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Karst subterranean estuaries (KSEs) are created from the two- and three-way mixing of saline groundwater, rain, and oceanic water in the subsurface on carbonate landscapes, and this hydrographic framework promotes unique physical processes, biogeochemical cycling, and biological communities. Here we provide evidence that the source and quantity of particulate organic matter (POM) that is delivered to the benthos strongly correlates to benthic habitat partitioning in the oxygenated marine sectors of KSEs. A dataset of benthic foraminifera at 128 different locations from several large flooded cave systems in Bermuda were compiled and evaluated against common environmental characteristics (e.g., tidal exposure, substrate particle size, bulk organic matter, C:N, total organic carbon, and δ13Corg). Benthic areas receiving more carbon isotopically depleted organic matter sources (mean δ13Corg values < −23.2‰, C:N ratios >11), most likely from the terrestrial surface and some marine plankton, were dominated by Trochammina inflata, Bolivina spp., and Helenina anderseni. In contrast, benthic areas receiving more carbon isotopically enriched organic matter sources (mean δ13Corg values > −21.6‰, C:N ratios Spirophthalmidium emaciatum, Spirillina vivipara, Patellina corrugata, and Rotaliella arctica. The benthic foraminifera most distal from any cave entrances were dominated by taxa also known from the deep-sea (e.g., Rotaliella, Spirophthalmidium) in sediment with the lowest bulk organic matter content (mean: 6%), or taxa that prefer hard substrates and are potentially living attached to cave walls (Patellina, Spirillina). While physical groundwater characteristics (e.g., salinity, dissolved oxygen) are expected drivers of benthic ecosystems in KSEs, these results suggest that POM source, quantity, and delivery mechanisms (e.g., groundwater-seawater circulation mechanisms, terrestrial flux) play an important role in benthic habitat partitioning and the spatial variability of biogeochemical cycles in the oxygenated marine sector of KSEs.

In the mid-20th century, an inland brackish pond from Bermuda, known as Eve’s Pond, was filled with marine sediment from an adjacent coastal lagoon. At this time, an eyewitness reported “…sediment billowing out of the Green Bay Cave for days…”, which is a marine-dominated anchialine cave located proximal to the former location of Eve’s Pond (~200 m). The purpose of this study was to evaluate the potential impact of this infilling event on cave sedimentation and benthic meiofaunal communities, as proxied by the unicellular protists foraminifera that remain preserved in the sediment record. Eight sediment cores were collected from an underwater passage in Green Bay Cave in a transect towards the location where Eve’s Pond was surveyed in 1901 CE. The sediment cores were analyzed for visual and density changes (photography, X-radiography), textural variability, benthic foraminifera fauna and diversity, and radiocarbon dating. The recovered sediment cores mostly sampled a late Holocene carbonate mud facies that had been described during previous research in the cave, with benthic foraminiferal assemblages post-dating the onset of seawater circulating between the saline groundwater flooding the cave and the adjacent Harrington Sound ~1,900 years ago. However, two cores located further into the cave (cores 13 and 17) contain a carbonate sand layer with lagoon foraminifera that is anomalous with respect to the Holocene depositional history of the cave and is most likely related to the mid-20th century infilling of Eve’s Pond. Examination of these two cores showed that after the infilling event, the community of benthic foraminifera rapidly reverted to pre-impact assemblages with foraminiferal stygophiles (e.g., Spirophthalmidium emaciatum, Sigmoilina tenuis), which were not displaced by new colonizers introduced into the cave by the dredge spoils. We caution that the results cannot be extrapolated to the pelagic crustacean community, but the results suggest that this physical sedimentary disturbance only minimally impacted the benthic foraminifera community in the cave passages that were sampled.