Habitat Partitioning in the Marine Sector of Karst Subterranean Estuaries and Bermuda's Marine Caves: Benthic Foraminiferal Evidence

Abstract

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 <10), most likely from marine plankton transported through marine cave openings cave from adjacent coastal waters, were dominated by 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.

Changes in Organic Matter Deposition Can Impact Benthic Marine Meiofauna in Karst Subterranean Estuaries

“…Comparing the measured δ 13 C org and C:N values with previously reported values of potential OM sources (Lamb et al, 2006) further indicates that organic carbon preserved in the Aerolito sediment is derived from two primary sources: vascular C3 plants and marine POM/algae (Figure 5A). In comparison to similar coastal caves in Bermuda (Cresswell and van Hengstum, 2021), sediment in Aerolito preserved less marine-sourced organic carbon, like from marine algae (Figure 5A). Interestingly, sedimentary OM closer to the marine opening (downstream) received greater contributions from vascular plant detritus than those near the terrestrial opening (upstream) (Figure 5).…”

“…As such, benthic foraminifera were the model organism used to provide a proxy for any long-term changes in the benthic meiofaunal community in the passages proximal to the ocean of Aerolito Cave. After an assessment of the radiocarbon results (discussed further below), our research design included: 1) sampling both the oldest (basal core samples) and youngest (core-tops) parts of the stratigraphic record (n 13), and 2) comparing Aerolito foraminiferal assemblages to the dataset of recent benthic foraminifera in Bermudian underwater caves (Cresswell and van Hengstum, 2021; n 125).…”

“…To promote counting efficiency, four samples were split into representative aliquots using a wet-splitter prior to census counting (Scott and Hermelin, 1993). Foraminifera were wet counted and identified to the species level, with taxonomic identification based on taxonomic literature (Cushman, 1933;Loeblich and Tappan, 1987) and previous studies on cave foraminifera in Bermuda (van Hengstum and Little and van Hengstum, 2019;Cresswell and van Hengstum, 2021). The raw foraminiferal database of census counts from the 13 samples from Aerolito included 40 taxonomic units identified to the species level from 60 genera.…”

“…The foraminiferal results from Aerolito (n 13) were then compared to dataset of recent foraminiferal distributions in surface samples from Bermudian marine caves (n 125, Cresswell and van Hengstum, 2021) to create a new composite dataset of 138 samples. Taxonomic differences were evaluated at the genus level in the composite dataset to 1) promote equitable comparison between regions, and 2) taxonomic bias imparted by separate microscopists.…”

“…Taxonomic differences were evaluated at the genus level in the composite dataset to 1) promote equitable comparison between regions, and 2) taxonomic bias imparted by separate microscopists. While the genera Patellina and Patellinoides were originally differentiated in the samples from Aerolito Cave, they were subsequently grouped into a pseudo-Patellina genus for comparison to Bermuda (see discussion in Cresswell and van Hengstum, 2021). Biodiversity indices [Shannon-Weiner Index (H') and Fisher's Alpha (α)] for each sample was calculated from the raw counts in the final database using the "vegan" package.…”