Dumped munition affects the benthic macrofauna depending on the bottom structure: comparison of two Baltic Sea areas

Dumped marine munition is a widespread pollutant that occurs in most coastal areas of the oceans. German waters alone contain around 1.6 million tons of dumped munition spread in the North and Baltic seas. The current study investigated the benthic macrofauna structure at two WWII-era dumpsites in the Baltic Sea, Falshöft and Pelzerhaken. A total of 60 stations were sampled using 0.1 m² van-Veen grab along six transects, sieved through a 1 mm sieve and fixed. All species were identified, counted and weighted. Additionally, the following environmental variables were analyzed: distance to individual munition items, depth and its morphological derivatives, grain size, and munition compounds content including trinitrotoluene (TNT), amino-dinitrotoluene (ADNT), diamino-nitrotoluene (DANT), dinitrobenzene (DNB), and hexahydro-1,3,5-trinitro-1,3,5-triazinane (RDX). Random Forests algorithm applied to the dataset revealed that depth and, to a lesser degree, slope degree and distance to munition items affected the abundance, biomass and diversity at both munition dumpsites - as was previously shown for another munition dumpsite in the Lübeck Bay. Specifically, the model predicted elevated abundance and biomass within 50 m of the munition objects in muddy sediments at Pelzerhaken. A possible reason for the distance influence is that munition shells and crates represent hard substrata, which is otherwise rare in the Lübeck Bay region. At Falshöft, this effect was less prominent, probably due to abundant hard substrate of a natural origin. Munition compounds in the sediment showed no significant influence on the overall structure of macrofauna. Remediation plans and replacement of the munition by artificial hard substrate is discussed.