Most deep seafloor communities, especially those from the ice-covered Arctic, are subject to severe food limitation as the prime source of food, particulate organic matter from the euphotic zone of the oceans, is increasingly attenuated with depth. One of the big questions in deep-sea ecology is how such communities are sustained despite this limited energy input. To address this issue, we have studied the benthic foodweb structure at HAUSGARTEN since 2004 via naturally occurring stable isotopes (δ¹⁵N, δ¹³C), which are enriched throughout the food chain.

Encompassing five trophic levels, the HAUSGARTEN foodweb is among the longest, indicating continuous recycling of organic material typical of food-limited deep-sea ecosystems. While the δ¹⁵N signatures ranged from 3.0% for Foraminifera to 21.4% for starfish (Poraniomorpha tumida) the majority of organisms occupies the 2^nd and 3^rd trophic level. Not surprisingly, predators/scavengers occupied the highest trophic level, followed by suspension feeders and deposit feeders. Ongoing foodweb modelling within the EU-project HERMIONE will allow us to discern energy flows at HAUSGARTEN.

Repeated analyses of muscle tissues of fish (Lycodes frigidus) from the HAUSGARTEN central station indicate a significant increase in δ¹⁵N and δ¹³C between 2004 and 2008. At the same time, the proportion of refractory phytodetrital matter in sediments increased, implying a relative decrease in fresh organic matter input and increased recycling. Such changes at the base of the foodweb may also lead to the increase in isotopic signatures observed in higher trophic levels. Time will tell if these trends are short-lived or related to the effects of climate forcing. 

Contact: M. Bergmann

Literature:

Bergmann, M., Dannheim, J., Bauerfeind, E. & M. Klages (2009). Trophic relationships along a bathymetric gradient at the deep-sea observatory HAUSGARTEN. Deep-Sea Research I 56 (3): 408-424.