Historical Ecology of the Northern Adriatic Sea

Identifying and evaluating environmental changes in the younger history of human civilisation is crucial for assessing the type, magnitude and rate of change and their underlying causes. It is also a prerequisite in defining historical baselines of "pristine" ecosystem conditions as targets for restoration and management efforts. The northern Adriatic is particularly suited to study ecosystem modifications under human pressure: it is among the most degraded marine ecosystems worldwide, with a long history of intense human impact that began in Roman times and culminated in strong eutrophication and heavy fishing pressure in recent decades. This makes it a model for studying the long-term evolution of (meta)community structure. This project is designed to investigate (1) the temporal ecological dynamic of benthic communities with a high biomass epifauna (HBE) over the last centuries, focusing on changes in taxonomic and functional composition, abundance and diversity, as well as (2) the preservation dynamic of hard-part producers (predominantly molluscs) in the surface sediments and in historical sediment layers. We will evaluate temporal changes in community composition by comparing living assemblages with surface death assemblages ("live-dead" analysis) and with consecutively older subsurface assemblages unaffected by major anthropogenic impacts. This approach will benefit from the unique opportunity to draw on older scientific surveys dating back to the 1930s. The sampling material will be gained from epifaunal surface samples as well as from 1m sediment cores taken at 7 stations in the northern Adriatic Sea. Assemblage age, temporal resolution (time averaging), and molluscan preservation rates will be quantified using radiocarbon-based dating coupled with amino acid racemization (AAR). This will yield the frequency distributions of post-mortem shell age for individual surface and subsurface death assemblages. Our study will be one of the first to reveal regional-scale and temporal variation in time-averaging. Taphonomic pathways of molluscan species will be quantifyied with standard taphofacies protocols. One goal is to identify the composition of pre-impact baseline communities, a current research frontier in taphonomy applied to conservation biology. We will date the timing of ecological turnover and test the impact of regional-scale nutrient increase and intense fishing on community structure and predation intensity. Furthermore we will identify the original distribution patterns of the HBE in pre-industrial times and determine the environmental conditions necessary to support HBE. This issue goes beyond regional interest to significantly improve our understanding of "retrograde" Paleozoic-type (i.e. macroepifauna-dominated) benthic communities worldwide. Based on these findings, we will identify regions that might serve as restoration baselines for the northern Adriatic Sea.

Marine sediments can preserve the shells and skeletons of organisms which lived in the sea decades to millennia ago. An investigation of these shell communities from various sediment layers thus opens a door into the history of marine ecosystems and can reveal major environmental turnovers which lead to changes in animal communities. In the frame of our project, we took more than 50 sediment cores from seven sites in the northern Adriatic Sea with a newly designed piston corer developed by an Austrian company in cooperation with the University of Vienna. They are all about 1.5m long and, depending on sedimentation rate, they cover vastly different time spans ranging from few decades to several millennia. Based on our quantitative data on the macro- and microfauna from these cores and on extensive age-dating of four bivalve species, we could identify major changes in the taxonomic and functional composition of benthic communities during the Holocene transgression. They are related to1) the rapidly rising sea level (until ca. 6000 y BP), 2) natural environmental changes under more or less stable sea level until a few hundred years ago, and 3) major changes over the past centuries, with strong amplification in the 20th century that we attribute to strong human impact (most notably input of nutrients, organic and inorganic pollutants, and destructive fishing techniques). The micro- and macrofauna showed similar responses to natural environmental changes in the course of the Holocene transgression and confirm eutrophication as the most important anthropogenic driver of community shifts. The macrofauna, however, more sensitively reflects the impact of low-oxygen conditions and destructive fishing gear. Distinct assemblages of sedentary animals living at the sediment surface developed only at certain periods and in certain regions of the study area, which are characterized by low sedimentation rates. Specifically, a conspicuous oyster bottom was visible in our sediment cores and on historical maps of the region, but due to the extensive use of destructive fishing gear this reef has meanwhile totally disappeared. Our results will help establish realistic baselines for restoration efforts for this ecologically particularly valuable bottom type. Based on outbreaks of Corbula gibba,an opportunistic bivalve that can cope with low-oxygen conditions on the seafloor, we could show that oxygen crises in the northern Adriatic Sea correlate with centennial-scale fluctuations in sea-surface temperature, indicating that such events were coupled with water-column stratification rather than with nutrient enrichment. These outbreaks represent long-term phenomena in the northern Adriatic ecosystem rather than novel states characteristic of the 20th century eutrophication. However, the late 20th century eutrophication further intensified the frequency of hypoxia and triggered local extinction of previously abundant suspension-feeders such as Turritella communis. Last but not least, invasive species brought by ships and aquaculture have been introduced in the Adriatic, further contributing to its abrupt change. This impact is likely older than usually assumed: we determined that an invasive bivalve arrived in the Adriatic Sea in the 1970s, although it was first detected in 2001. Such multi- decadal time-lags have important consequences for understanding how these species reached our seas and how they fit into and modify the new ecosystem.