Spatial distribution of secondary benthic invertebrate production in a north Norwegian fjord system

In line with principles of ecosystem based approach, ecosystem models are used as tools in marine management and conservation. For models aiming to quantify energy flows through ecosystems, production and productivity data are needed. Marine benthic invertebrates represent links in energy flow from primary producers to higher trophic levels, as well as in recycling of sedimentary organic material. Due to missing data, benthic invertebrates are often treated as single or few "bulk" groups in ecosystem models and this may hide important patterns for energy flow. An empirical model, based on correlations between secondary production and population features such as body size, living mode and taxon, in combination with temperature and depth, was used to estimate benthic productivity (P/B) and production in the North Norwegian Ullsfjord-Sørfjord system. This allows comparison between two systems closely coupled in geography, where a "map view" gives the impression that for the modelling and larger scale comparisons the extrapolation of data between the fjords is acceptable. However, different properties of the fjords affect their respective benthic communities. Ullsfjord represents the deeper outer part, with water-masses resembling the coastal current. A shallow sill separates it from Sørfjord, which is colder, shallower and more enclosed. Average production of the benthic fauna of Ullsfjord (from 2004) and Sørfjord (from 1999) was 118 and 185 kJ m-2yr-1, corresponding to a wet weight (WW) production of 89 and 78 g m-2yr-1, respectively. The community P/B ratio of Ullsfjord (0.69 y-1) is 1.6 times that of Sørfjord (0.42 y-1), whereas the biomass in Ullsfjord is much lower than in Sørfjord (172 kJ m-2 vs 441 kJ m-2). In Ullsfjord, the same stations were revisited in 2009. Using the same taxon specific P/B ratios as for 2004, WW production was estimated to 59 g m-2yr-1, which is 24% lower than 2009, based on a total biomass of 54 g WW. With P/B ratios closely linked to temperature, taxon and body size, relevant data have to be used for modeling purposes and caution should be taken when ratios are adopted from other systems. Biomass should be measured directly in the system of interest and, if possible, also for the time period in question. The benthic data have been included in ecosystem models for the Ullsfjord and Sørfjord systems using Ecopath. In a changing Arctic, community composition and temperature are likely to change and these models may aid in predicting ecosystem level effects.