No abstract

We investigated whether nutrient limitations of primary producers act upward through food webs only in terms of density effects or if there is a second pathway for nutrient limitation signals channelled upward to higher trophic levels. We used tritrophic food chains to assess the effects of nutrient-limited phytoplankters (the cryptophyte Rhodomonas salina) on herbivorous zooplankters (the calanoid copepod Acartia tonsa) and finally zooplanktivores (larval herring Clupea harengus) living on the herbivores. The primary producers' food quality had a significant effect on fish condition. Our experimental phosphorus-limited food chain resulted in larval fish with a significantly poorer condition than their counterparts reared under nitrogen-limited or nutrient-sufficient conditions. Our results show that mineral nutrient requirements of consumers have to be satisfied first before fatty acids can promote further growth. This challenges the match/mismatch hypothesis, which links larval fish survival probability solely to prey availability, and could imply that reduced nutrient releases into the environment may affect fish stocks even more severely than previously believed.

A series of in situ enrichment experiments was carried out at 1265 m water depth in the Sognefjord on the west coast of Norway in order to follow the short-term fate of freshly settled phytodetritus in a deep-sea sediment. For all experiments, a deep-sea benthic chamber lander system was used. In the lander chambers, a settling spring bloom was simulated by the injection of 0.2 g of freeze-dried Thalassiosira rotula, an equivalent of 1 g organic C m -2 . The algae were 98% 13 C-labeled, thus enabling us to follow the processing of the carbon by bacteria and macrofauna. Experiment duration varied from 8 h to 3 d. The total oxygen consumption of the sediments increased by approximately 25% due to particulate organic matter (POM) enrichment. Macrofauna organisms became immediately labeled with 13 C. After 3 d, 100% of the individuals sampled down to 10 cm sediment depth had taken up 13 C from the phytodetritus added. Bacterial uptake of the tracer was fast too, and even bacteria in deeper sediment layers had incorporated the fresh material within 3 d. Our study documents the rapid downward mixing of labile organic matter and the importance of macrofauna for this process. We present the first evidence for the immediate breakdown and incorporation of POM by bacteria even in deep sediment layers. Surprisingly, the initial processing of carbon was dominated by macrofauna, although the group comprises < 5% of the benthic biomass.Altogether, approximately 5% of the carbon added had been processed within 3 d, with the majority being released from the sediment as CO 2 . Due to the good comparability of our study site with midslope settings at continental margins, in general, we propose that the processes we observed are widespread at continental margins and are significant for the biogeochemical cycling of particulate matter on the slope.