Short-term post-mortality scavenging and longer term recovery after anoxia in the northern Adriatic Sea

M Blasnig,B Riedel,L Schiemer,M Zuschin,M Stachowitsch

doi:10.5194/bg-10-7647-2013

Abstract

Abstract. The northern Adriatic Sea is one of nearly 500 areas worldwide suffering widespread mortalities due to anoxia. The present study documents post-anoxia macrofauna dynamics after experimentally inducing small-scale anoxia in 24 m depth (2 plots, each 50 cm × 50 cm). Time-lapse camera deployments examined short-term scavenging of the moribund and dead organisms (multi-species clumps consisting of sponges and ascidians) over two 3-day periods (August 2009: 71.5 h, September 2009: 67.5 h). Longer term recovery (days to 2 yr) in the same two plots was examined with an independent photo series. Scavengers arrived quickly and in a distinct sequence: demersal (Gobius niger, Serranus hepatus) and benthopelagic fishes (Diplodus vulgaris, Pagellus erythrinus), followed by hermit crabs (Paguristes eremita, showing a clear day/night rhythm in presence) and gastropods (Hexaplex trunculus). This sequence is attributed to the relative speeds and densities of the organisms. The sessile fauna was largely removed or consumed within seven (August plot) and 13 (September plot) days after anoxia, confirming our first hypothesis that decaying organisms are quickly utilised. The scavengers remained in dense aggregations (e.g. up to 33 P. eremita individuals at one time) as long as dead organisms were available. No recovery of sessile macroepibenthos macroepibenthos occurred in the experimental plots one and two years after anoxia, undermining our second hypothesis that small denuded areas are more rapidly recolonised. This study underlines the sensitivity of this soft-bottom community and supports calls for reducing additional anthropogenic disturbances such as fishing practices that further impede recolonisation and threaten benthic community structure and function over the long term.

Highlights

Dayton et al (1995) established that coastal marine ecosystems are the world’s most endangered habitats, and the Mediterranean is no exception, with documented impacts on biodiversity (Danovaro and Pusceddu, 2007; Coll et al, 2010)
The northern Adriatic is one of nearly 500 eutrophicationassociated dead zones worldwide, with a global area of more than 245,000 km2 (Diaz and Rosenberg, 2008). It combines many of the features known to promote late summer hypoxia and anoxia
The activities of seven scavengers were examined. These encompass all the species that entered the experimental quadrats after the induced anoxia

Summary

Introduction

Dayton et al (1995) established that coastal marine ecosystems are the world’s most endangered habitats, and the Mediterranean is no exception, with documented impacts on biodiversity (Danovaro and Pusceddu, 2007; Coll et al, 2010). The northern Adriatic is one of nearly 500 eutrophicationassociated dead zones worldwide, with a global area of more than 245,000 km (Diaz and Rosenberg, 2008). It combines many of the features known to promote late summer hypoxia and anoxia (shallow depth, soft sediments, semi-enclosed water body, stratification, high riverine input, high productivity, long water-residence time; Stachowitsch and Avcin, 1988; Malej, 1995; Stachowitsch, 1991). Combined with anthropogenic eutrophication and massive marine snow events, the northern Adriatic Sea is a sensitive ecosystem (Stachowitsch, 1986) and among the most heavily exploited and degraded seas worldwide (Lotze et al, 2011a). Bottom-water oxygen content has decreased from the mid-20th century (Justic, 1991) until the early 1990s, associated with extensive mortalities of epibenthic communities (Fedra et al, 1976; Stachowitsch, 1991; Hrs-Brenko et al, 1994)

Methods

Results

Conclusion