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

Jacquelyn N Cresswell,Peter J Van Hengstum

doi:10.3389/fenvs.2020.594554

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 &lt; −23.2‰, C:N ratios &gt;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 &gt; −21.6‰, C:N ratios &lt;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.

Highlights

Karst subterranean estuaries (KSEs) are an environment 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
No new endemic species of foraminifera are described, but the identified benthic foraminiferal communities differ from those found in other Bermudian coastal environments
Seven communities of benthic foraminifera can be recognized at a dissimilarity index of 0.44 on the dendrogram produced from unconstrained Qmode cluster analysis (Figure 4), but this division increases to nine groupings if Community 5 is subdivided at a dissimilarity of index of ∼0.36

Summary

Introduction

Karst subterranean estuaries (KSEs) are an environment 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. Since the groundwater-ocean dynamics that create KSEs are linked to sea level, KSEs vertically migrate in the subsurface in response to orbitally-forced changes in eustatic sea level (van Hengstum et al, 2019) and this is represented in the fossil record (Proctor and Smart, 1991; van Hengstum et al, 2009; Rosso et al, 2015, 2018; Guido et al, 2017). Benthic foraminifera are eukaryotic protists that live among the meiofauna (0.062– 0.5 mm in size) in most marine settings, and they exhibit ecological zonation from physical environmental gradients (e.g., salinity, organic matter flux, dissolved oxygen). Individual foraminifera produce a simple shell, or test, that remains within the sediment after death, so statistically significant populations can be sampled by SCUBA divers during routine ecological surveys, or their fossil remains can preserve a record of prehistoric KSEs

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Conclusion