa; both were observed as singletons. Crustaceans represented 13.6% of species and 9.2% of individuals across samples. Molluscs represented 13.6% of species and 13.3% of individuals. Lophophorates (here, bryozoans and brachiopods) represented 6.8% of species and 9.2% of individuals.

Total abundance decreased significantly with distance from the alligator carcass (Table 1; Figure 3b). The mean abundance of inner cores was slightly more than double that of background cores. Mean abundance of inner cores was 8.83 individuals, decreasing to 7 in the outer cores and 4.17 in the background cores (Figure 3b). Species richness did not significantly vary with distance from the alligator carcass (Table 1; Figure 3c). However, there was a slight decreasing trend with distance; mean species richness was highest in the inner cores at 6.17, decreasing to 5 in the outer cores and 3.83 in the background cores (Figure 3c). Simpson's evenness scores did not significantly vary with distance from the alligator carcass (Table 1; Figure 3d).

Community structure varied significantly with distance group (Table S1; Figure 4; p = .0330, R² = 17.4%). Inner and background cores formed two extremes on the CAP plot, with the outer communities serving as an intermediate between the two (Figure 4). Inner communities were characterized by increased gastropod morphospecies (3) and polychaete morphospecies (27) and (4), outer communities were characterized by increased bryozoan morphospecies (7), and background communities were characterized by increased bivalve morphospecies (4) (Table S1). Variation in community structure remained significant across distance groups when running the CAP analysis on the presence/absence matrix, that is, accounting for abundance (p = .0390, R² = 18.5%). Gamma-diversity measured as richness (43 species) reflected a larger beta- (8.80) than alpha-diversity component (4.89, Figure 5).

4 DISCUSSION

We find that an intermediately sized (19.5 kg) reptilian food fall enriched the surrounding sediments in organic carbon, promoting gamma-diversity predominantly through changes in beta-diversity. Our findings suggest that food falls of intermediate size (10–1000 kg), non-marine origins, and which are non-mammalian can have quick (<51 days) and significant impacts on sediment macrofauna communities. This contrasts with the previous hypothesis that only large (>1000 kg) mammalian food falls individually play a strong role in structuring biodiversity outside of scavenging communities (Higgs et al., 2014).

Our findings add to a growing understanding of the high variability in the influence of food falls in maintaining diversity of deep-sea infauna. For example, in well-studied whale falls, some evidence suggests that surrounding sediments are not enriched with carbon and the abundance and diversity of infauna is not promoted (e.g., Smith et al., 1998). Yet, other studies document increases in the abundance of macrofauna near whale carcasses (e.g., Debenham et al., 2004). We find that macrofauna abundance, but not diversity, increased near the alligator fall. Differing results may reflect strong time-dependence of the effects of the food fall (Smith et al., 2014). Alternatively, the high heterogeneity (“patchiness”) of the deep sea floor in terms of carbon availability, sediment characteristics, disturbance, and currents (Grassle & Sanders, 1973; Levin et al., 2001) leads to substantial differences between localities. The addition of yet more patchiness, for example the food fall, may have varying effects depending on the prior baseline environment experienced by the macrofauna community.

The percent total organic carbon of background sediment cores (~2%) in this study was typical of many deep-sea sites, although regionally the Gulf of Mexico sediments may be more depleted in TOC (Escobar-Briones & García-Villalobos, 2009; Morse & Beazley, 2008). Sediment cores nearest the alligator fall represented a 2.5x increase in organic carbon availability over background cores. In the food-poor deep sea, carbon availability is strongly related to diversity and abundance (McClain et al., 2012; Rex et al., 2006; Wei et al., 2010; Woolley et al., 2016), and likewise here the significant increases in abundance near the carcass (2x the abundance of communities in the background distance group) mirrored patterns in organic carbon. The distribution of abundance among species did not vary across the distance groups as all communities were relatively even (~0.75 Simpson's Evenness; Figure 3d). This suggests that abundance increases were uniform across taxa, i.e., abundance increases did not occur in only a few dominant taxa.

Despite significant changes in abundance with distance from the alligator fall, no change in alpha-diversity occurred. However, alpha-diversity did highly correlate with abundance (R-sq = 0.93, p < .0001) and abundance correlated with TOC (R-sq = 0.13, p = .0142). We posit two possible mechanisms for this pattern. One, the experimental enrichment here occurred on a backdrop of existing TOC patch heterogeneity. Indeed, the background samples varied from 1.10% to 2.70% TOC. The measured TOC of each core then reflects both existing carbon heterogeneity and that additional carbon, which was introduced by the food fall. Both existing and introduced carbon may have subsequently affected abundance and richness, and the heterogeneity of TOC may have disrupted a pattern between alpha-diversity and the organic enrichment of the alligator fall. Second, the alligator fall may represent a disturbance suppressing alpha-diversity despite increases in carbon availability (Cosson-Sarradin et al., 1998; McClain & Barry, 2010; Smith et al., 1998). The counteracting forces of disturbance and enrichment with distance away from alligator fall may result in little net change to alpha-diversity. However, it should be noted that there is a trend of increased species richness closer to the carcass; the non-significance of this pattern may simply be reflective of low statistical power due to low sample size (n = 18).

Disturbance may be an important factor in structuring the sediment community around the food fall through two mechanisms. First, the food fall itself creates a disturbance, potentially burying macrofauna or interrupting feeding activities. The pulse of enrichment by food falls also leads to high sulfide and low oxygen, which represent physiological stressors for many species (Goffredi et al., 2004). Although we did not observe black patches of sediment consistent with the presence of sulfide at the time of sampling (Figure 2), it is possible that this stage occurred between deployment and observation. In this case, a temporary depression of alpha-diversity during the sulfidic stage would mitigate enhancement of alpha-diversity from enrichment alone, thus arriving at similar species richness as that of the background. Second, the food fall draws in scavengers and predators (McClain et al., 2019). These species may be agents of disturbance because enhanced abundance of higher trophic levels may lead to higher consumption of smaller species (Dayton & Hessler, 1972). In this way, it would be possible to maintain diversity while undergoing species turnover and shifts in abundance. Theory predicts that disturbance and productivity effects antagonistically impact community structure with intermediate levels of both facilitating the highest abundance and diversity (intermediate disturbance hypothesis (Connell, 1978), intermediate productivity model (Grime, 1973), dynamic equilibrium model (Huston, 1994; Kadmon & Benjamini, 2006; Kondoh, 2001; Svensson et al., 2007), and this pattern has been noted in the deep oceans (McClain & Barry, 2010; Paterson et al., 2011). Other counteracting forces are known to set up transition zones with enhanced beta-diversity but not necessarily alpha-diversity along deep-sea environmental gradients, for example the productivity and physiological stress gradients around methane seeps (Ashford et al., 2021).

Significant shifts occurred in community composition with distance from the food fall. Enhancement of gamma-diversity can be generated by increases in alpha-diversity (local species richness) and/or beta-diversity (changes in community composition). Our findings suggest that enrichment from food falls promote gamma-diversity primarily through beta-diversity. A “horseshoe pattern”, typical in multivariate spaces when strong environmental gradients result in substantial changes in beta-diversity (Beals, 1984; Gauch Jr, 1973), occurred among these macrofaunal communities (Figure 4). These compositional shifts may occur through the creation of novel patches of organic enrichments, sulfidic niches, and disturbances distinctive from the background. This creates a unique patch of which some species are better adapted to taking advantage than others, setting the stage for a suite of species unique from those in background patches.

Small-scale turnover is the foremost hypothesis for explaining elevated deep-sea diversity (Grassle & Sanders, 1973; McClain & Schlacher, 2015). Here, we show that intermediately sized reptilian food falls support increased gamma-diversity by potentially creating unique patches in the mosaic of the deep sea floor, filtering for unique suites of species, and promoting beta-diversity. At a large spatial scale, food falls may support macrofaunal species atypical of the background and enrich gamma-diversity. Although further work is required, the findings also indicate that species of Alligatoridae, abundant globally near coastlines (Balagera-Reina & Velasco, 2019), are a viable carbon pathway to the deep not only for megafauna (McClain et al., 2019) but also for macrofauna. This energetic connection between land and the deep sea indicates conservation initiatives of the deep oceans must connect with those working on conservation of relevant terrestrial megafauna and ecosystems.

CONFLICT OF INTEREST

The authors declare no conflict of interest.