Species diversity affects ecosystem structure and mass flows in fjords

Latitude and temperature have been hypothesized to influence species richness, diversity and trophic control (top-down vs. bottom-up) in marine ecosystems. Ecosystem structures and mass flows of two moderately exploited fjord ecosystems with different temperatures and species diversity were compared. The Ullsfjord and Sørfjord systems (69°-70° N) are located between the relatively simple, low-diversity arctic Barents Sea in the north (71°-80° N) and the more species-rich and temperate North Sea (51°-62° N) in the south. Ullsfjord is the outer part of the fjord system and is deeper, warmer and more diverse than Sørfjord. Ecopath mass-balance models containing 40 ecological groups were developed for Ullsfjord and Sørfjord for the time period 1993-96. To obtain input data, abundance and diet of top-predators, fish, pelagic and benthic invertebrates were investigated. In the more diverse Ullsfjord system, large krill and pelagic shrimps were abundant and lower trophic level groups (TL<3) had the highest keystoneness, suggesting importance of bottom-up control. In contrast, large cod had the highest keystoneness and a large top-down effect as predator on small fishes and the larger crustacean groups in Sørfjord. For the diverse benthic invertebrates, the warmer and faster system (Ullsfjord) had higher mortality rates, shorter life spans and lower biomass (6.3 vs. 9.2 g C m−2) than the colder system (Sørfjord), but production (3.2 vs. 3.6 g C m−2  year−1) was similar in the two systems suggesting bottom-up control of benthic invertebrates.