Description: Time-series studies of arctic marine ecosystems are rare. This is not surprising since polar regions are largely only accessible by means of expensive modern infrastructure and instrumentation. In 1999, the Alfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research (AWI) established the LTER (Long-Term Ecological Research) observatory HAUSGARTEN crossing the Fram Strait at about 79° N. Multidisciplinary investigations covering all parts of the open-ocean ecosystem are carried out at a total of 21 permanent sampling sites in water depths ranging between 250 and 5500 m. From the outset, repeated sampling in the water column and at the deep seafloor during regular expeditions in summer months was complemented by continuous year-round sampling and sensing using autonomous instruments in anchored devices (i.e., moorings and free-falling systems). The central HAUSGARTEN station at 2500 m water depth in the eastern Fram Strait serves as an experimental area for unique biological in situ experiments at the seafloor, simulating various scenarios in changing environmental settings. Long-term ecological research at the HAUSGARTEN observatory revealed a number of interesting temporal trends in numerous biological variables from the pelagic system to the deep seafloor. Contrary to common intuition, the entire ecosystem responded exceptionally fast to environmental changes in the upper water column. Major variations were associated with a Warm-Water-Anomaly evident in surface waters in eastern parts of the Fram Strait between 2005 and 2008. However, even after 15 years of intense time-series work at HAUSGARTEN, we cannot yet predict with complete certainty whether these trends indicate lasting alterations due to anthropologically-induced global environmental changes of the system, or whether they reflect natural variability on multiyear time-scales, for example, in relation to decadal oscillatory atmospheric processes.

Description: Seagrass meadows are among the most diverse and productive coastal ecosystems in the world. Currently, the accelerating loss of these habitats is recognized worldwide. In the southern Baltic Sea, a natural recovery of Zostera marina meadows has occurred after a dramatic reduction within the last century. The aim of this study is to understand if and how the recovering eelgrass meadows affect the functioning of benthic ecosystems. The trophic links within the benthic food webs in the seagrass meadows and bare sandy bottoms were depicted and compared. The trophic connections were examined by combining stable isotope (SI) composition (δ13C, δ15N) and fatty acid (FA) profiles of meio- and macrofauna consumers and of potential food sources (particulate organic matter, surface sediment organic matter, epiphytes, microphytobenthos/bacteria and macrophytes) in a Bayesian mixing model framework (MixSIAR). Significantly higher amounts of the FA bacterial marker (C18:1ɷ7) were observed in meiofauna (approximately 40%) than in the macrofauna (1% on average), suggesting that bacteria are an important part of the meiofauna diet. The mixing model results indicated that the benthic consumers in the vegetated habitat utilized more food sources (e.g., epiphytes in the diets of meiofauna and macrofaunal grazers) and thus had a more diverse diet. Macrofaunal omnivores relied to a larger degree on animal-derived organic matter in vegetated habitat, which could be linked to higher invertebrate prey availability. The results highlight the importance of recovering seagrass meadows in driving the mechanisms responsible for food web organization. Any type of change to the state of seagrass meadows is crucial to the functioning and stability of marine ecosystems.

Description: Seagrass meadows ecosystem engineering effects are correlated to their density (which is in turn linked to seasonal cycles) and often cannot be perceived below a given threshold level of engineer density. The density and biomass of seagrass meadows (Z. marina) together with associated macrophytes undergo substantial seasonal changes, with clear declines in winter. The present study aims to test whether the seasonal changes in the density of recovering seagrass meadows affect the benthic food webs of the southern Baltic Sea (Puck Bay). It includes meiofauna, macrofauna and fish of vegetated and unvegetated habitats in summer and winter seasons. Two levels of organization have been tested – species-specific diet preferences using stable isotopes (δ13C, δ15N) in Bayesian mixing models (MixSIAR) and the community-scale food web characteristics by means of isotopic niches (SIBER). Between-habitat differences were observed for grazers, as a greater food source diversity in species from vegetated habitats was noted in both seasons. Larger between-habitat differences in winter were documented for suspension/detritus feeders. The community-wide approach showed that the differences between the habitats were greater in winter than in summer (as indicated by the lower overlap of the respective isotope niches). Overall, the presence of seagrass meadows increased ecological stability (in terms of the range of food sources utilized by consumers) in the faunal assemblage, while invertebrates from unvegetated areas shifted their diet to cope with winter conditions. Therefore, as a more complex system, not sensitive to seasonal changes, Z. marina meadows create a stable habitat with high resilience potential.