Benthos and oxygen depletion
There is a growing interesst in historical datasets, because only the comparison with historical biomass data and species compositions can reveal longterm effects and trends. This may also lead to a better understanding of complex systems.
From comparison with historical datasets, Ragnar Elmgren followed radical changes in the energy flux in the central Baltic Sea since 1900 as a consequence of increasing biomass in the shallow areas and a biomass reduction in the deep basins. In the western Baltic similar changes can also be identified, for example a dominance shift in Kiel Bay in depths between 10 and 20m. In these strata, the priapulid worm Halicryptus spinulosus which is resistant to oxygen depletion replaced other species.
Historical data made it possible to document such a shift in species composition for all basins in the Baltic Sea within the last 60 years. In general, a sequence in community patterns has been revealed:
There is a transition from a community dominated by long living Bivalves and Echinoderms towards community of Bivalvia and Polychaetes with strong fluctuations and occasional occurence of Halicryptus and Priapulus during stress situations and finally towards a fading community with reduced biomass, mainly consistent of small Polychaetes (Scoloplos, Capitella, Polydora and Heteromastus). The last community is mainly followed by an azoic phase with only occasional presence of vagile epifauna (Harmothoe, Crustaceans).
In historical comparisons, eutrophication caused an increase in biomass above the salinity stratification while the fauna below the stratification faded and fluctuated a lot. This trend showed a strong west to east gradient. So far only the eastern basins were affected by this development, but in 1989 the Arkona Sea started to show the same pattern. Far spreading Beggiatoa bacteria planes -indicators of anoxic environment- could be found close to dead Makrofauna, like Arctica islandica.
The new discovery of these Beggiatoa meadows by imaging techniques in large areas of the Baltic Sea basisn could be explained by the fact that the grab gears previously used always physically blew away the surface layer and Beggiatoa therefore did not appear in the samples at all.
The main quality measure for the sea floor of the Baltic seams to be the presence and absence of macrofauna. These organisms can be regarded as "cleaners" as they clean up detritus that sinks down from the watercolumn or is drifted in from lateral sources by advection. The crossing from one level to the other is characterized by thick layers of detritus on top of the sediments. The flake size of detritus is increasing from west to east, an indicator of degrading environmental conditions. If all oxygen in the sediments is used up by bacterial degrading, the Beggiatoa meadows stabilise and consolidate the detritus layer. Beggiatoa can only survive at the border between H2S and water with low oxygen concentrations. This is a negative feedback loop that starts as soon as the macrofauna has vanished. The incoming detritus, formerly the food of macrofauna organisms, needs to be degraded by bacteria in the sediments thorugh oxygen consumption. The oxygen depletion then prevents benthic larvae from recolonisation. At the end of the 80ies these 'dead zones' could be found from the northern Bornholm deep, the Gdansk deep, the southern part of the Gotland deep and the Landsort deep. The complete area was quantified by scientists from Finland during the 70ies with 70 000-100 000 km2, but there is a high fluctuation.
