Live, dead, and fossil mollusks in Florida freshwater springs and spring-fed rivers: Taphonomic pathways and the formation of multisourced, time-averaged death assemblages

Abstract

Taphonomic processes are informative about the magnitude and timing of paleoecological changes but remain poorly understood with respect to freshwater invertebrates in spring-fed rivers and streams. We compared taphonomic alteration among freshwater gastropods in live, dead (surficial shell accumulations), and fossil (late Pleistocene–early Holocene in situ sediments) assemblages from two Florida spring-fed systems, the Wakulla and Silver/Ocklawaha Rivers. We assessed taphonomy of two gastropod species: the native Elimia floridensis (n = 2504) and introduced Melanoides tuberculata (n = 168). We quantified seven taphonomic attributes (aperture condition, color, fragmentation, abrasion, juvenile spire condition, dissolution, and exterior luster) and combined those attributes into a total taphonomic score (TT). Fossil E. floridensis specimens exhibited the greatest degradation (highest TT scores), whereas live specimens of both species were least degraded. Specimens of E. floridensis from death assemblages were less altered than fossil specimens of the same species. Within death assemblages, specimens of M. tuberculata were significantly less altered than specimens of E. floridensis, but highly degraded specimens dominated in both species. Radiocarbon dates on fossils clustered between 9792 and 7087 cal BP, whereas death assemblage ages ranged from 10,692 to 1173 cal BP. Possible explanations for the observed taphonomic patterns include: (1) rapid taphonomic shell alteration, (2) prolonged near-surface exposure to moderate alteration rates, and/or (3) introduction of reworked fossil shells into surficial assemblages. Combined radiocarbon dates and taphonomic analyses suggest that all these processes may have played a role in death assemblage formation. In these fluvial settings, shell accumulations develop as a complex mixture of specimens derived from multiple sources and characterized by multimillennial time-averaging. These findings suggest that, when available, fossil assemblages may be more appropriate than death assemblages for assessing preindustrial faunal associations and recent anthropogenic changes in freshwater ecosystems.

Long-Term Shifts in Faunal Composition of Freshwater Mollusks in Spring-Fed Rivers of Florida

“…Death assemblages, the loose collection of shell material that accumulates in scours, divots, or similar erosional depressions on the river bottom, are also commonly used in conservation paleobiological research (e.g., Brown et al, 2005). While molluscan death assemblages were collected for this study, their complicated, multi-sourced provenance (Kusnerik et al, 2020) led to their exclusion from further analyses.…”

“…Almost all fossil species present are shared between the two systems, with those absent from the Wakulla relatively rare in Silver/Ocklawaha samples. Given that radiocarbon dates on mollusk fossils coincide with the activation of spring flow across much of Florida (Balsillie and Donoghue, 2011;Donoghue, 2011;O'Donoughue, 2015;Kusnerik et al, 2020), the observed faunal resemblance of the two relatively distant river systems suggests that Florida springs and rivers were initially colonized by a similar suite of mollusks, reflected in the similarity of their fossil assemblages. Many fossil species identified in this survey have also been documented in freshwater fossil assemblages elsewhere in Florida (Karrow et al, 1996;Auffenberg et al, 2006), further suggesting that similar mollusk assemblages populated multiple Florida springs and rivers during the late Pleistocene-early Holocene.…”

“…Modern development around freshwater springs began as early as the 1820s, initiating a long interval of gradually intensifying human impacts on water quality and ecosystem health (Revels, 1990;King, 2004). Today, these spring-fed river systems are increasingly threatened by invasive/introduced species (Bogan, 2006;Wingard et al, 2008;Nico et al, 2009;Kusnerik et al, 2020), excessive nutrient inputs (Turner and Rabalais, 1991;Turner et al, 2006;Liu et al, 2009;Heffernan et al, 2010;Bricker et al, 2014), vegetation loss/shifts (Brainwood et al, 2006;Lauretta et al, 2019), decreasing waterflow (Weber and Perry, 2006), sea level rise (Donoghue, 2011;Hong et al, 2014), and salinity fluctuations (Donoghue, 2011;Hong et al, 2014).…”

“…Mollusks were used because their biomineralized shells have high potential for preservation in the freshwater fossil record, and their larger populations make collection of meaningful sample sizes feasible (Boardman et al, 1987;Kusnerik et al, 2020). Radiocarbon dates on individual fossil specimens (Kusnerik et al, 2020) revealed that freshwater mollusk shells from the studied rivers came predominantly from individuals that were late Pleistocene to early Holocene in age (18,217 to 7,087 cal BP), coinciding with hydrologic activation of flow in freshwater springs throughout Florida (Balsillie and Donoghue, 2011;Donoghue, 2011;O'Donoughue, 2015;Kusnerik et al, 2020). The radiocarbon dating efforts also indicate that mollusk assemblages were time-averaged over hundreds to thousands of years, constituting an averaged archive of long-term paleoecological conditions in the study system (Kusnerik et al, 2020).…”

“…Radiocarbon dates on individual fossil specimens (Kusnerik et al, 2020) revealed that freshwater mollusk shells from the studied rivers came predominantly from individuals that were late Pleistocene to early Holocene in age (18,217 to 7,087 cal BP), coinciding with hydrologic activation of flow in freshwater springs throughout Florida (Balsillie and Donoghue, 2011;Donoghue, 2011;O'Donoughue, 2015;Kusnerik et al, 2020). The radiocarbon dating efforts also indicate that mollusk assemblages were time-averaged over hundreds to thousands of years, constituting an averaged archive of long-term paleoecological conditions in the study system (Kusnerik et al, 2020). In more general terms, we focus here on mollusks because they are often used as environmental indicators (Williams et al, 2014), are sensitive to changes in water conditions (Montagna et al, 2008;Williams et al, 2014), can archive geochemical changes in aquatic habitats in their shell structure (Brown et al, 2005;Brainwood et al, 2006;Williams et al, 2014), and often provide ecosystem services including water filtration, substrate consolidation, and habitat structuring (Williams et al, 2014;Vaughn, 2018).…”