Mesophotic Depth Biogenic Accumulations (“Biogenic Mounds”) Offshore the Maltese Islands, Central Mediterranean Sea

Or M Bialik,Aaron Micallef,Andrea Giulia Varzi,Alessandra Savini,Ruth Durán,Timothy Le Bas,Adam Gauci

doi:10.3389/fmars.2022.803687

Abstract

The mesophotic domain is a poorly explored part of the oceans, notably in the Mediterranean Sea. Benthic communities in these depths are not well documented and as such are under higher risk from anthropogenic impacts. Hard substrate habitats in this depth window are not common and are a key ecotope. The Malta Plateau in the central Mediterranean, which is characterized by low sedimentation rates, offers a potentially unexplored domain for these features. Bathymetric and backscatter data offshore of the eastern coast of the island of Malta were used to identify &gt; 1,700 small structures in mesophotic depths. These structures were verified to be biogenic mounds by dives. The mounds extend from several meters to tens of meters in diameter and occur in two main depth windows −40 to 83 meters below present sea level (mbpsl) and 83–120 mbpsl—each formed probably in a different stage during the last glacial cycle. The mounds are composed of interlocking bioconstruction by encrusting organisms and are colonized by sponges and various cold water corals (most of which are protected; e.g., Madrepora oculata). This unique and important habitat is currently under grave threat by human activity, most immediately by trawling and anchoring activity.

Highlights

We have registered hundreds of small elevated features offshore eastern Malta which we identified as biogenic mounds
We propose that the 40–83 mbpsl cluster comprises drowned reefs formed upon terraces excavated in the last glacial cycle
These structures are ecologically relevant and a unique ecotope that would have potentially required prolonged

Summary

Introduction

Biogenic MoundsThe term “mound” is commonly used in literature to indicate biogenic-mediated features forming submerged positive reliefs (from a few to hundreds of meters in height), composed of a variety of dominant organisms capable of trapping or baffling fine sediments (e.g., cold-water corals, oysters, bryozoans, polychaetes). The terminology used to describe mound-like biogenic submarine landforms in cold water is inconsistent across literature. Mesophotic Mounds Offshore Malta mounds” and others are frequently used to refer to “mounded” bioconstructions developed in temperate seas (Greene et al, 1999; Wood, 2001; Riding, 2002; Bare et al, 2010). Comparative analysis of these features, is critical but difficult. In order to avoid adding confusion to the terminologies already in use, and to document the environmental relevance that biogenic landforms seem to hold in mesophotic depths (here defined as 30–150 mbpsl, meters below the present sea level), we use the general terms “biogenic mound” to describe the observations or “mounded bioconstruction” when referring to the growth process

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