Sympagic-pelagic coupling and vertical export in two seasonally ice-covered fjords. Study of the physical and biological drivers in the sub-Arctic Ramfjorden (Norwegian mainland) and the Arctic Van Mijenfjorden (Svalbard archipelago) during early spring bloom.

Abstract

Fjords are important high latitudes ecosystems, many have beside planctonic ones, a unique ecosystem associated with seasonal ice cover. During the development of the spring season conditions, two major type of primary producers contribute highly to the biomass production: sea-ice algae inhabiting brine channels and bottom surface of the ice (sympagic algae) and phytoplankton, living in the water column (pelagic algae). The biomass produced by algae is the base of the food web in the fjord ecosystem and is commonly exported to the benthic realm and sequestered at the sea floor. It is still not resolved 1) if autotrophic biomass in the sea-ice, and suspended in or exported from the water column differ in fjords on a short time scale, and 2) how physical processes drive the sympagic-pelagic coupling and sinking biomass in fjords on different latitudes. This study compared the seasonally ice-covered sub-Arctic, Ramfjorden (RMF, 69 °N, 19 °E) in March 2019 with the high Arctic, Van Mijenfjorden (VMF, 77 °N, 16 °E) in April 2019 during the early spring bloom to investigate these questions. Physical oceanographic and meteorologic data, ice cores and water column samples were collected as well as deployment of short-term sediment trap brought together with the chlorophyll a (Chl a), particulate organic carbon and nitrogen (POC and PON) concentration and the algal community composition in the ice and water column. This revealed that the two fjord systems differed with regard to sympagic-pelagic coupling and export of biomass. Ramfjorden was more impacted by river run-off than Van Mijenfjorden, the truly Arctic fjord. The sea-ice in Ramfjorden was fresher and hold a lower autotrophic biomass and a less rich sea-ice algae community, than the thicker and more saline sea-ice in Van Mijenfjorden. Thus, while the export in Ramfjorden was driven by pelagic species, in the high Arctic Van Mijenfjorden a tighter coupling between the sympagic, pelagic system and the vertical export was found. The short time scale, meteorological and hydrographical factors (e.g., air temperature and under-ice currents) seemed important drivers on the sea-ice, suspended and sinking biomass. In conclusion, the sympagic-pelagic coupling and the link to the vertical export of biomass seems to be very different in seasonally ice-covered fjords on different latitudes and in fjords with unlike freshwater impact.